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Nonlinear integrated quantum photonics with AlGaAs
Authors:
F. Baboux,
G. Moody,
S. Ducci
Abstract:
Integrated photonics provides a powerful approach for developing compact, stable and scalable architectures for the generation, manipulation and detection of quantum states of light. To this end, several material platforms are being developed in parallel, each providing its specific assets, and hybridization techniques to combine their strengths are now possible. This review focuses on AlGaAs, a I…
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Integrated photonics provides a powerful approach for developing compact, stable and scalable architectures for the generation, manipulation and detection of quantum states of light. To this end, several material platforms are being developed in parallel, each providing its specific assets, and hybridization techniques to combine their strengths are now possible. This review focuses on AlGaAs, a III-V semiconductor platform combining a mature fabrication technology, direct band-gap compliant with electrical injection, low-loss operation, large electro-optic effect, and compatibility with superconducting detectors for on-chip detection. We detail recent implementations of room-temperature sources of quantum light based on the high second- and third-order optical nonlinearities of the material, as well as photonic circuits embedding various functionalities ranging from polarizing beamsplitters to Mach-Zehnder interferometers, modulators and tunable filters. We then present several realizations of quantum state engineering enabled by these recent advances and discuss open perspectives and remaining challenges in the field of integrated quantum photonics with AlGaAs.
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Submitted 11 February, 2026;
originally announced February 2026.
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Characterization of Chromium Impurities in $β$-Ga$_2$O$_3$
Authors:
Mark E. Turiansky,
Sai Mu,
Lukas Razinkovas,
Kamyar Parto,
Sahil D. Patel,
Sean Doan,
Ganesh Pokharel,
Steven J. Gomez Alvarado,
Stephen D. Wilson,
Galan Moody,
Chris G. Van de Walle
Abstract:
Chromium is a common transition-metal impurity that is easily incorporated during crystal growth. It is perhaps best known for giving rise to the 694.3 nm (1.786 eV) emission in Cr-doped Al$_2$O$_3$, exploited in ruby lasers. Chromium has also been found in monoclinic gallium oxide, a wide-bandgap semiconductor being pursued for power electronics. In this work, we thoroughly characterize the behav…
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Chromium is a common transition-metal impurity that is easily incorporated during crystal growth. It is perhaps best known for giving rise to the 694.3 nm (1.786 eV) emission in Cr-doped Al$_2$O$_3$, exploited in ruby lasers. Chromium has also been found in monoclinic gallium oxide, a wide-bandgap semiconductor being pursued for power electronics. In this work, we thoroughly characterize the behavior of Cr in Ga$_2$O$_3$ through theoretical and experimental techniques. $β$-Ga$_2$O$_3$ samples are grown with the floating zone method and show evidence of a sharp photoluminescence signal, reminiscent of ruby. We calculate the energetics of formation of Cr from first principles, demonstrating that Cr preferentially incorporates as a neutral impurity on the octahedral site. Cr possesses a quartet ground-state spin and has an internal transition with a zero-phonon line near 1.8 eV. By comparing the calculated and experimentally measured luminescence lineshape function, we elucidate the role of coupling to phonons and uncover features beyond the Franck-Condon approximation. The combination of strong emission with a small Huang-Rhys factor of 0.05 and a technologically relevant host material render Cr in Ga$_2$O$_3$ attractive as a quantum defect.
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Submitted 31 December, 2024;
originally announced January 2025.
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Rational Design of Efficient Defect-Based Quantum Emitters
Authors:
Mark E. Turiansky,
Kamyar Parto,
Galan Moody,
Chris G. Van de Walle
Abstract:
Single-photon emitters are an essential component of quantum networks, and defects or impurities in semiconductors are a promising platform to realize such quantum emitters. Here we present a model that encapsulates the essential physics of coupling to phonons, which governs the behavior of real single-photon emitters, and critically evaluate several approximations that are commonly utilized. Emis…
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Single-photon emitters are an essential component of quantum networks, and defects or impurities in semiconductors are a promising platform to realize such quantum emitters. Here we present a model that encapsulates the essential physics of coupling to phonons, which governs the behavior of real single-photon emitters, and critically evaluate several approximations that are commonly utilized. Emission in the telecom wavelength range is highly desirable, but our model shows that nonradiative processes are greatly enhanced at these low photon energies, leading to a decrease in efficiency. Our results suggest that reducing the phonon frequency is a fruitful avenue to enhance the efficiency.
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Submitted 13 February, 2024;
originally announced February 2024.
