Abstract While the existing terahertz (THz) fiber optics components suffer from high loss and dispersion, low technological reliability, poor environmental resistance and radiation strength, as well as large cross-section, THz applications in different fields still require hardware for the sensing and exposure of hard-to-access objects. To mitigate this difficulty, we develop the two variants of hollow-core THz waveguides, those exploit the antiresonant reflecting optical waveguiding (ARROW) mechanism and use (as a key element) a few-millimeter-diameter sapphire tube produced by the edge-defined film-fed growth (EFG) technique. In the all-dielectric arrangement, the outer surface of this tube is coated by a sub-millimeter-thick polytetrafluoroethylene (PTFE) film, while in the metal-coated one—by a sub-micrometer-thick reflecting copper layer. These coatings increase the guiding efficiency and underlie different performance of the two geometries. Both waveguides are studied numerically and experimentally in the 0.56–0.7 THz frequency range. The observed discrepancies between the theoretical and measured propagation loss are attributed to fluctuation of the cross-section geometry over the waveguide length. In narrow frequency bands, the metal-coated waveguide offers the propagation loss as small as 5.0 dB/m, which is significantly lower than that of the all-dielectric one. Furthermore, the outer metal coating completely prevents mode leakage, whereas in an all-dielectric waveguide, some of the evanescent field extends into the surrounding space and still can be de-coupled. Our findings highlight that the ARROW sapphire THz waveguides provide a reasonable compromise between the guiding efficiency and the cross-section dimensions, thus, forming a favorable platform for the THz sensing and exposure.

Broadband experimental data in the THz-IR region are used to analyze the temperature evolution of optical parameters of sapphire in the temperature range of 77–300 K. Temperature dependences of the refractive index of ordinary and extraordinary rays were obtained and, based on these data, temperature dependence of birefringence in the THz range was calculated. The parameters of optical phonon modes and many-particle processes in the THz-IR region were obtained by using classical oscillator and four-parameter generalized models. Estimates of the contributions of optical phonon modes and many-particle processes to absorption in the THz range in the temperature range of 77–300 K are presented. The obtained data made it possible to establish the operating frequency region and dynamic range of sapphire optical elements for application in THz devices at low and cryogenic temperatures.

Thin conductive films deposited on the surface of the heat-sensitive element of the radiation receiver make it possible to absorb up to 50% of the incident radiation. This increases the efficacy of detectors in the terahertz (THz) frequency range. The absorbing conductive coating on the array of a multi-pixel detector is an ordered structure with dimensions comparable to the wavelength of THz radiation. Diffraction of radiation on this structure leads to a change in the wavefront of both transmitted and reflected waves, leading to image distortion in a multi-pixel detector. Based on experimental data obtained using pulsed THz and IR Fourier spectroscopy, the transmission and conductivity spectra of solid films and capacitive Al meshes of different thicknesses were analyzed. The optimal thickness of the Al coating for maximum absorption of THz radiation has been determined. The transmission spectra of capacitive grids with an Al film thickness corresponding to the maximum absorption indicate the disappearance of diffraction effects in the THz range.

Solid immersion microscopy is a near‐field imaging modality that overcomes the Abbe diffraction limit by focusing the light beam behind a high refractive index lens. It offers high energy efficiency, thanks to the absence of any sub‐wavelength probes or apertures in the optical path. A favorable combination of superresolution and high optical throughput opens up a variety of imaging applications in different branches of science and technology. The spatial resolution of solid immersion microscopy is mostly limited by the refractive index value of the lens, with optically denser lenses offering higher resolutions. In this paper, bulk rutile (TiO 2 ) crystal is used as a material for the solid immersion lens, which offers an impressive refractive index of ≈10 in the terahertz range. This is the highest value of refractive index ever used in solid immersion microscopy. A continuous wave impact ionization avalanche transit‐time diode‐emitter at the 0.2 THz frequency (the λ = 1.5 mm wavelength) and a Golay detector are used for building a solid immersion microscope. Numerical and experimental studies reveal 0.06–0.11λ resolution of the developed microscope. This is the highest normalized resolution ever reported for any solid immersion imaging systems.

