Showing 1–29 of 29 results for author: Scalari, G

Absence of a Superradiant Phase Transition in Dirac Landau Polaritons

Authors: Elsa Jöchl, Felix Helmrich, Frieder Lindel, Lucy Hale, Lorenzo Graziotto, Mona Jarrahi, Tobia F. Nova, Jérôme Faist, Giacomo Scalari

Abstract: One of the most striking predictions in cavity quantum electrodynamics is the condensation of photons into a macroscopically populated ground state, the so-called superradiant phase transition (SRPT). SRPTs are theorized to occur in light-matter coupled systems above a critical coupling strength, yet have not been experimentally realized in equilibrium. On the contrary, the very existence of SRPTs… ▽ More One of the most striking predictions in cavity quantum electrodynamics is the condensation of photons into a macroscopically populated ground state, the so-called superradiant phase transition (SRPT). SRPTs are theorized to occur in light-matter coupled systems above a critical coupling strength, yet have not been experimentally realized in equilibrium. On the contrary, the very existence of SRPTs has been largely disputed by No-Go theorems. In cavity-coupled electronic systems with Dirac dispersion, the diamagnetic $\vec{A}^2$-term crucial to No-go theorems is not present at leading order, making graphene Landau level transitions ultrastrongly coupled to terahertz cavities good candidates for SRPTs. In this work, we present the first terahertz spectroscopic measurements of an hBN-encapsulated monolayer graphene flake coupled to a highly sub-wavelength resonator mode. By tuning the graphene carrier density, we drive the resulting Landau polaritons into the ultrastrong coupling regime, with the normalized coupling reaching $\approx 40 \%$, approaching criticality. In this regime, the continuous SRPT would lead to a unique spectroscopic polariton softening, which we consistently rule out. The full polariton dispersion is instead quantitatively reproduced by a Hopfield Hamiltonian using a quasistatic near-field model that accounts for the sub-wavelength character of the cavity. △ Less

Submitted 19 June, 2026; v1 submitted 26 May, 2026; originally announced May 2026.

Profiling THz Beams With Off-Label Use of Infrared Microbolometric Cameras

Authors: Gabriel Nagamine, Carlo Vicario, Tariq Leinen, Guy Matmon, Marco Raffa, Mattias Beck, Giacomo Scalari, Adrian L. Cavalieri, Flavio Giorgianni

Abstract: Visualizing the spatial profile of light beams is essential for evaluating irradiance, characterizing beam quality, and achieving precise alignment. In the optical spectral range, this is readily performed using silicon-based CCD and CMOS cameras. In the terahertz (THz) range, however, it typically requires specialized detectors with prohibitive costs. Here, we show that an infrared (IR) camera ca… ▽ More Visualizing the spatial profile of light beams is essential for evaluating irradiance, characterizing beam quality, and achieving precise alignment. In the optical spectral range, this is readily performed using silicon-based CCD and CMOS cameras. In the terahertz (THz) range, however, it typically requires specialized detectors with prohibitive costs. Here, we show that an infrared (IR) camera can be used outside of its labeled specifications to achieve similar performance as a dedicated microbolometric THz camera, at under 1% of the THz camera's cost. We compared the cameras by characterizing THz beam profiles from two sources: a pulsed broadband THz beam produced through optical rectification in organic crystals, and a narrowband quasi-continuous-wave (quasi-CW) THz beam emitted by a quantum cascade laser. For the broadband THz radiation, the beam width measured by the two cameras differed by only ~ 6%, well within the pixel resolution limit, and in the narrowband quasi-CW case by just ~ 1.3%. Additionally, the IR camera exhibits a lower minimum detectable power (down to 1.5 THz) than the THz camera, while also maintaining a linear and polarization-independent responsivity. These results expand the applicability of conventional IR cameras to the THz range, suggesting that they will become routine tools for high-fidelity THz beam diagnostics and imaging in scientific and industrial applications. △ Less

Submitted 30 June, 2026; v1 submitted 23 February, 2026; originally announced February 2026.

Comment on "Electrostatics-induced breakdown of the integer quantum Hall effect in cavity QED''

Authors: C. Ciuti, G. Scalari, J. Faist

Abstract: We comment on the preprint arXiv:2511.04744 by Andolina et al. We comment on the preprint arXiv:2511.04744 by Andolina et al. △ Less

Submitted 21 January, 2026; originally announced January 2026.

Comments: Comment on arXiv:2511.04744

Gate-tunable single terahertz meta-atom ultrastrong light-matter coupling

Authors: Elsa Jöchl, Anna-Lydia Vieli, Lucy Hale, Felix Helmrich, Deniz Turan, Mona Jarrahi, Mattias Beck, Jérôme Faist, Giacomo Scalari

Abstract: We study the electrical tunability of ultrastrong light-matter interactions between a single terahertz circuit-based complementary split ring resonator (cSRR) and a two-dimensional electron gas. For this purpose, transmission spectroscopy measurements are performed under the influence of a strong magnetic field at different set points for the electric gate bias. The resulting Landau polariton disp… ▽ More We study the electrical tunability of ultrastrong light-matter interactions between a single terahertz circuit-based complementary split ring resonator (cSRR) and a two-dimensional electron gas. For this purpose, transmission spectroscopy measurements are performed under the influence of a strong magnetic field at different set points for the electric gate bias. The resulting Landau polariton dispersion depends on the applied electric bias, as the gating technique confines the electrons in-plane down to extremely sub-wavelength dimensions as small as d = 410 nm. This confinement allows for the excitation of standing plasma waves at zero magnetic field and an effective tunability of the electron number coupled to the THz resonator. This allows the normalized coupling strength to be tuned in-situ from $η$ = 0.46 down to $η$ = 0.18. This is the first demonstration of terahertz far-field spectroscopy of an electrically tunable interaction between a single terahertz resonator and electrons in a GaAs quantum well heterostructure. △ Less

Submitted 5 November, 2025; originally announced November 2025.