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Heterogeneous integration of superconducting thin films and epitaxial semiconductor heterostructures with Lithium Niobate
Authors:
Michelle Lienhart,
Michael Choquer,
Emeline D. S. Nysten,
Matthias Weiß,
Kai Müller,
Jonathan J. Finley,
Galan Moody,
Hubert J. Krenner
Abstract:
We report on scalable heterointegration of superconducting electrodes and epitaxial semiconductor quantum dots on strong piezoelectric and optically nonlinear lithium niobate. The implemented processes combine the sputter-deposited thin film superconductor niobium nitride and III-V compound semiconductor membranes onto the host substrate. The superconducting thin film is employed as a zero-resisti…
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We report on scalable heterointegration of superconducting electrodes and epitaxial semiconductor quantum dots on strong piezoelectric and optically nonlinear lithium niobate. The implemented processes combine the sputter-deposited thin film superconductor niobium nitride and III-V compound semiconductor membranes onto the host substrate. The superconducting thin film is employed as a zero-resistivity electrode material for a surface acoustic wave resonator with internal quality factors $Q \approx 17000$ representing a three-fold enhancement compared to identical devices with normal conducting electrodes. Superconducting operation of $\approx 400\,\mathrm{MHz}$ resonators is achieved to temperatures $T>7\,\mathrm{K}$ and electrical radio frequency powers $P_{\mathrm{rf}}>+9\,\mathrm{dBm}$. Heterogeneously integrated single quantum dots couple to the resonant phononic field of the surface acoustic wave resonator operated in the superconducting regime. Position and frequency selective coupling mediated by deformation potential coupling is validated using time-integrated and time-resolved optical spectroscopy. Furthermore, acoustoelectric charge state control is achieved in a modified device geometry harnessing large piezoelectric fields inside the resonator. The hybrid quantum dot - surface acoustic wave resonator can be scaled to higher operation frequencies and smaller mode volumes for quantum phase modulation and transduction between photons and phonons via the quantum dot. Finally, the employed materials allow for the realization of other types of optoelectronic devices, including superconducting single photon detectors and integrated photonic and phononic circuits.
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Submitted 30 April, 2023; v1 submitted 6 February, 2023;
originally announced February 2023.
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Surface Acoustic Wave Cavity Optomechanics with WSe$_2$ Single Photon Emitters
Authors:
Sahil D. Patel,
Kamyar Parto,
Michael Choquer,
Sammy Umezawa,
Landon Hellman,
Daniella Polishchuk,
Galan Moody
Abstract:
Surface acoustic waves (SAWs) are a versatile tool for coherently interfacing with a variety of solid-state quantum systems spanning microwave to optical frequencies, including superconducting qubits, spins, and quantum emitters. Here, we demonstrate SAW cavity optomechanics with quantum emitters in 2D materials, specifically monolayer WSe$_2$, on a planar lithium niobate SAW resonator driven by s…
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Surface acoustic waves (SAWs) are a versatile tool for coherently interfacing with a variety of solid-state quantum systems spanning microwave to optical frequencies, including superconducting qubits, spins, and quantum emitters. Here, we demonstrate SAW cavity optomechanics with quantum emitters in 2D materials, specifically monolayer WSe$_2$, on a planar lithium niobate SAW resonator driven by superconducting electronics. Using steady-state photoluminescence spectroscopy and time-resolved single-photon counting, we map the temporal dynamics of modulated 2D emitters under coupling to different SAW cavity modes, showing energy-level splitting consistent with deformation potential coupling of 30 meV/%. We leverage the large anisotropic strain from the SAW to modulate the excitonic fine-structure splitting on a nanosecond timescale, which may find applications for on-demand entangled photon-pair generation from 2D materials. Cavity optomechanics with SAWs and 2D quantum emitters provides opportunities for compact sensors and quantum electro-optomechanics in a multi-functional integrated platform that combines phononic, optical, and superconducting electronic quantum systems.
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Submitted 28 November, 2022;
originally announced November 2022.
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Prospects and challenges of quantum emitters in 2D materials
Authors:
Shaimaa I. Azzam,
Kamyar Parto,
Galan Moody
Abstract:
The search for an ideal single-photon source has generated significant interest in discovering novel emitters in materials as well as developing new manipulation techniques to gain better control over the emitters' properties. Quantum emitters in atomically thin two-dimensional (2D) materials have proven very attractive with high brightness, operation under ambient conditions, and the ability to b…
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The search for an ideal single-photon source has generated significant interest in discovering novel emitters in materials as well as developing new manipulation techniques to gain better control over the emitters' properties. Quantum emitters in atomically thin two-dimensional (2D) materials have proven very attractive with high brightness, operation under ambient conditions, and the ability to be integrated with a wide range of electronic and photonic platforms. This perspective highlights some of the recent advances in quantum light generation from 2D materials, focusing on hexagonal boron nitride and transition metal dichalcogenides (TMDs). Efforts in engineering and deterministically creating arrays of quantum emitters in 2D materials, their electrical excitation, and their integration with photonic devices are discussed. Lastly, we address some of the challenges the field is facing and the near-term efforts to tackle them. We provide an outlook towards efficient and scalable quantum light generation from 2D materials towards controllable and addressable on-chip quantum sources.
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Submitted 14 April, 2021;
originally announced April 2021.
