ABSTRACT. When light-matter interaction cannot be treated perturbatively in terms of emission and absorption, polariton quasi-particles become the proper concept to describe the system excitations. Intersubband polaritons possess many peculiar features related to their intrinsic many-body nature. These allow new physics regimes, as well as possible exciting device implementations. This talk will give a tutorial introduction to intersubband polaritons and discuss the hottest research presently developing.

ABSTRACT. We demonstrate spectroscopy of IR intersubband (ISB) polaritons in an isolated subwavelength size nanoantenna (NA) using near-field FTIR spectroscopy. The detected evanescent field reveals a distinctive polariton splitting of the NA resonance both in the amplitude and phase spectra. Using this approach and NAs of different sizes, we map the ISB polariton dispersion. This nano-FTIR spectroscopy approach opens doors for investigations of ISB polaritons in the single nanoantenna regime.

ABSTRACT. We present a novel architecture for THz metamaterials relying on three-dimensional meta-atoms embedding a semiconductor active region. The meta-atoms behave as inductor-capacitive (LC) resonators having nano-scale capacitive parts, with an extremely sub-wavelength mode volume (10-6λ0). We study their ultra-strong coupling with a two-dimensional electron gas inserted inside the capacitive elements, and detail strategies to enhance the coupling to free space radiation towards perfect absorption.

ABSTRACT. Intersubband (ISB) transitions in semiconductor multi-quantum well (MQW) structures are promising candidates for the development of saturable absorbers at terahertz (THz) frequencies. Here, we demonstrate saturation (polaritonic bleaching) of intersubband polaritons at THz frequencies both by exploiting ultrafast amplitude and phase-resolved two-dimensional (2D) THz spectroscopy and pump and probe reflectivity measurements with a continuous wave high power quantum cascade laser.

ABSTRACT. We demonstrate strong coupling between Mie modes of dielectric nanoresonators in a metasurface and intersubband transitions in semiconductor quantum wells that are embedded inside the resonator. The ability to achieve such light-matter coupling creates the possibility to realize ultrathin nonlinear optical devices that are free from complex phase matching requirements and have very high nonlinear conversion efficiencies.

ABSTRACT. We introduce a novel approach for THz light-matter coupled systems by treating the electronic and photonic components equally instead of optimizing them separately. We exploit the electronic excitation to confine the vacuum mode and achieve WR/wc = 1.43 for cyclotron resonances ultrastrongly coupled to metamaterials. The vacuum ground state exhibits a record population of 0.37 virtual photons per resonator, facilitating the envisaged detection of vacuum radiation by diabatic switching of WR/wc.

ABSTRACT. We show the crucial role of the electronic component of THz polaritons in the magneto-transport of a cavity-embedded 2DEG: the DC resistivity is substantially modified by the coupling to the cavity without any external irradiation. New experiments where the polaritonic linewidth is studied as a function of the sub-micrometric gap of the split rings resonators are also presented.

ABSTRACT. We experimentally demonstrate the existence of bound excitonic resonances in doped quantum wells, whose electron and hole are bind by the exchange of virtual cavity photons.

ABSTRACT. We present a linear-response theory of the electronic conductance along the growthdirection of a semiconductor heterostructure embedded in a single-mode cavity resonator in the absence of illumination. The conductance depends on the ground-state properties and virtual collective polaritonic excitations that have been determined via a bosonic treatment. We show that the cavity vacuum effects can modify significantly the dark vertical conductance with respect to the bare heterostructure.

ABSTRACT. In this talk I will review some among the most exciting perspectives of the field of intersubband polaritons in the direction of novel coherent light sources based on Bose-Einstein condensation of intersubband polaritons, of novel and complex nonlinear spatio-temporal dynamics of polariton fluids, and the observation of quantum optical effects such as the emission of parametrically entangled photon pairs and of antibunched light in the MIR/FIR.