Cavity modification of magnetoplasmon mode through coupling with intersubband polaritons

Authors: Lucy L. Hale, Daniele De Bernardis, Stephan Lempereur, Lianhe H. Li, A. Giles Davies, Edmund H. Linfield, Trevor Blaikie, Chris Deimert, Zbigniew R. Wasilewski, Iacopo Carusotto, Jean-Michel Manceau, Mathieu Jeannin, Raffaele Colombelli, Jérôme Faist, Giacomo Scalari

Abstract: We investigate the coupling of a multi-mode metal-insulator-metal cavity to a two-dimensional electron gas (2DEG) in a quantum well in the presence of a strong magnetic field. The TM cavity mode is strongly hybridized with an intersubband transition of the 2DEG, forming a polaritonic mode in the ultrastrong coupling regime, while the TE mode remains an almost purely cavity mode. The magnetoplasmon… ▽ More We investigate the coupling of a multi-mode metal-insulator-metal cavity to a two-dimensional electron gas (2DEG) in a quantum well in the presence of a strong magnetic field. The TM cavity mode is strongly hybridized with an intersubband transition of the 2DEG, forming a polaritonic mode in the ultrastrong coupling regime, while the TE mode remains an almost purely cavity mode. The magnetoplasmon excitation emerging from the presence of the magnetic field couples with both TM and TE modes, exhibiting different coupling strengths and levels of spatial field inhomogeneity. While the strong homogeneity of the bare TE mode gives rise to the standard anticrossing of strong coupling, the inhomogeneous polaritonic TM mode is shown to activate an observable Coulombic effect in the spectral response, often referred to as non-locality. This experiment demonstrates a cavity-induced modification of the 2DEG response and offers a new route to probing the effect of Coulomb interactions in ultrastrongly coupled systems via reshaping of their cavity mode profiles. △ Less

Submitted 5 March, 2026; v1 submitted 21 October, 2025; originally announced October 2025.

Multi-mode Deep Strong Coupling in a Multi Quantum Well Fabry-Perot Cavity

Authors: Lucy Hale, Johan Andberger, Ethan Koskas, Frieder Lindel, Mattias Beck, Giacomo Scalari, Jérôme Faist

Abstract: We present multi-mode deep-strong coupling in a multi-quantum well (N=166) heterostructure. The heterostructure itself acts as a Fabry-Perot cavity, for which the even cavity modes strongly couple to the cyclotron resonance to form Landau polaritons. The experimentally observed vacuum Rabi splitting is larger than the mode spacing and well into the deep-strong coupling regime ($η>1$) resulting in… ▽ More We present multi-mode deep-strong coupling in a multi-quantum well (N=166) heterostructure. The heterostructure itself acts as a Fabry-Perot cavity, for which the even cavity modes strongly couple to the cyclotron resonance to form Landau polaritons. The experimentally observed vacuum Rabi splitting is larger than the mode spacing and well into the deep-strong coupling regime ($η>1$) resulting in a rich multi-mode polaritonic spectrum which is accurately reproduced by an all-to-all multi-photonic, multi-electronic Hopfield coupling model. Remarkably, light-matter decoupling is observed across the whole measurable spectrum, including in the low frequency limit ($λ>>L_{cav}$) where the normalized coupling strength reaches $η=8.1$. The system demonstrates a robust platform for exploring extreme coupling regimes and its chiral nature holds potential for chiral cavity and chiral mirror applications. △ Less

Submitted 2 December, 2025; v1 submitted 27 August, 2025; originally announced August 2025.

Comments: 34 pages, 7 figures

Hybrid interfaces at the single quantum level in fluorescent molecules

Authors: Daniele De Bernardis, Hugo Levy-Falk, Elena Fanella, Rocco Duquennoy, Valerio Digiorgio, Giacomo Scalari, Maja Colautti, Costanza Toninelli

Abstract: We theoretically investigate a single fluorescent molecule as a hybrid quantum optical device, in which multiple external laser sources exert control of the vibronic states. In the high-saturation regime, a coherent interaction is established between the vibrational and electronic degrees of freedom, and molecules can simulate several cavity QED models, whereby a specific vibrational mode plays th… ▽ More We theoretically investigate a single fluorescent molecule as a hybrid quantum optical device, in which multiple external laser sources exert control of the vibronic states. In the high-saturation regime, a coherent interaction is established between the vibrational and electronic degrees of freedom, and molecules can simulate several cavity QED models, whereby a specific vibrational mode plays the role of the cavity mode. Focusing on the specific example where the system is turned into an analogue simulator of the quantum Rabi model, the steady state exhibits vibrational bi-modality resulting in a statistical mixture of highly non-classical vibronic cat states. Applying our paradigm to molecules with prominent spatial asymmetry and combining an optical excitation with a THz(IR) driving, the system can be turned into a single photon transducer. Two possible implementations are discussed based on the coupling to a subwavelength THz patch antenna or a resonant metamaterial. In a nutshell, this work assesses the role of molecules as an optomechanical quantum toolbox for creating hybrid entangled states of electrons, photons, and vibrations, hence enabling frequency conversion over very different energy scales. △ Less

Submitted 23 April, 2025; v1 submitted 26 March, 2025; originally announced March 2025.