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Twist Angle Dependent Interlayer Exciton Lifetimes in van der Waals Heterostructures
Authors:
Junho Choi,
Matthias Florian,
Alexander Steinhoff,
Daniel Erben,
Kha Tran,
Dong Seob Kim,
Liuyang Sun,
Jiamin Quan,
Robert Claassen,
Somak Majumder,
Jennifer A. Hollingsworth,
Takashi Taniguchi,
Kenji Watanabe,
Keiji Ueno,
Akshay Singh,
Galan Moody,
Frank Jahnke,
Xiaoqin Li
Abstract:
In van der Waals (vdW) heterostructures formed by stacking two monolayers of transition metal dichalcogenides, multiple exciton resonances with highly tunable properties are formed and subject to both vertical and lateral confinement. We investigate how a unique control knob, the twist angle between the two monolayers, can be used to control the exciton dynamics. We observe that the interlayer exc…
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In van der Waals (vdW) heterostructures formed by stacking two monolayers of transition metal dichalcogenides, multiple exciton resonances with highly tunable properties are formed and subject to both vertical and lateral confinement. We investigate how a unique control knob, the twist angle between the two monolayers, can be used to control the exciton dynamics. We observe that the interlayer exciton lifetimes in $\text{MoSe}_{\text{2}}$/$\text{WSe}_{\text{2}}$ twisted bilayers (TBLs) change by one order of magnitude when the twist angle is varied from 1$^\circ$ to 3.5$^\circ$. Using a low-energy continuum model, we theoretically separate two leading mechanisms that influence interlayer exciton radiative lifetimes. The shift to indirect transitions in the momentum space with an increasing twist angle and the energy modulation from the moiré potential both have a significant impact on interlayer exciton lifetimes. We further predict distinct temperature dependence of interlayer exciton lifetimes in TBLs with different twist angles, which is partially validated by experiments. While many recent studies have highlighted how the twist angle in a vdW TBL can be used to engineer the ground states and quantum phases due to many-body interaction, our studies explore its role in controlling the dynamics of optically excited states, thus, expanding the conceptual applications of "twistronics".
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Submitted 26 January, 2021; v1 submitted 29 July, 2020;
originally announced July 2020.
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Dephasing of InAs quantum dot p-shell excitons using two-dimensional coherent spectroscopy
Authors:
Takeshi Suzuki,
Rohan Singh,
Galan Moody,
Marc Aßmann,
Manfred Bayer,
Arne Ludwig,
Andreas D. Wieck,
and Steven T. Cundiff
Abstract:
The dephasing mechanisms of p-shell and s-shell excitons in an InAs self-assembled quantum dot ensemble are examined using two-dimensional coherent spectroscopy (2DCS). 2DCS provides a comprehensive picture of how the energy level structure of dots affects the exciton dephasing rates and recombination lifetimes. We find that at low temperatures, dephasing of s-shell excitons is lifetime limited, w…
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The dephasing mechanisms of p-shell and s-shell excitons in an InAs self-assembled quantum dot ensemble are examined using two-dimensional coherent spectroscopy (2DCS). 2DCS provides a comprehensive picture of how the energy level structure of dots affects the exciton dephasing rates and recombination lifetimes. We find that at low temperatures, dephasing of s-shell excitons is lifetime limited, whereas p-shell excitons exhibit significant pure dephasing due to scattering between degenerate spin states. At elevated temperatures, quadratic exciton-phonon coupling plays an important role in both s-shell and p-shell exciton dephasing. We show that multiple p-shell states are also responsible for stronger phonon dephasing for these transitions
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Submitted 22 August, 2018;
originally announced August 2018.
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Microsecond Valley Lifetime of Defect-Bound Excitons in Monolayer WSe$_2$
Authors:
Galan Moody,
Kha Tran,
Xiaobo Lu,
Travis Autry,
James M. Fraser,
Richard P. Mirin,
Li Yang,
Xiaoqin Li,
Kevin L. Silverman
Abstract:
In atomically thin two-dimensional semiconductors such as transition metal dichalcogenides (TMDs), controlling the density and type of defects promises to be an effective approach for engineering light-matter interactions. We demonstrate that electron-beam irradiation is a simple tool for selectively introducing defect-bound exciton states associated with chalcogen vacancies in TMDs. Our first-pri…
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In atomically thin two-dimensional semiconductors such as transition metal dichalcogenides (TMDs), controlling the density and type of defects promises to be an effective approach for engineering light-matter interactions. We demonstrate that electron-beam irradiation is a simple tool for selectively introducing defect-bound exciton states associated with chalcogen vacancies in TMDs. Our first-principles calculations and time-resolved spectroscopy measurements of monolayer WSe2 reveal that these defect-bound excitons exhibit exceptional optical properties including a recombination lifetime approaching 200 ns and a valley lifetime longer than 1 $μ$s. The ability to engineer the crystal lattice through electron irradiation provides a new approach for tailoring the optical response of TMDs for photonics, quantum optics, and valleytronics applications.
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Submitted 10 July, 2018;
originally announced July 2018.