While terahertz (THz) technology offers a variety of applications in medical diagnosis, nondestructive testing, and quality control, its acceptance in these practical fields is hampered by the absence of endoscopic systems, capable of sensing the complex refractive index of the hard-to-access objects. In this paper, we develop the THz endoscope based on the hollow-core antiresonant waveguide, formed by a polytetrafluoroethylene (PTFE)-coated sapphire tube with the outer end closed by a monolithic sapphire window. The endoscope is attached to the backward wave oscillator spectrometer to measure the sample reflectivity. By studying the well-known liquid and solid samples, we demonstrate that analysis of the Fabry–Pérot resonance in the measured reflection spectra makes it possible to quantify the complex refractive index of an analyte. Thanks to the advanced chemical inertness and thermal strength of sapphire and PTFE, the developed endoscope is capable of operation in harsh environments, which broadens the range of its applications. Our findings pave the way for the THz technology use in a number of demanding practical fields.

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.

A rutile crystal is used to machine a solid immersion lens, boasting a very high refractive index of ~10 in the terahertz range. We further show experimentally that a solid immersion microscope using such a lens achieves a record-breaking spatial resolution of 0.06–0.11 wavelength.

Abstract Terahertz (THz) technology offers a variety of applications in label-free medical diagnosis and therapy, majority of which rely on the effective medium theory that assumes biological tissues to be optically isotropic and homogeneous at the scale posed by the THz wavelengths. Meanwhile, most recent research discovered mesoscale ( $$\sim \lambda $$ ∼ λ ) heterogeneities of tissues; $$\lambda $$ λ is a wavelength. This posed a problem of studying the related scattering and polarization effects of THz-wave–tissue interactions, while there is still a lack of appropriate tools and instruments for such studies. To address this challenge, in this paper, quantitative polarization-sensitive reflection-mode THz solid immersion (SI) microscope is developed, that comprises a silicon hemisphere-based SI lens, metal-wire-grid polarizer and analyzer, a continuous-wave 0.6 THz ( $$\lambda = 500$$ λ = 500 µm) backward-wave oscillator (BWO), and a Golay detector. It makes possible the study of local polarization-dependent THz response of mesoscale tissue elements with the resolution as high as $$0.15 \lambda $$ 0.15 λ . It is applied to retrieve the refractive index distributions over the freshly-excised rat brain for the two orthogonal linear polarizations of the THz beam, aimed at uncovering the THz birefringence (structural optical anisotropy) of tissues. The most pronounced birefringence is observed for the Corpus callosum, formed by well-oriented and densely-packed axons bridging the cerebral hemispheres. The observed results are verified by the THz pulsed spectroscopy of the porcine brain, which confirms higher refractive index of the Corpus callosum when the THz beam is polarized along axons. Our findings highlight a potential of the quantitative polarization THz microscopy in biophotonics and medical imaging.

Various mechanisms of absorption including both electric-dipole excitations and magnetic ones in a broadband (5–5000 cm−1) polarized transmission and reflection spectra were studied in orthoferrite TbFeO3 using a coherent submillimeter, pulsed THz, and IR Fourier transform spectroscopy. The classical oscillator model and generalized four-parameter model were used to analyze the spectra obtained, and the parameters of the models were determined. We analyzed main absorption processes, which include contributions of both phonons and multi-phonon processes as well as electron transitions in Tb3+ ions and two antiferromagnetic resonance modes in the Fe subsystem. As a result, the spectra of real and imaginary parts of permeability and permittivity were obtained. A distinctive feature of the studied electrodynamic response in TbFeO3 is a significant absorption in the THz range due to multi-phonon processes, which exceeds optical phonon contribution by an order of magnitude at room temperature.