Cavity QED Control of Quantum Hall Stripes

Authors: Lorenzo Graziotto, Josefine Enkner, Sambuddha Chattopadhyay, Jonathan B. Curtis, Ethan Koskas, Christian Reichl, Werner Wegscheider, Giacomo Scalari, Eugene Demler, Jérôme Faist

Abstract: Controlling quantum phases of materials with vacuum field fluctuations in engineered cavities is a novel route towards the optical control of emergent phenomena. We demonstrate, using magnetotransport measurements of a high-mobility two-dimensional electron gas, striking cavity-induced anisotropies in the electronic transport, including the suppression of the longitudinal resistance well below the… ▽ More Controlling quantum phases of materials with vacuum field fluctuations in engineered cavities is a novel route towards the optical control of emergent phenomena. We demonstrate, using magnetotransport measurements of a high-mobility two-dimensional electron gas, striking cavity-induced anisotropies in the electronic transport, including the suppression of the longitudinal resistance well below the resistivity at zero magnetic field. Our cavity-induced effects occur at ultra-low temperatures (< 200 mK) when the magnetic field lies between quantized Hall plateaus. We interpret our results as arising from the stabilization of thermally-disordered quantum Hall stripes. Our work presents a clear demonstration of the cavity QED control of a correlated electronic phase. △ Less

Submitted 21 February, 2025; originally announced February 2025.

Comments: Main: 7 pages, 4 figures. Supplementary Material: 31 pages, 19 figures, 1 table

Cavity-Driven Attractive Interactions in Quantum Materials

Authors: F. Helmrich, H. S. Adlong, I. Khanonkin, M. Kroner, G. Scalari, J. Faist, A. Imamoglu, T. F. Nova

Abstract: Many-body phenomena in quantum materials emerge from the interplay among a broad continuum of electronic states, and controlling these interactions is critical for engineering novel phases. One promising approach exploits fluctuations of the vacuum electromagnetic field confined within optical cavities to tailor electronic properties. Here, we demonstrate that cavity photons can mediate attractive… ▽ More Many-body phenomena in quantum materials emerge from the interplay among a broad continuum of electronic states, and controlling these interactions is critical for engineering novel phases. One promising approach exploits fluctuations of the vacuum electromagnetic field confined within optical cavities to tailor electronic properties. Here, we demonstrate that cavity photons can mediate attractive interactions in a tunable van der Waals material and reorganize a continuum of electron-hole transitions into an exciton-like state. We introduce a broadband, sub-wavelength time-domain microscope that integrates exfoliated, dual-gated two-dimensional quantum materials into a terahertz cavity. This approach enables the first-ever measurement of the field-tunable bandgap of bilayer graphene at terahertz frequencies while revealing ultrastrong coupling with a vacuum Rabi frequency exceeding $Ω_{Rabi}/ω\approx 40\%$ of the bare photon energy. Crucially, we identify a novel cavity-induced resonance emerging from the interband continuum that resembles Coulomb-bound excitons and remains stable across a broad temperature range. By uniting longstanding theoretical predictions with advanced experimental techniques, our findings open new avenues for designing and probing unique light-matter states and realizing hybrid correlated phases in quantum materials. △ Less

Submitted 23 April, 2025; v1 submitted 31 July, 2024; originally announced August 2024.

Enhanced fractional quantum Hall gaps in a two-dimensional electron gas coupled to a hovering split-ring resonator

Authors: Josefine Enkner, Lorenzo Graziotto, Dalin Boriçi, Felice Appugliese, Christian Reichl, Giacomo Scalari, Nicolas Regnault, Werner Wegscheider, Cristiano Ciuti, Jérôme Faist

Abstract: The magnetotransport of a high-mobility two-dimensional electron gas coupled to a hovering split-ring resonator with controllable distance is studied in the quantum Hall regime. The measurements reveal an enhancement by more than a factor 2 of the quantum Hall energy gaps at the fractional filling factors 4/3, 5/3, and 7/5, alongside a concurrent reduction in exchange splitting at odd integer fill… ▽ More The magnetotransport of a high-mobility two-dimensional electron gas coupled to a hovering split-ring resonator with controllable distance is studied in the quantum Hall regime. The measurements reveal an enhancement by more than a factor 2 of the quantum Hall energy gaps at the fractional filling factors 4/3, 5/3, and 7/5, alongside a concurrent reduction in exchange splitting at odd integer filling factors. Theoretically, we show the strength of both effects to be quantitatively compatible with the emergence of an effective electron-electron long-range attractive interaction mediated by the exchange of virtual cavity photons in the presence of significant spatial gradients of the cavity electric vacuum fields. These results unveil a compelling interplay between cavity quantum electrodynamics and electronic correlations in two-dimensional systems, with profound implications for the manipulation and control of quantum phases in 2D materials. △ Less

Submitted 28 May, 2024; originally announced May 2024.

