We investigate the coupling of a multimode 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 nonlocality. This experiment demonstrates a cavityinduced modification of the 2DEG response and offers a route to probing the effect of Coulomb interactions in ultrastrongly coupled systems via reshaping of their cavity mode profiles.

Mid-infrared (mid-IR) frequency combs are powerful tools for molecular sensing, industrial monitoring, and precision spectroscopy, yet their development beyond 5 um remains limited. Electro-optic modulation offers a promising path toward compact, agile comb generation, but extending this approach into the mid-infrared has been hindered by the lack of practical, high-performance modulators. Here we present an approach that leads to efficient generation of mid infrared frequency combs around 9 um, by employing ultrafast, room temperature, free space electro optic intensity modulators. By driving a single modulator with short electrical pulse trains, we realize both single- and dual-comb operation from a continuous-wave quantum cascade laser, providing a compact and versatile platform for mid-IR spectroscopy. This scheme produces combs with tunable repetition rates down to the megahertz range with direct observation on an electrical spectrum analyzer without any interferometer. As a proof of concept, we perform single- and dual-comb spectroscopy of a germanium etalon and an ammonia cell, achieving resolution far beyond that of conventional Fourier-transform infrared (FTIR) spectrometers and highlighting the potential of this approach for precise measurements in the long-wavelength molecular-fingerprint region. These results establish high-performance mid-IR modulators as a promising route toward practical, energy-efficient frequency-comb systems for sensing and spectroscopy.

We demonstrate mid-IR integrated race-track resonators on a III-V semiconductor platform: InGaAs core epitaxially grown on InP. We have performed a complete characterization of the optical propagation losses at lambda = 4.6 mu m and lambda = 8.5 mu m, two representative wavelengths for the first and second atmospheric transparency windows, respectively. We measured losses of alpha similar to 1 dB/cm (TE polarization) and alpha similar to 1.3 dB/cm (TM polarization) at 8.5 mu m. Substantially lower losses alpha similar to 0.3 dB/cm were measured at lambda = 4.6 mu m (TM polarization). We then implemented racetrack resonators with straight evanescent couplers. We obtained loaded quality factors larger than 600 000 at lambda = 4.6 mu m. These results are promising toward the development of non-linear mid-IR integrated devices with Q factors beyond 10(6), where the onset of stimulated parametric processes could be reachable.

We report experimental evidence of the interaction between intersubband polaritons and longitudinal optical phonons in non-dispersive mid-infrared cavities, under resonant optical injection. Light emission originating from spontaneous polariton-phonon scattering is observed at a frequency corresponding to an energy shift of one phonon below the pump frequency. Given the extremely low spontaneous scattering rate, we employ a custom-developed quantum mechanical model to numerically demonstrate the feasibility of stimulating such a process using a pump-probe scheme. Based on this theoretical framework, we identify a set of experimental conditions under which optical gain may be realized in an intersubband polaritonic system. (c) 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

We demonstrate a free-space amplitude modulator for mid-infrared radiation (lambda=9.6 um) that operates at room temperature up to at least 20 GHz (above the -3dB cutoff frequency measured at 8.2 GHz). The device relies on the ultra-fast transition between weak and strong-coupling regimes induced by the variation of the applied bias voltage. Such transition induces a modulation of the device reflectivity. It is made of a semiconductor heterostructure enclosed in a judiciously designed array of metal-metal optical resonators, that - all-together - behave as an electrically tunable surface. At negative bias, it operates in the weak light-matter coupling regime. Upon application of an appropriate positive bias, the quantum wells populate with electrons and the device transitions to the strong-coupling regime. The modulator transmission keeps linear with input RF power in the 0dBm - 9dBm range. The increase of optical powers up to 25 mW exhibit a weak beginning saturation a little bit below.

We present an experimental technique to accurately predict the formation of vibro-polaritons from a molecular polymeric film embedded in a resonant mid-infrared cavity. Using simple Fourier-transform reflectance measurement, we extract the complex dielectric function of a polyethylene film using Kramers-Kronig relations. The fitted dielectric function can be plugged into a numerical code to predict the strength and dispersion of the strong light-matter coupling regime between the quantized electromagnetic modes of a microcavity and the vibrational bands of the molecules. As a demonstration, we experimentally resolve the simultaneous formation of multiple vibro-polariton modes issued from the strong coupling of some vibrational bands of the methylene group (CH 2 ) in a 2.5-mu m-thick polyethylene film embedded in a microcavity. We measure a Rabi splitting of 6.3 THz for the stretching doublet around 87.5 THz and a Rabi splitting of 1.1 THz for the scissoring doublet around 43.7 THz, in excellent agreement with numerical predictions.

We demonstrate scalable ultra-strong light-matter coupling with intersubband polaritons in a truly harmonic confining potential. The harmonicity grants immunity from electron-electron interactions, a protection guaranteed by the Kohn theorem, allowing the intersubband transition frequency to be lowered while keeping the light-matter interaction strength constant. In principle, this procedure permits increasing the relative coupling strength (eta = Omega(Rabi)/omega(12)) at will. We measure a record low intersubband transition at 1.24 THz and a lower polaritonic mode at 920 GHz, below the barrier of 1 THz. The system exhibits a eta ratio of 0.24, fully in the ultra-strong coupling regime, and remains stable up to 78 K. This approach is valuable for future non-adiabatic quantum electrodynamic experiments at long wavelengths.

