We investigate terahertz (THz) single-pixel imaging under spatially non-uniform illumination using a quantum cascade laser source with structured beam profiles. The reconstruction problem is formulated as a self-calibrated framework. The unknown illumination field is estimated directly from single-pixel measurements. It is then incorporated into the sensing model for object reconstruction. The approach is evaluated with computational ghost imaging and with compressive sensing methods, including OMP and TVAL3. All quality metrics are reported as distributions over random subsets of the recorded measurements, and the method comparisons are supported by paired significance tests. We find that the benefit of self-calibration depends on the degree of illumination inhomogeneity. Under strongly non-uniform illumination, the self-calibrated TVAL3 method provides the highest reconstruction quality and significantly outperforms all single-stage methods. Under weakly non-uniform illumination, the error of the beam estimate outweighs the gain from the illumination correction, and single-stage reconstruction remains preferable. The transition between the two regimes is characterized by the intensity coefficient of variation within the object region. The proposed framework enables THz imaging without beam homogenization optics, which is particularly relevant for compact THz systems based on quantum cascade lasers.

All-optical diffractive deep neural networks (D2NNs) offer significant advantages in processing speed and power consumption, thereby accelerating the development of optical computing and artificial intelligence (AI). Integrating multiple degrees of freedom (multi-DoF) into D2NNs is a pivotal role in improving information processing and task-loading capacity, an enormous challenge in current all-optical diffractive computing/processors. Here, a multi-DoF diffractive processor is proposed and experimentally demonstrated that leverages a metasurfaces-based approach to integrate polarization, distance, and rotation channels for versatile inference tasks and information encryption. The approach is validated using three-layer metasurfaces that enable high task-capacity tasks, including single-/dual-digit and single-/dual-fashion-product classification, logic operators, and image transformation. Moreover, by mapping large volumes of input data into multi-DoF channels and encoding the information in Morse code with our D2NNs framework, a high-security information transmission system is experimentally implemented. The integration of polarization, distance, and rotation channels into an all-optical diffractive processor with multifunctional capabilities paves the way for multifunctional integrated devices and communication.

Terahertz (THz) dual-comb spectroscopy (THz-DCSS) is a competitive spectral technique due to its high frequency resolution. However, because the pump and detection femtosecond (fs) pulses are from two stabilized fs lasers, the perfect mutual coherence is difficult to reach, which prohibits long time data average and improving frequency resolution. Here, two frequency noise sources, the residual unlocked and transferred ones are investigated for a THz dual comb spectrometer (THz-DCSM). In time domain, the amplitude noise plays a dominate role on the performance of THz-DCSS; the repetition-frequency noises are negligible within Fourier-transform frequency resolution. In frequency domain, the relative synchronization error originating from the instantaneous frequency differences between the two fs lasers is 4 x 10(-13) (Delta f/f); the relative frequency resolution is similar to 4 x 10(-10), limited by the radio frequency reference. Our investigations are helpful for improving the performance of THz-DCSS and developing high frequency resolution THz spectroscopy.

The article describes a new method for determining the complex dielectric permittivity of films, whose thickness is much smaller than the terahertz (THz) radiation wavelength. This method is based on the simultaneous measurement of THz waveforms reflected by and transmitted through the sample-containing films (dual-mode configuration). We present analytical and numerical solutions of the Maxwell equations, which allow to precisely and easily determine the complex material parameters of the films for transverse electric (TE) and transverse magnetic (TM) polarization cases for the arbitrary incident angle of the THz beam. The proposed method reduces the noise contribution related to laser fluctuation, which improves the precision determination of the dielectric permittivity.

The work is focused on the development and implementation of the technique of terahertz quartz-enhanced photo-acoustic spectroscopy and opens up new opportunities for the detection of target substances in gas mixtures. The relevance of this technique is due, on the one hand, to the advantages of terahertz spectroscopy, which allows unambiguously determining the presence of target substances in a mixture. On the other hand, the use of a quartz tuning fork (QTF) as a supersensitive receiver of a photoacoustic signal makes it possible to determine the substances in low concentrations. The paper considers various factors affecting the resonant characteristics of a QTF and proposes an approach that ensures the stability of registration of a useful signal regardless of changes in the parameters of the environment. Despite only a discrete set of absorption coefficients due to the limited resolution of the terahertz quantum-cascade laser (QCL) used, high-resolution spectra were reconstructed using the developed approach based on a deep fully connected neural network. Two fully synthetic datasets of 1 million spectra each have been prepared, taking into account the discreteness of the input experimental data. For testing the developed model, the experimental data subjected the procedure of baseline shift compensation. The results of experiments on photo-acoustic spectroscopy using a terahertz multi-frequency QCL, where the frequency can be ‘tuned’ to the characteristic features of the absorption spectrum, demonstrate the potential for the development of a quartz-enhanced photo-acoustic spectroscopy in a terahertz range.