Giant ultra-broadband photoconductivity in twisted graphene heterostructures

Authors: Hitesh Agarwal, Krystian Nowakowski, Andres Forrer, Alessandro Principi, Riccardo Bertini, Sergi Batlle-Porro, Antoine Reserbat-Plantey, Parmeshwar Prasad, Lorenzo Vistoli, Kenji Watanabe, Takashi Taniguchi, Adrian Bachtold, Giacomo Scalari, Roshan Krishna Kumar, Frank H. L. Koppens

Abstract: The requirements for broadband photodetection are becoming exceedingly demanding in hyperspectral imaging. Whilst intrinsic photoconductor arrays based on mercury cadmium telluride represent the most sensitive and suitable technology, their optical spectrum imposes a narrow spectral range with a sharp absorption edge that cuts their operation to < 25 um. Here, we demonstrate a giant ultra-broadban… ▽ More The requirements for broadband photodetection are becoming exceedingly demanding in hyperspectral imaging. Whilst intrinsic photoconductor arrays based on mercury cadmium telluride represent the most sensitive and suitable technology, their optical spectrum imposes a narrow spectral range with a sharp absorption edge that cuts their operation to < 25 um. Here, we demonstrate a giant ultra-broadband photoconductivity in twisted double bilayer graphene heterostructures spanning a spectral range of 2 - 100 um with internal quantum efficiencies ~ 40 % at speeds of 100 kHz. The giant response originates from unique properties of twist-decoupled heterostructures including pristine, crystal field induced terahertz band gaps, parallel photoactive channels, and strong photoconductivity enhancements caused by interlayer screening of electronic interactions by respective layers acting as sub-atomic spaced proximity screening gates. Our work demonstrates a rare instance of an intrinsic infrared-terahertz photoconductor that is complementary metal-oxide-semiconductor compatible and array integratable, and introduces twist-decoupled graphene heterostructures as a viable route for engineering gapped graphene photodetectors with 3D scalability. △ Less

Submitted 4 September, 2023; originally announced September 2023.

Report number: 17, 1047-1053

Journal ref: Nature Photonics 2023

Terahertz chiral sub-wavelength cavities breaking time-reversal symmetry via ultra-strong light-matter interaction

Authors: Johan Andberger, Lorenzo Graziotto, Luca Sacchi, Mattias Beck, Giacomo Scalari, Jérôme Faist

Abstract: We demonstrate terahertz chiral sub-wavelength cavities that break time-reversal symmetry by coupling the degenerate linearly polarized modes of two orthogonal sets of nano-antenna arrays using the inter-Landau level transition of a two-dimensional electron gas in a perpendicular magnetic field, realizing normalized light-matter coupling rates up to $Ω_R/ω_{\mathrm{cav}} = 0.78$ with a dispersion… ▽ More We demonstrate terahertz chiral sub-wavelength cavities that break time-reversal symmetry by coupling the degenerate linearly polarized modes of two orthogonal sets of nano-antenna arrays using the inter-Landau level transition of a two-dimensional electron gas in a perpendicular magnetic field, realizing normalized light-matter coupling rates up to $Ω_R/ω_{\mathrm{cav}} = 0.78$ with a dispersion that is modified by the parasitic capacitive coupling between the orthogonal antennas. The deep sub-wavelength confinement of the nano-antennas means that the ultra-strong coupling regime can be reached even with a small number of carriers compared to Fabry-Perot cavities, making it viable to be used with a variety of 2D materials. The non-degenerate circularly polarized ground state was only obtained after carefully optimizing the optical design to minimize the parasitic coupling to linearly polarized light. △ Less

Submitted 27 April, 2025; v1 submitted 6 August, 2023; originally announced August 2023.

Comments: 17 pages, 8 figures

An engineered planar plasmonic reflector for polaritonic mode confinement

Authors: Shima Rajabali, Josefine Enkner, Erika Cortese, Mattias Beck, Simone De Liberato, Jérôme Faist, Giacomo Scalari

Abstract: It was recently demonstrated that, in deep subwavelength gap resonators coupled to two-dimensional electron gases, coupling to propagating plasmons can lead to energy leakage and prevent the formation of polaritonic resonances. This process, akin to Landau damping, limits the achievable field confinement and thus the value of light-matter coupling strength. In this work, we show how plasmonic subw… ▽ More It was recently demonstrated that, in deep subwavelength gap resonators coupled to two-dimensional electron gases, coupling to propagating plasmons can lead to energy leakage and prevent the formation of polaritonic resonances. This process, akin to Landau damping, limits the achievable field confinement and thus the value of light-matter coupling strength. In this work, we show how plasmonic subwavelength reflectors can be used to create an artificial energy stopband in the plasmon dispersion, confining them and enabling the recovery of the polaritonic resonances. Using this approach we demonstrate a normalized light-matter coupling ratio of Ω/ω = 0.35 employing a single quantum well with a gap size of λ/2400 in vacuum. △ Less

Submitted 27 December, 2022; originally announced December 2022.

Comments: 13 pages, 3 figures

Journal ref: Optical Materials Express, Vol. 13, Issue 10, pp. 2944-2952 (2023)

An ultrastrongly coupled single THz meta-atom

Authors: Shima Rajabali, Sergej Markmann, Elsa Jöchl, Mattias Beck, Christian A. Lehner, Werner Wegscheider, Jérôme Faist, Giacomo Scalari

