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 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.

Imaging at terahertz frequencies (0.1-10 THz, wavelengths 3 mm-30 µm) has proven to be useful in the biomedical field. Still, the acquisition time is an important hurdle. Here, we discuss recent developments toward achieving real-time THz imaging. First, we demonstrate a spectral encoding algorithm to reconstruct a 4500-pixels image with 45 measurements. Second, we improve the image resolution using a super-resolution algorithm specifically developed for the THz. Third, we discuss our most recent work on the fabrication of an THz photoconductive antenna array for imaging. These works pave the way for future applications of THz imaging in biomedical science.

Terahertz (THz) technology is promising in several applications such as imaging, spectroscopy and communications. Among several methods in the generation and detection of THz waves, a THz time-domain system that is developed using photoconductive antennas (PCA) as emitter and detector presents several advantages such as simple alignment, low cost, high performance etc. In this work, we report the design, fabrication and characterization of a 2-D PCA array that is capable of detecting both the amplitude and phase of the THz pulse. The PCA array is fabricated using LT-GaAs and has 8 channels with 64 pixels ( $8 \times 8$ ). A novel approach using a spatial light modulator (SLM) to steer and focus the infrared probe beam towards pixels of the PCA array is presented. Each channel records the photocurrent generated by the THz signal (amplitude and phase) separately and frequencies up to 1.4 THz can be detected. Furthermore, the parameters such as directional time delay of the THz pulse, crosstalk between the channels etc., were characterized. Finally, we show that the proposed 2D PCA array design is flexible and can be used for accelerated THz spectral image acquisition.

We report on a terahertz (THz) detector based on a photoconductive antenna (PCA) utilizing an artificially strained undoped InGaAs/InAlAs superlattice (SL). Using our laboratory pulsed THz time-domain spectrometer, we demonstrate the advancement of the strain-induced SL-based PCA detector (SID) when operating with an optical probe power of P-opt > 6 mWover the PCA detector based on a lattice-matched InGaAs/InAlAs SL (LMD). Both detectors demonstrate a broad frequency bandwidth of 3.5 THz at the excitation wavelength of 780 nm with a signal-tonoise ratio (SNR) of similar to 70 dB. The experimental results demonstrate a change in the behavior of two detectors: At P-opt = 1 mW, the LMD shows a bit increased SNR compared to that for SID, while vice versa at P-opt = 10 mW. Also, SID shows a quadratic dependence of its SNR on Popt while the SNR for LMD starts saturating at Popt similar to 5 mW. Moreover, the noise floor in SID is almost independent on probe power, while the noise floor for LMD demonstrates a rapid growth with an increase of Popt. We believe that SID coupled to a fiber telecommunication wavelength laser could open a pathway toward the development and fabrication of portable and cost-effective THz photoconductive devices.

The fabrication and investigation of single and multilayered structures have become an essential issue in the past decades since these structures directly define valuable properties and efficiency of widely used terahertz (THz) emitters and detectors. Since the development of molecular-beam epitaxy, as well as other crystal growth techniques, a variety of structural designs has appeared and has been proposed. Since that, an enormous progress has been achieved beginning from the pioneering work on photoconductivity in silicon toward different multilayered heterostructures. The last are now commonly utilized as base components in photoconductive THz emitters/detectors, quantum-cascade lasers for pulsed and continuous-wave THz spectroscopic and imaging systems providing critical fundamental and practical applications at the forefront of scientific knowledge (sensors, flexible electronics, security systems, biomedicine, and others). This review summarizes the developments in different approaches and crystal growth techniques, emphasizing the importance of using single and multilayered arsenides-and related III-V materials-based (phosphides, antimonides, bismuthides) structures to accomplish the needs of modern and existing instruments of THz science and technology.

This review highlights recent and novel trends focused on metallic (plasmonic) and dielectric metasurfaces in photoconductive terahertz (THz) devices. We demonstrate the great potential of its applications in the field of THz science and technology, nevertheless indicating some limitations and technological issues. From the state-of-the-art, the metasurfaces are, by far, able to force out previous approaches like photonic crystals and are capable of significantly increasing the performance of contemporary photoconductive devices operating at THz frequencies. (C) 2019 Society of Photo-Optical Instrumentation Engineers (SPIE)