Showing 1–50 of 75 results for author: Pacuski, W

Thickness-dependent degradation and optical access in epitaxial 2H-MoTe2 protected by metallic capping layers

Authors: Wojciech Ryś, Iaroslav Lutsyk, Michał Piskorski, Maxime Le Ster, Maciej Rogala, Paweł Dąbrowski, Paweł Krukowski, Katarzyna Ranoszek-Soliwoda, Jarosław Grobelny, Zuzanna Ogorzałek-Sory, Wojciech Pacuski, Janusz Sadowski, Marta Gryglas-Borysiewicz, Karol Szałowski, Paweł J. Kowalczyk

Abstract: We investigate degradation pathways and surface protection strategies for epitaxial 2H-MoTe2 films grown by molecular beam epitaxy on GaAs(111)B substrates. Using X-ray photoelectron spectroscopy, scanning tunneling microscopy, atomic force microscopy (AFM), Kelvin probe microscopy (KPM), Raman spectroscopy, and density functional theory (DFT) calculations, we analyse the structural, chemical, and… ▽ More We investigate degradation pathways and surface protection strategies for epitaxial 2H-MoTe2 films grown by molecular beam epitaxy on GaAs(111)B substrates. Using X-ray photoelectron spectroscopy, scanning tunneling microscopy, atomic force microscopy (AFM), Kelvin probe microscopy (KPM), Raman spectroscopy, and density functional theory (DFT) calculations, we analyse the structural, chemical, and electronic evolution of MoTe2 films protected by Co and Ni capping layers. XPS shows that the metallic caps effectively suppress oxidation during short-term air transfer, while the films exhibit a pronounced Te-rich near-surface composition. However, the caps become increasingly difficult to remove with exposure time, suggesting the gradual development of interfacial bonding, likely promoted by excess tellurium and defect-rich MoTe2 interfaces. AFM and KPM reveal strong thickness-dependent changes with time (ageing), with ultrathin regions showing markedly different contact-potential evolution from thicker films. DFT calculations for pristine and oxidized MoTe2 based structures qualitatively support the sensitivity of the work function and density of states to thickness and surface chemistry. Raman measurements performed through approximately 20 nm thick metallic caps demonstrate that such layers can provide partial optical access to the protected material. Additional AFM and Raman observations suggest that Te-rich nanostructures may form locally under laser illumination or near edges of the sample and mechanically formed chips. These results establish practical guidelines for the short-term protection, transfer, delamination, and optical characterization of air-sensitive MoTe2 and related van der Waals materials. △ Less

Submitted 19 August, 2026; originally announced August 2026.

Comments: Main text: 29 pages, 5 figures; supporting information: 4 pages, 3 figures, 2 tables

Moire based strain analysis in wurtzite GaAs -- rock-salt (Pb,Sn)Te core-shell nanowires grown by molecular beam epitaxy

Authors: Maciej Wojcik, Sania Dad, Piotr Dziawa, Slawomir Kret, Wojciech Pacuski, Janusz Sadowski

Abstract: We investigate core/shell GaAs/(Pb,Sn)Te nanowire nanoheterostructures with wurtzite (wz) GaAs cores and (Pb,Sn)Te topological crystalline insulator shells. The nanostructures have been grown by molecular beam epitaxy using two distinct MBE systems dedicated to III-V, and IV-VI semiconductors. The interface structure of wz-GaAs/(Pb,Sn)Te nanowires is investigated using high resolution transmission… ▽ More We investigate core/shell GaAs/(Pb,Sn)Te nanowire nanoheterostructures with wurtzite (wz) GaAs cores and (Pb,Sn)Te topological crystalline insulator shells. The nanostructures have been grown by molecular beam epitaxy using two distinct MBE systems dedicated to III-V, and IV-VI semiconductors. The interface structure of wz-GaAs/(Pb,Sn)Te nanowires is investigated using high resolution transmission electron microscopy, scanning transmission electron microscopy and geometric phase analysis. Misfit dislocations and moiré fringes are observed as a direct result of the lattice mismatch between the core and the shell materials, and used to estimate strain in crystalline topological insulator shells. Our results point to a possibility of using moiré patterns analysis as an alternative, for estimating strain in the core-shell nanowire structures. △ Less

Submitted 8 May, 2026; v1 submitted 7 May, 2026; originally announced May 2026.

Comments: 11 pages, 5 figures

Strain-driven spin mixing and dark-exciton recombination in a neutral Ni2+ doped quantum dot

Authors: K. E. Polczynska, S. Karouaz, W. Pacuski, L. Besombes

Abstract: We investigate the optical properties of neutral excitons in CdTe/ZnTe quantum dots containing a single Ni2+ ion. We show that the photoluminescence spectra provide a direct spectroscopic signature of strain induced mixing of the Ni2+ spin states. A misalignment between the principal axis of the local strain tensor and the quantum dot growth direction reorients the spin quantization axis of the ma… ▽ More We investigate the optical properties of neutral excitons in CdTe/ZnTe quantum dots containing a single Ni2+ ion. We show that the photoluminescence spectra provide a direct spectroscopic signature of strain induced mixing of the Ni2+ spin states. A misalignment between the principal axis of the local strain tensor and the quantum dot growth direction reorients the spin quantization axis of the magnetic ion, reducing the hole Ni2+ exchange interaction at low magnetic field and giving rise to photoluminescence replicas around the partially linearly polarized bright-exciton transitions. A longitudinal magnetic field restores the circularly polarized optical selection rules, allowing the three spin projections S_z = 0, +-1 of the Ni2+ ion to be spectrally resolved. Dark exciton emission appears on the low energy side of the spectra and is dominated at low field by transitions involving spin flips of the magnetic ion. An effective spin Hamiltonian including strain orientation and valence band mixing reproduces the magnetic field evolution of both bright and dark exciton spectra. These results highlight the key role of the local strain environment in determining the spin exciton coupling of transition metal dopants in semiconductor quantum dots. △ Less

Submitted 18 March, 2026; originally announced March 2026.

Exciton coherence propagation measured with non-local four-wave mixing micro-spectroscopy

Authors: Mateusz Raczyński, Amadeusz Dydniański, Karolina Ewa Połczyńska, Gabriela Szwed, Adam Szczerba, Jin-Woo Jung, Gilles Nogues, Wolfgang Langbein, Piotr Kossacki, Wojciech Pacuski, Jacek Kasprzak

Abstract: Coherence transfer is a multi-disciplinary topic of interest, including chemistry, biology and physics. In quantum technologies, achieving non-local coherent coupling between solid-state qubits is of the utmost importance. Here, we demonstrate that excitons - i.e. electron-hole pairs bound by the Coulomb force within a quantum well - can act as a medium for mesoscopic optical coherence transfer in… ▽ More Coherence transfer is a multi-disciplinary topic of interest, including chemistry, biology and physics. In quantum technologies, achieving non-local coherent coupling between solid-state qubits is of the utmost importance. Here, we demonstrate that excitons - i.e. electron-hole pairs bound by the Coulomb force within a quantum well - can act as a medium for mesoscopic optical coherence transfer in semiconductors. To this end, we use a femtosecond laser pulse to resonantly generate excitons within the light cone. These excitons can then either recombine radiatively or scatter out of the light cone, gaining an in-plane momentum in the process. In samples without disorder, such as the CdTe quantum wells used here, the resulting fast excitons can diffuse over mesoscopic distances before recombining radiatively. Using coherent nonlinear micro-spectroscopy, we carry out exciton time-of-flight measurements. Specifically, we monitor the spatio-temporal propagation of launched exciton wave packets, selectively observing their coherence or density on a scale of up to 10$\,μ$m. Our proof-of-principle experiment demonstrates that free excitons inherit a phase modulation from the optical pulsed excitation and can generate coherent links within excitonic circuits, offerring a higher level of miniaturisation and compactness than photonic or polaritonic architectures. △ Less

Submitted 12 January, 2026; originally announced January 2026.

Hole to Electron Crossover in a (Cd,Mn)Te Quantum Well through Surface Metallization

Authors: Amadeusz Dydniański, Mateusz Raczyński, Aleksandra Łopion, Tomasz Kazimierczuk, Jacek Kasprzak, Karolina Ewa Połczyńska, Wojciech Pacuski, Piotr Kossacki

Abstract: In this work we look into how the contact material influences the local charge properties of a p-type CdTe-based quantum well. We study five metals deposited as 10 nm layers on the sample surface: Au, Ag, Cr, Ni and Ti. We use magneto-spectroscopy to discriminate their charge states through monitoring the Zeeman shifts at singlet-triplet transitions. Most tested metals retain the original p-type o… ▽ More In this work we look into how the contact material influences the local charge properties of a p-type CdTe-based quantum well. We study five metals deposited as 10 nm layers on the sample surface: Au, Ag, Cr, Ni and Ti. We use magneto-spectroscopy to discriminate their charge states through monitoring the Zeeman shifts at singlet-triplet transitions. Most tested metals retain the original p-type of the QW, while gold and nickel coverage flips the local doping to n-type. This is attributed to a robust bonding of these two metals to the semiconductor, efficiently passivating its surface and thus improving electron diffusion from the metal to the quantum well. △ Less

Submitted 4 December, 2025; originally announced December 2025.

Magnetic field induced polarization enhancement in the photoluminescence of MBE-grown WSe$_2$ layers

Authors: Maksymilian Kuna, Mateusz Raczyński, Julia Kucharek, Takashi Taniguchi, Kenji Watanabe, Tomasz Kazimierczuk, Wojciech Pacuski, Piotr Kossacki

Abstract: We report an experimental study of the magnetic-field dependence of the optically pumped valley polarization in an epitaxial tungsten diselenide (WSe$_2$) monolayer grown by molecular-beam epitaxy (MBE) on a hexagonal boron nitride (hBN) substrate. Circularly polarized photoluminescence (PL) measurements reveal that applying a weak out-of-plane magnetic field, on the order of 0.1 T, dramatically i… ▽ More We report an experimental study of the magnetic-field dependence of the optically pumped valley polarization in an epitaxial tungsten diselenide (WSe$_2$) monolayer grown by molecular-beam epitaxy (MBE) on a hexagonal boron nitride (hBN) substrate. Circularly polarized photoluminescence (PL) measurements reveal that applying a weak out-of-plane magnetic field, on the order of 0.1 T, dramatically increases the effectiveness of the optical orientation of the emission associated with defect-bound localized excitons. We compare the obtained results with the earlier studies on the reference exfoliated monolayers, discussing both qualitative similarity as well as quantitative differences. Our observations are further supplemented by the results of time-resolved PL measurements, which confirm the pseudospin relaxation time of approximately 25 ps, a value significantly shorter than the $\approx$100 ps previously reported for mechanically exfoliated samples. △ Less

Submitted 2 December, 2025; originally announced December 2025.