We report on the development and fabrication of a plasmonic metasurfaces for PCA based on InAlAs/InGaAs superlattice heterostructures. As a result of THz time-domain measurements, it was found that for PCA emitters with a 150 nm thick plasmonic metasurface, the maximum recorded overall THz power was 5.3 (the optical-THz conversion efficiency was 0.19 %) under laser pumping with an average power of 2.8 mW and a bias voltage of 8.6 V. The increase in the signal-to-noise ratio due to the excitation of plasmon modes exceeds ~80 dB. The experiments have shown that the increase of metallization thickness provides the increase of the emitted power. The results could open a pathway towards the development of the Russian THz spec-troscopic and imaging systems.

The reflection spectra of pure single-phase pressed samples of saccharides are measured by the methods of coherent submillimeter, pulsed terahertz, and broadband Fourier-transform infrared spectroscopy; the parameters of the absorption bands of samples are estimated using the additive classical oscillator model. The possibility of analyzing the vibrational spectrum with correction for scattering using calibration data for the visible and terahertz ranges is shown. The sensitivity and accuracy of this approach are limited by low reflection coefficients and, as a result, by a low signal-to-noise ratio, as well as by the asymmetric broadening of bands due to anharmonicity of vibrations and overlap of absorption bands.

Plasmonic metasurfaces for photoconductive antennas (PCAs) based on InAlAs/InGaAs superlattice heterostructures are proposed and fabricated. It is found by the measurements of the PCAs in a terahertz pulsed spectrometer that the maximum detected integrated terahertz power is 5.3 μW (the optical-to-THz conversion efficiency is 0.19%) under laser pumping at an average power of 2.8 mW and a bias voltage of 8.6 V. The signal increment caused by the excitation of plasmonic modes makes it possible to obtain a high signal-to-noise ratio (~80 dB). The experiments show that the emitted power density increases with the metasurface metallization height (thickness). With the parameters obtained, the developed antennas can open a path towards the development of THz spectroscopic and imaging systems in Russia.

The engineering of dielectrics such that they have low dielectric constants is an important challenge in the semiconductor industry. The IR absorption and THz relaxation make a significant contribution to the low-frequency dielectric constant of porous organosilicate glass (OSG) thin films used in the interconnects of integrated circuits as low-k dielectrics. Determination of the dielectric contributions with the aid of the electric dipole absorption bands in the THz region has certain methodological limitations owing to the effect of the substrate. To overcome this problem, we used a c-cut sapphire plate as a substrate, which is optically isotropic and transparent in the THz range. The parameters of both the IR and THz absorption bands in the OSG films were measured using Fourier IR and pulsed THz spectroscopy and analysed using the classical oscillator model. The contribution of the THz relaxation is responsible for approximately 15%–17% of the value of the low-frequency dielectric constant.

The reflection spectra of pure single-phase pressed samples of saccharides were measured using coherent submillimeter, pulsed terahertz, and broadband Fourier IR spectroscopy, and the parameters of their absorption bands were estimated using an additive classical oscillator model. The possibility of analyzing the vibrational spectrum with the correction of scattering from calibration data in the visible and terahertz ranges is shown. The sensitivity and accuracy of this approach are limited by the low values of reflection coefficient and, as a consequence, the low signal-to-noise ratio, as well as the asymmetric broadening of the bands associated with the anharmonicity of the vibrations and the overlap of the absorption lines.

The spatial resolution of an optical fiber bundle is limited by the size of a mode propagating in an individual fiber, which can be as small as a single wavelength approximately lambda for conventional low-to -medium refractive-index optical fibers. High-refractive-index optical fiber bundles have a potential for the subwavelength-resolution imaging due to strong mode confinement in a fiber core, but they suffer from inconvenient image readout from the bundle output end, which is done by energy-inefficient near-field probes. To address this issue, this work introduces a tapered high-refractive-index terahertz (THz) optical fiber bundle. It comprises the subwavelength-diameter sapphire optical fibers, that are stacked tightly in the object plane to sample the near field with a subwavelength resolution. The fibers diverge from the object plane and, thus, stretch the captured near field for its further read out from the output bundle end using conventional diffraction-limited lens. Such a tapered fiber bundle was investigated numerically and then implemented experimentally at 0.33 THz. The subwavelength resolution of this bundle was confirmed, and varies over its aperture and can be as high as 0.35 lambda. The developed imaging principle allows the <^> 0.5 lambda Abbe resolution limit of a free-space focusing to be overcome and almost any common diffraction-limited optics for the near-field applications to be adapted.

