Vadim S. Nozdrin

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.

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 E_a. 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 E_a ≃ ~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.

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.

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.

Reduction of interconnect capacitance is one of the key ways to increase the speed of an ultra-large-scale integrated circuit (ULSI). A porous organosilicate glass (OSG) thin film on a platinum sublayer was used as a test vehicle for modelling the conductor–insulator electrodynamic properties in an ULSI. Investigation of the structure and residual effect on the dielectric contributions of the IR absorption bands is a direct approach to control the dielectric properties of the film. The absorption band dispersion was simulated by the Drude–Lorentz equations within a framework consisting of a two-layer medium model. The contributions to the dielectric permittivity of IR absorption bands in thin OSG films with different volume porosities were determined. It was shown that the total dielectric permittivity in the IR region was approximately 10% lower than that at low frequencies of the order of 100 kHz. An additional contribution can arise as a result of low-energy excitations and H ₂ O electrodipole relaxation in the microwave—THz range.

Polarized THz radiation is increasingly being investigated and applied in the new interdisciplinary field “TeraNano.” We report the generation of terahertz (THz) radiation by a linearly polarized femtosecond optical pulse in black phosphorus (BP) crystallites grown by chemical vapor deposition. Characterization of THz radiation is performed by time-resolved THz spectroscopy. It was found that BP emits elliptically polarized THz radiation with an ellipticity of ε = 0.77 ± 0.03. A saturation was found in the dependence of the THz intensity on the intensity of the incident optical pump for the latter higher than 6 mJ / cm². The results of our work can provide new fundamental knowledge needed to create detectors and generators based on BP in the THz range.

Creation of new dielectric materials for insulating layers of interconnects with low losses at high frequencies (low-k) is one of the main directions of modern microelectronics. Currently, various modifications of standard for modern integrated circuits dielectric structures based on SiO2, differing in composition and morphological characteristics, are being investigated. In this paper, the dielectric losses of thin-film SiO2 samples on an Al substrate are studied by terahertz (THz) and IR spectroscopy. Significant differences in the spectra of such structures in comparison with bulk samples of fused quartz, including berriman resonance modes, were found.

Atmospheric water adsorbed by porous materials may significantly change its electrodynamic response in a wide frequency range. Mechanical and chemical stabilities of silicon dioxide along with a proven manufacturing method of porous SiO₂ samples made it convenient for studying the effects of moisture adsorption on various parameters of the porous media. We report the dielectric properties of SiO₂-based nanoporous glass in the frequency range of 20 Hz–400 THz at ambient atmospheric conditions and at a low residual pressure of ≤1 mbar. We observed a significant low-frequency dispersion of the complex dielectric permittivity and enhancement of dielectric loss for the porous sample exposed to the ambient moisture. In the terahertz range, the change in dielectric response is smaller and correlates with the moisture saturation of the sample.

The transmission spectra of a single crystal, artificial and natural fluorite ceramics were recorded in the areas of material transparency in THz and the average IR range at room temperature. In the region of high absorption, the reflectance spectra were measured. Differences in the optical characteristics of a single crystal and optical ceramics are insignificant.