Comments: 6 pages, 3 figures, 53rd International School & Conference on the Physics of Semiconductors "Jaszowiec 2025" Proceeding in Solid State Communications, pre-print

Journal ref: Solid State Communications 409C (2026) 116263

Multiplexed back focal plane imaging with on-chip integrated microlens array

Authors: Magdalena Furman, Marcin Muszyński, Przemysław Oliwa, Łukasz Zinkiewicz, Aleksander Bogucki, Jacek Szczytko, Piotr Wasylczyk, Wojciech Pacuski, Mateusz Król, Barbara Piętka

Abstract: On-chip optical architectures that enable angle-resolved spectroscopy are essential for advancing photonic platforms towards low-volume, scalable, and cryo-compatible devices. Here, we introduce spatially resolved momentum-space imaging using arrays of 3D-printed microlenses directly integrated onto semiconductor optical microcavities. Each microlens functions as an independent optical element wit… ▽ More On-chip optical architectures that enable angle-resolved spectroscopy are essential for advancing photonic platforms towards low-volume, scalable, and cryo-compatible devices. Here, we introduce spatially resolved momentum-space imaging using arrays of 3D-printed microlenses directly integrated onto semiconductor optical microcavities. Each microlens functions as an independent optical element with a high numerical aperture (approx. 0.95), enabling parallel back focal plane imaging across, in our implementation, 64 distinct locations. This approach eliminates the need for bulky microscope objectives while maintaining broad wavevector access, even under cryogenic conditions. We demonstrate its versatility across various cavity systems, including dielectric planar resonators, GaAs-based polariton microcavities, and CdTe platforms supporting nonequilibrium Bose-Einstein condensation of exciton-polaritons. The microlenses not only enhance collection efficiency but also enable tightly focused excitation, yielding an order of magnitude reduction in condensation thresholds compared with conventional setups. Our results establish 3D-printed microlens arrays as a compact, versatile, integrated platform for next-generation angle-resolved spectroscopy in nanophotonics and quantum materials. △ Less

Submitted 10 October, 2025; originally announced October 2025.

Comments: 23 pages, 16 figures

Wurtzite MnSe as a barrier for CdSe quantum wells with built-in electric field

Authors: M. J. Grzybowski, W. Pacuski, J. Suffczyński

Abstract: Altermagnetic materials have attracted a lot of attention recently due to the numerous effects, which have an application potential and occur due to the spin-split band structure coexisting with the compensated magnetic order. Incorporation of such intriguing compounds into low-dimensional structures represents an important avenue towards exploiting and enhancing their functionalities. Prominent e… ▽ More Altermagnetic materials have attracted a lot of attention recently due to the numerous effects, which have an application potential and occur due to the spin-split band structure coexisting with the compensated magnetic order. Incorporation of such intriguing compounds into low-dimensional structures represents an important avenue towards exploiting and enhancing their functionalities. Prominent examples of this group are semiconductors well suited to the band-gap engineering strategies. Here, we present for the first time visible-light-emitting CdSe quantum wells, in which wurtzite MnSe as an alermagnetic candidate plays the role of a barrier. Photoluminescence experiments with temporal resolution demonstrate that in such quantum wells, a built-in electric field is present and strongly influences the energies of the emitted photons, the dynamics of recombination, and excitation power dependence. Numerical simulations allow us to estimate that the magnitude of the electric field is 14MV/m. We anticipate that such quantum wells offer potential to probe the barrier properties and that wurtzite MnSe is an interesting platform to study the interplay of the altermagnetism and built-in electric field. △ Less

Submitted 25 August, 2025; originally announced August 2025.

Influence of local strain on the optical probing of a Ni$^{2+}$ spin in a charged self-assembled quantum dot

Authors: K. E. Polczynska, S. Karouaz, W. Pacuski, L. Besombes

Abstract: This study explores the optical properties of quantum dots doped with a Ni$^{2+}$ ion that interacts with a charged exciton. Systematic magneto-optical analysis reveals that the strain distribution at the Ni$^{2+}$ site significantly influences its spin structure. In positively charged dots dominated by in-plane biaxial strain, the three spins states of the Ni$^{2+}$ (S$_z$=0, S$_z$=$\pm$1) can be… ▽ More This study explores the optical properties of quantum dots doped with a Ni$^{2+}$ ion that interacts with a charged exciton. Systematic magneto-optical analysis reveals that the strain distribution at the Ni$^{2+}$ site significantly influences its spin structure. In positively charged dots dominated by in-plane biaxial strain, the three spins states of the Ni$^{2+}$ (S$_z$=0, S$_z$=$\pm$1) can be observed and the magneto-optical spectra enables a local strain anisotropy to be determined. However, in most of the dots, lower-symmetry strain mixes all the Ni$^{2+}$ spin states, thereby increasing the number of observed optical transitions. In charged dots, we identify optical transitions that share a common excited state. They form a series of $Λ$ levels systems that can be individually addressed optically to determine the energy level structure. Magneto-optical measurements demonstrate that the hole-Ni$^{2+}$ exchange interaction is antiferromagnetic and considerably stronger than the electron-Ni$^{2+}$ interaction. A spin-effective model that incorporates local strain orientation can successfully reproduce key experimental results. Furthermore, we demonstrate that low-symmetry terms in the hole-Ni$^{2+}$ exchange interaction must be considered in order to accurately describe the emission spectra details in a magnetic field. △ Less

Submitted 20 August, 2025; originally announced August 2025.

Emission enhanced exciton-polariton condensates with optical feedback

Authors: R. Mirek, M. Furman, A. Opala, M. Król, W. Pacuski, J. Szczytko, H. Sigurðsson, B. Piętka

Abstract: Optical feedback is a well-known method of controlling laser dynamics, which has been widely studied in photonic systems to induce complex behaviors such as chaos or enhanced coherence. However, its application to systems in the strong light-matter coupling regime remains unexplored. In this work, we introduce a delayed optical feedback loop into a nonresonantly pumped polariton condensate. By fee… ▽ More Optical feedback is a well-known method of controlling laser dynamics, which has been widely studied in photonic systems to induce complex behaviors such as chaos or enhanced coherence. However, its application to systems in the strong light-matter coupling regime remains unexplored. In this work, we introduce a delayed optical feedback loop into a nonresonantly pumped polariton condensate. By feeding part of the emission back into the cavity to seed the next condensate, we observe a strong increase in the output intensity, up to 110%. We explain this effect using a classical rate equation model for the condensate coupled to excitonic reservoirs. Our results evidence that polariton condensates can respond strongly to optical feedback, congruent with well known polariton amplification techniques using resonant pump-probe setups. Our method opens new possibilities for using polariton feedback to connect multiple condensates and can be an essential step toward neuromorphic computing based on recurrent signaling in photonic systems. △ Less

Submitted 27 July, 2025; originally announced July 2025.

$WSe_2$ Monolayers Grown by Molecular Beam Epitaxy on hBN

Authors: Julia Kucharek, Mateusz Raczyński, Rafał Bożek, Anna Kaleta, Bogusława Kurowska, Marta Bilska, Sławomir Kret, Takashi Taniguchi, Kenji Watanabe, Piotr Kossacki, Mateusz Goryca, Wojciech Pacuski

Abstract: A three-step process was developed for growing high-quality, optically uniform WSe2 monolayers by molecular beam epitaxy (MBE) with advantage of using hexagonal boron nitride (hBN). The process was optimized to maximize the efficiency of photoluminescence and promote formation of hexagonal WSe2 domains. Atomic force microscopy (AFM) was employed to estimate the dispersion of WSe2 hexagonal domains… ▽ More A three-step process was developed for growing high-quality, optically uniform WSe2 monolayers by molecular beam epitaxy (MBE) with advantage of using hexagonal boron nitride (hBN). The process was optimized to maximize the efficiency of photoluminescence and promote formation of hexagonal WSe2 domains. Atomic force microscopy (AFM) was employed to estimate the dispersion of WSe2 hexagonal domains orientation. Monolayer character of the film was identified using optical methods and verified with high-resolution transmission electron microscopy (TEM) cross-section. Temperature-and-magnetic-field-dependent studies revealed the behaviour of exciton complexes to be analogical to that of exfoliated counterparts. Direct growth on hBN combined with uniform optical response proves this WSe2 superior to mechanically exfoliated WSe2 in terms of convenience of use and reproducibility. Provided results establish a significant progress in optical quality of epitaxially grown transition metal dichalcogenides (TMDs) monolayers and fabrication of large-scale functional devices. △ Less

Submitted 12 July, 2025; originally announced July 2025.

Comments: Article and SI, 18 pages in total, 10 figures in total

Journal ref: Nano Letters 2025

Photoluminescence efficiency of MBE-grown MoSe$_2$ monolayers featuring sharp excitonic lines and diverse grain structures

Authors: Mateusz Raczyński, Julia Kucharek, Kacper Oreszczuk, Aleksander Rodek, Tomasz Kazimierczuk, Rafał Bożek, Takashi Taniguchi, Kenji Watanabe, Wojciech Pacuski, Piotr Kossacki

Abstract: Recent studies have demonstrated that using h-BN as a substrate for the growth of transition metal dichalcogenides can significantly reduce excitonic linewidths. However, many other optical parameters still require optimization. In this work, we present a detailed study of the low-temperature photoluminescence efficiency of MBE-grown MoSe$_2$ monolayers on h-BN substrates, comparing them to state-… ▽ More Recent studies have demonstrated that using h-BN as a substrate for the growth of transition metal dichalcogenides can significantly reduce excitonic linewidths. However, many other optical parameters still require optimization. In this work, we present a detailed study of the low-temperature photoluminescence efficiency of MBE-grown MoSe$_2$ monolayers on h-BN substrates, comparing them to state-of-the-art exfoliated monolayers encapsulated in h-BN. We demonstrate that a quantitative comparison between samples requires accounting for interference effects and Purcell enhancement or suppression of the emission. By accounting for these effects in both photoluminescence and Raman signals, we show that the overall intrinsic luminescence efficiency is proportional to the sample coverage. Consequently, we find that exciton diffusion and edge effects are negligible in spectroscopy of MBE-grown samples, even for nanometer-sized crystals. △ Less

Submitted 20 August, 2025; v1 submitted 24 February, 2025; originally announced February 2025.