ABSTRACT. We present a novel approach towards engineering the infrared dielectric response of polaritonic materials by using atomic-scale superlattices of polar semiconductors. For layer thicknesses approaching the atomic scale, new optic phonons arise in this structure due to interface bonding and confinement, resulting in a unique and strongly anisotropic polaritonic response. We thus introduce a new platform for engineered infrared nanophotonic materials using well-established semiconductors.

ABSTRACT. We report QWIP detectors operating in strong light-matter coupling regime. It is an ideal system to elucidate the problem of the injection of electrons (single-particles) into polaritonic modes (dressed states). We obtained experimental information on the transfer function between a polaritonic system and an electronic reservoir. This approach opens avenues in view of adding previously unavailable functionalities to quantum well detectors (mid-IR/THz) and improving their performance.

ABSTRACT. We introduce a Gross-Pitaevskii approach to the driven-dissipative dynamics of an intersubband polariton system consisting of a quantum well embedded in a patterned planar microcavity. The patterned mirror yields non-Markovian features of the radiative decay. These are well captured by our model, which further allows to study the nonlinear polariton-polariton scattering dynamics and consequent optical parametric oscillation processes. The latter are key to build polariton-based laser sources.

ABSTRACT. Strong coupling between light and matter represent a milestone towards the development of polariton devices based on intersubband transitions. In this work we investigate a strongly coupled device, by time domain spectroscopy (TDS) to probe reflectivity changes with different k vectors. We show how a low k-vector close to k=0, the polaritons are mostly predominately matter and show the effect of optical pumping on the polariton reflectivity at these small k-vectors

ABSTRACT. We demonstrate room temperature intersubband (ISB) transitions in the THz range using continuously-graded AlGaAs parabolic quantum wells. Below 100K, the linewidth of the transition reaches a record-low value of 5.7% of the central frequency.

ABSTRACT. We present a THz QCL spectroscopy technique, for analyzing deuteration reactions in gases. This is an important analytical chemistry technique, underpinning mechanistic studies. We use a self-mixing interferometry approach, in which radiation from a multimode QCL is fed-back into the device, via a gas cell, causing perturbations to the QCL voltage. This enables “detector-free” sensing over a 17 GHz range, allowing speciated measurements of reactions involving H2O, D2O, HDO, CH3OH and CH2DOD.

ABSTRACT. A quantum cascade (QC) structure is incorporated into a plasmonic semiconductor metamaterial while preserving its negative refraction. Transmission spectra with gain were simulated using the transfer matrix method. We show that gain from an active QC structure partially compensates the plamonic loss in the metamaterial. Thus QC-incorporated plasmonic metamaterials are a promising platform for realizing negative refraction with loss compensation.

ABSTRACT. We previously proposed InGaSb-well-based THz-QCLs to improve performance such as operation temperature. For realization of the InGaSb-well-based THz-QCLs, we performed molecular beam epitaxy (MBE) growth of InGaSb layers on a GaSb substrate with introducing a graded buffer layer. The threading dislocation density in the In0.2Ga0.8Sb layer was improved much compared to that grown on a GaAs substrate.

ABSTRACT. We apply double microwave injections onto terahertz QCLs emitting around 4.2 THz to achieve broadband on-chip dual-comb radiation sources. With weak radio frequency (RF) powers, the optical comb bandwidth is significantly broadened from 86 to 166 GHz.

ABSTRACT. We present our open-source framework for simulations of the quantum cascade laser (QCL) dynamics. This framework serves as basis for investigations of the mechanisms behind the different mode locking approaches in QCLs.

ABSTRACT. We investigate the intersubband relaxation dynamics in n-type Ge/SiGe asymmetric coupled quantum wells (ACQW) after optical excitation by means of a resonant THz picosecond pulse. In each subband the particle number, the carrier temperature and energy has been calculated as a function of time by means of an energy balance model. Moreover, numerical results have been used to interpret differential transmission data acquired with a pump-probe set-up at the free electron laser FELBE in Dresden.

