Vanadium dioxide ( $$\hbox {VO}_2$$ ) is a favorable material platform of modern optoelectronics, since it manifests the reversible temperature-induced insulator-metal transition (IMT) with an abrupt and rapid changes in the conductivity and optical properties. It makes possible applications of such a phase-change material in the ultra-fast optoelectronics and terahertz (THz) technology. Despite the considerable interest to this material, data on its broadband electrodynamic response in different states are still missing in the literature. This hampers the design and implementation of the $$\hbox {VO}_2$$ -based devices. In this paper, we combine the Fourier-transform infrared (FTIR) spectroscopy, THz pulsed spectroscopy (TPS), and four-contact probe method to study the $$\hbox {VO}_2$$ films prepared by magnetron sputtering on a c-cut sapphire substrate. Considering different temperatures of a substrate and pressures of atmosphere, we reconstruct complex dielectric permittivity of $$\hbox {VO}_2$$ film in the frequency range of 0.2–150 THz, along with its static conductivity. The dielectric response is modeled using Lorentz and Drude kernels, which make possible splitting contributions from vibrational modes and free charge carriers to the total dynamic conductivity. By studying $$\hbox {VO}_2$$ at different substrate temperatures and atmosphere pressures, we show that IMT appears to be pressure-dependent, which we attribute to the different thermostatic conditions of a sample. Finally, we estimate somewhat optimal thickness and temperature of the $$\hbox {VO}_2$$ film in metallic phase for the THz optoelectronic applications. Our finding should be useful for further developments of the $$\hbox {VO}_2$$ -based devices and technologies.

Experimental observation of the enhanced terahertz (THz) emission in a large-area photoconductive antenna-emitter (LAE), boosted by an array of cylindrical sapphire-fiber-based microlenses, is reported. The observed enhancement is achieved, thanks to the sharp focusing of a pump laser beam near the semiconductor surface, for which the high-refractive-index sapphire lenses are used. We predict numerically and confirm experimentally a considerable enhancement in the emitted THz spectral power for such a sapphire-fiber-coupled LAE, as compared to an ordinary one with an equal electrode topology. In fact, a ≃8.5-fold THz power boost is achieved, resulting in a +9.3 dB increase in the dynamic range. The results of our findings can be used to improve the performance of large-area THz devices, aimed at meeting the demands of rapidly developed THz spectroscopy, imaging, sensing, and exposure technologies.

Phase change materials (PCM) forms a platform for multiple applications in modern optics, photonics, microelectronics. In this work, we performed broadband dielectric spectroscopy of Ge 2 Sb 2 Te 5 (GST) and VO 2 phase change materials in a form of thin films on a sapphire substrate. Broadband dielectric response of three different GST phases was retrieved and thoroughly analyzed. Reversible metal-insulator transition (MIT) of VO 2 film was studied over the broad range of temperatures covering typical hysteresis loop of the material. The factor of residual pressure in the sample chamber was considered. Spectroscopy results were supplemented with the electrical conductivity measurements. Reconstructed THz-IR response was analyzed using classical models of complex dielectric permittivity for all the studied phases of PCMs.

In this work, we present the first experimental observation of the enhanced THz radiation in plasmonic photoconductive antenna (PCA) based on $\mathrm{Bi}_{2-\mathrm{x}} \mathrm{Sb}_{\mathbf{x}} \mathrm{Se}_{\mathbf{y}} \mathrm{Te}_{3 \text {-y }}$ topological insulator (TI). By using numerical simulations, we optimized the plasmonic grating geometry to maximize optical light transmission, stimulating efficient surface plasmon-polariton (SPP) excitations along the TI/grating interface. We fabricated two equal PCAs, the one of which contains high-aspect ratio plasmonic grating. The THz spectral measurements showed a $\sim 9$-fold enhancement of the emitted THz power in the plasmonic TI-based PCA. The results of our study can open a pathway toward time-domain plasmonics-enhanced THz setups utilizing innovative photoconductive materials.