Transparent semiconducting oxides are widely used as conductive electrodes in optoelectronic devices in the near-infrared and visible ranges. However, their applications in the THz frequency range devices are limited because of the absorption by free carriers in this range and the low-frequency tail of the optical phonon modes. In this study, we investigated the optical and electrodynamic parameters of lanthanum nickelate films using contactless and nondestructive methods, including submillimeter coherent spectroscopy, terahertz pulsed spectroscopy, and infrared Fourier transform spectroscopy. Evidently, the film transmission deviates from the Hagen–Rubens relation by as much as 30%, and the temperature dependence of the conductivity exhibits a dominantly semiconducting behavior. A decrease in the plasma frequency of the free carriers to approximately 2000 cm−1 (0.25 eV) increases the intensity of the vibrational absorption bands of the film. Further, films with a reduced conductivity and a thickness of 100–200 nm are expected to transmit at least half of the incident radiation in the THz range. These results demonstrate the prospect of employing lanthanum nickelate films with decreased conductivity as electrode layers in optoelectronic converters in the THz frequency range.

Transformations of the low-energy vibrational spectra are associated with structural changes in an analyte and closely related to the instability of weak chemical bounds. Terahertz (THz)/far-infrared optical spectroscopy is commonly used to probe such transformation, aimed at characterization of the underlying solid-phase chemical reactions in organic compounds. However, such studies usually provide quite qualitative information about the temperature- and time-dependent parameters of absorption peaks in dielectric spectra of an analyte. In this paper, an approach for quantitative analyses of the solid-phased chemical reactions based on the THz pulsed spectroscopy was developed. It involves studying an evolution of the sample optical properties, as a function of the analyte temperature and reaction time, and relies on the classical oscillator model, the sum rule, and the Arrhenius theory. The method allows one to determine the temperature-dependent reaction rate V1(T) and activation energy Ea. To demonstrate the practical utility of this method, it was applied to study α-lactose monohydrate during its temperature-induced molecular decomposition. Analysis of the measured THz spectra revealed the increase of the reaction rate in the range of V1 ≃ ~9 × 10-4-10-2 min-1, when the analyte temperature rises from 313 to 393 K, while the Arrhenius activation energy is Ea ≃ ~45.4 kJ/mol. Thanks to a large number of obtained physical and chemical parameters, the developed approach expands capabilities of THz spectroscopy in chemical physics, analytical chemistry, and pharmaceutical industry.

Solid Immersion (SI) microscopy is a modern imaging modality that overcomes the Abbe diffraction limit and offers novel applications in various branches of visible, infrared, terahertz, and millimeter-wave optics. Despite the widespread use, SI microscopy usually results in qualitative imaging. Indeed, it presents only the raw distributions (in the image plane) of the backscattered field intensity, while unlocking the information about the physical properties of an imaged object, such as its complex refractive index (RI) distribution, requires resolving the inverse problem and remains a daunting task. In this paper, a method for resolving the SI microscopy inverse problem is developed, capable of reconstructing the RI distribution at the object imaging plane with subwavelength spatial resolution, while performing only intensity measurements. The sample RI is retrieved via minimization of the error function that characterizes discrepancy between the experimental data and the predictions of analytical model. This model incorporates all the key features of the electromagnetic-wave interaction with the SI lens and an imaged object, including contributions of the evanescent and ordinary-reflected waves, as well as effects of light polarization and wide beam aperture. The model is verified numerically, using the finite-element frequency-domain method, and experimentally, using the in-house reflection-mode continuous-wave terahertz SI microscope. Spatial distributions of the terahertz RIs of different low-absorbing optical materials and highly absorbing biological objects were studied and compared to a priori known data to demonstrate the potential of the novel SI microscopy modality. Given the linear nature of the Maxwell’s equations, the developed method can be applied for subwavelength-resolution SI microscopy at other spectral ranges.