arXiv subjects

Jerome Faist

Publications and source records attributed to Jerome Faist.

At least 19 recordsLinked to original sources

arXiv2026

Fluctuation engineering in cavity quantum materials

Coupling tailored electromagnetic fluctuations to materials provides a resource for controlling correlated quantum matter. By structuring the frequency, spatial, and modal distribution of fluctuations through a new generation of cavity quantum materials, vacuum and thermal spectra can shift phase boundaries and stabilize or suppress orders. This review organizes the field around a fluctuation-focused perspective, surveying a practical design toolbox and recent milestones, and outlining theory-experiment challenges in realistic, multimode, beyond-long-wavelength regimes. We highlight photonic observables and map opportunities for equilibrium and driven control across superconducting, magnetic, moire, and topological platforms.

cond-mat.mes-hall↗

arXiv2026

Short mode-locked pulses from planarized Y-coupled THz lasers

Short THz pulses are highly attractive both for fundamental research and practical applications in this underdeveloped region of the electromagnetic spectrum. Typically, THz pulses are generated using nonlinear optical methods, starting from a high-power visible or near-IR mode-locked source, but usually suffer from low conversion efficiencies. Here, we present a direct on-chip THz source of coherent short pulse trains based on active mode-locking of an inverse-designed planarized Y-coupled THz quantum cascade laser. By employing quasi-resonant microwave modulation of the asymmetric laser cavity, mode-locked pulses as short as 2.3 ps are generated, with emission bandwidths spanning 500 and 700 GHz at a central frequency of 2.9 THz.

physics.optics↗

arXiv2026

Epitaxial growth optimization, measurement and theoretical analysis of strain-compensated QCL grown on (511)A InP

Interface roughness scattering is an important limiting factor for achieving high performance Quantum Cascade Lasers. Following recent results, we study the growth conditions for a strain-compensated QCL emitting around 4.6 {\mu}m grown on a (511)A InP substrate using AFM and XRD measurements. We find that modulating the arsenic flux and correctly tailoring the III/V ratio is fundamental to achieve a good quality material. We report the first lasing device on such a platform with a current density threshold of 1.34 kA/cm2 and a slope efficiency of 1.1 W/A, which result suboptimal compared to the (100) reference. Finally, we find a 7% redshift of the (511)A spectrum which we attribute to an impurity scattering due to the increased incorporation along the exposed (111) direction. We validate this statement by verifying that the change in CBO and effective electron mass due to strain along a non-trivial direction cannot cause such a shift by using the k-p method generalized to arbitrary growth directions

physics.app-ph↗

arXiv2025

Metasurface-Enhanced Mid-Infrared Imaging Spectroscopy with Broadband Quantum Cascade Lasers

Mid-infrared (mid-IR) spectroscopy offers unparalleled opportunities in sensing through chemically specific detection of molecular absorption fingerprints. Yet, its practical applications are limited by the weak light-matter interaction in the mid-IR range and low brightness of mid-IR light sources. Surface-enhanced infrared absorption (SEIRA) spectroscopy addresses the sensitivity limitations by leveraging resonant photonic structures, in particular, plasmonic and frequency-selective dielectric metasurfaces. However, current implementations of SEIRA approach mainly rely on complex instruments and scanning components such as Fourier-transform infrared spectroscopy and tunable external cavity quantum cascade lasers (EC QCLs). Here, we present a compact and high-throughput imaging-based SEIRA platform that combines broadband gradient metasurfaces with a radiofrequency-modulated QCL that generates remarkably broad instantaneous emission spectrum (250 cm$^{-1}$) covering absorption bands of multiple distinct molecular vibrational modes. By matching the resonance spectrum of the compact (1 mm$^2$) broadband gradient metasurface with the laser emission projected on its surface through a dispersive element, we ensure that every QCL spectral component is uniquely addressed for an efficient targeted enhancement of the electromagnetic field. This enables us to use a low-cost and room-temperature mid-IR camera, acquiring in a single frame the enhanced absorption signatures of analytes deposited on the metasurface as a barcode image, thus reducing the acquisition time by up to 3 orders of magnitude compared to the FTIR and EC QCL based measurements. Eliminating the need for tunable light sources, bulky spectrometers, and expensive low-temperature detectors, our approach enables high-throughput, miniaturized, and highly specific molecular diagnostics for diverse chemical and biological applications.