Abstract: Free-space coupling to strongly subwavelength individual optical elements is a central theme in quantum optics, as it allows to control and manipulate the properties of quantum systems. In this work, we show that by combining an asymmetric immersion lens setup and complementary design of metasurfaces we are able to perform THz time-domain spectroscopy of an individual, strongly subwavelength (d/λ0… ▽ More Free-space coupling to strongly subwavelength individual optical elements is a central theme in quantum optics, as it allows to control and manipulate the properties of quantum systems. In this work, we show that by combining an asymmetric immersion lens setup and complementary design of metasurfaces we are able to perform THz time-domain spectroscopy of an individual, strongly subwavelength (d/λ0=1/20) meta-atom. We unravel the linewidth dependence of planar metamaterials as a function of the meta-atom number indicating quenching of the Dicke superradiance. On these grounds, we investigate ultrastrongly coupled Landau polaritons at the single resonator level, measuring a normalized coupling ratio of Ω/ω=0.60 resulting from coupling of the fundamental mode to a few thousand electrons. Similar measurements on a low loss, less doped two dimensional electron gas yield a coupling ratio Ω/ω=0.33 with a cooperativity C=4g^2/κγ= 94. Interestingly, the coupling strength of a coupled single resonator is the same as of a coupled array. Our findings pave the way towards the control of light-matter interaction in the ultrastrong coupling regime at the single electron/single resonator level. The proposed technique is way more general and can be useful to characterize the complex conductivity of micron-sized samples in the THz and sub-THz domain. △ Less

Submitted 19 October, 2021; originally announced October 2021.

Comments: 28 pages, 5 figures

Breakdown of the topological protection by cavity vacuum fields in the integer quantum Hall effect

Authors: Felice Appugliese, Josefine Enkner, Gian Lorenzo Paravicini-Bagliani, Mattias Beck, Christian Reichl, Werner Wegscheider, Giacomo Scalari, Cristiano Ciuti, Jérôme Faist

Abstract: The control of the electronic properties of materials via the vacuum fields of cavity electromagnetic resonators is one of the emerging frontiers of condensed matter physics. We show here that the enhancement of vacuum field fluctuations in subwavelength split-ring resonators dramatically affects arguably one of the most paradigmatic quantum protectorates, namely the quantum Hall electron transpor… ▽ More The control of the electronic properties of materials via the vacuum fields of cavity electromagnetic resonators is one of the emerging frontiers of condensed matter physics. We show here that the enhancement of vacuum field fluctuations in subwavelength split-ring resonators dramatically affects arguably one of the most paradigmatic quantum protectorates, namely the quantum Hall electron transport in high-mobility 2D electron gases. The observed breakdown of the topological protection of the integer quantum Hall effect is interpreted in terms of a long-range cavity-mediated electron hopping where the anti-resonant terms of the light-matter coupling finally result into a finite resistivity induced by the vacuum fluctuations. The present experimental platform can be used for any 2D material and provides new ways to manipulate electron phases in matter thanks to vacuum-field engineering △ Less

Submitted 29 July, 2021; originally announced July 2021.

Journal ref: Science 375, 6584 (2022)

Polaritonic non-locality in ultrastrong light-matter coupling

Authors: Shima Rajabali, Erika Cortese, Mattias Beck, Simone De Liberato, Jérôme Faist, Giacomo Scalari

Abstract: Sub-wavelength electromagnetic field localization has been central in photonic research in the last decade, allowing to enhance sensing capabilities as well as increasing the coupling between photons and material excitations. The ultrastrong light-matter coupling regime in the THz range with split-ring resonators coupled to magnetoplasmons has been widely investigated, achieving successive world-r… ▽ More Sub-wavelength electromagnetic field localization has been central in photonic research in the last decade, allowing to enhance sensing capabilities as well as increasing the coupling between photons and material excitations. The ultrastrong light-matter coupling regime in the THz range with split-ring resonators coupled to magnetoplasmons has been widely investigated, achieving successive world-records for the largest light-matter coupling ever achieved. Ever shrinking resonators have allowed to approach the regime of few electrons strong coupling, in which single-dipole properties can be modified by the vacuum field. Here we demonstrate, theoretically and experimentally, the existence of a limit to the possibility of arbitrarily increasing electromagnetic confinement in polaritonic systems. Strongly sub-wavelength fields can excite a continuum of high-momenta propagative magnetoplasmons. This leads to peculiar nonlocal polaritonic effects, as certain polaritonic features disappear and the system enters in the regime of bound-to-continuum strong coupling. Emerging nonlinearities due to the local breaking of Kohn's theorem are also reported. △ Less

Submitted 21 January, 2021; originally announced January 2021.

Comments: 24 pages, 4 figures

Room temperature operation of n-type Ge/SiGe terahertz quantum cascade lasers predicted by non-equilibrium Green's functions

Authors: T. Grange, D. Stark, G. Scalari, J. Faist, L. Persichetti, L. Di Gaspare, M. De Seta, M. Ortolani, D. J. Paul, G. Capellini, S. Birner, M. Virgilio

Abstract: n-type Ge/SiGe terahertz quantum cascade laser are investigated using non-equilibrium Green's functions calculations. We compare the temperature dependence of the terahertz gain properties with an equivalent GaAs/AlGaAs QCL design. In the Ge/SiGe case, the gain is found to be much more robust to temperature increase, enabling operation up to room temperature. The better temperature robustness with… ▽ More n-type Ge/SiGe terahertz quantum cascade laser are investigated using non-equilibrium Green's functions calculations. We compare the temperature dependence of the terahertz gain properties with an equivalent GaAs/AlGaAs QCL design. In the Ge/SiGe case, the gain is found to be much more robust to temperature increase, enabling operation up to room temperature. The better temperature robustness with respect to III-V is attributed to the much weaker interaction with optical phonons. The effect of lower interface quality is investigated and can be partly overcome by engineering smoother quantum confinement via multiple barrier heights. △ Less

Submitted 30 November, 2018; originally announced November 2018.