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Moiré Excitons in Van der Waals Heterostructures
Authors:
Kha Tran,
Galan Moody,
Fengcheng Wu,
Xiaobo Lu,
Junho Choi,
Akshay Singh,
Jacob Embley,
André Zepeda,
Marshall Campbell,
Kyounghwan Kim,
Amritesh Rai,
Travis Autry,
Daniel A. Sanchez,
Takashi Taniguchi,
Kenji Watanabe,
Nanshu Lu,
Sanjay K. Banerjee,
Emanuel Tutuc,
Li Yang,
Allan H. MacDonald,
Kevin L. Silverman,
Xiaoqin Li
Abstract:
In van der Waals (vdW) heterostructures formed by stacking two monolayer semiconductors, lattice mismatch or rotational misalignment introduces an in-plane moiré superlattice. While it is widely recognized that a moiré superlattice can modulate the electronic band structure and lead to novel transport properties including unconventional superconductivity and insulating behavior driven by correlati…
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In van der Waals (vdW) heterostructures formed by stacking two monolayer semiconductors, lattice mismatch or rotational misalignment introduces an in-plane moiré superlattice. While it is widely recognized that a moiré superlattice can modulate the electronic band structure and lead to novel transport properties including unconventional superconductivity and insulating behavior driven by correlations, its influence on optical properties has not been investigated experimentally. We present spectroscopic evidence that interlayer excitons are confined by the moiré potential in a high-quality MoSe2/WSe2 heterobilayer with small rotational twist. A series of interlayer exciton resonances with either positive or negative circularly polarized emission is observed in photoluminescence, consistent with multiple exciton states confined within the moiré potential. The recombination dynamics and temperature dependence of these interlayer exciton resonances are consistent with this interpretation. These results demonstrate the feasibility of engineering artificial excitonic crystals using vdW heterostructures for nanophotonics and quantum information applications.
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Submitted 10 July, 2018;
originally announced July 2018.
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Demonstrating sub-3 ps temporal resolution in a superconducting nanowire single-photon detector
Authors:
B. A. Korzh,
Q-Y. Zhao,
S. Frasca,
J. P. Allmaras,
T. M. Autry,
E. A. Bersin,
M. Colangelo,
G. M. Crouch,
A. E. Dane,
T. Gerrits,
F. Marsili,
G. Moody,
E. Ramirez,
J. D. Rezac,
M. J. Stevens,
E. E. Wollman,
D. Zhu,
P. D. Hale,
K. L. Silverman,
R. P. Mirin,
S. W. Nam,
M. D. Shaw,
K. K. Berggren
Abstract:
Improving the temporal resolution of single photon detectors has an impact on many applications, such as increased data rates and transmission distances for both classical and quantum optical communication systems, higher spatial resolution in laser ranging and observation of shorter-lived fluorophores in biomedical imaging. In recent years, superconducting nanowire single-photon detectors (SNSPDs…
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Improving the temporal resolution of single photon detectors has an impact on many applications, such as increased data rates and transmission distances for both classical and quantum optical communication systems, higher spatial resolution in laser ranging and observation of shorter-lived fluorophores in biomedical imaging. In recent years, superconducting nanowire single-photon detectors (SNSPDs) have emerged as the highest efficiency time-resolving single-photon counting detectors available in the near infrared. As the detection mechanism in SNSPDs occurs on picosecond time scales, SNSPDs have been demonstrated with exquisite temporal resolution below 15 ps. We reduce this value to 2.7$\pm$0.2 ps at 400 nm and 4.6$\pm$0.2 ps at 1550 nm, using a specialized niobium nitride (NbN) SNSPD. The observed photon-energy dependence of the temporal resolution and detection latency suggests that intrinsic effects make a significant contribution.
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Submitted 18 April, 2018;
originally announced April 2018.
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Trion Valley Coherence in Monolayer Semiconductors
Authors:
Kai Hao,
Lixiang Xu,
Fengcheng Wu,
Philipp Nagler,
Kha Tran,
Xin Ma,
Christian Schüller,
Tobias Korn,
Allan H. MacDonald,
Galan Moody,
Xiaoqin Li
Abstract:
The emerging field of valleytronics aims to exploit the valley pseudospin of electrons residing near Bloch band extrema as an information carrier. Recent experiments demonstrating optical generation and manipulation of exciton valley coherence (the superposition of electron-hole pairs at opposite valleys) in monolayer transition metal dichalcogenides (TMDs) provide a critical step towards control…
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The emerging field of valleytronics aims to exploit the valley pseudospin of electrons residing near Bloch band extrema as an information carrier. Recent experiments demonstrating optical generation and manipulation of exciton valley coherence (the superposition of electron-hole pairs at opposite valleys) in monolayer transition metal dichalcogenides (TMDs) provide a critical step towards control of this quantum degree of freedom. The charged exciton (trion) in TMDs is an intriguing alternative to the neutral exciton for control of valley pseudospin because of its long spontaneous recombination lifetime, its robust valley polarization, and its coupling to residual electronic spin. Trion valley coherence has however been unexplored due to experimental challenges in accessing it spectroscopically. In this work, we employ ultrafast two-dimensional coherent spectroscopy to resonantly generate and detect trion valley coherence in monolayer MoSe$_2$ demonstrating that it persists for a few-hundred femtoseconds. We conclude that the underlying mechanisms limiting trion valley coherence are fundamentally different from those applicable to exciton valley coherence. Based on these observations, we suggest possible strategies for extending valley coherence times in two-dimensional materials.
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Submitted 10 November, 2016;
originally announced November 2016.