ABSTRACT. The fast modulation characteristics of quantum cascade lasers (QCLs) up to the MHz-/GHz-range give insight into their dynamical properties and act as a prerequisite for experiments like the injection locking of frequency combs[1], spectroscopic measurements[2] or optical free-space telecommunication applications[3]. In this paper we present the first analysis of the high frequency modulation characteristics of mid-IR DFB-ring QCLs in comparison to DFB-ridge QCLs.

ABSTRACT. In quantum cascade laser-based dual comb spectroscopy, the best signal-to-noise ratios (SNRs) are obtained by coherent averaging. This requires a phase reference, but such a reference is usually unavailable. We present an algorithm capable of correcting arbitrary dual comb signals, and show that it is able to measure the power of comb teeth nearly as accurately as coherent averaging. The power in this technique lies in the fact that each tooth is coherently corrected by every other tooth.

ABSTRACT. We have optimized mid-IR QCL frequency comb structures in terms of spectral gain width, using expensive but accurate simulation models. We find an increase of the predicted spectral width from 25 meV to more than 60 meV.

ABSTRACT. We present an electromagnetic model of a THz QCL, integrated with a micro-machined waveguide and dual diagonal feedhorns, enabling simultaneous access to both facets of the QCL. A hybrid finite-element/Fourier transform approach enables analysis of both the near and far-fields in agreement with experimental observations. The far-field pattern shows enhancement of the beam profile when compared with an unmounted QCL, in terms of beam divergence and side-lobe suppression ratio.

ABSTRACT. We present a 3D numerical model for simulating the optical performance of patch antenna structure on terahertz quantum well infrared photo detectors (QWIPs). The model is proved to be able to provide an accurate prediction on resonant frequency of antenna structure. Moreover, a decreasing of antenna cavity quality factor caused by connection wires among patch mesas is investigated. With the help of this numerical model, we design and fabricate a 3.3 THz patch antenna QWIP.

ABSTRACT. We characterize the mid-infrared spectral response of optical antennas fabricated on thin epsilon-near-zero substrates. Reflection measurements demonstrate pining of the fundamental antenna mode and subsequent antenna modes to a narrow spectral band, ∆v≈30 cm-1. Additionally, coupling between the antenna modes and the thin film Berreman mode near the epsilon equals zero frequency is observed. The experimental results agree well with numerical and analytical models.

ABSTRACT. Surface phonon polariton modes in ZnO nanoparticles (NPs) are identified and characterized using both simulation and experiment. Two distinct localized modes observed in transmission measurements using mid-infrared optical spectroscopy are found to be in agreement with Mie theory results.

ABSTRACT. Here we utilize inelastically tunneling electrons to demonstrate an electrically-driven light emitter operating at room temperature. We find that the emission spectrum is not given by the quantum condition where the emission frequency would be proportional to the applied voltage, but the spectrum is determined by the spectral overlap between the energy-dependent tunnel current and the modal dispersion of the plasmon.

ABSTRACT. We have developed low temperature, low pressure pulsed laser deposition of VO2 on III-V heterostructures. We have implemented mid-IR meta-surfaces on the VO2 deposited layers.The reflectivity response as a function of the sample temperature exhibits a change of the reflectivity maximum. It also reveals a maximum frequency shift of hundreds of cm-1 of the resonance between 60°C and 72°C. We implemented VO2 thin film on Mid-IR QCL surface it shows room temperature operation at 7.7µm wavelength.

ABSTRACT. A method for obtaining the dispersion of terahertz quantum cascade lasers is presented. By measuring the relative phase of the center burst and first satellite peak within the interferogram of a sub-threshold THz QCL emitting cavity inside a Fourier Transform Infrared Spectrometer, the electroluminescence spectrum is determined by Fourier Transform and the group velocity dispersion can be calculated. This method is applicable to any QCL – here shown for Fabry-Pérot ridge laser as well as VECSEL.

ABSTRACT. Developing devices (detectors, emitters) operating in the terahertz (THz) range is important because of perspective applications in metrology, healthcare and security. Most of the currently available THz detectors are thermal devices with slow response times. We fabricated and measured tunable sub-wavelength terahertz photo-detectors based on a split ring geometry (RLC circuit). Ultrasmall THz quantum cascade lasers were conceived and measured with the same architecture.