Terahertz (THz) wave manipulation based on laser filaments-plasma channels formed by femtosecond laser-induced air ionization-has emerged as a promising platform for free-space THz applications. However, in-situ characterization of the spatially confined THz modes within filaments faces significant challenges due to the plasma's ultra-high intensity, which not only hinders direct near-field probing but also limits reliance on indirect far-field reconstruction. Here, we introduce a non-invasive near-field modulation scheme where a metal plate approaches the filament at submillimeter distances (comparable to THz wavelengths), perturbing the dielectric environment to convert the symmetric annular THz mode into an asymmetric state. This controlled transition enables far-field detection of broadband calculus behaviors (first- and second-order differentiation/integration) on time-domain THz waveforms and characteristic spectral transfer functions with 1/f, 1/f^2, f or f^2 dependency (where f is the THz frequency), thereby diagnosing the near-field THz mode confinement. Hence, the proposed approach synergizes near-field modulation efficiency with far-field detection robustness, advancing fundamental understanding of plasma-THz interactions and enabling novel all-optical signal processing for filament-based THz technologies.

Advanced sensing devices based on metasurfaces have emerged as a revolutionary platform for innovative label-free biosensors, holding promise for early diagnostics and the detection of low-concentration analytes. Here, we developed a chip-based ultrasensitive terahertz (THz) metasensor, leveraging a quasi-bound state in the continuum (q-BIC) to address the challenges associated with intricate operations in trace biochemical detection. The metasensor design features an open-ring resonator metasurface, which supports magnetic dipole q-BIC combining functionalized gold nanoparticles (AuNPs) bound with a specific antibody. The substantial enhancement in THz-analyte interactions, facilitated by the potent near-field enhancement enabled by the q-BICs, results in a substantial boost in biosensor sensitivity by up to 560 GHz/refractive index units. This methodology allows for the detection of conjugated antibody-AuNPs for cardiac troponin I at concentrations as low as 0.5 pg/ml. These discoveries deliver valuable insight for AuNP-based trace biomolecule sensing and pave the path for the development of chip-scale biosensors with profound light-matter interactions.

Background. Up to 30% of combatants are diagnosed with combat syndrome (post-traumatic stress disorder, PTSD), and 15% of veterans show symptoms even 10 years after the end of the war. Not only during hostilities, but also in peacetime, about 60% of people at different periods of their lives encounter traumatic events that can provoke a disorder that, in its totality of symptoms, resembles “combat syndrome”. Objective. PTSD is a multimodal disorder, the diagnosis and treatment of which requires an interdisciplinary approach. The article is devoted to a brief review of methods of psychophysiological (instrumental) diagnosis and neurorehabilitation of PTSD. Results. The key areas of psychophysiological research into the mechanisms of PTSD formation are highlighted. A review of the achievements and prospects of clinical psychophysiology in the development of instrumental methods for the diagnosis and neurorehabilitation of PTSD is presented. Conclusions. Methods of psychophysiology in combination with methods of psycho- and pharmacotherapy increase the effectiveness of treatment of PTSD and are indispensable in situations where patients do not trust the methods of conventional medicine or show insensitivity to traditional therapy. The most promising directions in the development of methods for instrumental diagnosis and correction of PTSD are the development of neurofeedback techniques and adaptive neural interfaces, TES and TMS methods, and the study of the contribution of genetic and epigenetic factors to the etiology of PTSD.

In this work, we use T-shaped graphene as the channel of terahertz (THz) detector, and adopt metallic metamaterials near the source. Graphene plasmons at the source side are enhanced by the metallic metamaterials, increasing THz absorption and thus enhancing THz response generated by photo-thermoelectric effect. Because of the non-degraded absorption of graphene plasmons, samples with large metallic metamaterials show non-degraded response within 0.3 THz, and samples with small metallic metamaterials show non-degraded response within 0.75 THz. Series connection instead of parallel connection of each T-shaped graphene channel is expected to yield higher responsivity. Higher responsivity can also be achieved by applying back gate voltage. Samples using graphene with lower mobility show weaker response due to reduced THz absorption, and it is expected that higher response can be obtained by using graphene with higher mobility.

Quartz-enhanced photoacoustic spectroscopy technique (QEPAS) in terahertz frequency range is applied for hydrogen sulfide detection. A tunable distributed feedback multifrequency terahertz quantum cascade laser is used as an excitation source and a standard quartz tuning fork - as a QEPAS sensor. Data analysis is carried out using physically-informed machine learning. Direct numerical simulations are carried out for absorption estimation on terahertz laser lines showing a good correspondence to experimental data. A clear dependence of the signal from QEPAS sensor on a target analyte is seen showing a feasibility for recognition of the chosen gases.

Optical properties of phase-change materials (PCM) GeTe and GeTe 2 were studied. Active planar metamaterials based on PCM with ultrafast switching of THz field characteristics were developed. A new technique for obtaining the complex refractive index of metamaterials based on simultaneous measurement of reflection and transmission of THz radiation was proposed and experimentally confirmed. Such anisotropic effects as optical activity and circular dichroism in the interaction of THz field with active metamaterials were investigated.