Compact and cost-effective spectrometers and imaging systems in the terahertz (THz) frequency range based on optical-THz photoconductive converters of ultrashort laser pulses (photoconductive antennas PCAs) are actively being developed and widely used to solve fundamental and applied problems in a variety of fields of science and technology. This high activity of research and development is associated with the PCAs' reliability and compact size, the easy scalability of a single element to 1D and 2D arrays, and PCAs' ability to provide a wide spectral range and high dynamic range of recorded THz signals without cooling. Recently, systems for multi-pixel detection of THz radiation based on matrix PCA detectors, designed to greatly increase the speed of THz imaging, have been of particular interest. This review presents the latest trends in the development of PCA-based THz devices, PCA-based methods of THz pulsed spectroscopy and imaging, as well as alternative approaches to THz pulse recording and THz imaging.

Double-barrier GaAs/AlAs resonant tunneling diodes (RTDs) have become the promising elements for the development of sub-mm and THz emitters. We report on the fabrication of the RTD samples that were characterized via RF-reflectometry to determine the parameters of its equivalent circuit. By using numerical simulation we show that the coplanar transmission line with the RTD under study provides an amplification up to 8 GHz. Keywords: Resonant tunneling diodes, active microstrip transmission lines, distributed emitters, diodes with double metal contacts.

The paper proposes designs of distributed resonant tunneling diodes (RTDs) with a double metal waveguide (DMW), in the top electrode of which a periodic pattern (metasurface) is formed based on holes in a hexagonal array and a chain of slotted (split-ring) resonators. This solution will make it possible to control the phase velocity of the propagating wave and increase the wave impedance in order to reduce ohmic losses in the waveguide, as well as to create a generation mode in it. For the proposed designs of metasurfaces, electromagnetic modeling was carried out and the input impedances of the resonators were calculated.

We have demonstrated a quantum cascade laser (QCL) with a generation frequency of about 3.8 THz, grown by metal-organic vapor phase epitaxy. The multilayer heterostructure for QCLs consists of 185 repetitions of an active module containing four GaAs/Al0.15Ga0.85As quantum wells. The threshold current and threshold voltage of the fabricated QCL were 2.25 kA/cm2 and 19.7 V, respectively. The QCL oscillations were carried out in the multimode regime, and the detection of terahertz radiation continued with an increase in the laser temperature up to 60 K.

In this paper, we present a new approach for the enhancement of pulsed terahertz (THz) generation in quantum dot (QD) based photoconductive antennas (PCA).We demonstrate the benefits of the combination of a QD substrate based PCA and an interdigitated electrodes topology which allows the photocarriers to reach the antenna terminals in a quasi-ballistic regime and immediately contribute to the THz emission.A 50fold increase in the generated THz power is observed.Such enhancement is made possible by unique combination of QD substrate properties, such as very high electric and thermal breakdown ruggedness, high carrier mobility, and yet short carrier lifetimes, compared to typical low temperature grown materials.We expect this solution to become favourable for development of powerful compact THz emitters.

Direct laser writing/ultra-hyperdoping technology was harnessed to fabricate a universal CMOS-compatible Si-based broadband (UV-THz) n-p junction detector, with its spectral response controlled by external liquid helium-nitrogen cooling. Near-far IR (2-40 mu m) temperature-dependent photoconductivity (PC) was revealed in a biased n-p junction, made of n-type sulfur-ultrahyperdoped Si (uh-Si, sulfur content similar to 1 at. %) sub-micron-thick layer on a surface of a p-doped Si substrate. The observed broadband photoconductivity is provided by a dense quasi-continuum series of sulfur-impurity donor states near the conduction band bottom (so-called "intermediate band", the bandwidth approximate to 0.6 eV), corresponding to IR-absorbing neutral and singly-ionized substitutional atomic and cluster centers of sulfur. The "intermediate" donor band supports the gradual "blue" spectral migration of PC maximum due to thermal ionization of deeper states at the temperature increasing in the range of 5-250 K. As a result, in the temperature range of 77-300 K complementary THz (wavenumbers <100 cm(-1)) spectroscopy indicates the predominating Drude-like conduction-band response of electrons and their gradually raising density similar to 10(19)-10(20) cm(-3). Illustrating the thermal-ionization depletion of the donor sulfur states from the THz-probed plasma side, this unveils the unprecedently high concentration of electrically-active donor sulfur centers similar to 10(20) cm 3 and related donor-state density similar to 10(20) states/eV.cm(3) in the uh-Si sample. Overall, these advances enable in situ laser writing of universal liquid helium/nitrogen cooled Si nano/micro/macrodetectors with broad - near-far IR and even THz - spectral response, crucial for photovoltaics, thermal and bio-imaging.