Comments: 5 pages, 5 figures

Magneto-transport controlled by Landau polariton states

Authors: Gian L. Paravicini-Bagliani, Felice Appugliese, Eli Richter, Federico Valmorra, Janine Keller, Mattias Beck, Nicola Bartolo, Clemens Rössler, Thomas Ihn, Klaus Ensslin, Cristiano Ciuti, Giacomo Scalari, Jerome Faist

Abstract: Hybrid excitations, called polaritons, emerge in systems with strong light-matter coupling. Usually, they dominate the linear and nonlinear optical properties with applications in quantum optics. Here, we show the crucial role of the electronic component of polaritons in the magneto-transport of a cavity-embedded 2D electron gas in the ultrastrong coupling regime. We show that the linear dc resist… ▽ More Hybrid excitations, called polaritons, emerge in systems with strong light-matter coupling. Usually, they dominate the linear and nonlinear optical properties with applications in quantum optics. Here, we show the crucial role of the electronic component of polaritons in the magneto-transport of a cavity-embedded 2D electron gas in the ultrastrong coupling regime. We show that the linear dc resistivity is significantly modified by the coupling to the cavity even without external irradiation. Our observations confirm recent predictions of vacuum-induced modification of the resistivity. Furthermore, photo-assisted transport in presence of a weak irradiation field at sub-THz frequencies highlights the different roles of localized and delocalized states. △ Less

Submitted 17 October, 2018; v1 submitted 2 May, 2018; originally announced May 2018.

Comments: Accepted for Nature Physics

Journal ref: Nat. Phys. 15, 186-190 (2019)

Landau polaritons in highly non-parabolic 2D gases in the ultra-strong coupling regime

Authors: Janine Keller, Giacomo Scalari, Felice Appugliese, Shima Rajabali, Mattias Beck, Johannes Haase, Christian A. Lehner, Werner Wegscheider, Michele Failla, Maksym Myronov, David R. Leadley, James Lloyd-Hughes, Pierre Nataf, Jerome Faist

Abstract: We probe ultra-strong light matter coupling between metallic terahertz metasurfaces and Landau-level transitions in high mobility 2D electron and hole gases. We utilize heavy-hole cyclotron resonances in strained Ge and electron cyclotron resonances in InSb quantum wells, both within highly non-parabolic bands, and compare our results to well known parabolic AlGaAs/GaAs quantum well (QW) systems.… ▽ More We probe ultra-strong light matter coupling between metallic terahertz metasurfaces and Landau-level transitions in high mobility 2D electron and hole gases. We utilize heavy-hole cyclotron resonances in strained Ge and electron cyclotron resonances in InSb quantum wells, both within highly non-parabolic bands, and compare our results to well known parabolic AlGaAs/GaAs quantum well (QW) systems. Tuning the coupling strength of the system by two methods, lithographically and by optical pumping, we observe a novel behavior clearly deviating from the standard Hopfield model previously verified in cavity quantum electrodynamics: an opening of a lower polaritonic gap. △ Less

Submitted 30 December, 2019; v1 submitted 25 August, 2017; originally announced August 2017.

Journal ref: Phys. Rev. B 101, 075301 (2020)

THz ultrastrong light-matter coupling

Authors: Giacomo Scalari, Curdin Maissen, Sara Cibella, Roberto Leoni, Christian Reichl, Werner Wegscheider, Mattias Beck, Jérôme Faist

Abstract: Cavity photon resonators with ultrastrong light-matter interactions are attracting interest both in semiconductor and superconducting systems displaying the capability to manipulate the cavity quantum electrodynamic ground state with controllable physical properties. Here we review a series of experiments aimed at probing the ultrastrong light-matter coupling regime, where the vacuum Rabi splittin… ▽ More Cavity photon resonators with ultrastrong light-matter interactions are attracting interest both in semiconductor and superconducting systems displaying the capability to manipulate the cavity quantum electrodynamic ground state with controllable physical properties. Here we review a series of experiments aimed at probing the ultrastrong light-matter coupling regime, where the vacuum Rabi splitting $Ω$ is comparable to the bare transition frequency $ω$ . We present a new platform where the inter-Landau level transition of a two-dimensional electron gas (2DEG) is strongly coupled to the fundamental mode of deeply subwavelength split-ring resonators operating in the mm-wave range. Record-high values of the normalized light-matter coupling ratio $\fracΩω= 0.89$ are reached and the system appears highly scalable far into the microwave range. △ Less

Submitted 28 November, 2016; originally announced November 2016.

Journal ref: Il Nuovo Saggiatore, Vol 31, 3-4, pag.4-14, 2015

Sub-cycle measurement of intensity correlations in the Terahertz range

Authors: Ileana-Cristina Benea-Chelmus, Curdin Maissen, Giacomo Scalari, Mattias Beck, Jérôme Faist

Abstract: The Terahertz frequency range bears intriguing opportunities, beyond very advanced applications in spectroscopy and matter control. Peculiar quantum phenomena are predicted to lead to light emission by non-trivial mechanisms. Typically, such emission mechanisms are unraveled by temporal correlation measurements of photon arrival times, as demonstrated in their pioneering work by Hanbury Brown and… ▽ More The Terahertz frequency range bears intriguing opportunities, beyond very advanced applications in spectroscopy and matter control. Peculiar quantum phenomena are predicted to lead to light emission by non-trivial mechanisms. Typically, such emission mechanisms are unraveled by temporal correlation measurements of photon arrival times, as demonstrated in their pioneering work by Hanbury Brown and Twiss. So far, the Terahertz range misses an experimental implementation of such technique with very good temporal properties and high sensitivity. In this paper, we propose a room-temperature scheme to measure photon correlations at THz frequencies based on electro-optic sampling. The temporal resolution of 146 fs is faster than one cycle of oscillation and the sensitivity is so far limited to ~1500 photons. With this technique, we measure the photon statistics of a THz quantum cascade laser. The proposed measurement scheme allows, in principle, the measurement of ultrahigh bandwidth photons and paves the way towards THz quantum optics. △ Less

Submitted 16 December, 2015; v1 submitted 7 December, 2015; originally announced December 2015.