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Neutral and Charged Inter-Valley Biexcitons in Monolayer MoSe$_2$
Authors:
Kai Hao,
Lixiang Xu,
Judith F. Specht,
Philipp Nagler,
Kha Tran,
Akshay Singh,
Chandriker Kavir Dass,
Christian Schüller,
Tobias Korn,
Marten Richter,
Andreas Knorr,
Xiaoqin Li,
Galan Moody
Abstract:
In atomically thin transition metal dichalcogenides (TMDs), reduced dielectric screening of the Coulomb interaction leads to strongly correlated many-body states, including excitons and trions, that dominate the optical properties. Higher-order states, such as bound biexcitons, are possible but are difficult to identify unambiguously using linear optical spectroscopy methods alone. Here, we implem…
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In atomically thin transition metal dichalcogenides (TMDs), reduced dielectric screening of the Coulomb interaction leads to strongly correlated many-body states, including excitons and trions, that dominate the optical properties. Higher-order states, such as bound biexcitons, are possible but are difficult to identify unambiguously using linear optical spectroscopy methods alone. Here, we implement polarization-resolved two-dimensional coherent spectroscopy to unravel the complex optical response of monolayer MoSe$_2$ and identify multiple higher-order correlated states. Decisive signatures of neutral and charged inter-valley biexcitons appear in cross-polarized two-dimensional spectra as distinct resonances with respective ~20 meV and ~5 meV binding energies--similar to recent calculations using variational and Monte Carlo methods. A theoretical model taking into account the valley-dependent optical selection rules reveals the specific quantum pathways that give rise to these states. Inter-valley biexcitons identified here, comprised of neutral and charged excitons from different valleys, offer new opportunities for creating exotic exciton-polariton condensates and for developing ultrathin biexciton lasers and polarization-entangled photon sources.
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Submitted 7 September, 2016;
originally announced September 2016.
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Optical amplitude and phase modulation dynamics at the single-photon level in a quantum dot ridge waveguide
Authors:
Galan Moody,
Corey McDonald,
Ari Feldman,
Todd Harvey,
Richard P. Mirin,
Kevin L. Silverman
Abstract:
The amplitude and phase of a material's nonlinear optical response provide insight into the underlying electronic dynamics that determine its optical properties. Phase-sensitive nonlinear spectroscopy techniques are widely implemented to explore these dynamics through demodulation of the complex optical signal field into its quadrature components; however, complete reconstruction of the optical re…
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The amplitude and phase of a material's nonlinear optical response provide insight into the underlying electronic dynamics that determine its optical properties. Phase-sensitive nonlinear spectroscopy techniques are widely implemented to explore these dynamics through demodulation of the complex optical signal field into its quadrature components; however, complete reconstruction of the optical response requires measuring both the amplitude and phase of each quadrature, which is often lost in standard detection methods. Here, we implement a heterodyne-detection scheme to fully reconstruct the amplitude and phase response of spectral hole-burning from InAs/GaAs charged quantum dots. We observe an ultra-narrow absorption profile and a corresponding dispersive lineshape of the phase, which reflect the nanosecond optical coherence time of the charged exciton transition. Simultaneously, the measurements are sensitive to electron spin relaxation dynamics on a millisecond timescale, as this manifests as a magnetic-field dependent delay of the amplitude and phase modulation. Appreciable amplitude modulation depth and nonlinear phase shift up to 0.09$\timesπ$ radians (16$°$) are demonstrated, providing new possibilities for quadrature modulation at faint photon levels with several independent control parameters, including photon number, modulation frequency, detuning, and externally applied fields.
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Submitted 26 August, 2016;
originally announced August 2016.
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Coherent and Incoherent Coupling Dynamics between Neutral and Charged Excitons in Monolayer MoSe2
Authors:
Kai Hao,
Lixiang Xu,
Philipp Nagler,
Akshay Singh,
Kha Tran,
Chandriker Kavir Dass,
Christian Schüller,
Tobias Korn,
Xiaoqin Li,
Galan Moody
Abstract:
The optical properties of semiconducting transition metal dichalcogenides are dominated by both neutral excitons (electron-hole pairs) and charged excitons (trions) that are stable even at room temperature. While trions directly influence charge transport properties in optoelectronic devices, excitons may be relevant through exciton-trion coupling and conversion phenomena. In this work, we reveal…
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The optical properties of semiconducting transition metal dichalcogenides are dominated by both neutral excitons (electron-hole pairs) and charged excitons (trions) that are stable even at room temperature. While trions directly influence charge transport properties in optoelectronic devices, excitons may be relevant through exciton-trion coupling and conversion phenomena. In this work, we reveal the coherent and incoherent nature of exciton-trion coupling and the relevant timescales in monolayer MoSe2 using optical two-dimensional coherent spectroscopy. Coherent interaction between excitons and trions is definitively identified as quantum beating of cross-coupling peaks that persists for a few hundred femtoseconds. For longer times up to 10 ps, surprisingly, the relative intensity of the cross-coupling peaks increases, which is attributed to incoherent energy transfer likely due to phonon-assisted up-conversion and down-conversion processes that are efficient even at cryogenic temperature.
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Submitted 26 May, 2016;
originally announced May 2016.
