arXiv subjects

Raffaele Colombelli

Publications and source records attributed to Raffaele Colombelli.

At least 19 recordsLinked to original sources

arXiv2026

Mid-IR single- and dual-electro-optic comb generation with an ultrafast modulator

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.

physics.optics↗

arXiv2026

Plasmonic-cavity Modulator for the Mid-IR with a Semi-transparent and Nonlinear Heavily-doped Semiconductor Mirror

We present a free-space plasmonic modulator based on a single heavily-doped semiconductor layer. We investigate its ability to modulate both the linear and nonlinear response at mid-infrared frequencies slightly below the plasma frequency of the semiconductor. We demonstrate electric control of the linear transmittance and reflectance, and of the efficiency of third-harmonic generation with a field-effect gate structure. We discuss further performance optimization of the device in terms of modulation speed and depth towards a fast modulator with very simple active material requirements. Our results establish a viable route toward practical plasmonic modulators and mixers operating in the mid-infrared atmospheric window available for free-space communications at wavelengths between 8 and 12 um.

physics.optics↗

arXiv2025

Cavity modification of magnetoplasmon mode through coupling with intersubband polaritons

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.

cond-mat.mes-hall↗

arXiv2025

Intersubband polariton -- LO phonon interaction in mid-infrared non-dispersive cavities: experimental demonstration of spontaneous scattering and perspectives towards polariton lasing

We report experimental evidence of the interaction between intersubband polaritons and longitudinal optical phonons in non-dispersive mid-infrared cavities, under resonant optical injection. The 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 to stimulate such process using a pump-probe scheme. Based on this analysis, we identify a set of experimental conditions under which optical gain may be realized in a mid-infrared intersubband polaritonic system.

physics.optics↗

arXiv2025

Mid-infrared integrated resonators on a III-V platform with Q-factors beyond half a million

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 wavelength = 4.6 um and wavelength = 8.5 um, two representative wavelengths for the 1st and 2nd atmospheric transparency windows. We measured losses of 1 dB/cm (TE polarization) and 1.3 dB/cm (TM polarization) at 8.5 um. Substantially lower losses, 0.28 dB/cm, were measured at wavelength = 4.6 um (TM polarization). We then implemented racetrack resonators with straight evanescent couplers. We obtained loaded quality factors larger than 600.000 at wavelength = 4.6 um. These results are promising towards the development of non-linear mid-IR integrated devices with Q factors beyond a million, where the onset of stimulated parametric processes could be reachable.

physics.optics↗

arXiv2024

Ultrafast (10 GHz) mid-IR modulator based on ultra-fast electrical switching of the light-matter coupling

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.

physics.app-ph↗

arXiv2024

Origin of optical nonlinearity in plasmonic semiconductor nanostructures

The development of nanoscale nonlinear elements in photonic integrated circuits is hindered by the physical limits to the nonlinear optical response of dielectrics, which requires that the interacting waves propagate in transparent volumes for distances much longer than their wavelength. Here we present experimental evidence that optical nonlinearities in doped semiconductors are due to free-electron and their efficiency could exceed by several orders of magnitude that of conventional dielectric nonlinearities. Our experimental findings are supported by comprehensive computational results based on the hydrodynamic modeling, which naturally includes nonlocal effects, of the free-electron dynamics in heavily doped semiconductors. By studying third-harmonic generation from plasmonic nanoantenna arrays made out of heavily n-doped InGaAs with increasing levels of free-carrier density, we discriminate between hydrodynamic and dielectric nonlinearities. As a result, the value of maximum nonlinear efficiency as well as its spectral location can now be controlled by tuning the doping level. Having employed the common material platform InGaAs/InP that supports integrated waveguides, our findings pave the way for future exploitation of plasmonic nonlinearities in all-semiconductor photonic integrated circuits.

physics.optics↗

arXiv2023

THz ultra-strong light-matter coupling up to 200K with continuously-graded parabolic quantum wells

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 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 200K. 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 $2.10^{-6} \lambda_0^3$, 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.

physics.optics↗

arXiv2022

Low intensity saturation of an ISB transition by a mid-IR quantum cascade laser

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 $\mu$m. 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.

physics.optics↗

arXiv2022

Detection of strong light-matter interaction in a single nano-cavity with a thermal transducer