physics.optics↗

arXiv2025

Quantum Walk Comb in a Dual Waveguide Quantum Cascade Laser

Ring quantum cascade lasers (QCLs) proved to be a versatile tool for generating tunable and stable frequency combs in the mid infrared range in the form of quantum walk combs. By homogeneously integrating a racetrack QCL with a passive waveguide, which lays on top of the active region plane and therefore can be designed to be fully independent from the laser geometry, we improve the light outcoupling from the ring by more than 2 orders of magnitude reaching a maximum output power of 120 mW. In addition, we show that it is possible to achieve quantum walk comb operation in the devices under analysis. Finally, we prove that we can change the light dispersion by tuning the parameters of the passive waveguide, with a direct impact on the behavior of the generated comb.

quant-ph↗

arXiv2024

On-chip, inverse-designed active wavelength division multiplexer at THz frequencies

The development of photonic integrated components for terahertz has become an active and growing research field. Despite its numerous applications, several challenges are still present in hardware design. We demonstrate an on-chip active wavelength division multiplexer (WDM) operating at THz frequencies. The WDM architecture is based on an inverse design topology optimization, which is applied in this case to the active quantum cascade heterostructure material embedded within a polymer in a planarized double metal cavity. Such an approach enables the fabrication of a strongly subwavelength device, with a normalized volume of only $V/\lambda^3 \simeq 0.5$. The WDM input is integrated with a THz quantum cascade laser frequency comb, providing three broadband output ports, ranging from 2.2 THz to 3.2 THz, with $\approx$ 330 GHz bandwidth and a maximum crosstalk of -6 dB. The three ports are outcoupled via integrated broadband patch array antennas with surface emission. Such a device can be also function as a stand-alone element, unlocking complex on-chip signal processing in the THz range

physics.optics↗

arXiv2024

Quench dynamics of Wannier-Stark states in an active synthetic photonic lattice

Photonic emulators have facilitated the investigation of numerous solid-state phenomena and have contributed to the development of optical devices inspired by quantum mechanics. Although current photonic emulators are constrained to bosonic behavior with local interactions, the utilization of active synthetic lattices holds promise for surpassing these limitations. In this study, we propose employing the modulated ring fast-gain laser as a foundation for emulating quench dynamics within a synthetic lattice that conforms to equal density filling of its reciprocal space. To illustrate the effectiveness of this emulation platform, we subject a dispersed Wannier-Stark ladder to quenching and directly observe oscillations, enabled by the fast-gain, along with their coherent stabilization to a single Wannier stark state. These coherent dynamics stem directly from our lasers liquid state of light, a characteristic resulting from fast-gain and explained by the rapid decay of fluctuations occurring on the system's shortest timescale. Additionally, by adequately biasing the lattice through detuning the modulation from the cavity resonance, this process supports oscillatory dynamics within the synthetic space.

physics.optics↗

arXiv2024

Controlling radiative heat flow through cavity electrodynamics

Cavity electrodynamics is emerging as a promising tool to control chemical processes and quantum material properties. In this work we develop a formalism to describe the cavity mediated energy exchange between a material and its electromagnetic environment. We show that coplanar cavities can significantly affect the heat load on the sample if the cavity resonance lies within the frequency region where free-space radiative heat dominates, typically the mid-IR at ambient temperature, while spectral filtering is necessary for having an effect with lower frequency cavities.

physics.optics↗

arXiv2023

Quantum Cascade Lasers as Broadband Sources via Strong RF Modulation

In this work, we demonstrate that in a regime of strong modulation, by generating pulses of the length of the order of a few cavity lifetimes (hundreds of ps), a broadband quantum cascade laser can be driven to lase on a bandwidth (250cm-1) limited by the gain. In addition, the amplitude noise of the radiation was shown to be limited by the detector. A laser linewidth study has been performed under different operating conditions finding values spanning from 20MHz to 800MHz, indicating a trade-off between emission bandwidth, amplitude stability and coherence.

