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.

We show that the sensitivity of antenna-coupled field-effect transistors (FETs) to terahertz (THz) radiation improves continuously with decreasing temperature. The noise-equivalent power (NEP) of 540 GHz patch-antenna-coupled FETs decreases as temperature reduces to 20K. We project NEP values approaching 1-2pW/Hz---root under efficient power coupling conditions (e.g., using a superstrate Si-lens), which is comparable to superconducting niobium transition-edge sensorsat 4K. Building on these findings, a compact, low-noise, liquid-nitrogen-cooled (77K) FET-based direct (incoherent) THz-power sensing system for spectroscopy applications was realized. Here, an 8 & times;8 pixel-binned detector array fabricated in a commercial 65-nm Si-CMOS process, was optimized for operation in the 2.85-3.4THz band. Characterization was performed in the focal plane of a 2.85-THz quantum-cascade laser delivering similar to 2 mW of THz power. A linear dynamic range exceeding 67dB was achieved without saturation (for 1 Hz-detection bandwidth). The system provides a -3dB readout bandwidth of 5MHz, exceeding that of conventional thermal detectors (typically 1kHz). Combined with its broad temperature operability (20-300K) and compact design, the system is particularly well suited for space- and payload-constrained platforms such as balloon- and satellite-based missions, where deep cryogenic coolingis impractical.

Terahertz imaging holds great potential for non-destructive material inspection, but practical implementation has been limited by resolution constraints. In this study, we present a single-pixel THz imaging system based on a confocal microscope architecture, utilizing a quantum cascade laser as both transmitter and phase-sensitive receiver. We demonstrate, for the first time to the best of our knowledge, that laser feedback interferometry-based imaging systems achieve enhanced lateral and axial resolution compared to conventional confocal imaging. Specifically, our approach yields a twofold improvement in lateral resolution, reaching λ/2, and a two-order-of-magnitude enhancement in axial resolution, from 25λ to beyond λ/5, through interferometric phase detection. The system can produce a 0.5 megapixel image in under three minutes, surpassing both raster-scanning single-pixel and multipixel focal-plane array-based imagers. Coherent operation enables simultaneous amplitude and phase image acquisition, and a custom visualization method links amplitude to image saturation and phase to hue, enhancing material characterization. A 3D tomographic analysis of a silicon chip reveals subwavelength features, demonstrating the system's potential for high-resolution THz imaging and material analysis. This work overcomes the resolution limits of conventional lens-based imaging systems, by enabling rapid, high-fidelity imaging of subwavelength features beyond the diffraction limit.

Recent advancements in renewable energy technology highlight photovoltaic (PV) power generation as crucial for sustainable energy. This paper examines the effects of partial shading and temperature on PV systems, focusing on series configurations of two and three modules. Using MATLAB for modeling and simulation, the study optimizes PV performance under varying environmental conditions. Results demonstrate the model's reliability and accuracy in simulating output characteristics, with a simple structure that facilitates easy simulation of different configurations. This versatile model is a valuable tool for exploring PV system designs and offers significant potential for future research.

The photogalvanic effect can probe the surface states of topological insulators. At optical photon energies surface states near the Dirac point are excited into bulk bands, whereas at energies commensurate with THz excitation, surface states are excited within the Dirac cone leading to a potential enhancement in sensitivity to topological surface states (TSS) and Berry phase. Here, we demonstrate measurements of the circular photogalvanic effect (CPGE) in the topological insulator Bi2Se3 with both optical and THz frequencies at room temperature. Furthermore, we also probe this effect at millikelvin temperatures with THz light. We find the CPGE increases rapidly below 2 K, and is potentially correlated with the weak antilocalization, which itself is dependent on phase coherence lengths. This work lays the foundation for studying both TI and TSS using CPGE at ultra-low temperatures.

High-speed free-space optical (FSO) communication in the terahertz (THz) spectrum is gaining attention as a solution to the rising demand for wireless data. THz frequencies (0.1-10 THz) offer ample bandwidth and can deliver fiber-equivalent data rates wirelessly, making them suitable for beyond-5G backhaul [1]. Terahertz quantum cascade lasers (THz QCLs) offer multi-gigabit modulation speed in FSO links. The THz FSO communication systems >1 THz remain largely unexplored due to limitations in source and photodetector limitations. However, significant progress in sources, modulators, and receivers has improved output power, bandwidth, and sensitivity. [2]. A THz QCL link at 20 Mbit/s was demonstrated [3], and a recent link achieved 1 Mbit/s with a room-temperature graphene receiver [4]. However, achieving higher data rates in a THz FSO communication link remains a significant goal.