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Exciton Dynamics in Monolayer Transition Metal Dichalcogenides
Authors:
Galan Moody,
John Schaibley,
Xiaodong Xu
Abstract:
Since the discovery of semiconducting monolayer transition metal dichalcogenides, a variety of experimental and theoretical studies have been carried out seeking to understand the intrinsic exciton population decay and valley relaxation dynamics. Reports of the exciton decay time range from hundreds of femtoseconds to ten nanoseconds, while the valley depolarization time can exceed one nanosecond.…
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Since the discovery of semiconducting monolayer transition metal dichalcogenides, a variety of experimental and theoretical studies have been carried out seeking to understand the intrinsic exciton population decay and valley relaxation dynamics. Reports of the exciton decay time range from hundreds of femtoseconds to ten nanoseconds, while the valley depolarization time can exceed one nanosecond. At present, however, a consensus on the microscopic mechanisms governing exciton radiative and non-radiative recombination is lacking. The strong exciton oscillator strength resulting in up to 20% absorption for a single monolayer points to ultrafast radiative recombination. However, the low quantum yield and large variance in the reported lifetimes suggest that non-radiative Auger-type processes obscure the intrinsic exciton radiative lifetime. In either case, the electron-hole exchange interaction plays an important role in the exciton spin and valley dynamics. In this article, we review the experiments and theory that have led to these conclusions and comment on future experiments that could complement our current understanding.
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Submitted 14 February, 2016;
originally announced February 2016.
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Electronic Coherence Control in a Charged Quantum Dot
Authors:
Galan Moody,
Corey McDonald,
Ari Feldman,
Todd Harvey,
Richard P. Mirin,
Kevin L. Silverman
Abstract:
Minimizing decoherence due to coupling of a quantum system to its fluctuating environment is at the forefront of quantum information science and photonics research. Nature sets the ultimate limit, however, given by the strength of the system's coupling to the electromagnetic field. Here, we establish the ability to electronically control this coupling and $\textit{enhance}$ the coherence time of a…
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Minimizing decoherence due to coupling of a quantum system to its fluctuating environment is at the forefront of quantum information science and photonics research. Nature sets the ultimate limit, however, given by the strength of the system's coupling to the electromagnetic field. Here, we establish the ability to electronically control this coupling and $\textit{enhance}$ the coherence time of a quantum dot excitonic state. Coherence control is demonstrated on the positively charged exciton transition (an electron Coulomb-bound with two holes) in quantum dots embedded in a photonic waveguide by manipulating the electron and hole wavefunctions through an applied lateral electric field. With increasing field up to 15 kV cm$^{-1}$, the coherence time increases by a factor of two from $\sim1.4$ ns to $\sim2.7$ ns. Numerical calculations reveal that longer coherence arises from the separation of charge carriers by up to $\sim6$ nm, which leads to a $30\%$ weaker transition dipole moment. The ability to electrostatically control the coherence time and transition dipole moment opens new avenues for quantum communication and novel coupling schemes between distant qubits.
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Submitted 19 October, 2015;
originally announced October 2015.
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Direct Measurement of Exciton Valley Coherence in Monolayer WSe$_2$
Authors:
Kai Hao,
Galan Moody,
Fengcheng Wu,
Chandriker Kavir Dass,
Lixiang Xu,
Chang-Hsaio Chen,
Ming-Yang Li,
Lain-Jong Li,
Allan H. MacDonald,
Xiaoqin Li
Abstract:
In crystals, energy band extrema in momentum space can be identified by their valley index. The internal quantum degree of freedom associated with valley pseudospin indices can act as a useful information carrier analogous to electronic charge or spin. Interest in valleytronics has been revived in recent years following the discovery of atomically thin materials such as graphene and transition met…
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In crystals, energy band extrema in momentum space can be identified by their valley index. The internal quantum degree of freedom associated with valley pseudospin indices can act as a useful information carrier analogous to electronic charge or spin. Interest in valleytronics has been revived in recent years following the discovery of atomically thin materials such as graphene and transition metal dichalcogenides. However, the valley coherence time, a key quantity for manipulating the valley pseudospin, has never been measured in any material. In this work, we use a sequence of laser pulses to resonantly generate a coherent superposition of excitons (Coulomb-bound electron-hole pairs) in opposite valleys of monolayer WSe$_2$. The imposed valley coherence persists for approximately one hundred femtoseconds. We propose that the electron-hole exchange interaction provides an important decoherence mechanism in addition to exciton population decay. Our work provides critical insight into the requirements and strategies for optical manipulation of the valley pseudospin for future valleytronics applications.
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Submitted 29 September, 2015;
originally announced September 2015.
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Trion Formation Dynamics in Monolayer Transition Metal Dichalcogenides
Authors:
Akshay Singh,
Galan Moody,
Kha Tran,
Marie Scott,
Vincent Overbeck,
Gunnar Berghäuser,
John Schaibley,
Edward J. Seifert,
Dennis Pleskot,
Nathaniel M. Gabor,
Jiaqiang Yan,
David G. Mandrus,
Marten Richter,
Ermin Malic,
Xiaodong Xu,
Xiaoqin Li
Abstract:
We report charged exciton (trion) formation dynamics in doped monolayer transition metal dichalcogenides (TMDs), specifically molybdenum diselenide (MoSe2), using resonant two-color pump-probe spectroscopy. When resonantly pumping the exciton transition, trions are generated on a picosecond timescale through exciton-electron interaction. As the pump energy is tuned from the high energy to low ener…
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We report charged exciton (trion) formation dynamics in doped monolayer transition metal dichalcogenides (TMDs), specifically molybdenum diselenide (MoSe2), using resonant two-color pump-probe spectroscopy. When resonantly pumping the exciton transition, trions are generated on a picosecond timescale through exciton-electron interaction. As the pump energy is tuned from the high energy to low energy side of the inhomogeneously broadened exciton resonance, the trion formation time increases by ~ 50%. This feature can be explained by the existence of both localized and delocalized excitons in a disordered potential and suggests the existence of an exciton mobility edge in TMDs. The quasiparticle formation and conversion processes are important for interpreting photoluminescence and photoconductivity in TMDs.