Recently, the concept of strong light-matter coupling has been demonstrated in semiconductor structures, and it is poised to revolutionize the design and implementation of components, including solid state lasers and detectors. We demonstrate an original nanospectroscopy technique that permits to study the light-matter interaction in single subwavelength-sized nano-cavities where far-field spectroscopy is not possible using conventional techniques. We inserted a thin ($\approx$ 150 nm) polymer layer with negligible absorption in the mid-IR (5 $\mu$m < $\lambda$ < 12 $\mu$m) inside a metal-insulator-metal resonant cavity, where a photonic mode and the intersubband transition of a semiconductor quantum well are strongly coupled. The intersubband transition peaks at $\lambda$ = 8.3 $\mu$m, and the nano-cavity is overall 270 nm thick. Acting as a non-perturbative transducer, the polymer layer introduces only a limited alteration of the optical response while allowing to reveal the optical power absorbed inside the concealed cavity. Spectroscopy of the cavity losses is enabled by the polymer thermal expansion due to heat dissipation in the active part of the cavity, and performed using an atomic force microscope (AFM). This innovative approach allows the typical anticrossing characteristic of the polaritonic dispersion to be identified in the cavity loss spectra at the single nano-resonator level. Results also suggest that near-field coupling of the external drive field to the top metal patch mediated by a metal-coated AFM probe tip is possible, and it enables the near-field mapping of the cavity mode symmetry including in the presence of strong light-matter interaction.

physics.optics↗

arXiv2022

Magnetic-field-induced cavity protection for intersubband polaritons

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.

cond-mat.mes-hall↗

arXiv2022

A 5.7 THz GaN/AlGaN quantum cascade detector based on polar step quantum wells

We report on a GaN/AlGaN quantum cascade detector operating in the terahertz spectral range. The device was grown by metal organic chemical vapor deposition on a c-sapphire substrate and relies on polar GaN/AlGaN step quantum wells. The active region thickness is in micrometer range. The structural, electrical and optical investigations attest of high structural quality of the synthetized nitride material. The detector exhibits a peak photocurrent at 5.7 THz (23.6 meV) with a responsivity of 0.1 mA/W at 10 K under surface normal irradiation through a 10 um period grating. The photocurrent persists up to 20 K.

physics.optics↗

arXiv2021

Direct polariton-to-electron tunneling in quantum cascade detectors operating in the strong light-matter coupling regime

We demonstrate mid-infrared quantum cascade detectors (QCD) operating in the strong light-matter coupling regime. They operate around $\lambda = 10~\mu m$ with a minimum Rabi splitting of 9.3 meV. A simple model based on the usual description of transport in QCDs does not reproduce the polaritonic features in the photo-current spectra. On the contrary, a more refined approach, based on the semi-classical coupled modes theory, is capable to reproduce both optical and electrical spectra with excellent agreement. By correlating absorption/photo-response with the simulations, we demonstrate that - in this system - resonant tunneling from the polaritonic states is the main extraction mechanism. The dark intersubband states are not involved in the process, contrary to what happens in electrically injected polaritonic emitters.

physics.app-ph↗

arXiv2021

Multiple vibro-polaritons formation from a thin polyethylene film embedded in a resonant mid-infrared cavity

We experimentally resolve the dispersion of multiple vibro-polariton modes issued from the strong coupling of different vibrational bands of the methylene group (CH2) in a 2.56$\mu$m thick polyethylene film with the confined modes of a mid-infrared Fabry-Perot micro-cavity. We measure a Rabi frequency of 111 cm$^{-1}$ for the stretching doublet around 2950 cm$^{-1}$ and a Rabi frequency of 29 cm$^{-1}$ for the scissoring doublet around 1460 cm$^{-1}$. This simple experimental approach offers the possibility to accurately fit the measured molecular film dielectric function. We show that the polariton dispersion and Rabi splitting can be precisely predicted from numerical simulations, offering a valuable tool for the design of strongly coupled system and the development of novel molecular films with crystalline organization.