Comments: 9 pages, 9 figures

Optical bound states in the continuum in subwavelength gratings made of an epitaxial van der Waals material

Authors: Emilia Pruszyńska-Karbownik, Tomasz Fąs, Katarzyna Brańko, Dmitriy Yavorskiy, Bartłomiej Stonio, Rafał Bożek, Piotr Karbownik, Jerzy Wróbel, Tomasz Czyszanowski, Tomasz Stefaniuk, Wojciech Pacuski, Jan Suffczyński

Abstract: High refractive index (4.4 at 1100 nm), negligibly small absorption in near-infrared spectral range, and ease of processing make MoSe$_2$ a perfect material for applications in near-infrared photonics. So far, implementation of MoSe$_2$-based photonic structures has been hindered by the lack of large surface MoSe$_2$ substrates. The use of molecular beam epitaxy allows the production of homogeneou… ▽ More High refractive index (4.4 at 1100 nm), negligibly small absorption in near-infrared spectral range, and ease of processing make MoSe$_2$ a perfect material for applications in near-infrared photonics. So far, implementation of MoSe$_2$-based photonic structures has been hindered by the lack of large surface MoSe$_2$ substrates. The use of molecular beam epitaxy allows the production of homogeneous layers of MoSe$_2$ with a few-inch surface and a thickness controlled at the sub-nm level. In the present work, we design by theoretical calculations and fabricate by a simple lithography process an ultrathin subwavelength grating out of 42-nm thick, epitaxially-grown MoSe$_2$ layer. Our polarization-resolved reflectivity measurements confirm that the gratings host a peculiar type of a confined optical mode that is a bound state in the continuum. Moreover, the fabricated structures enhance the efficiency of the third harmonic generation by over three orders of magnitude as compared to the unstructured MoSe$_2$ layer. The presented results are promising for the realization of flat, ultra-compact devices for lasing, wavefront control, and higher-order topological states of the light. △ Less

Submitted 5 February, 2025; originally announced February 2025.

Carrier localization in defected areas of (Cd, Mn)Te quantum well investigated via Optically Detected Magnetic Resonance employed in the microscale

Authors: Amadeusz Dydniański, Aleksandra Łopion, Mateusz Raczyński, Tomasz Kazimierczuk, Karolina Ewa Połczyńska, Wojciech Pacuski, Piotr Kossacki

Abstract: In this work, we study the impact of carrier localization on three quantities sensitive to carrier gas density at the micrometer scale: charged exciton (X+) oscillator strength, local free carrier conductivity, and the Knight shift. The last two are observed in a micrometer-scale, spatially resolved optically detected magnetic resonance experiment (ODMR). On the surface of MBE-grown (Cd,Mn)Te quan… ▽ More In this work, we study the impact of carrier localization on three quantities sensitive to carrier gas density at the micrometer scale: charged exciton (X+) oscillator strength, local free carrier conductivity, and the Knight shift. The last two are observed in a micrometer-scale, spatially resolved optically detected magnetic resonance experiment (ODMR). On the surface of MBE-grown (Cd,Mn)Te quantum well we identify defected areas in the vicinity of dislocations. We find that these areas show a much lower conductivity signal while maintaining the same Knight shift values as the pristine areas of the quantum well. We attribute this behavior to carrier localization in the defected regions. △ Less

Submitted 13 December, 2024; originally announced December 2024.

Journal ref: Solid State Communications (2024): 115755

Single vanadium ion magnetic dopant in an individual CdTe/ZnTe quantum dot

Authors: Karolina Ewa Połczyńska, Tomasz Kazimierczuk, Piotr Kossacki, Wojciech Pacuski

Abstract: We present the basic properties of a new physical system: an individual V2+ ion embedded into an individual quantum dot. The system is realized utilizing molecular beam epitaxy and it is observed using a low-temperature polarization-resolved magneto-photoluminescence. The nature of the system is confirmed by observation of the excitonic lines split due to the interactions of a vanadium ion with ca… ▽ More We present the basic properties of a new physical system: an individual V2+ ion embedded into an individual quantum dot. The system is realized utilizing molecular beam epitaxy and it is observed using a low-temperature polarization-resolved magneto-photoluminescence. The nature of the system is confirmed by observation of the excitonic lines split due to the interactions of a vanadium ion with carriers confined in a CdTe/ZnTe quantum dot. Observed data are explained by the numerical modeling which includes s,p-d exchange interaction, Zeeman splitting of the exciton and the ion, diamagnetic shift, and the presence of shear strain within the quantum dot. The fundamental state of vanadium exhibits a spin +/- 1/2 making this system a textbook localized qubit. △ Less

Submitted 11 March, 2026; v1 submitted 4 September, 2024; originally announced September 2024.

Comments: License updated

Journal ref: Phys. Rev. B 111, 085428 (2025)

Colossal magneto-excitonic effects in 2D van der Waals magnetic semiconductor CrSBr

Authors: R. Komar, M. Goryca, A. Łopion, M. Rybak, T. Woźniak, M. Raczyński, K. M. Gałązka, K. Mosina, A. Söll, Z. Sofer, W. Pacuski, C. Faugeras, M. Birowska, P. Kossacki, T. Kazimierczuk

Abstract: 2D magnetic semiconductors, which intrinsically couple a rich landscape of magnetic orders with tightly bound electron-hole pairs (excitons), present an exciting platform to investigate the interplay between optical and magnetic phenomena at the atomic scale. In such systems, the strength of magneto-optical effects determines how deeply the magnetic properties can be revealed. Here, we report the… ▽ More 2D magnetic semiconductors, which intrinsically couple a rich landscape of magnetic orders with tightly bound electron-hole pairs (excitons), present an exciting platform to investigate the interplay between optical and magnetic phenomena at the atomic scale. In such systems, the strength of magneto-optical effects determines how deeply the magnetic properties can be revealed. Here, we report the observation of remarkably strong magneto-excitonic effects in the 2D magnetic semiconductor CrSBr that allow probing its magnetic order with unprecedented sensitivity. By investigating optical transitions above the fundamental exciton energy, we discover a massive spectral shift approaching 100 meV under applied magnetic fields - an order of magnitude larger than previously observed magneto-excitonic responses. Our comprehensive magneto-optical experiments accompanied by detailed DFT calculations indicate the possible origin of the transitions exhibiting such intriguing behavior. These findings open avenues for exploiting magneto-excitonic phenomena at newly accessible regimes, enabling novel opto-spintronic applications previously limited by weak magnetic responses. △ Less

Submitted 8 June, 2026; v1 submitted 30 August, 2024; originally announced September 2024.

Comments: list of authors updated; in review in Nature Communications, Supplementary Material included

Impact of the Hole Gas on Optically Detected Magnetic Resonance in (Cd,Mn)Te Based Quantum Well

Authors: Aleksandra Łopion, Aleksander Bogucki, Mateusz Raczyński, Zuzanna Śnioch, Karolina E. Połczyńska, Wojciech Pacuski, Tomasz Kazimierczuk, Andrzej Golnik, Piotr Kossacki

Abstract: Optically detected magnetic resonance (ODMR) is a useful technique for studying interactions between local spins (magnetic ions) and carrier gas. We present the ODMR study of single (Cd,Mn)Te/(Cd,Mg)Te quantum wells (QWs) with the hole gas. We observe different characteristics of the ODMR signals obtained simultaneously using the optical signals of neutral and positively charged exciton. From that… ▽ More Optically detected magnetic resonance (ODMR) is a useful technique for studying interactions between local spins (magnetic ions) and carrier gas. We present the ODMR study of single (Cd,Mn)Te/(Cd,Mg)Te quantum wells (QWs) with the hole gas. We observe different characteristics of the ODMR signals obtained simultaneously using the optical signals of neutral and positively charged exciton. From that, we infer an existence of local fluctuations of carrier gas density resulting in separate populations of Mn$^{2+}$ ions. At the same time, the shape of the ODMR signal contains information about the temperature of the magnetic ions involved in the absorption of the MW. Studying it in detail provides even more information about the interactions with charge carriers. In the QW, two separate ensembles of ions are thermalized differently in the presence of carriers. △ Less

Submitted 24 March, 2025; v1 submitted 10 July, 2024; originally announced July 2024.

Twisted MoSe2 Homobilayer Behaving as a Heterobilayer

Authors: Arka Karmakar, Abdullah Al-Mahboob, Natalia Zawadzka, Mateusz Raczyński, Weiguang Yang, Mehdi Arfaoui, Gayatri, Julia Kucharek, Jerzy T. Sadowski, Hyeon Suk Shin, Adam Babiński, Wojciech Pacuski, Tomasz Kazimierczuk, Maciej R Molas

Abstract: Heterostructures (HSs) formed by the transition-metal dichalcogenides (TMDCs) materials have shown great promise in next-generation optoelectronic and photonic applications. An artificially twisted HS, allows us to manipulate the optical, and electronic properties. With this work, we introduce the understanding of the complex energy transfer (ET) process governed by the dipolar interaction in a tw… ▽ More Heterostructures (HSs) formed by the transition-metal dichalcogenides (TMDCs) materials have shown great promise in next-generation optoelectronic and photonic applications. An artificially twisted HS, allows us to manipulate the optical, and electronic properties. With this work, we introduce the understanding of the complex energy transfer (ET) process governed by the dipolar interaction in a twisted molybdenum diselenide (MoSe2) homobilayer without any charge-blocking interlayer. We fabricated an unconventional homobilayer (i.e., HS) with a large twist angle by combining the chemical vapor deposition (CVD) and mechanical exfoliation (Exf.) techniques to fully exploit the lattice parameters mismatch and indirect/direct (CVD/Exf.) bandgap nature. This effectively weaken the charge transfer (CT) process and allows the ET process to take over the carrier recombination channels. We utilize a series of optical and electron spectroscopy techniques complementing by the density functional theory calculations, to describe a massive photoluminescence enhancement from the HS area due to an efficient ET process. Our results show that the electronically decoupled MoSe2 homobilayer is coupled by the ET process, mimicking a 'true' heterobilayer nature. △ Less

Submitted 7 June, 2024; v1 submitted 23 April, 2024; originally announced April 2024.