Journal ref: Phys. Rev. A 93, 043812 (2016)

InGaAs/AlInGaAs THz Quantum Cascade Lasers operating up to 195 K in strong magnetic field

Authors: Federico Valmorra, Giacomo Scalari, Keita Ohtani, Mattias Beck, Jerome Faist

Abstract: Terahertz quantum cascade lasers based on InGaAs wells and quaternary AlInGaAs barriers were measured in magnetic field. This study was carried out on a four quantum well active region design with photon energy of 14.3 meV processed both with Au and Cu waveguides. The heterostructure operates up to 148 K at B=0 T and in a Cu waveguide. The complete magneto-spectroscopic study allowed the compariso… ▽ More Terahertz quantum cascade lasers based on InGaAs wells and quaternary AlInGaAs barriers were measured in magnetic field. This study was carried out on a four quantum well active region design with photon energy of 14.3 meV processed both with Au and Cu waveguides. The heterostructure operates up to 148 K at B=0 T and in a Cu waveguide. The complete magneto-spectroscopic study allowed the comparison of emission and transport data. Increasing the magnetic field, the low effective mass of the InGaAs wells allowed us to reach the very strong confinement regime. At B=12 T, where the cyclotron transition is almost resonant with the LO-phonon, we recorded a maximum operating temperature of 195 K for the devices with Cu waveguide. Additional lasing at 5.9 meV was detected for magnetic fields between 7.3 and 7.7 T. △ Less

Submitted 20 February, 2015; v1 submitted 6 November, 2014; originally announced November 2014.

Journal ref: New J. Phys. 17 (2015) 023050

Ultrastrong Coupling in the Near-field of Complementary Split Ring Resonators

Authors: Curdin Maissen, Giacomo Scalari, Federico Valmorra, Sara Cibella, Roberto Leoni, Christian Reichl, Christohpe Charpentier, Werner Wegscheider, Mattias Beck, Jérôme Faist

Abstract: The ultrastrong light-matter interaction regime was investigated in metallic and superconducting complementary split ring resonators coupled to the cyclotron transition of two dimensional electron gases. The sub-wavelength light confinement and the large optical dipole moment of the cyclotron transition yield record high normalized coupling rates of up to $\frac{Ω_R}{ω_c}=$ 0.87. We observed a blu… ▽ More The ultrastrong light-matter interaction regime was investigated in metallic and superconducting complementary split ring resonators coupled to the cyclotron transition of two dimensional electron gases. The sub-wavelength light confinement and the large optical dipole moment of the cyclotron transition yield record high normalized coupling rates of up to $\frac{Ω_R}{ω_c}=$ 0.87. We observed a blue-shift of both polaritons due to the diamagnetic term of the interaction Hamiltonian. △ Less

Submitted 15 August, 2014; originally announced August 2014.

Octave-spanning semiconductor laser

Authors: Markus Rösch, Giacomo Scalari, Mattias Beck, Jérôme Faist

Abstract: We present here a semiconductor injection laser operating in continuous wave with an emission covering more than one octave in frequency, and displaying homogeneous power distribution among the lasing modes. The gain medium is based on a heterogeneous quantum cascade structure operating in the THz range. Laser emission in continuous wave takes place from 1.64 THz to 3.35 THz with optical powers in… ▽ More We present here a semiconductor injection laser operating in continuous wave with an emission covering more than one octave in frequency, and displaying homogeneous power distribution among the lasing modes. The gain medium is based on a heterogeneous quantum cascade structure operating in the THz range. Laser emission in continuous wave takes place from 1.64 THz to 3.35 THz with optical powers in the mW range and more than 80 modes above threshold. Free-running beatnote investigations on narrow waveguides with linewidths of 980 Hz limited by jitter indicate frequency comb operation on a spectral bandwidth as wide as 624 GHz, making such devices ideal candidates for octave-spanning semiconductor-laser-based THz frequency combs. △ Less

Submitted 11 July, 2014; originally announced July 2014.

Superconducting complementary metasurfaces for THz ultrastrong light-matter coupling

Authors: G. Scalari, C. Maissen, S. Cibella, R. Leoni, P. Carelli, F. Valmorra, M. Beck, J. Faist

Abstract: A superconducting metasurface operating in the THz range and based on the complementary metamaterial approach is discussed. Experimental measurements as a function of temperature and magnetic field display a modulation of the metasurface with a change in transmission amplitude and frequency of the resonant features. Such a metasurface is successively used as a resonator for a cavity quantum electr… ▽ More A superconducting metasurface operating in the THz range and based on the complementary metamaterial approach is discussed. Experimental measurements as a function of temperature and magnetic field display a modulation of the metasurface with a change in transmission amplitude and frequency of the resonant features. Such a metasurface is successively used as a resonator for a cavity quantum electrodynamic experiment displaying ultrastrong coupling to the cyclotron transition of a 2DEG. A finite element modeling is developed and its results are in good agreement with the experimental data. In this system a normalized coupling ratio of $\fracΩ{ω_c}=0.27$ is measured and a clear modulation of the polaritonic states as a function of the temperature is observed. △ Less

Submitted 1 November, 2013; originally announced November 2013.