Polaritons in two-dimensional layered crystals offer an effective solution to confine, enhance and manipulate terahertz (THz) frequency electromagnetic waves at the nanoscale. Recently, strong THz field confinement has been achieved in a graphene-insulator-metal structure, exploiting THz plasmon polaritons (PPs) with strongly reduced wavelength ( λ p ≈ λ 0 /66) compared to the photon wavelength λ 0 . However, graphene PPs propagate isotropically, complicating the directional control of the THz field, which, on the contrary, can be achieved exploiting anisotropic layered crystals, such as orthorhombic black-phosphorus. Here, we detect PPs, at THz frequencies, in hBN-encapsulated black phosphorus field effect transistors through THz near-field photocurrent nanoscopy. The real-space mapping of the thermoelectrical near-field photocurrents reveals deeply sub-wavelength THz PPs ( λ p ≈ λ 0 /76), with dispersion tunable by electrostatic control of the carrier density. The in-plane anisotropy of the dielectric response results into anisotropic polariton propagation along the armchair and zigzag crystallographic axes of black-phosphorus. The achieved directional subwavelength light confinement makes this material system a versatile platform for sensing and quantum technology based on nonlinear optics.

Optical frequency combs (OFCs), which establish a rigid phase-coherent link between the microwave and optical domains of the electromagnetic spectrum, are emerging as key high-precision tools for the development of quantum technology platforms. These include potential applications for communication, computation, information, sensing, and metrology and can extend from the near-infrared with micro-resonator combs, up to the technologically attractive terahertz (THz) frequency range, with powerful and miniaturized quantum cascade laser (QCL) FCs. The recently discovered ability of the QCLs to produce a harmonic frequency comb (HFC)—a FC with large intermodal spacings—has attracted new interest in these devices for both applications and fundamental physics, particularly for the generation of THz tones of high spectral purity for high data rate wireless communication networks, for radio frequency arbitrary waveform synthesis, and for the development of quantum key distributions. The controlled generation of harmonic states of a specific order remains, however, elusive in THz QCLs. Here, and by design, we devise a strategy to obtain broadband HFC emission of a pre-defined order in a QCL. By patterning n regularly spaced defects on the top surface of a double-metal Fabry–Perot QCL, we demonstrate harmonic comb emission with modes spaced by an (n+1) free spectral range and with an optical power/mode of ∼270µW.

AbstractOptical resonators are fundamental building blocks of photonic systems, enabling meta-surfaces, sensors, and transmission filters to be developed for a range of applications. Sub-wavelength size (< λ/10) resonators, including planar split-ring resonators, are at the forefront of research owing to their potential for light manipulation, sensing applications and for exploring fundamental light-matter coupling phenomena. Near-field microscopy has emerged as a valuable tool for mode imaging in sub-wavelength size terahertz (THz) frequency resonators, essential for emerging THz devices (e.g. negative index materials, magnetic mirrors, filters) and enhanced light-matter interaction phenomena. Here, we probe coherently the localized field supported by circular split ring resonators with single layer graphene (SLG) embedded in the resonator gap, by means of scattering-type scanning near-field optical microscopy (s-SNOM), using either a single-mode or a frequency comb THz quantum cascade laser (QCL), in a detectorless configuration, via self-mixing interferometry. We demonstrate deep sub-wavelength mapping of the field distribution associated with in-plane resonator modes resolving both amplitude and phase of the supported modes, and unveiling resonant electric field enhancement in SLG, key for high harmonic generation.