We study the band designs of terahertz quantum–cascade lasers (THz QCLs) with an active region of GaAs/Al x Ga 1− x As quantum wells (QWs) by using the solution of the Schrödinger equation with allowance for dephasing of quantum states, as well as solving a system of closed balance equations. For two-QW designs with increased Al x Ga 1− x As potential barrier height ( x = 0.20, 0.25, and 0.30), temperature dependences of the peak gain are calculated. It is shown that by increasing the aluminum content in the barrier layers compared to the conventional x = 0.15, it becomes possible to increase the operating temperatures of THz QCLs by more than 220 K. Two new designs of laser transitions are proposed to increase the output power and operating temperature of THz QCLs. To increase the output power, a design with a two-photon scheme of laser transitions was proposed, which causes an approximately twofold slower drop in the nonlinear gain with increasing photon density. To increase operating temperatures, it is suggested to use weakly localized electron states with wave functions extending over two or more periods of the structure. The matrix element of dipole transitions in such structures is shown to be greatly increased, while the lower laser level has a larger energy gap with the injector, is less populated, and is more temperature stable compared to the conventional designs. In this case, the calculated value of the maximum operating temperature is about 250 K.

Phase-change alloy Ge2Sb2Te5 (GST) forms a favorable material platform for modern optics, photonics, and electronics thanks to a pronounced increase in conductivity with thermally induced phase transitions from amorphous (a-GST) into cubic (c-GST) and then hexagonal (h-GST) crystalline states at the temperatures of ≃150 and ≃300°C, respectively. Nevertheless, the data on broadband electrodynamic response of distinct GST phases are still missing, which hamper the design and implementation of related devices and technologies. In this paper, a-, c-, and h-GST films on a sapphire substrate are studied using broadband dielectric spectroscopy. For all GST phases, complex dielectric permittivity is retrieved using Drude and Lorentz models in the frequency range of 0.06–50 THz or the wavelength range of ≃5000–6 μm. A contribution from the free charge-carriers conductivity and vibrational modes to the broadband response of an analyte is quantified. In this way, the Drude model allows for estimation of the static (direct current—DC) and dynamic (at 1.0 THz) conductivity values, caused by motions of free charges only, which are as high as σDC≃15 and 40 S/cm and σ1.0THz≃8.8 and 28.6 S/cm for the c- and h-GSTs, respectively. This overall agrees with the results of electrical measurements of GST conductivity using the four-point probe technique. The broadband electrodynamic response models obtained for the three GST phases are important for further research and developments of GST-based devices and technologies.

Objectives. The study aims to improve the efficiency of a large-area photoconductive terahertz (THz) emitter based on an optical-to-terahertz converter (OTC) having a radiating area of 0.3 × 0.3 mm2 for generating high-power THz radiation by using an array of close-packed profiled sapphire fibers having a diameter in the range of 100–300 μm as focusing optics. Methods. As a photoconductive substrate, we used a semi-infinite LT-GaAs layer (low-temperature grown GaAs; GaAs layer grown by molecular beam epitaxy at a low growth temperature). Additional Si3N4 and Al2O3 layers are intended for reducing leakage currents in the OTC and reducing the reflection of the laser pump pulse from the air/semiconductor interface (Fresnel losses), respectively, at a gap width of 10 μm. For forming the antenna electrodes and feed strips, the Ti/Au metal system was used. The simulation was carried out by the finite element method in the COMSOL Multiphysics environment. Results. The use of a profiled sapphire fiber whose diameter has been optimized with respect to the gap parameters to significantly increase the concentration of charge carriers in the immediate vicinity of the electrodes of an OTC is demonstrated. The integrated efficiency of a large-area photoconductive THz emitter was determined taking into account the microstrip topology of the array with a characteristic size of feed strips proportional to the gap width in the OTC and with the upper (masking) metal layer. The maximum localization of the electromagnetic field in close proximity to the edges of electrodes at the “fiber–semiconductor” interface is achieved with a profiled sapphire fiber diameter of 220 μm. Conclusions. By optimizing the diameter of the sapphire fiber, the possibility of improving the localization of incident electromagnetic waves in close proximity to the edges of the OTC electrodes by ~40 times compared to the case without fiber, as well as increasing the overall efficiency of a large-area emitter by up to ~7–10 times, was demonstrated.