Journal ref: New J. Phys. 16 033005 (2014)

Room temperature terahertz polariton emitter

Authors: Markus Geiser, Giacomo Scalari, Fabrizio Castellano, Mattias Beck, Jérôme Faist

Abstract: The strong-coupling regime between an electronic transition and the photonic mode of a optical resonator manifests itself in the lifting of the degeneracy between the two modes and the creation of two polariton states with mixed optical and electronic character. This phenomenon has been studied in atoms, excitons in semiconductors and quantum electrodynamics circuits based on Josephson junctions.… ▽ More The strong-coupling regime between an electronic transition and the photonic mode of a optical resonator manifests itself in the lifting of the degeneracy between the two modes and the creation of two polariton states with mixed optical and electronic character. This phenomenon has been studied in atoms, excitons in semiconductors and quantum electrodynamics circuits based on Josephson junctions. Recently, there is also strong interest to study similar effects using intersubband transitions in quantum wells in the terahertz, where the ultra strong coupling regime can be reached and new physical effects have been predicted. An other interesting feature of this system is that, in contrast to systems based on superconductors, the ultra strong coupling regime can be maintained up to room temperature. In this work, we demonstrate that parabolic quantum wells coupled to LC circuit resonators in the ultra strong coupling regime can achieve terahertz emission up to room temperature. △ Less

Submitted 9 January, 2012; originally announced January 2012.

Ultra strong coupling regime and plasmon-polaritons in parabolic semiconductor quantum wells

Authors: Markus Geiser, Fabrizio Castellano, Giacomo Scalari, Mattias Beck, Laurent Nevou, Jérôme Faist

Abstract: Ultra strong coupling is studied in a modulation-doped parabolic potential well coupled to an inductance-capacitance resonant circuit. In this system, in accordance to Kohn's theorem, strong reduction of the energy level separation caused by the electron-electron interaction compensates the depolarization shift. As a result, a very large ratio of 27% of the Rabi frequency to the center resonance f… ▽ More Ultra strong coupling is studied in a modulation-doped parabolic potential well coupled to an inductance-capacitance resonant circuit. In this system, in accordance to Kohn's theorem, strong reduction of the energy level separation caused by the electron-electron interaction compensates the depolarization shift. As a result, a very large ratio of 27% of the Rabi frequency to the center resonance frequency as well as a polariton gap of width 2? ? 670GHz are observed, suggesting parabolic quantum wells as the system of choice in order to explore the ultra-strong coupling regime. △ Less

Submitted 13 December, 2011; v1 submitted 30 November, 2011; originally announced November 2011.

Ultrastrong coupling of the cyclotron transition of a two-dimensional electron gas to a THz metamaterial

Authors: Giacomo Scalari, Curdin Maissen, Dana Turcinková, David Hagenmüller, Simone De Liberato, Cristiano Ciuti, Dieter Schuh, Christian Reichl, Werner Wegscheider, Mattias Beck, Jérôme Faist

Abstract: Artificial cavity photon resonators with ultrastrong light-matter interactions are attracting interest both in semiconductor and superconducting systems, due to the possibility of manipulating the cavity quantum electrodynamic ground state with controllable physical properties. We report here experiments showing ultrastrong light-matter coupling in a terahertz metamaterial where the cyclotron tran… ▽ More Artificial cavity photon resonators with ultrastrong light-matter interactions are attracting interest both in semiconductor and superconducting systems, due to the possibility of manipulating the cavity quantum electrodynamic ground state with controllable physical properties. We report here experiments showing ultrastrong light-matter coupling in a terahertz metamaterial where the cyclotron transition of a high mobility two-dimensional electron gas is coupled to the photonic modes of an array of electronic split-ring resonators. We observe a normalized coupling ratio $\fracΩ{ω_c}=0.58$ between the vacuum Rabi frequency $Ω$ and the cyclotron frequency $ω_c$. Our system appears to be scalable in frequency and could be brought to the microwave spectral range with the potential of strongly controlling the magnetotransport properties of a high-mobility 2DEG. △ Less

Submitted 4 May, 2012; v1 submitted 10 November, 2011; originally announced November 2011.

Journal ref: Science. 2012 Mar 16;335(6074):1323-6

Observation of Zone-Folded Acoustic Phonons in Terahertz Quantum Cascade Lasers using Picosecond Ultrasonics

Authors: Axel Bruchhausen, Mike Hettich, James Lloyd-Hughes, Milan Fischer, Mattias Beck, Giacomo Scalari, Jérôme Faist, Thomas Dekorsy

Abstract: We have investigated the time-resolved vibrational properties of terahertz quantum cascade lasers by means of ultra-fast laser spectroscopy. By the observation of the acoustic folded branches, and by analyzing the involved phonon modes it is possible to extract accurate structural information of these devices, which are essential for their design and performance. We have investigated the time-resolved vibrational properties of terahertz quantum cascade lasers by means of ultra-fast laser spectroscopy. By the observation of the acoustic folded branches, and by analyzing the involved phonon modes it is possible to extract accurate structural information of these devices, which are essential for their design and performance. △ Less

Submitted 6 December, 2010; originally announced December 2010.

Comments: 8 pages, 5 figures