physics.optics↗

arXiv2022

Sensitive dependence of the linewidth enhancement factor on electronic quantum effects in quantum cascade lasers

The linewidth enhancement factor (LEF) describes the coupling between amplitude and phase fluctuations in a semiconductor laser, and has recently been shown to be a crucial component for frequency comb formation in addition to linewidth broadening. It necessarily arises from causality, as famously formulated by the Kramers-Kronig relation, in media with non-trivial dependence of the susceptibility on intensity variations. While thermal contributions are typically slow, and thus can often be excluded by suitably designing the dynamics of an experiment, the many quantum contributions are harder to separate. In order to understand and, ultimately, design the LEF to suitable values for frequency comb formation, soliton generation, or narrow laser linewidth, it is therefore important to systematically model all these effects. In this comprehensive work, we introduce a general scheme for computing the LEF, which we employ with a non-equilibrium Green's function model. This direct method, based on simulating the system response under varying optical intensity, and extracting the dependence of the susceptibility to intensity fluctuations, can include all relevant electronic effects and predicts the LEF of an operating quantum cascade laser to be in the range of 0.1 - 1, depending on laser bias and frequency. We also confirm that many-body effects, off-resonant transitions, dispersive (Bloch) gain, counter-rotating terms, intensity-dependent transition energy, and precise subband distributions all significantly contribute and are important for accurate simulations of the LEF.

physics.optics↗

arXiv2022

Investigation of Lasing in Highly Strained Germanium at the Crossover to Direct Band Gap

Efficient and cost-effective Si-compatible lasers are a long standing wish of the optoelectronic industry. In principle, there are two options. For many applications, lasers based on III-V compounds provide compelling solutions, even if the integration is complex and therefore costly. However, where low costs and also high integration density are crucial, group-IV-based lasers - made of Ge and GeSn, for example - could be an alternative, provided their performance can be improved. Such progresses will come with better materials but also with the development of a profounder understanding of their optical properties. In this work, we demonstrate, using Ge microbridges with strain up to 6.6%, a powerful method for determining the population inversion gain and the material and optical losses of group IV lasers. This is made by deriving the values for the injection carrier densities and the cavity losses from the measurement of the change of the refractive index and the mode linewidth, respectively. We observe a laser threshold consistent with optical gain and material loss values obtained from a tight binding calculation. Lasing in Ge - at steady-state - is found to be limited to low temperatures in a narrow regime of tensile strain at the crossover to the direct band gap bandstructure. We explain this observation by parasitic intervalence band absorption that increases rapidly with higher injection densities and temperature. N-doping seems to reduce the material loss at low excitation but does not extend the lasing regime. We also discuss the impact of the optically inactive carriers in the L-valley on the linewidth of group IV lasers.

physics.optics↗

arXiv2022

Intensity correlations in quantum cascade laser harmonic frequency combs

A novel study on harmonic frequency combs emitted by Quantum Cascade Lasers (QCLs) is here presented, demonstrating the presence of intensity correlations between twin modes characterising the emission spectra. These originate from a Four-Wave Mixing (FWM) process driven by the active medium's third-order non-linearity. The study of such correlations is essential for the engineering of a new generation of semiconductor devices with the potential of becoming integrated emitters of light with quantum properties, such as squeezing and entanglement. Starting from experimental results, the limits of state-of-the-art technology are discussed as well as the possible methodologies that could lead to the detection of non-classical phenomena, or alternatively improve the design of QCLs, in the compelling perspective of generating quantum correlations in mid-infrared light.

physics.optics↗

arXiv2021

Regenerative Terahertz Quantum Detectors

Because of the ultrafast and photon-driven nature of the transport in their active region, we demonstrate that quantum cascade lasers can be operated as resonantly amplified terahertz detectors. Tunable responsivities up to 50 V/W and noise equivalent powers down to 100 pW/sqrt(Hz) are demonstrated at 4.7 THz. Constant peak responsivities with respect to the detector temperature are observed up to 80K. Thanks to the sub-ps intersubband lifetime electrical bandwidths larger than 20 GHz can be obtained, allowing the detection of optical beatnotes from quantum cascade THz frequency combs.