Comments: 4 figures

Optical probing of the carriers-mediated coupling of the spin of two Co atoms in a quantum dot

Authors: L. Besombes, J. Kobak, W. Pacuski

Abstract: We report on the optical spectroscopy of the spin of two Co atoms in a quantum dot and interacting with a single exciton. The spectrum of quantum dots containing two Co atoms are exchange interaction and by the strain at the location of the magnetic atoms. A wide range of spectrum can be obtained depending on the relative coupling of each atom to the confined exciton. We obtained a comprehensive i… ▽ More We report on the optical spectroscopy of the spin of two Co atoms in a quantum dot and interacting with a single exciton. The spectrum of quantum dots containing two Co atoms are exchange interaction and by the strain at the location of the magnetic atoms. A wide range of spectrum can be obtained depending on the relative coupling of each atom to the confined exciton. We obtained a comprehensive interpretation of the experimental data with a spin Hamiltonian model. We show that the two Co atoms spins can be orientated by the injection of spin polarized carriers at zero magnetic field. This induces a correlation between the two spins that is observed in the intensity distribution of the emission spectra. The optical absorption in the phonon sideband of quantum dots doped with two Co reveals resonant absorptions which strongly depend on a transverse magnetic field. We show that these characteristic absorptions result from an interplay between the mixing of Co spin states induced by the presence of in-plane strain anisotropy at the magnetic atoms location and the transverse field. △ Less

Submitted 15 April, 2024; originally announced April 2024.

Strain-induced speed-up of Mn$^{2+}$ spin-lattice relaxation in (Cd,Mn)Te/(Cd,Mg)Te quantum wells: a time-resolved ODMR study

Authors: Aleksander Bogucki, Aleksandra Łopion, Karolina Ewa Połczyńska, Wojciech Pacuski, Tomasz Kazimierczuk, Andrzej Golnik, Piotr Kossacki

Abstract: This study examines the spin-lattice relaxation rate of Mn$^{2+}$ ions in strained diluted magnetic semiconductor (Cd,Mn)Te/(Cd,Mg)Te quantum wells using the optically detected magnetic resonance (ODMR) technique. By adjusting the magnesium (Mg) content in the buffer layer, we created samples with different strain levels. Our time-resolved ODMR results show that the spin-lattice relaxation time be… ▽ More This study examines the spin-lattice relaxation rate of Mn$^{2+}$ ions in strained diluted magnetic semiconductor (Cd,Mn)Te/(Cd,Mg)Te quantum wells using the optically detected magnetic resonance (ODMR) technique. By adjusting the magnesium (Mg) content in the buffer layer, we created samples with different strain levels. Our time-resolved ODMR results show that the spin-lattice relaxation time becomes faster as strain increases. We also found that the relaxation rate increases with both magnetic field and temperature, showing a power-law behavior. To understand these observations, we used a theoretical model based on six-level rate equations with non-equal level separations. This model suggests that the main factor affecting relaxation in our samples is a "direct" mechanism. The model's predictions match well with our experimental data. Overall, our findings give insights into spin-lattice relaxation in strained quantum wells and could be important for the development of future quantum and spintronic devices. △ Less

Submitted 31 January, 2024; originally announced February 2024.

Molecular beam epitaxy growth of cadmium telluride structures on hexagonal boron nitride

Authors: Adam Krzysztof Szczerba, Julia Kucharek, Jan Pawłowski, Takashi Taniguchi, Kenji Watanabe, Wojciech Pacuski

Abstract: We investigate the feasibility of epitaxial growth of a three-dimensional semiconductor on a two-dimensional substrate. In particular, we report for the first time on molecular beam epitaxy growth of cadmium telluride (CdTe) quantum wells on hexagonal boron nitride (hBN). The presence of the quantum wells is confirmed by photoluminescence measurements conducted at helium temperatures. Growth of qu… ▽ More We investigate the feasibility of epitaxial growth of a three-dimensional semiconductor on a two-dimensional substrate. In particular, we report for the first time on molecular beam epitaxy growth of cadmium telluride (CdTe) quantum wells on hexagonal boron nitride (hBN). The presence of the quantum wells is confirmed by photoluminescence measurements conducted at helium temperatures. Growth of quantum wells on two dimensional, almost perfectly flat hBN appears to be very different from growth on bulk substrates, in particular it requires 70-100$^\circ$C lower temperatures. △ Less

Submitted 8 November, 2023; originally announced November 2023.

Comments: submitted to ACS Omega, 7 pages, 8 figures

Journal ref: ACS Omega 2023, 8 (47), 44745-44750

Wurtzite vs rock-salt MnSe epitaxy: electronic and altermagnetic properties

Authors: Michał J. Grzybowski, Carmine Autieri, Jarosław Domagała, Cezary Krasucki, Anna Kaleta, Sławomir Kret, Katarzyna Gas, Maciej Sawicki, Rafał Bożek, Jan Suffczyński, Wojciech Pacuski

Abstract: Newly discovered altermagnets are magnetic materials exhibiting both compensated magnetic order, similar to antiferromagnets, and simultaneous non-relativistic spin-splitting of the bands, akin to ferromagnets. This characteristic arises from the specific symmetry operations that connect the spin sublattices. In this report, we show with ab initio calculations that the semiconductive MnSe exhibits… ▽ More Newly discovered altermagnets are magnetic materials exhibiting both compensated magnetic order, similar to antiferromagnets, and simultaneous non-relativistic spin-splitting of the bands, akin to ferromagnets. This characteristic arises from the specific symmetry operations that connect the spin sublattices. In this report, we show with ab initio calculations that the semiconductive MnSe exhibits altermagnetic spin-splitting in the wurtzite phase as well as a critical temperature well above room temperature. It is the first material from such space group identified to possess altermagnetic properties. Furthermore, we demonstrate experimentally through structural characterization techniques that it is possible to obtain thin films of both the intriguing wurtzite phase of MnSe and the more common rock-salt MnSe using molecular beam epitaxy on GaAs substrates. The choice of buffer layers plays a crucial role in determining the resulting phase and consequently extends the array of materials available for the physics of altermagnetism. △ Less

Submitted 13 September, 2023; v1 submitted 12 September, 2023; originally announced September 2023.

Journal ref: Nanoscale,16, 6259-6267 (2024)

Raman scattering excitation in monolayers of semiconducting transition metal dichalcogenides

Authors: M. Zinkiewicz, M. Grzeszczyk, T. Kazimierczuk, M. Bartos, K. Nogajewski, W. Pacuski, K. Watanabe, T. Taniguchi, A. Wysmołek, P. Kossacki, M. Potemski, A. Babiński, M. R. Molas

Abstract: Raman scattering excitation (RSE) is an experimental technique in which the spectrum is made up by sweeping the excitation energy when the detection energy is fixed. We study the low-temperature ($T$=5~K) RSE spectra measured on four high quality monolayers (ML) of semiconducting transition metal dichalcogenides (S-TMDs), $i.e.$ MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$, encapsulated in hexagonal BN.… ▽ More Raman scattering excitation (RSE) is an experimental technique in which the spectrum is made up by sweeping the excitation energy when the detection energy is fixed. We study the low-temperature ($T$=5~K) RSE spectra measured on four high quality monolayers (ML) of semiconducting transition metal dichalcogenides (S-TMDs), $i.e.$ MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$, encapsulated in hexagonal BN. The outgoing resonant conditions of Raman scattering reveal an extraordinary intensity enhancement of the phonon modes, which results in extremely rich RSE spectra. The obtained spectra are composed not only of Raman-active peaks, $i.e.$ in-plane E$'$ and out-of-plane A$'_1$, but the appearance of 1$^{st}$, 2$^{nd}$, and higher-order phonon modes is recognised. The intensity profiles of the A$'_1$ modes in the investigated MLs resemble the emissions due to neutral excitons measured in the corresponding PL spectra for the outgoing type of resonant Raman scattering conditions. Furthermore, for the WSe$_2$ ML, the A$'_1$ mode was observed when the incoming light was in resonance with the neutral exciton line. The strength of the exciton-phonon coupling (EPC) in S-TMD MLs strongly depends on the type of their ground excitonic state, $i.e.$ bright or dark, resulting in different shapes of the RSE spectra. Our results demonstrate that RSE spectroscopy is a powerful technique for studying EPC in S-TMD MLs. △ Less

Submitted 16 August, 2023; originally announced August 2023.

Comments: 9 pages, 6 figures, ESI

Journal ref: npj 2D Materials and Applications 8, 2 (2024)

Natural exceptional points in the excitation spectrum of a light-matter system

Authors: Andrzej Opala, Magdalena Furman, Mateusz Król, Rafał Mirek, Krzysztof Tyszka, Bartłomiej Seredyński, Wojciech Pacuski, Jacek Szczytko, Michał Matuszewski, Barbara Piętka

Abstract: We observe natural exceptional points in the excitation spectrum of an exciton-polariton system by optically tuning the light-matter interactions. The observed exceptional points do not require any spatial or polarization degrees of freedom and result solely from the transition from weak to strong light-matter coupling. We demonstrate that they do not coincide with the threshold for photon lasing,… ▽ More We observe natural exceptional points in the excitation spectrum of an exciton-polariton system by optically tuning the light-matter interactions. The observed exceptional points do not require any spatial or polarization degrees of freedom and result solely from the transition from weak to strong light-matter coupling. We demonstrate that they do not coincide with the threshold for photon lasing, confirming previous theoretical predictions [Phys. Rev. Lett. 122, 185301 (2019), Optica 7, 1015 (2020) ]. Using a technique where a strong coherent laser pump induces up-converted excitations, we encircle the exceptional point in the parameter space of coupling strength and particle momentum. Our method of local optical control of light-matter coupling paves the way to investigation of fundamental phenomena including dissipative phase transitions and non-Hermitian topological states. △ Less

Submitted 2 June, 2023; originally announced June 2023.

Molecular Beam Epitaxy Growth of Transition Metal Dichalcogenide (Mo,Mn)Se$_2$ on 2D, 3D and polycrystalline substrates

Authors: Julia Kucharek, Rafał Bożek, Wojciech Pacuski

Abstract: Magnetic doping of 2D materials such as Transition Metal Dichalcogenides is promising for the enhancement of magneto-optical properties, as it was previously observed for 3D diluted magnetic semiconductors. To maximize the effect of magnetic ions, they should be incorporated into the crystal lattice of 2D material rather than form separated precipitates. This work shows a study on incorporating ma… ▽ More Magnetic doping of 2D materials such as Transition Metal Dichalcogenides is promising for the enhancement of magneto-optical properties, as it was previously observed for 3D diluted magnetic semiconductors. To maximize the effect of magnetic ions, they should be incorporated into the crystal lattice of 2D material rather than form separated precipitates. This work shows a study on incorporating magnetic manganese ions into the MoSe$_2$ monolayers using molecular beam epitaxy. We test growth on various substrates with very different properties: polycrystalline SiO$_2$ on Si, exfoliated 2D hexagonal Boron Nitride flakes (placed on SiO$_2$ / Si), monocrystalline sapphire, and exfoliated graphite (on tantalum foil). Although atomic force microscopy images indicate the presence of MnSe precipitates, but at the same time, various techniques reveal effects related to alloying MoSe$_2$ with Mn: Raman scattering and photoluminescence measurements show energy shift related to the presence of Mn, scanning transmission microscopy shows Mn induced partial transformation of 1H to 1T^\prime phase. Above effects evidence partial incorporation of Mn into the MoSe$_2$ layer. △ Less

Submitted 14 September, 2023; v1 submitted 29 March, 2023; originally announced April 2023.