Harmonic generation is a result of a strong non-linear interaction between light and matter. It is a key technology for optics, as it allows the conversion of optical signals to higher frequencies. Owing to its intrinsically large and electrically tunable non-linear optical response, graphene has been used for high harmonic generation but, until now, only at frequencies < 2 THz, and with high-power ultrafast table-top lasers or accelerator-based structures. Here, we demonstrate third harmonic generation at 9.63 THz by optically pumping single-layer graphene, coupled to a circular split ring resonator (CSRR) array, with a 3.21 THz frequency quantum cascade laser (QCL). Combined with the high graphene nonlinearity, the mode confinement provided by the optically-pumped CSRR enhances the pump power density as well as that at the third harmonic, permitting harmonic generation. This approach enables potential access to a frequency range (6-12 THz) where compact sources remain difficult to obtain, owing to the Reststrahlenband of typical III-V semiconductors.

We demonstrate that asynchronous optical sampling (ASOPS) can be used to measure the propagation of terahertz (THz) bandwidth pulses in a coplanar waveguide device with integrated photoconductive switches used for signal excitation and detection. We assess the performance of the ASOPS technique as a function of measurement duration, showing the ability to acquire full THz time-domain traces at rates up to 100 Hz. We observe a peak dynamic range of 40 dB for the shortest measurement duration of 10 ms, increasing to 88 dB with a measurement time of 500 s. Our work opens a route to real-time video-rate imaging via modalities using scanned THz waveguides, as well as real-time THz sensing of small volume analytes; we benchmark our on-chip ASOPS measurements against previously published simulations of scanning THz sensor devices, demonstrating sufficient dynamic range to underpin future video-rate THz spectroscopy measurements with these devices.

We present measurements of the coherence times of excited states of hydrogen-like arsenic impurities in germanium (Ge:As) using a table-top two-dimensional time-domain spectroscopy (2D-TDS) system. We show that this laboratory system is capable of resolving the coherence lifetimes of atomic-like excited levels of impurity centers in semiconductors, such as those used in solid-state quantum information technologies, on a subpicosecond time scale. By fitting the coherent nonlinear response of the system with the known intracenter transition frequencies, we are able to monitor coherent population transfer and decay of the transitions from the 2p0 and 2p± states for different low excitation pulse fields. Furthermore, by examining the off-diagonal resonances in the 2D frequency-domain map, we are able to identify coherences between excited electronic states that are not visible via conventional single-frequency pump-probe or Hahn-echo measurements.

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.

Terahertz (THz) imaging has long held promise for skin cancer detection but has been hampered by the lack of practical technological implementation. In this article, we introduce a technique for discriminating several skin pathologies using a coherent THz confocal system based on a THz quantum cascade laser. High resolution in vivo THz images (with diffraction limited to the order of 100 μm) of several different lesion types were acquired and compared against one another using the amplitude and phase values. Our system successfully separated pathologies using a combination of phase and amplitude information and their respective surface textures. The large scan field (50 × 40 mm) of the system allows macroscopic visualization of several skin lesions in a single frame. Utilizing THz imaging for dermatological assessment of skin lesions offers substantial additional diagnostic value for clinicians. THz images contain information complementary to the information contained in the conventional digital images.

Surface plasmon polaritons (SPPs) are electromagnetic waves that have attracted significant interest owing to their subwavelength confinement and the strong field enhancement that they provide. Yet in the terahertz (THz) frequency region of the spectrum, which is well below the plasma frequency of metals, these surface waves are characterized by extremely weak confinement that has severely limited their exploitation for information processing and sensing. One means to circumvent this limitation is through subwavelength structuring of a metallic surface, which can thereby be engineered to support the propagation of spoof surface plasmon polaritons (SSPPs) that closely mimic the properties of SPPs. In this work, we report the design and experimental characterization of an ultra-thin metamaterial planar waveguide that supports SSPPs at THz frequencies. Finite-element method simulations are shown to predict the excitation of SSPPs on the surface of our devices under free-space illumination at 3.45 THz. We investigate these structures experimentally using THz scattering-type scanning near-field microscopy (THz-s-SNOM) to map directly the out-of-plane electric field associated with the propagation of SSPPs on the surface of the waveguides. Our work paves the way for the future development of plasmonic integrated circuit technologies and components operating in the THz frequency band.