ABSTRACT. The ignition of Quantum Cascade Lasers can occur from a state of oscillating field domains. The interplay between lasing and the kinetics of traveling domain boundaries provides complex oscillation scenarios, where our simulations reveal chaotic behavior.

ABSTRACT. Piezoelectric polarization effects on GaN/AlGaN THz-QCLs with two-well resonant-phonon scheme is studied by using nonequilibrium Green’s functions. Its scattering on the optical gain is quite small, but it perturbs the resonant tunneling seriously. In experiments, strain relaxation along 200 periods of GaN/Al0.15Ga0.85N quantum wells is shown which are grown on Si(111) by MBE. Finally, possible strain-balance strategy in growth is discussed.

ABSTRACT. This work presents a 4-well terahertz (THz) quantum cascade laser (QCL) design in which resonant tunneling and scattering-assisted injection are combined within a single structure. In comparison to a previously reported 3-well structure, this design uses an extra excited state as a hybrid injection channel, and can minimize the appearance of an early negative differential resistance (NDR) before the lasing threshold. The final NDR is observable up to 220K.

ABSTRACT. Long-wave infrared laser beams (λ∼14-20μm) are used for benzene derivative detection and UF6 selective multiphoton dissociation. As a representative, a new 16μm Quantum Cascade Laser (QCL) design based on GaAs/Al0.33Ga0.67As is presented.

ABSTRACT. We report a new active region design aimed at maximizing the superluminescence power and reducing the coherence length of quantum cascade superluminescent emitters for imaging applications. We engineer the structure by introducing doping into the active region to broaden the spectrum and suppressing population inversion in the device.

ABSTRACT. We present the development of a highly sensitive and compact HEB/QCL heterodyne detection system. The HEB mixer is a NbN nano-bridge. The local oscillator is a low power consumption, single mode, 3rd DFB THz QC laser. A new quasi-optical coupling scheme between mixer and local oscillator has been developed that allows a better transmission of the RF signal and a more compact detection system. The lowest uncorrected double side band receiver noise temperature of this system is 880 K.

ABSTRACT. We present an optically pumped terahertz gas laser at 66 µm, which is based on a distributed-feedback mid-infrared quantum-cascade laser as a pump source and 15NH3 as a gain medium.

ABSTRACT. All-dielectric asymmetric metasurfaces have previously shown high-quality (high-Q) factor resonances due to breaking symmetry-protected bound states in the continuum. We simulated an all-dielectric metasurface comprised of free-standing cylindrical structures for a varying asymmetry parameter. We found that several modes are observed for this metasurface and the high-Q factor exceeds 10^8. This metamaterial shows great promise in increased sensitivity and specificity in biosensing detection.

ABSTRACT. We demonstrate the acquisition of terahertz polarization sensitive images using a 3.4 terahertz (THz) quantum-cascade (QC) vertical external cavity surface-emitting laser (VECSEL) source and a fast rotating polarizer for polarization modulation with lock-in detection. Using this source in a single-pixel imaging configuration, images with noise equivalent polarization rotation of 0.9 mrad·Hz-1/2 are obtained.

ABSTRACT. A deep analysis of the strong interaction between graphene surface plasmons and the intersubband transition in a quantum well is carried out by means of simulations. By varying the doping in the graphene and in the well, the generation of the intersubband polariton has been optimized to achieve high contrast and strong coupling, achieving what we named “Clear Strong Coupling”.

ABSTRACT. We propose an optical heterodyne detection of quasi continuouse wave terahertz wave from a quantum cascade laser by using frequency up-conversion in a nonlinear crystal. In our experiment, the up-converted signal beams by mixing the terahertz wave with the intense pumping beam are parametrically amplified by the MgO:LiNbO3 optical parametric amplifier, then, visualized and measured the frequency and the intensity from the position and the intensity depending on the phase matching condition.