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Submitted 16 December, 2015; v1 submitted 16 July, 2015;
originally announced July 2015.
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Intrinsic Exciton Linewidth in Monolayer Transition Metal Dichalcogenides
Authors:
Galan Moody,
Chandriker Kavir Dass,
Kai Hao,
Chang-Hsiao Chen,
Lain-Jong Li,
Akshay Singh,
Kha Tran,
Genevieve Clark,
Xiaodong Xu,
Gunnar Bergauser,
Ermin Malic,
Andreas Knorr,
Xiaoqin Li
Abstract:
Monolayer transition metal dichalcogenides feature Coulomb-bound electron-hole pairs (excitons) with exceptionally large binding energy and coupled spin and valley degrees of freedom. These unique attributes have been leveraged for electrical and optical control of excitons for atomically-thin optoelectronics and valleytronics. The development of such technologies relies on understanding and quant…
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Monolayer transition metal dichalcogenides feature Coulomb-bound electron-hole pairs (excitons) with exceptionally large binding energy and coupled spin and valley degrees of freedom. These unique attributes have been leveraged for electrical and optical control of excitons for atomically-thin optoelectronics and valleytronics. The development of such technologies relies on understanding and quantifying the fundamental properties of the exciton. A key parameter is the intrinsic exciton homogeneous linewidth, which reflects irreversible quantum dissipation arising from system (exciton) and bath (vacuum and other quasiparticles) interactions. Using optical coherent two-dimensional spectroscopy, we provide the first experimental determination of the exciton homogeneous linewidth in monolayer transition metal dichalcogenides, specifically tungsten diselenide (WSe2). The role of exciton-exciton and exciton-phonon interactions in quantum decoherence is revealed through excitation density and temperature dependent linewidth measurements. The residual homogeneous linewidth extrapolated to zero density and temperature is ~1.5 meV, placing a lower bound of approximately 0.2 ps on the exciton radiative lifetime. The exciton quantum decoherence mechanisms presented in this work are expected to be ubiquitous in atomically-thin semiconductors.
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Submitted 12 October, 2014;
originally announced October 2014.
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Homogeneous Linewidth Narrowing of the Charged Exciton via Nuclear Spin Screening in an InAs/GaAs Quantum Dot Ensemble
Authors:
G. Moody,
M. Feng,
C. McDonald,
R. P. Mirin,
K. L. Silverman
Abstract:
In semiconductor quantum dots, the electron hyperfine interaction with the nuclear spin bath is the leading source of spin decoherence at cryogenic temperature. Using high-resolution two-color differential transmission spectroscopy, we demonstrate that such electron-nuclear coupling also imposes a lower limit for the positively charged exciton dephasing rate, γ, in an ensemble of InAs/GaAs quantum…
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In semiconductor quantum dots, the electron hyperfine interaction with the nuclear spin bath is the leading source of spin decoherence at cryogenic temperature. Using high-resolution two-color differential transmission spectroscopy, we demonstrate that such electron-nuclear coupling also imposes a lower limit for the positively charged exciton dephasing rate, γ, in an ensemble of InAs/GaAs quantum dots. We find that the dephasing rate is sensitive to the strength of the hyperfine interaction, which can be controlled through the application of an external magnetic field in the Faraday configuration. At zero applied field, strong electron-nuclear coupling induces additional dephasing beyond the radiative limit and γ= 230 MHz (0.95 μeV). Screening of the hyperfine interaction is achieved for an external field of ~1 T, resulting in γ= 172 MHz (0.71 μeV) limited only by spontaneous recombination. On the other hand, application of a Voigt magnetic field mixes the spin eigenstates, which increases the dephasing rate by up to 75%. These results are reproduced with a simple and intuitive model that captures the essential features of the electron hyperfine interaction and its influence on γ.
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Submitted 29 September, 2014;
originally announced September 2014.
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Coherent Electronic Coupling in Atomically Thin MoSe2
Authors:
Akshay Singh,
Galan Moody,
Sanfeng Wu,
Yanwen Wu,
Nirmal J. Ghimire,
Jiaqiang Yan,
David G. Mandrus,
Xiaodong Xu,
Xiaoqin Li
Abstract:
We report the first direct spectroscopic evidence for coherent electronic coupling between excitons and trions in atomically thin transition metal dichalcogenides, specifically monolayer MoSe2. Signatures of coupling appear as isolated cross-peaks in two-color pump-probe spectra, and the lineshape of the peaks reveals that the coupling originates from many-body interactions. Excellent agreement be…
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We report the first direct spectroscopic evidence for coherent electronic coupling between excitons and trions in atomically thin transition metal dichalcogenides, specifically monolayer MoSe2. Signatures of coupling appear as isolated cross-peaks in two-color pump-probe spectra, and the lineshape of the peaks reveals that the coupling originates from many-body interactions. Excellent agreement between the experiment and density matrix calculations suggests the formation of a correlated exciton-trion state due to their coupling.