This study investigated the use of terahertz (THz) imaging as a rapid, high-fidelity technique for discriminating between genetic variants of the Allium genus based on cellular water dynamics. It has been demonstrated earlier that plant genetic variations can be related to the biochemical and biomechanical alterations of the cell and that in turn affect the water dynamics within the cell. In this article, we show that the water dynamics, when considered in the form of the temporal evolution of the trajectory of the plant's response to THz radiation probe, and measured by a coherent THz transceiver, provides unique signature of the genetic makeup of the plant. Therefore, by exploring these trajectories, we discriminate between closely related variants of the same genus. The technique used for THz probing was the laser feedback interferometry with THz quantum cascade lasers, which enabled fast acquisition of high-resolution THz amplitude and phase images, which were processed into evaporation profiles describing the time-dependent dehydration of the samples. The trajectory of this profile in amplitude-phase reflectivity domain discriminates between different members of the Allium genus. This enables real-time genetic discrimination in agricultural and genome conservation applications.

Near-field photocurrent nanoscopy is applied to investigate the real-space propagation of plasmon polaritons in hBN-encapsulated black phosphorus at terahertz (THz) frequencies. We introduce a novel measurement configuration based on four-gate field effect transistors to resolve the polariton wavefront anisotropy, expected in biaxial crystals such as black-phosphorus. This new method enabled us to map the propagation of strongly confined THz plasmon polaritons of bP with elliptic wavefront and gate tunable dispersion. The attained directional subwavelength confinement of THz light anticipates promising applications in sensing, nonlinear optics and waveguiding. Whereas the presented photocurrent nanoscopy scheme shows promise for inspecting THz polaritons in anisotropic crystals.

Assessing the nature of topological quantum materials, and in particular probing the existence of topological surface states, is a very challenging task. Terahertz (THz) frequency scattering near‐field optical microscopy has emerged as an effective technique to investigate the presence of massless surface carriers by locally probing collective surface excitations, i.e., plasmon polaritons, whose dispersion critically depends on the density and nature of surface carriers. Here, thin (14–19 nm) films of Bi 2 Se 3 are experimentally investigated through a combination of x‐ray diffraction, Hall‐bar magneto‐transport, and near‐field detectorless optical holography at THz frequencies, from 2 to 4.3 THz. The dispersion of surface plasmon polaritons are determined for different Bi 2 Se 3 film thicknesses, proving the presence of massless surface carriers. The results open intriguing opportunities in THz nano‐plasmonics and topological nano‐photonics including the development of superlenses and metasurfaces, making use of plasmon polaritons.

In this article, we report our work on the development of a non- invasive, rapid, robust, and high-fidelity technique that can be used to discriminate between genetic variants. Our study focused on terahertz (THz) spectroscopy and imaging to distinguish between genetic variants of the Allium genus rapidly and accurately. This was done by measuring the cellular water dynamics of the samples by measuring their evaporation profiles using laser feedback interferometry (LFI) with THz quantum cascade lasers (QCL). The evaporation profiles of the samples were then processed to create trajectories in the amplitude-phase domain, which correlated with cell age, cell type, and the amount of water bound to biomolecules. This technique can differentiate between the members of the Allium genus. The presence of outliers was also studied to determine the effectiveness of the technique for different samples and to negate external influence. This was done to discern the extent of influence of cell biomechanics and biochemistry between genetic variants. We found that within a genus, different species would have different degree of interaction between cellular water and cell biochemistry, which could be clearly mapped out using THz-QCL-based LFI. Based on our observations, we propose that this method could be appropriate for observing minute alterations in cellular water dynamics in real-time, and in the future, has the potential to be employed for rapid and effective genetic discrimination in agricultural and genome conservation applications.

We have performed an investigation into the optical conductivity and magnetotransport properties of top-gated devices patterned on the topological insulator Bi 2 Se 3 in order to determine the relative effects of the different carrier species that exist within these novel materials. We find that the topologically protected surfaces within our samples are partially screened from the action of the gate by trivial band-bending states formed at the top surface of the topological insulator. Despite this, the mobility of the topological surface carriers is significantly affected by the application of an external gate bias. Additionally, we find that the optical conductivity response is dominated by the topologically protected surface states, and that the optical conductivity is particularly sensitive to the scattering caused by the topological surfaces coupling to trivial states, arising from the bulk or band-bending induced surface states. These results will have interesting applications to the design of future plasmonic devices that incorporate topological materials.