ABSTRACT. We demonstrate optical tuneability of terahertz quantum cascade random lasers by illumination with an additional, external 813 nm source. Due to the broadband nature of the multiple scattering feedback mechanism, the unterlying mode structure can be modified. Our results pave the way towards a continuosly tuneable terahertz light source.

ABSTRACT. In this contribution, we will investigate the effects arising, when coupling a metamaterial surface supporting a superradiant energy loss to intersubband transitions in the terahertz regime.

ABSTRACT. We study the influence of Coulomb interactions among carriers confined to a quantum well on the response to an optical pulse resonant with the intersubband transition. We go beyond a perturbative treatment and solve the full time dependent Poisson-Schrödinger equation and extract the excitation spectrum from the dipole response. For low pulse powers we observe the well-established depolarization shift, while for higher excitations the resonance moves back towards its non-interacting value.

ABSTRACT. We demonstrate dual wavelength InAs-based DFB QCLs with a double metal (DM) waveguide. The lasers exhibit simultaneous single frequency emission near 17.5 µm and 19.5 µm. These first dual-wavelength far-infrared QCLs with a DM waveguide operate in pulsed mode up to 200 K.

ABSTRACT. We demonstrate a fibre-coupled, fully monolithic plasmonic terahertz field detector on the silicon-photonics platform. The detection scheme leverages on the electro-optic effect introduced by a highly confined terahertz field to a volume of $10^{-8}\lambda/2$. The versatile design allows a maximal electro-optic bandwidth of 2.5 THz with a 30 dB field amplitude dynamical range at only 5 $\mu$m interaction length, as well as customized design at target frequencies.

ABSTRACT. Level broadening in AlGaN/AlGaN superlattice structures originated from dipole scattering is investigated by utilizing NEGF. For this investigation, dipole scattering self-energy is derived after adopting Debye screening model. Simulation results show that when dipole momentum increases as a function of Al composition, level broadening originated from the dipole scattering should be considered for the analysis of intersubband transitions since it shows quite large broadening.

ABSTRACT. We present an amplifier device with a silicon horn integrated into the fabrication process of a terahertz QCL cavity. The silicon horn structure provides an impedance transformation of the lasing mode from the metal-metal waveguides to free space thus reducing the reflection coefficient at the laser facet and suppressing lasing. This fabrication approach yields a continuous and very adiabatic horn with a taper angle below 2o at the throat of the structure.

ABSTRACT. We present a master equation model to simulate for the transport dynamics and gain of a quantum dot cascade laser (QDCL), demonstrating the importance of coupling to LO and LA phonons.

ABSTRACT. We report recent development of scanning near-field optical microscopy (SNOM) for thermal imaging with sub-wavelength spatial resolution. The SNOM system is developed as a capability for the thermal profiling of MWIR quantum cascade lasers (QCLs) during pulsed and continuous wave (CW) operation, targeting QCL design optimization

ABSTRACT. Here, we investigate the temporal dynamics of ultrafast quantum dot laser (QDL) frequency combs using quadratic and linear autocorrelation techniques. In the passively mode-locked regime, the QDL emits a train of 10 ps long pulses, confirmed independently by both autocorrelation techniques. By carefully tuning the driving conditions of the QDL, we find another frequency comb regime, which was previously observed in mid-infrared semiconductor laser frequency combs and appears to be universal.

ABSTRACT. Plasmonic metasurfaces coupled to intersubband transitions (ISTs) in semiconductor heterostructures were extensively studied for realizing various ultrathin devices in the mid and long infrared (IR). Here, we leverage the very large conduction band offset of InAs and AlSb and demonstrate coupling between a plasmonic metasurface and ISTs at near infrared wavelengths. Our work paves the way for scaling coupled metasurface-intersubband based devices to technologically relevant near IR wavelengths.

ABSTRACT. We examine broadband mid-infrared angle resolved transmission spectroscopy of highly porous catalytic support for chemical processing and energy applications