physics.optics↗

arXiv2020

Mid-infrared quantum cascade laser frequency combs based on multi-section waveguides

We present quantum cascade laser (QCL) frequency comb devices with engineered waveguides for managing the dispersion. The QCL waveguide consists of multiple sections with different waveguide widths. The narrow and wide sections of the waveguide are designed in a way to compensate the group velocity dispersion (GVD) of each other and thereby produce a flat and slightly negative GVD for the QCL. The QCL exhibits continuous comb operation over a large part of the dynamic range of the laser. Strong and narrow-linewidth intermode beatnotes are achieved in more than 300 mA wide operation current range. The comb device features also considerably high output power (>380 mW) and wide optical bandwidth (>55 cm-1)

physics.optics↗

arXiv2020

Quantum Electrodynamic Control of Matter: Cavity-Enhanced Ferroelectric Phase Transition

The light-matter interaction can be utilized to qualitatively alter physical properties of materials. Recent theoretical and experimental studies have explored this possibility of controlling matter by light based on driving many-body systems via strong classical electromagnetic radiation, leading to a time-dependent Hamiltonian for electronic or lattice degrees of freedom. To avoid inevitable heating, pump-probe setups with ultrashort laser pulses have so far been used to study transient light-induced modifications in materials. Here, we pursue yet another direction of controlling quantum matter by modifying quantum fluctuations of its electromagnetic environment. In contrast to earlier proposals on light-enhanced electron-electron interactions, we consider a dipolar quantum many-body system embedded in a cavity composed of metal mirrors, and formulate a theoretical framework to manipulate its equilibrium properties on the basis of quantum light-matter interaction. We analyze hybridization of different types of the fundamental excitations, including dipolar phonons, cavity photons, and plasmons in metal mirrors, arising from the cavity confinement in the regime of strong light-matter interaction. This hybridization qualitatively alters the nature of the collective excitations and can be used to selectively control energy-level structures in a wide range of platforms. Most notably, in quantum paraelectrics, we show that the cavity-induced softening of infrared optical phonons enhances the ferroelectric phase in comparison with the bulk materials. Our findings suggest an intriguing possibility of inducing a superradiant-type transition via the light-matter coupling without external pumping. We also discuss possible applications of the cavity-induced modifications in collective excitations to molecular materials and excitonic devices.

cond-mat.mes-hall↗

arXiv2019

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

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.

cond-mat.mes-hall↗

arXiv2019

Pulses from a mid-infrared quantum cascade laser frequency comb using an external compressor

A Martinez-type stretcher-compressor is used to modify the spectral phases of a high-power (~1 W) QCL comb emitted at 8.2 μm with more than 100 cm-1 spectral bandwidth. Using this scheme, we demonstrate a compression of the QCL output from a 134 ps continuous wave waveform, to a train of pulses of width 12 ps, and a power with peak to average ratio of 40.7. An evaluation of the phase noise of the free-running device yields an integrated timing jitter of 335 fs over the frequency range 20 kHz - 100 MHz, and a pulse-to-pulse jitter of 2.0 fs.

physics.optics↗

arXiv2018

Coexisting frequency combs spaced by an octave in a monolithic quantum cascade laser

Quantum cascade lasers are proving to be instrumental in the development of compact frequency comb sources at mid-infrared and terahertz frequencies. Here we demonstrate a heterogeneous terahertz quantum cascade laser with two active regions spaced exactly by one octave. Both active regions are based on a four-quantum well laser design and they emit a combined 3~mW peak power at 15~K in pulsed mode. The two central frequencies are 2.3~THz (bandwidth 300~GHz) and 4.6~THz (bandwidth 270~GHz). The structure is engineered in a way that allows simultaneous operation of the two active regions in the comb regime, serving as a double comb source as well as a test bench structure for all waveguide internal self-referencing techniques. Narrow RF beatnotes ($\sim$ 15~kHz) are recorded showing the simultaneous operation of the two combs, whose free-running coherence properties are investigated by means of beatnote spectroscopy performed both with an external detector and via self-mixing. Comb operation in a highly dispersive region (4.6~THz) relying only on gain bandwidth engineering shows the potential for broad spectral coverage with compact comb sources.

physics.optics↗