Comments: 13 pages, 8 figures

Journal ref: Materials Science in Semiconductor Processing 163, 107550 (2023)

Magneto-optical induced supermode switching in quantum fluids of light

Authors: Magdalena Furman, Rafał Mirek, Mateusz Król, Wojciech Pacuski, Helgi Sigurðsson, Jacek Szczytko, Barbara Piętka

Abstract: The insensitivity of photons towards external magnetic fields forms one of the hardest barriers against efficient magneto-optical control, aiming at modulating the polarization state of light. However, there is even scarcer evidence of magneto-optical effects that can spatially modulate light. Here, we demonstrate the latter by exploiting strongly coupled states of semimagnetic matter and light in… ▽ More The insensitivity of photons towards external magnetic fields forms one of the hardest barriers against efficient magneto-optical control, aiming at modulating the polarization state of light. However, there is even scarcer evidence of magneto-optical effects that can spatially modulate light. Here, we demonstrate the latter by exploiting strongly coupled states of semimagnetic matter and light in planar semiconductor microcavities. We nonresonantly excite two spatially adjacent exciton-polariton condensates which, through inherent ballistic near field coupling mechanism, spontaneously synchronise into a dissipative quantum fluidic supermode of definite parity. Applying a magnetic field along the optical axis, we continuously adjust the light-matter composition of the condensate exciton-polaritons, inducing a supermode switch into a higher order mode of opposite parity. Our findings set the ground towards magnetic spatial modulation of nonlinear light. △ Less

Submitted 1 August, 2024; v1 submitted 18 April, 2023; originally announced April 2023.

Comments: Author's Accepted Manuscript: 9 pages, 6 figures

Journal ref: Commun. Phys. 6, 196 (2023)

Coherent imaging and dynamics of excitons in MoSe$_2$ monolayers epitaxially grown on hexagonal boron nitride

Authors: Karolina Ewa Połczyńska, Simon Le Denmat, Takashi Taniguchi, Kenji Watanabe, Marek Potemski, Piotr Kossacki, Wojciech Pacuski, Jacek Kasprzak

Abstract: Using four-wave mixing microscopy, we measure the coherent response and ultrafast dynamics of excitons and trions in MoSe$_2$ monolayers grown by molecular beam epitaxy on thin films of hexagonal boron nitride. We assess inhomogeneous and homogeneous broadenings in the transition spectral lineshape. The impact of phonons on the homogeneous dephasing is inferred via the temperature dependence of th… ▽ More Using four-wave mixing microscopy, we measure the coherent response and ultrafast dynamics of excitons and trions in MoSe$_2$ monolayers grown by molecular beam epitaxy on thin films of hexagonal boron nitride. We assess inhomogeneous and homogeneous broadenings in the transition spectral lineshape. The impact of phonons on the homogeneous dephasing is inferred via the temperature dependence of the dephasing. Four-wave mixing mapping, combined with the atomic force microscopy, reveals spatial correlations between exciton oscillator strength, inhomogeneous broadening and the sample morphology. The quality of coherent optical response of the epitaxially grown transition metal dichalcogenides becomes now comparable with the samples produced by mechanical exfoliation, enabling coherent nonlinear spectroscopy of innovative materials, like magnetic layers or Janus semiconductors. △ Less

Submitted 19 March, 2023; originally announced March 2023.

Spin polarization of exciton-polariton condensate in a photonic synthetic effective magnetic field

Authors: R. Mirek, M. Furman, M. Król, B. Seredyński, K. Łempicka-Mirek, K. Tyszka, W. Pacuski, M. Matuszewski, J. Szczytko, B. Piętka

Abstract: We investigate the spin polarization of localized exciton-polariton condensates. We demonstrate the presence of an effective magnetic field leading to the formation of elliptically polarized condensates. We show that this synthetic field has an entirely photonic origin, which we believe is unique for the CdTe-based microcavities. Moreover, the degree of spin polarization of localized polariton con… ▽ More We investigate the spin polarization of localized exciton-polariton condensates. We demonstrate the presence of an effective magnetic field leading to the formation of elliptically polarized condensates. We show that this synthetic field has an entirely photonic origin, which we believe is unique for the CdTe-based microcavities. Moreover, the degree of spin polarization of localized polariton condensates in samples with magnetic ions depends on the excitation power or polarization of the non-resonant excitation laser. In an external magnetic field, the semimagnetic condensate spontaneously builds up strong spin polarization. Based on the magnetic field behavior of the condensate in the presence of magnetic ions, we apply a model that allows us to estimate the polariton-polariton interaction strength in a CdTe-system to approx. 0.8 $μ\text{eV}\!\cdot\!μ\text{m}^2$. △ Less

Submitted 4 November, 2022; originally announced November 2022.

Structural properties of TaAs Weyl semimetal thin films grown by molecular beam epitaxy on GaAs(001) substrates

Authors: Janusz Sadowski, Jarosław Z. Domagała, Wiktoria Zajkowska, Sławomir Kret, Bartłomiej Seredyński, Marta Gryglas-Borysiewicz, Zuzanna Ogorzałek, Rafał Bożek, Wojciech Pacuski

Abstract: Thin crystalline layers of TaAs Weyl semimetal (9 and 18 nm thick) are grown by molecular beam epitaxy on GaAs(001) substrates. The (001) planes of the tetragonal TaAs lattice are parallel to the GaAs(001) substrate, but the corresponding in-plane crystallographic directions of the substrate and the layer are rotated by 45°. In spite of a substantial lattice mismatch (about 19%) between GaAs(001)… ▽ More Thin crystalline layers of TaAs Weyl semimetal (9 and 18 nm thick) are grown by molecular beam epitaxy on GaAs(001) substrates. The (001) planes of the tetragonal TaAs lattice are parallel to the GaAs(001) substrate, but the corresponding in-plane crystallographic directions of the substrate and the layer are rotated by 45°. In spite of a substantial lattice mismatch (about 19%) between GaAs(001) substrate and TaAs epilayer no misfit dislocations are observed at the GaAs(001)/TaAs(001) interface. Only stacking fault defects in TaAs are detected with transmission electron microscopy. Thorough X-ray diffraction measurements and analysis of the in-situ reflection high energy electron diffraction images indicates that TaAs layers are fully relaxed already at the initial deposition stage. Atomic force microscopy imaging reveals the columnar structure of the layers, with lateral (parallel to the layer surface) columns about 20 nm wide and 200 nm long. Both X-ray diffraction and transmission electron microscopy measurements indicate that the columns share the same orientation and crystalline structure. △ Less

Submitted 4 September, 2022; v1 submitted 1 September, 2022; originally announced September 2022.

Comments: 23 pages, 6 figures

Magnetic ion relaxation time distribution within a quantum well

Authors: Aleksandra Łopion, Aleksander Bogucki, Wiktor Kraśnicki, Karolina E. Połczyńska, Wojciech Pacuski, Tomasz Kazimierczuk, Andrzej Golnik, Piotr Kossacki

Abstract: Time-resolved optically detected magnetic resonance (ODMR) is a valuable technique to study the local deformation of the crystal lattice around magnetic ion as well as the ion spin relaxation time. Here we utilize selective Mn-doping to additionally enhance the inherent locality of the ODMR technique. We present the time-resolved ODMR studies of single {(Cd,Mg)Te/(Cd,Mn)Te} quantum wells (QWs) wit… ▽ More Time-resolved optically detected magnetic resonance (ODMR) is a valuable technique to study the local deformation of the crystal lattice around magnetic ion as well as the ion spin relaxation time. Here we utilize selective Mn-doping to additionally enhance the inherent locality of the ODMR technique. We present the time-resolved ODMR studies of single {(Cd,Mg)Te/(Cd,Mn)Te} quantum wells (QWs) with manganese ions located at different positions along the growth axis -- in the center or on the sides of the quantum well. We observe that spin-lattice relaxation of Mn$^{2+}$ significantly depends on the ion-carrier wavefunction overlap at low-magnetic fields. Interestingly, the effect is clearly observed in spite of very low carrier density, which suggests the potential for control of the Mn$^{2+}$ ion relaxation rate by means of the electric field in future experiments. △ Less

Submitted 20 July, 2023; v1 submitted 5 May, 2022; originally announced May 2022.

Coherent dynamics of a single Mn-doped quantum dot revealed by four-wave mixing spectroscopy

Authors: Jacek Kasprzak, Daniel Wigger, Thilo Hahn, Tomasz Jakubczyk, Łukasz Zinkiewicz, Paweł Machnikowski, Tilmann Kuhn, Jean-François Motte, Wojciech Pacuski

Abstract: For future quantum technologies the combination of a long quantum state lifetime and an efficient interface with external optical excitation are required. In solids, the former is for example achieved by individual spins, while the latter is found in semiconducting artificial atoms combined with modern photonic structures. One possible combination of the two aspects is reached by doping a single q… ▽ More For future quantum technologies the combination of a long quantum state lifetime and an efficient interface with external optical excitation are required. In solids, the former is for example achieved by individual spins, while the latter is found in semiconducting artificial atoms combined with modern photonic structures. One possible combination of the two aspects is reached by doping a single quantum dot, providing a strong excitonic dipole, with a magnetic ion, that incorporates a characteristic spin texture. Here, we perform four-wave mixing spectroscopy to study the system's quantum coherence properties. We characterize the optical properties of the undoped CdTe quantum dot and find a strong photon echo formation which demonstrates a significant inhomogeneous spectral broadening. Incorporating the Mn$^{2+}$ ion introduces its spin-5/2 texture to the optical spectra via the exchange interaction, manifesting as six individual spectral lines in the coherent response. The random flips of the Mn-spin result in a special type of spectral wandering between the six transition energies, which is fundamentally different from the quasi-continuous spectral wandering that results in the Gaussian inhomogeneous broadening. Here, the discrete spin-ensemble manifests in additional dephasing and oscillation dynamics. △ Less

Submitted 3 January, 2022; originally announced January 2022.