physics.optics↗

arXiv2021

A "Janus" double sided mid-IR photodetector based on a MIM architecture

We present a mid-IR ($\lambda \approx$ 8.3 $\mu$m) quantum well infrared photodetector (QWIP) fabricated on a mid-IR transparent substrate, allowing photodetection 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) cavity and hosted on a mid-IR transparent ZnSe substrate. Metallic stripes are symmetrically patterned by e-beam lithography on both sides of the active region. The detector spectral coverage spans from $\lambda \approx 7.15$ $\mu$m to $\lambda \approx 8.7$ $\mu$m by changing the stripe width L - from L = 1.0 $\mu$m to L = 1.3 $\mu$m - thus frequency-tuning the optical cavity mode. Both micro-FTIR passive optical characterizations and photocurrent measurements of the two-port system are carried out. They reveal a similar spectral response for the two detector ports, with an experimentally measured T$_{BLIP}$ of $\approx$ 200K.

physics.app-ph↗

arXiv2020

Unified description of saturation and bistability of intersubband transitions in the weak and strong light-matter coupling regimes

We propose a unified description of intersubband absorption saturation for quantum wells inserted in a resonator, both in the weak and strong light-matter coupling regimes. We demonstrate how absorption saturation can be engineered. In particular we show that the saturation intensity increases linearly with the doping in the strong coupling regime, while it remains doping independent in weak coupling. Hence, countering intuition, the most suitable region to exploit low saturation intensities is not the ultra-strong coupling regime, but is instead at the onset of the strong light-matter coupling. We further derive explicit conditions for the emergence of bistability. This work sets the path towards yet unexisting ultrafast mid-infrared semiconductor saturable absorption mirrors (SESAMs) and bistable systems. As an example, we show how to design a mid-infrared SESAM with a three orders of magnitude reduction in saturation intensity, down to about 5 kW/cm2.

cond-mat.mes-hall↗

arXiv2020

Ultra-fast amplitude modulation of mid-IR free-space beams at room-temperature

Applications relying on mid-infrared radiation (Mid-IR, $\lambda\sim$ 3-30 $\mu$m) have progressed at a very rapid pace in recent years, stimulated by scientific and technological breakthroughs. Mid-IR cameras have propelled the field of thermal imaging. And the invention of the quantum cascade laser (QCL) has been a milestone, making compact, semiconductor-based mid-IR lasers available to a vast range of applications. All the recent breakthrough advances stemmed from the development of a transformative technology. In addition to the generation and detection of light, a key functionality for most photonics systems is the electrical control of the amplitude and/or phase of an optical beam at ultra-fast rates (GHz or more). However, standalone, broadband, integrated modulators are missing from the toolbox of present mid-IR photonics integrated circuits and systems developers. We have developed a free-space amplitude modulator for mid-IR radiation ($\lambda\sim$ 10 $\mu$m) that can operate up to at least 1.5 GHz (-3dB cut-off at $\sim$ 750 MHz) and at room-temperature. The device relies on a semiconductor hetero-structure enclosed in a judiciously designed metal-metal optical resonator. At zero bias, it operates in the strong light-matter coupling regime up to 300K. By applying an appropriate bias, the device transitions to the weak coupling regime. The large change in reflectivity due to the disappearance of the polaritonic states is exploited to modulate the intensity of a mid-IR continuous-wave laser up to speeds of more than 1.5 GHz.

physics.optics↗

arXiv2019

Excitons bound by photon exchange

In contrast to interband excitons in undoped quantum wells, doped quantum wells do not display sharp resonances due to excitonic bound states. In these systems the effective Coulomb interaction between electrons and holes typically only leads to a depolarization shift of the single-electron intersubband transitions. Non-perturbative light-matter interaction in solid-state devices has been investigated as a pathway to tune optoelectronic properties of materials. A recent theoretical work [Cortese et al., Optica 6, 354 (2019)] predicted that, when the doped quantum wells are embedded in a photonic cavity, emission-reabsorption processes of cavity photons can generate an effective attractive interaction which binds electrons and holes together, leading to the creation of an intraband bound exciton. Spectroscopically, this bound state manifests itself as a novel discrete resonance which appears below the ionisation threshold only when the coupling between light and matter is increased above a critical value. Here we report the first experimental observation of such a bound state using doped GaAs/AlGaAs quantum wells embedded in metal-metal resonators whose confinement is high enough to permit operation in strong coupling. Our result provides the first evidence of bound states of charged particles kept together not by Coulomb interaction, but by the exchange of transverse photons. Light-matter coupling can thus be used as a novel tool in quantum material engineering, tuning electronic properties of semiconductor heterostructures beyond those permitted by mere crystal structures, with direct applications to mid-infrared optoelectronics.

cond-mat.mes-hall↗