Continuously graded parabolic quantum wells with excellent optical performances are used to overcome the low-frequency and thermal limitations of square quantum wells at terahertz (THz) frequencies. The formation of microcavity intersubband polaritons at frequencies as low as 1.8 THz is demonstrated, with a sustained ultra-strong coupling regime up to a temperature of 200 K. Thanks to the excellent intersubband transition linewidth, polaritons present quality factors up to 17. It is additionally shown that the ultra-strong coupling regime is preserved when the active region is embedded in sub-wavelength resonators, with an estimated relative strength eta = Omega(R)/omega(0) = 0.12. This represents an important milestone for future studies of quantum vacuum radiation because such resonators can be optically modulated at ultrafast rates, possibly leading to the generation of non-classical light via the dynamic Casimir effect. Finally, with an effective volume of 2x10(-6)lambda(3)(0), it is estimated that fewer than 3000 electrons per resonator are ultra-strongly coupled to the quantized electromagnetic mode, proving it is also a promising approach to explore few-electron polaritonic systems operating at relatively high temperatures.

We demonstrate that absorption saturation of a mid-infrared intersubband transition can be engineered to occur at moderate light intensities of the order of 10-20 kW.cm^-2 and at room temperature. The structure consists of an array of metal-semiconductor-metal patches hosting a judiciously designed 253 nm thick GaAs/AlGaAs semiconductor heterostructure. At low incident intensity the structure operates in the strong light-matter coupling regime and exhibits two absorption peaks at wavelengths close to 8.9 μm. Saturation appears as a transition to the weak coupling regime - and therefore to a single-peaked absorption - when increasing the incident intensity. Comparison with a coupled mode theory model explains the data and permits to infer the relevant system parameters. When the pump laser is tuned at the cavity frequency, the reflectivity decreases with increasing incident intensity. When instead the laser is tuned at the polariton frequencies, the reflectivity non-linearly increases with increasing incident intensity. At those wavelengths the system therefore mimics the behavior of a saturable absorption mirror (SESAM) in the mid-IR range, a technology that is currently missing.

We theoretically study the coherent nonlinear response of electrons confined in semiconductor quantum wells under the effect of an electromagnetic radiation close to resonance with an intersubband transition. Our approach is based on the time-dependent Schrödinger-Poisson equation stemming from a Hartree description of Coulomb-interacting electrons. This equation is solved by standard numerical tools and the results are interpreted in terms of approximated analytical formulas. For growing intensity, we observe a redshift of the effective resonance frequency due to the reduction of the electric dipole moment and the corresponding suppression of the depolarization shift. The competition between coherent nonlinearities and incoherent saturation effects is discussed. The strength of the resulting optical nonlinearity is estimated across different frequency ranges from mid-IR to THz with an eye to ongoing experiments on Bose-Einstein condensation of intersubband polaritons and to the speculative exploration of quantum optical phenomena such as single-photon emission in the mid-IR and THz windows.

We present a mid-IR ($\lambda \approx$ 8.3 μm) quantum well infrared photodetector (QWIP) fabricated on a transparent substrate, allowing photo detection with illumination from either the front surface or through the substrate. The device is based on a 400 nm-thick GaAs/AlGaAs semiconductor QWIP heterostructure enclosed in a metal-insulator-metal (MIM) optical cavity. For two different excitation configurations - backside illumination through the substrate and frontside direct illumination - we present passive optical characterizations (reflectivity and transmissivity), electric transport measurements and photocurrent generation, showing similar broadband spectral response for the two detector ports and an experimental $T_{\text{blip}}$ of $\approx$ 200K.

We analyse the effect of a strong perpendicular magnetic field on an intersubband transition in a disordered doped quantum well strongly coupled to an optical cavity. The magnetic field changes the lineshape of the intersubband optical transition due to the interface roughness of the quantum well from a Lorentzian to a Gaussian one. In this regime, a novel form of magnetic-field-induced cavity protection sets in, which strongly reduces the polariton linewidth to the cavity contribution only. Implications of our results for fundamental studies of nonlinear polariton dynamics and for technological applications to polariton lasers are finally highlighted.

We report on an ultra-low threshold saturable absorber operating in the mid-infrared spectral region (λ ~ 9 μm). The device relies on intersubband transitions in semiconductor quantum wells embedded in metallic microcavities. It operates in the strong light-matter coupling regime. The absorption change stems from the collapse of the polaritons resonances upon bleaching of the transition. We have developed an analytical formalism based on temporal coupled mode theory to model the device non-linear behavior. Using a commercial TEC-cooled quantum cascade laser as source, we demonstrate saturation with a peak pumping power of 150 mW (corresponding to an intensity on the sample of 10 kW.cm -2 ).

This work studies the impact of the nonparabolicity on the formation of multisubband plasmons. We explore three semiclassical optical response models and compare their predictions to temperature-dependent absorption measurements from three structures: all doped GaAs/AlGaAs quantum wells with continuously varied parabolic binding potentials. We show that qualitative improvement in the prediction of the plasmon absorption peak can only be obtained by including both the energy and wave-function dependence on the in-plane wave vector. Our model, developed to include both these dependencies, uses a <(k)over the arrow > center dot <(p)over the arrow >-derived current density operator (instead of the usual scalar effective mass one). The model should be readily generalizable to a wide set of nanostructures, such as asymmetric half-parabolic wells or narrow band materials nanostructures beyond the quasi-Kohn regime.

Abstract - Non-collinear pump-probe spectroscopy with phase-stable multi-THz pulses reveals the ultrafast scattering dynamics of intersubband polaritons in a dispersive microcavity. For resonant excitation of the lower polariton at a finite in-plane momentum, $k_{||}$, a narrowband maximum emerges in the probe reflectivity at the polaritonic ground state at $k_{||}=0$. A quantum mechanical model reproduces the experimental key results quantitatively and confirms the underlying microscopic process as stimulated polariton-polariton scattering. These results are an important milestone on the way to bosonic lasing in custom tailored polaritonic systems in the multi-THz and THz spectral regions.