Journal ref: ACS Photonics 9 (3), 1033-1041 (2022)

Neural networks based on ultrafast time-delayed effects in exciton-polaritons

Authors: Rafał Mirek, Andrzej Opala, Magdalena Furman, Mateusz Król, Krzysztof Tyszka, Bartłomiej Seredyński, Wojciech Pacuski, Jan Suffczyński, Jacek Szczytko, Michał Matuszewski, Barbara Piętka

Abstract: We demonstrate that time-delayed nonlinear effects in exciton-polaritons can be used to construct neural networks where information is coded in optical pulses arriving consecutively on the sample. The highly nonlinear effects are induced by time-dependent interactions with the excitonic reservoir. These nonlinearities allow to create a nonlinear XOR logic gate that can perform operations on the pi… ▽ More We demonstrate that time-delayed nonlinear effects in exciton-polaritons can be used to construct neural networks where information is coded in optical pulses arriving consecutively on the sample. The highly nonlinear effects are induced by time-dependent interactions with the excitonic reservoir. These nonlinearities allow to create a nonlinear XOR logic gate that can perform operations on the picosecond timescale. An optoelectronic neural network based on the constructed logic gate performs classification of spoken digits with a high accuracy rate. △ Less

Submitted 1 January, 2022; originally announced January 2022.

Molecular Beam Epitaxy growth of MoTe$_2$ on Hexagonal Boron Nitride

Authors: Bartłomiej Seredyński, Rafał Bożek, Jan Suffczyński, Justyna Piwowar, Janusz Sadowski, Wojciech Pacuski

Abstract: Hexagonal boron nitride has already been proven to serve as a decent substrate for high quality epitaxial growth of several 2D materials, such as graphene, MoSe$_{\tiny{\textrm{2}}}$, MoS$_{\tiny{\textrm{2}}}$ or WSe$_{\tiny{\textrm{2}}}$. Here, we present for the first time the molecular beam epitaxy growth of MoTe$_{\tiny{\textrm{2}}}$ on atomically smooth hexagonal boron nitride (hBN) substrate… ▽ More Hexagonal boron nitride has already been proven to serve as a decent substrate for high quality epitaxial growth of several 2D materials, such as graphene, MoSe$_{\tiny{\textrm{2}}}$, MoS$_{\tiny{\textrm{2}}}$ or WSe$_{\tiny{\textrm{2}}}$. Here, we present for the first time the molecular beam epitaxy growth of MoTe$_{\tiny{\textrm{2}}}$ on atomically smooth hexagonal boron nitride (hBN) substrate. Occurrence of MoTe$_{\tiny{\textrm{2}}}$ in various crystalline phases such as distorted octahedral 1T' phase with semimetal properties or hexagonal 2H phase with semiconducting properties opens a possibility of realisation of crystal-phase homostructures with tunable properties. Atomic force microscopy studies of MoTe$_{\tiny{\textrm{2}}}$ grown in a single monolayer regime enable us to determine surface morphology as a function of the growth conditions. The diffusion constant of MoTe$_{\tiny{\textrm{2}}}$ grown on hBN can be altered 5 times by annealing after the growth, reaching about 5 $\cdot$ 10$^{-6}$ cm$^{2}$/s. Raman spectroscopy results suggest a coexistence of both 2H and 1T' MoTe$_{\tiny{\textrm{2}}}$ phases in the studied samples. △ Less

Submitted 5 July, 2023; v1 submitted 24 November, 2021; originally announced November 2021.

Comments: 8 pages, 4 figures

Journal ref: Journal of Crystal Growth 596, 126806 (2022)

Heteroepitaxial growth of high optical quality, wafer-scale van der Waals heterostrucutres

Authors: Katarzyna Ludwiczak, Aleksandra Krystyna Dąbrowska, Johannes Binder, Mateusz Tokarczyk, Jakub Iwański, Bogusława Kurowska, Jakub Turczyński, Grzegorz Kowalski, Rafał Bożek, Roman Stępniewski, Wojciech Pacuski, Andrzej Wysmołek

Abstract: Transition metal dichalcogenides (TMDs) are materials that can exhibit intriguing optical properties like a change of the bandgap from indirect to direct when being thinned down to a monolayer. Well-resolved narrow excitonic resonances can be observed for such monolayers, however only for materials of sufficient crystalline quality, so far mostly available in the form of micrometer-sized flakes. A… ▽ More Transition metal dichalcogenides (TMDs) are materials that can exhibit intriguing optical properties like a change of the bandgap from indirect to direct when being thinned down to a monolayer. Well-resolved narrow excitonic resonances can be observed for such monolayers, however only for materials of sufficient crystalline quality, so far mostly available in the form of micrometer-sized flakes. A further significant improvement of optical and electrical properties can be achieved by transferring the TMD on hexagonal boron nitride (hBN). To exploit the full potential of TMDs in future applications, epitaxial techniques have to be developed that not only allow to growlarge-scale, high-quality TMD monolayers, but allow to perform the growth directly on large-scale epitaxial hBN. In this work we address this problem and demonstrate that MoSe2 of high optical quality can be directly grown on epitaxial hBN on an entire two-inch wafer. We developed a combined growth theme for which hBN is first synthesized at high temperature by Metal Organic Vapor Phase Epitaxy (MOVPE) and as a second step MoSe2 is deposited on top by Molecular Beam Epitaxy (MBE) at much lower temperatures. We show that this structure exhibits excellent optical properties, manifested by narrow excitonic lines in the photoluminescence spectra. Moreover, the material is homogeneous on the area of the whole two-inch wafer, with only +/-0.14 meV deviation of excitonic energy. Our mixed growth technique may guide the way for future large-scale production of high quality TMD/hBN heterostructures. △ Less

Submitted 4 September, 2021; originally announced September 2021.

Journal ref: ACS Appl. Mater. Interfaces 2021

Molecular Beam Epitaxy of a 2D material nearly lattice matched to a 3D substrate: $NiTe_{2}$ on $GaAs$

Authors: Bartłomiej Seredyński, Zuzanna Ogorzałek, Wiktoria Zajkowska, Rafał Bożek, Mateusz Tokarczyk, Jan Suffczyński, Sławomir Kret, Janusz Sadowski, Marta Gryglas-Borysiewicz, Wojciech Pacuski

Abstract: The lattice mismatch between interesting 2D materials and commonly available 3D substrates is one of the obstacles in the epitaxial growth of monolithic 2D/3D heterostructures, but a number of 2D materials have not yet been considered for epitaxy. Here we present the first molecular beam epitaxy growth of NiTe$_{2}$ 2D transition metal dichalcogenide. Importantly, the growth is realized on a nearl… ▽ More The lattice mismatch between interesting 2D materials and commonly available 3D substrates is one of the obstacles in the epitaxial growth of monolithic 2D/3D heterostructures, but a number of 2D materials have not yet been considered for epitaxy. Here we present the first molecular beam epitaxy growth of NiTe$_{2}$ 2D transition metal dichalcogenide. Importantly, the growth is realized on a nearly lattice matched GaAs(111)B substrate. Structural properties of the grown layers are investigated by electron diffraction, X-ray diffraction, and scanning tunnelling microscopy. Surface coverage and atomic scale order is evidenced by images obtained with atomic force, scanning electron, and transmission electron microscopy. Basic transport properties were measured confirming that NiTe$_{2}$ layers are metallic, with the Hall concentration of $10^{20}$cm$^{-3}$ to $10^{23}$cm$^{-3}$, depending on the growth conditions. △ Less

Submitted 16 August, 2021; originally announced August 2021.

Journal ref: Cryst. Growth Des. 21, 5773 (2021)

Angle-resolved optically detected magnetic resonance as a tool for strain determination in nanostructures

Authors: A. Bogucki, M. Goryca, A. Łopion, W. Pacuski, K. E. Połczyńska, J. Domagała, M. Tokarczyk, T. Fąs, A. Golnik, P. Kossacki

Abstract: In this paper, we apply the angle-resolved Optically Detected Magnetic Resonance (ODMR) technique to study series of strained (Cd, Mn)Te/(Cd, Mg)Te quantum wells (QWs) produced by molecular beam epitaxy. By analyzing characteristic features of ODMR angular scans, we determine strain-induced axial-symmetry spin Hamiltonian parameter D with neV precision. Furthermore, we use low-temperature optical… ▽ More In this paper, we apply the angle-resolved Optically Detected Magnetic Resonance (ODMR) technique to study series of strained (Cd, Mn)Te/(Cd, Mg)Te quantum wells (QWs) produced by molecular beam epitaxy. By analyzing characteristic features of ODMR angular scans, we determine strain-induced axial-symmetry spin Hamiltonian parameter D with neV precision. Furthermore, we use low-temperature optical reflectivity measurements and X-ray diffraction scans to evaluate the local strain present in QW material. In our analysis, we take into account different thermal expansion coefficients of GaAs substrate and CdTe buffer. The additional deformation due to the thermal expansion effects has the same magnitude as deformation origination from the different compositions of the samples. Based on the evaluated deformations and values of strain-induced axial-symmetry spin Hamiltonian parameter D, we find strain spin-lattice coefficient G11 = (72.2 +- 1.9) neV for Mn2+ in CdTe and shear deformation potential b = (-0.94 +- 0.11) eV for CdTe. △ Less

Submitted 16 June, 2024; v1 submitted 11 June, 2021; originally announced June 2021.

Polariton lasing and energy-degenerate parametric scattering in non-resonantly driven coupled planar microcavities

Authors: Krzysztof Sawicki, Thomas J. Sturges, Maciej Ściesiek, Tomasz Kazimierczuk, Kamil Sobczak, Andrzej Golnik, Wojciech Pacuski, Jan Suffczyński

Abstract: Multi-level exciton-polariton systems offer an attractive platform for studies of non-linear optical phenomena. However, studies of such consequential non-linear phenomena as polariton condensation and lasing in planar microcavities have so far been limited to two-level systems, where the condensation takes place in the lowest attainable state. Here, we report non-equilibrium Bose-Einstein condens… ▽ More Multi-level exciton-polariton systems offer an attractive platform for studies of non-linear optical phenomena. However, studies of such consequential non-linear phenomena as polariton condensation and lasing in planar microcavities have so far been limited to two-level systems, where the condensation takes place in the lowest attainable state. Here, we report non-equilibrium Bose-Einstein condensation of exciton-polaritons and low threshold, dual-wavelength polariton lasing in vertically coupled, double planar microcavities. Moreover, we find that the presence of the non-resonantly driven condensate triggers interbranch exciton-polariton transfer in the form of energy-degenerate parametric scattering. Such an effect has so far been observed only under excitation that is strictly resonant in terms of the energy and incidence angle. We describe theoretically our time-integrated and time-resolved photoluminescence investigations by a set of rate equations involving an open-dissipative Gross-Pitaevskii equation. Our platform's inherent tunability is promising for construction of planar lattices, enabling three-dimensional polariton hopping and realization of photonic devices, such as two-qubit polariton-based logic gates. △ Less

Submitted 12 February, 2021; originally announced February 2021.