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Submitted 9 January, 2014;
originally announced January 2014.
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Exciton-Trion Coherent Interactions in a CdTe/CdMgTe Quantum Well
Authors:
G. Moody,
I. A. Akimov,
H. Li,
R. Singh,
D. R. Yakovlev,
G. Karczewski,
M. Wiater,
T. Wojtowicz,
M. Bayer,
S. T. Cundiff
Abstract:
We present a collection of zero-, one- and two-quantum two-dimensional coherent spectra of excitons and trions in a CdTe/(Cd,Mg)Te quantum well. The set of spectra provides a unique and comprehensive picture of the exciton and trion nonlinear optical response. Exciton-exciton and exciton-trion coherent coupling is manifest as distinct peaks in the spectra, whereas signatures of trion-trion interac…
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We present a collection of zero-, one- and two-quantum two-dimensional coherent spectra of excitons and trions in a CdTe/(Cd,Mg)Te quantum well. The set of spectra provides a unique and comprehensive picture of the exciton and trion nonlinear optical response. Exciton-exciton and exciton-trion coherent coupling is manifest as distinct peaks in the spectra, whereas signatures of trion-trion interactions are absent. Excellent agreement using density matrix calculations is obtained, which highlights the essential role of many-body effects on coherent interactions in the quantum well.
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Submitted 9 August, 2013;
originally announced August 2013.
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Coherent Excitonic Coupling in an Asymmetric Double InGaAs Quantum Well Arises from Many-Body Effects
Authors:
Gael Nardin,
Galan Moody,
Rohan Singh,
Travis M. Autry,
Hebin Li,
Francois Morier-Genoud,
Steven T. Cundiff
Abstract:
We study an asymmetric double InGaAs quantum well using optical two-dimensional coherent spectroscopy. The collection of zero-quantum, one-quantum, and two-quantum two-dimensional spectra provides a unique and comprehensive picture of the double well coherent optical response. Coherent and incoherent contributions to the coupling between the two quantum well excitons are clearly separated. An exce…
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We study an asymmetric double InGaAs quantum well using optical two-dimensional coherent spectroscopy. The collection of zero-quantum, one-quantum, and two-quantum two-dimensional spectra provides a unique and comprehensive picture of the double well coherent optical response. Coherent and incoherent contributions to the coupling between the two quantum well excitons are clearly separated. An excellent agreement with density matrix calculations reveals that coherent interwell coupling originates from many-body interactions.
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Submitted 29 January, 2014; v1 submitted 7 August, 2013;
originally announced August 2013.
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Correlation and dephasing effects on the non-radiative coherence between bright excitons in an InAs QD ensemble measured with 2D spectroscopy
Authors:
Galan Moody,
Rohan Singh,
Hebin Li,
Ilya A. Akimov,
Manfred Bayer,
Dirk Reuter,
Andreas D. Wieck,
Steven T. Cundiff
Abstract:
Exchange-mediated fine-structure splitting of bright excitons in an ensemble of InAs quantum dots is studied using optical two-dimensional Fourier-transform spectroscopy. By monitoring the non-radiative coherence between the bright states, we find that the fine-structure splitting decreases with increasing exciton emission energy at a rate of 0.1 $μ$eV/meV. Dephasing rates are compared to populati…
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Exchange-mediated fine-structure splitting of bright excitons in an ensemble of InAs quantum dots is studied using optical two-dimensional Fourier-transform spectroscopy. By monitoring the non-radiative coherence between the bright states, we find that the fine-structure splitting decreases with increasing exciton emission energy at a rate of 0.1 $μ$eV/meV. Dephasing rates are compared to population decay rates to reveal that pure dephasing causes the exciton optical coherences to decay faster than the radiative limit at low temperature, independent of excitation density. Fluctuations of the bright state transition energies are nearly perfectly-correlated, protecting the non-radiative coherence from interband dephasing mechanisms.
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Submitted 31 December, 2012;
originally announced December 2012.
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Fifth-order nonlinear optical response of excitonic states in an InAs quantum dot ensemble measured with 2D spectroscopy
Authors:
G. Moody,
R. Singh,
H. Li,
I. A. Akimov,
M. Bayer,
D. Reuter,
A. D. Wieck,
S. T. Cundiff
Abstract:
Exciton, trion and biexciton dephasing rates are measured within the inhomogeneous distribution of an InAs quantum dot (QD) ensemble using two-dimensional Fourier-transform spectroscopy. The dephasing rate of each excitonic state is similar for all QDs in the ensemble and the rates are independent of excitation density. An additional spectral feature -- too weak to be observed in the time-integrat…
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Exciton, trion and biexciton dephasing rates are measured within the inhomogeneous distribution of an InAs quantum dot (QD) ensemble using two-dimensional Fourier-transform spectroscopy. The dephasing rate of each excitonic state is similar for all QDs in the ensemble and the rates are independent of excitation density. An additional spectral feature -- too weak to be observed in the time-integrated four-wave mixing signal -- appears at high excitation density and is attributed to the $χ^{\textrm{(5)}}$ biexcitonic nonlinear response.
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Submitted 30 October, 2012;
originally announced October 2012.