Double-barrier GaAs/AlAs resonant tunneling diodes (RTDs) have become the promising elements for the development of sub-mm and THz emitters. We report on the fabrication of the RTD samples that were characterized via RF-reflectometry to determine the parameters of its equivalent circuit. By using numerical simulation we show that the coplanar transmission line with the RTD under study provides an amplification up to 8 GHz.

We propose the design of a photoconductive antenna (PCA) emitter with a plasmonic grating featuring a very high plasmonic Au electrode with a thickness of 170 nm. As we show numerically, the increase in h significantly changes the electric field distribution, owing to the excitation of higher-order plasmon guided modes in the Au slit waveguides, leading to an additional increase in the emitted THz power. We develop the plasmonic grating geometry with respect to maximal transmission of the incident optical light, so as to expect the excitation of higher-order plasmon guided Au modes. The fabricated PCA can efficiently work with low-power laser excitation, demonstrating an overall THz power of 5.3 μW over an ∼4.0 THz bandwidth, corresponding to a conversion efficiency of 0.2%. We believe that our design can be used to meet the demands of modern THz spectroscopic and high-speed imaging applications.

The design of a terahertz (THz) quantum cascade laser (QCL) with an active module based on three GaAs/Al0.18Ga0.82As quantum wells for high-temperature generation at a frequency of about 3.3 THz is optimized. A heterostructure based on the developed design with an active region thickness of 10 μm is grown by molecular-beam epitaxy with a deviation of the active-module thickness from the nominal of less than 1%. The fabricated THz QCLs with a double metal waveguide demonstrate lasing up to a temperature of 125 K. Investigations of the I–V characteristics, the dependences of the integrated emission on the current, and the lasing spectra show good agreement with the calculated characteristics.

In this work, we conduct research of spectral and power characteristics of quantum cascade lasers (QCLs) based on a GaAs/Al0.15Ga0.85As active region emitting at 2.3 (A), 3.2 (B) and 4.1 (C) THz. The QCL devices had a double-metal Au waveguide and operated in pulsed mode with 1.5–9 us pulses at 20 Hz repetition rate. Using the integral output power curves measured with different pulse durations, we consider the potential mechanisms of QCL temperature degradation using Arrhenius plots. Moreover, we present the spectra of the lasers measured at fixed operating points for devices A, С and with current scanning for device B in a wide temperature range from 5 to 120 K. We hope that our results will prove useful for research concerning QCL maximum operating temperatures.

We report a seminal approach for localization of photocarriers in a photoconductive antenna (PCA)-emitter via a focusing element comprising the sapphire fiber. Using numerical simulation, we showed that at a certain ratio between the fiber diameter and antenna gap size (d/g~ 22.5), one can attain a ~ 35-fold enhancement of laser irradiation in the vicinity of the PCA electrodes. This provides the formation of subwavelength electromagnetic wave caustics located at the edges of the PCA electrodes, which potentially promotes an increase in the optical-to-terahertz conversion efficiency. Keywords: terahertz frequency, terahertz emitters, semiconductors, photoconductive antenna, IR radiation.

We report on our recent advancement in the development and fabrication of the state-of-the art photoconductive emitters and detectors of THz waves. Thanks to band structure engineering and seminal approaches, we demonstrate that these devices can be used to fulfil the demands of modern THz instruments, spectroscopic systems and high-speed imaging applications.