Comments: 8 pages, 5 figures

Journal ref: Cryst. Growth Des. 17, 3716 (2017)

Long-Distance Coupling and Energy Transfer between Exciton States in Magnetically Controlled Microcavities

Authors: Maciej Ściesiek, Krzysztof Sawicki, Wojciech Pacuski, Kamil Sobczak, Tomasz Kazimierczuk, Andrzej Golnik, Jan Suffczyński

Abstract: Coupling of quantum emitters in a semiconductor relies, generally, on short-range dipole-dipole or electronic exchange type interactions. Consistently, energy transfer between exciton states, that is, electron-hole pairs bound by Coulomb interaction, is limited to distances of the order of 10~nm. Here, we demonstrate polariton-mediated coupling and energy transfer between excitonic states over a d… ▽ More Coupling of quantum emitters in a semiconductor relies, generally, on short-range dipole-dipole or electronic exchange type interactions. Consistently, energy transfer between exciton states, that is, electron-hole pairs bound by Coulomb interaction, is limited to distances of the order of 10~nm. Here, we demonstrate polariton-mediated coupling and energy transfer between excitonic states over a distance exceeding 2~$μ$m. We accomplish this by coupling quantum well-confined excitons through the delocalized mode of two coupled optical microcavities. Use of magnetically doped quantum wells enables us to tune the confined exciton energy by the magnetic field and in this way to control the spatial direction of the transfer. Such controlled, long-distance interaction between coherently coupled quantum emitters opens possibilities of a scalable implementation of quantum networks and quantum simulators based on solid-state, multi-cavity systems. △ Less

Submitted 17 September, 2020; originally announced September 2020.

Comments: 4 figures

Readout of a dopant spin in the anisotropic quantum dot with a single magnetic ion

Authors: Aleksander Rodek, Tomasz Kazimierczuk, Aleksander Bogucki, Tomasz Smoleński, Wojciech Pacuski, Piotr Kossacki

Abstract: Owing to exchange interaction between the exciton and magnetic ion, a quantum dot embedding a single magnetic ion is a great platform for optical control of individual spin. In particular, a quantum dot provides strong and sharp optical transitions, which give experimental access to spin states of an individual magnetic ion. We show, however, that physics of quantum dot excitons also complicate sp… ▽ More Owing to exchange interaction between the exciton and magnetic ion, a quantum dot embedding a single magnetic ion is a great platform for optical control of individual spin. In particular, a quantum dot provides strong and sharp optical transitions, which give experimental access to spin states of an individual magnetic ion. We show, however, that physics of quantum dot excitons also complicate spin readout and optical spin manipulation in such a system. This is due to electron-hole exchange interaction in anisotropic quantum dots, which affects the polarization of the emission lines. One of the consequences is that the intensity of spectral lines in a single spectrum are not simply proportional to the population of various spin states of magnetic ion. In order to provide a solution of the above problem, we present a method of extracting both the spin polarisation degree of a neutral exciton and magnetic dopant inside a semiconductor quantum dot in an external magnetic field. Our approach is experimentally verified on a system of CdSe/ZnSe quantum dot containing a single Fe$^{2+}$ ion. Both the resonant and non-resonant excitation regimes are explored resulting in a record high optical orientation efficiency of dopant spin in the former case. The proposed solutions can be easily expanded to any other system of quantum dots containing magnetic dopants. △ Less

Submitted 7 March, 2019; originally announced March 2019.

Comments: 7 pages, 4 figures

Journal ref: J. Phys.: Condens. Matter 31 (2019) 455301

Direct measurement of hyperfine shifts and radiofrequency manipulation of the nuclear spins in individual CdTe/ZnTe quantum dots

Authors: G. Ragunathan, J. Kobak, G. Gillard, W. Pacuski, K. Sobczak, J. Borysiuk, M. S. Skolnick, E. A. Chekhovich

Abstract: We achieve direct detection of electron hyperfine shifts in individual CdTe/ZnTe quantum dots. For the previously inaccessible regime of strong magnetic fields $B_z\gtrsim0.1$ T, we demonstrate robust polarization of a few-hundred-particle nuclear spin bath, with optical initialization time of $\sim$ 1 ms and polarization lifetime exceeding $\sim$ 1 s. Nuclear magnetic resonance spectroscopy of in… ▽ More We achieve direct detection of electron hyperfine shifts in individual CdTe/ZnTe quantum dots. For the previously inaccessible regime of strong magnetic fields $B_z\gtrsim0.1$ T, we demonstrate robust polarization of a few-hundred-particle nuclear spin bath, with optical initialization time of $\sim$ 1 ms and polarization lifetime exceeding $\sim$ 1 s. Nuclear magnetic resonance spectroscopy of individual dots reveals strong electron-nuclear interactions characterized by the Knight fields $|B_e|\gtrsim50$ mT, an order of magnitude stronger than in III-V semiconductor quantum dots. Our studies confirm II-VI semiconductor quantum dots as a promising platform for hybrid electron-nuclear spin quantum registers, combining the excellent optical properties comparable to III-V dots, and the dilute nuclear spin environment similar to group-IV semiconductors. △ Less

Submitted 19 July, 2018; originally announced July 2018.

Journal ref: Phys. Rev. Lett. 122, 096801 (2019)

Giant spin Meissner effect in a non-equilibrium exciton-polariton gas

Authors: M. Król, R. Mirek, D. Stephan, K. Lekenta, J. -G. Rousset, W. Pacuski, A. V. Kavokin, M. Matuszewski, J. Szczytko, B. Piętka

Abstract: The suppression of Zeeman energy splitting due to spin-dependent interactions within a Bose-Einstein condensate (the spin Meissner effect) was predicted to occur up to a certain value of magnetic field strength. We report a clear observation of this effect in semimagnetic microcavities which exhibit the giant Zeeman energy splitting between two spin-polarised polariton states as high as 2 meV, and… ▽ More The suppression of Zeeman energy splitting due to spin-dependent interactions within a Bose-Einstein condensate (the spin Meissner effect) was predicted to occur up to a certain value of magnetic field strength. We report a clear observation of this effect in semimagnetic microcavities which exhibit the giant Zeeman energy splitting between two spin-polarised polariton states as high as 2 meV, and demonstrate that partial suppression of energy difference occurs already in the uncondensed phase in a striking similarity to the up-critical superconductors in the fluctuation dominated regime. These observations are explained quantitatively by a kinetic model accounting for both the condensed and uncondensed polaritons and taking into account the non-equilibrium character of the system. △ Less

Submitted 8 March, 2019; v1 submitted 17 July, 2018; originally announced July 2018.

Comments: 12 pages, 10 figures

Journal ref: Phys. Rev. B 99, 115318 (2019)

Spin polarized semimagnetic exciton-polariton condensate in magnetic field

Authors: Mateusz Król, Rafał Mirek, Katarzyna Lekenta, Jean-Guy Rousset, Daniel Stephan, Michał Nawrocki, Michał Matuszewski, Jacek Szczytko, Wojciech Pacuski, Barbara Piętka

Abstract: Owing to their integer spin, exciton-polaritons in microcavities can be used for observation of non-equilibrium Bose-Einstein condensation in solid state. However, spin-related phenomena of such condensates are difficult to explore due to the relatively small Zeeman effect of standard semiconductor microcavity systems and the strong tendency to sustain an equal population of two spin components, w… ▽ More Owing to their integer spin, exciton-polaritons in microcavities can be used for observation of non-equilibrium Bose-Einstein condensation in solid state. However, spin-related phenomena of such condensates are difficult to explore due to the relatively small Zeeman effect of standard semiconductor microcavity systems and the strong tendency to sustain an equal population of two spin components, which precludes the observation of condensates with a well defined spin projection along the axis of the system. The enhancement of the Zeeman splitting can be achieved by introducing magnetic ions to the quantum wells, and consequently forming semimagnetic polaritons. In this system, increasing magnetic field can induce polariton condensation at constant excitation power. Here we evidence the spin polarization of a semimagnetic polaritons condensate exhibiting a circularly polarized emission over 95% even in a moderate magnetic field of about 3 T. Furthermore, we show that unlike nonmagnetic polaritons, an increase on excitation power results in an increase of the semimagnetic polaritons condensate spin polarization. These properties open new possibilities for testing theoretically predicted phenomena of spin polarized condensate. △ Less

Submitted 29 April, 2018; v1 submitted 2 January, 2018; originally announced January 2018.

Comments: 9 pages, 5 figures

Journal ref: Scientific Reports 8, 6694 (2018)

Effect of magnetic field on the lasing threshold of a semimagnetic polariton condensate

Authors: J. -G. Rousset, B. Piętka, M. Król, R. Mirek, K. Lekenta, J. Szczytko, W. Pacuski, M. Nawrocki

Abstract: We evidence magnetic field triggered polariton lasing in a microcavity containing semimagnetic quantum wells. This effect is associated with a decrease of the polariton lasing threshold power in magnetic field. The observed magnetic field dependence of the threshold power systematically exhibits a minimum which only weakly depends on the zero-field photon-exciton detuning. These results are interp… ▽ More We evidence magnetic field triggered polariton lasing in a microcavity containing semimagnetic quantum wells. This effect is associated with a decrease of the polariton lasing threshold power in magnetic field. The observed magnetic field dependence of the threshold power systematically exhibits a minimum which only weakly depends on the zero-field photon-exciton detuning. These results are interpreted as a consequence of the polariton giant Zeeman splitting which in magnetic field: leads to a decrease of the number of accessible states in the lowest polariton branch by a factor of two, and substantially changes the photon-exciton detuning. △ Less

Submitted 25 August, 2017; v1 submitted 2 July, 2017; originally announced July 2017.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 96, 125403 (2017)

Antireflective photonic structure for coherent nonlinear spectroscopy of single magnetic quantum dots

Authors: W. Pacuski, J. -G. Rousset, V. Delmonte, T. Jakubczyk, K. Sobczak, J. Borysiuk, K. Sawicki, E. Janik, J. Kasprzak

Abstract: This work presents epitaxial growth and optical spectroscopy of CdTe quantum dots (QDs) in (Cd,Zn,Mg)Te barriers placed on the top of (Cd,Zn,Mg)Te distributed Bragg reflector. The formed photonic mode in our half-cavity structure permits to enhance the local excitation intensity and extraction efficiency of the QD photoluminescence, while suppressing the reflectance within the spectral range cover… ▽ More This work presents epitaxial growth and optical spectroscopy of CdTe quantum dots (QDs) in (Cd,Zn,Mg)Te barriers placed on the top of (Cd,Zn,Mg)Te distributed Bragg reflector. The formed photonic mode in our half-cavity structure permits to enhance the local excitation intensity and extraction efficiency of the QD photoluminescence, while suppressing the reflectance within the spectral range covering the QD transitions. This allows to perform coherent, nonlinear, resonant spectroscopy of individual QDs. The coherence dynamics of a charged exciton is measured via four-wave mixing, with the estimated dephasing time $T_2=(210\,\pm\,40)$ ps. The same structure contains QDs doped with single Mn$^{2+}$ ions, as detected in photoluminescence spectra. Our work therefore paves the way toward investigating and controlling an exciton coherence coupled, via $s$,$p$-$d$ exchange interaction, with an individual spin of a magnetic dopant. △ Less

Submitted 23 March, 2017; originally announced March 2017.

Comments: 6 pages, 5 figures

Design and control of mode interaction in coupled ZnTe optical microcavities

Authors: M. Ściesiek, W. Pacuski, J-G. Rousset, M. Parlińska-Wojtan, A. Golnik, J. Suffczyński

Abstract: The photonics involving II-VI epitaxial layers was limited so far to structures based on a single planar microcavity. Here, we present double, vertically coupled, ZnTe optical microcavities in planar and 3-D photonic molecule geometry. We design the structures with the help of transfer matrix method calculations and we establish their fabrication technology by molecular beam epitaxy. We characteri… ▽ More The photonics involving II-VI epitaxial layers was limited so far to structures based on a single planar microcavity. Here, we present double, vertically coupled, ZnTe optical microcavities in planar and 3-D photonic molecule geometry. We design the structures with the help of transfer matrix method calculations and we establish their fabrication technology by molecular beam epitaxy. We characterize the samples by reflectivity spatial mapping and study them by measurements of angle-integrated and angle-resolved photoluminescence and reflectivity. We efficiently tailor the interaction strength of the cavities optical modes by an adjustment of the spatial separation between the microcavities, their thickness ratio and by the size of micropillars etched out of the planar structure. Coupling constants extracted from our measurements agree well with those determined in calculations in the frame of a tight-binding approach applied to one-dimensional photonic structures. △ Less

Submitted 9 March, 2017; originally announced March 2017.

Origin of luminescence quenching in structures containing CdSe/ZnSe quantum dots with a few Mn$^{2+}$ ions

Authors: Kacper Oreszczuk, Mateusz Goryca, Wojciech Pacuski, Tomasz Smoleński, Michał Nawrocki, Piotr Kossacki

Abstract: We present a detailed spectroscopic study of the photoluminescence quenching in an epitaxial structures containing CdSe/ZnSe quantum dots doped with low concentration of Mn$^{2+}$ ions. Our time-resolved and time-integrated experiments reveal the origin of the quenching observed in macro-photoluminescence studies of ensembles of such dots. We show that incorporation of even a few ions to an indivi… ▽ More We present a detailed spectroscopic study of the photoluminescence quenching in an epitaxial structures containing CdSe/ZnSe quantum dots doped with low concentration of Mn$^{2+}$ ions. Our time-resolved and time-integrated experiments reveal the origin of the quenching observed in macro-photoluminescence studies of ensembles of such dots. We show that incorporation of even a few ions to an individual dot does not quench its luminescence effectively, although some fingerprints of expected spin-dependent quenching are visible. At the same time, the presence of Mn$^{2+}$ ions in the sample significantly affects the luminescence intensity of the wetting layer, resulting in a quenching of the global luminescence from studied structure. On the other hand, the luminescence decay dynamics is found to be independent of the presence of Mn$^{2+}$ ions, which suggests that the observed quenching occurs for the excited excitonic states. △ Less

Submitted 9 November, 2017; v1 submitted 22 February, 2017; originally announced February 2017.

Journal ref: Phys. Rev. B 96, 205436 (2017)

Fine structure of an exciton coupled to a single Fe2+ ion in a CdSe/ZnSe quantum dot

Authors: T. Smoleński, T. Kazimierczuk, M. Goryca, W. Pacuski, P. Kossacki

Abstract: We present a polarization-resolved photoluminescence study of the exchange interaction effects in a prototype system consisting of an individual Fe2+ ion and a single neutral exciton confined in a CdSe/ZnSe quantum dot. Maximal possible number of eight fully linearly-polarized lines in the bright exciton emission spectrum is observed, evidencing complete degeneracy lifting in the investigated syst… ▽ More We present a polarization-resolved photoluminescence study of the exchange interaction effects in a prototype system consisting of an individual Fe2+ ion and a single neutral exciton confined in a CdSe/ZnSe quantum dot. Maximal possible number of eight fully linearly-polarized lines in the bright exciton emission spectrum is observed, evidencing complete degeneracy lifting in the investigated system. We discuss conditions required for such a scenario to take place: anisotropy of the electron-hole interaction and the zero-field splitting of the Fe2+ ion spin states. Neglecting of either of these components is shown to restore partial degeneracy of the transitions, making the excitonic spectrum similar to those previously reported for all other systems of quantum dots with single magnetic dopants. △ Less

Submitted 20 February, 2017; originally announced February 2017.

Comments: 7 pages, 4 figures, 1 table

Journal ref: Phys. Rev. B 96, 155411 (2017)

Direct determination of zero-field splitting for single Co$^{2+}$ ion embedded in a CdTe/ZnTe quantum dot

Authors: J. Kobak, A. Bogucki, T. Smoleński, M. Papaj, M. Koperski, M. Potemski, P. Kossacki, A. Golnik, W. Pacuski

Abstract: When Co$^{2+}$ impurity is embedded in semiconductor structure, crystal strain strongly influences zero-filed splitting between Co$^{2+}$ states with spin projection $S_z = \pm 3/2$ and $S_z = \pm 1/2$. Experimental evidences of this effect have been given in previous studies, however direct measurement of the strain induced zero-field splitting has been inaccessible so far. Here this splitting is… ▽ More When Co$^{2+}$ impurity is embedded in semiconductor structure, crystal strain strongly influences zero-filed splitting between Co$^{2+}$ states with spin projection $S_z = \pm 3/2$ and $S_z = \pm 1/2$. Experimental evidences of this effect have been given in previous studies, however direct measurement of the strain induced zero-field splitting has been inaccessible so far. Here this splitting is determined thanks to magneto-optical studies of individual Co$^{2+}$ ion in epitaxial CdTe quantum dot in ZnTe barrier. Using partially allowed optical transitions we measure strain induced zero-field splitting of Co$^{2+}$ ion directly on excitonic photoluminescence spectrum. Moreover, by observation of anticrossing of $S_z = + 3/2$ and $S_z = - 1/2$ Co$^{2+}$ spin states in magnetic field, we determine axial and in-plane components of crystal field acting on Co$^{2+}$. Proposed technique can be applied for optical determination of zero-field splitting of other transition metal ions in quantum dots. △ Less

Submitted 18 October, 2016; originally announced October 2016.

Comments: 7 pages, 1 table, 5 figures

Journal ref: Phys. Rev. B 97, 045305 (2018)

Angular dependence of giant Zeeman effect for semi-magnetic cavity polaritons

Authors: R. Mirek, M. Król, K. Lekenta, J. -G. Rousset, M. Nawrocki, M. Kulczykowski, M. Matuszewski, J. Szczytko, W. Pacuski, B. Piętka

Abstract: The observation of spin-related phenomena of microcavity polaritons has been limited due to weak Zeeman effect of non-magnetic semiconductors. We demonstrate that the incorporation of magnetic ions into quantum wells placed in a non-magnetic microcavity results in enhanced effects of magnetic field on exciton-polaritons. We show that in such a structure the Zeeman splitting of exciton-polaritons s… ▽ More The observation of spin-related phenomena of microcavity polaritons has been limited due to weak Zeeman effect of non-magnetic semiconductors. We demonstrate that the incorporation of magnetic ions into quantum wells placed in a non-magnetic microcavity results in enhanced effects of magnetic field on exciton-polaritons. We show that in such a structure the Zeeman splitting of exciton-polaritons strongly depends on the photon - exciton detuning and polariton wavevector. Our experimental data are explained by a model where the impact of magnetic field on the lower polariton state is directly inherited from the excitonic component, and the coupling strength to cavity photon is modified by external magnetic field. △ Less

Submitted 20 December, 2016; v1 submitted 1 September, 2016; originally announced September 2016.

Journal ref: Phys. Rev. B 95, 085429 (2017)

Anisotropy of in-plane hole g-factor in CdTe/ZnTe quantum dots

Authors: A. Bogucki, T. Smoleński, M. Goryca, T. Kazimierczuk, J. Kobak, W. Pacuski, P. Wojnar, P. Kossacki

Abstract: Optical studies of a bright exciton provide only limited information about the hole anisotropy in a quantum dot. In this work we present a universal method to study heavy hole anisotropy using a dark exciton in a moderate in-plane magnetic field. By analysis of the linear polarization of the dark exciton photoluminescence we identify both isotropic and anisotropic contributions to the hole g-facto… ▽ More Optical studies of a bright exciton provide only limited information about the hole anisotropy in a quantum dot. In this work we present a universal method to study heavy hole anisotropy using a dark exciton in a moderate in-plane magnetic field. By analysis of the linear polarization of the dark exciton photoluminescence we identify both isotropic and anisotropic contributions to the hole g-factor. We employ this method for a number of individual self-assembled CdTe/ZnTe quantum dots, demonstrating a variety of behaviors of in-plane hole g-factor: from almost fully anisotropic to almost isotropic. We conclude that, in general, both contributions play an important role and neither contribution can be neglected. △ Less

Submitted 9 April, 2016; originally announced April 2016.

Comments: 7 pages, 4 figures