Ultrafast pulse switching is one of the key elements for ultrahigh speed communication technology. We study the terahertz (THz) induced birefringence response on the laser pulse through the quartz with different THz electric field strength. The magnitude of the observed Pockels signals scales linearly with the THz field amplitude, while the Kerr signals scale quadratically with the THz field amplitude. We demonstrate that the quartz is a good candidate for polarization modulation of 800 nm laser pulse, which has the advantages of low-cost, large bandgap, and negligible dispersion. Furthermore, our investigation finds application beyond ultrafast polarization switching, and the THz-induced polarization gating technique works as a tool for intense THz pulse detection.

In my lecture I will talk about Stimulated Emission, one of the interesting optical phenomena and technologies. In the microwave wavelength range, stimulated emission is well described theoretically. In my lecture I show that stimulated emission in this wavelength range can have the character of maser generation. This effect is based on the fact that coherent phonon states can be observed in crystalline media, which form a two-level maser-type system. In my lecture I will talk about the stimulated formation of such media under femtosecond excitation. The temporal dynamics of such maser-like generation allows the formation of pulsed terahertz radiation with a variable pulse shape.

Exhaled air contains volatile molecular compounds of endogenous origin, being products of current metabolic pathways. It can be used for medical express diagnostics through control of these compounds in the patient’s breath using molecular absorption spectroscopy. The fundamental problem in this field is that the composition of exhaled air or other gas mixtures of natural origin is unknown, and content analysis of such spectra by conventional iterative methods is unpredictable. Machine learning methods enable the establishment of latent dependencies in spectral data and the conducting of their qualitative and quantitative analysis. This review is devoted to the most effective machine learning methods of exhaled air sample absorption spectra qualitative and content analysis. The focus is on interpretable machine learning methods, which are important for reliable medical diagnosis. Also, the steps additional to the standard machine learning pipeline and important for medical decision support are discussed.

The associations of the exhaled air IR absorption spectra with clinical parameters characterising the state of COVID-19 patients were studied at the time of their hospitalization. The clinical parameters included standard blood tests characteristics and the presence of comorbidities. The exhaled air IR absorption spectra were measured by laser photo-acoustic gas-analyzer based on a tunable CO2 laser. The measured absorption spectra analysis was based on their splitting on subgroups in dependence on clinical parameters values and the further comparison of these subgroups with the same for healthy participants using principal component analysis. In the result, informative clinical parameters, which allowed distinguishing exhaled air absorption spectra from COVID-19 patients and healthy participants, were established. Revealed correspondence of changes in exhaled air spectral characteristics of COVID-19 patients and clinical parameters demonstrates that pathological processes occurring in the body of COVID-19 patients and success of their therapy can be revealed not only by the results of laboratory tests but also by the exhaled air spectral analysis.

The pulsed laser deposition (PLD) method is widely used in the production of different thin films. PLD has several advantages: flexible adjustment of parameters and controllability of processes and ease of synthesis of materials. The deposition of zinc oxide droplets during the synthesis of zinc oxide thin films by PLD is considered and the temperature and initial speed of droplets, which can boil on the thin film surface, are determined. The study of the process of the deposition of droplets from a ZnO target was carried out during the growth of thin films according to the traditional scheme for the PLD method. The process of the deposition of ZnO droplets during the PLD of ZnO films under vacuum conditions is considered. For the first time, it has been revealed that ZnO drops are boiling on the thin ZnO film surface. For the first time, the temperature and speed of solid and liquid drops of ZnO, at which they can boil when they hit the substrate, have been calculated taking into account the processes of heat loss through radiation and the heat of phase transition. Thus, for the first time, it has been established experimentally and confirmed in calculations that at the moment of collision some drops of ZnO are boiling. The role of the heat of phase transition and heat loss due to radiation during the flight of droplets during the PLD of ZnO thin films in a vacuum has been clarified. The temperature of a ZnO drop after a collision with a substrate was calculated in a wide range of initial speed and temperature taking into account heat losses due to radiation and heating from impact.

Terahertz (THz) electromagnetic interference (EMI) shielding materials is crucial for ensuring THz electromagnetic protection and information confidentiality technology. Here, it is demonstrated that high electrical conductivity and strong absorption of THz electromagnetic radiation by type-II Dirac semimetal PdTe2 film make it a promising material for EMI shielding. Compared to MXene film, a commonly used metallic 2D material, the PdTe2 film demonstrates a remarkable 40.36% increase in average EMI shielding efficiency per unit thickness within a broadband THz frequency range. Furthermore, it is demonstrated that a photoinduced long life-time THz transparency in Dirac semimetal PdTe2 films is attributed to the formation of small polarons due to the strong electron-phonon coupling. A 15 nm-thick PdTe2 film exhibits a photoinduced change of EMI SE of 1.1 dB, a value exceeding three times that measured on MXene film with a similar pump fluence. This work provides insights into the fundamental photocarrier properties in type-II Dirac semimetals that are essential for designing advanced THz optoelectronic devices.

Plasma is the optimal choice for acquiring and modulating the extremely high visible and near-infrared light. However, few attempts have been made to apply this strategy for the terahertz (THz) wave modulation in an ex situ manner. Here, we show a laser-driven plasma-based THz modulator (PTM) to ex situ control the incident THz waves. The presented PTM allows for the amplification or extinction of the incident THz waves covering 0.1-2.0 THz within a few picoseconds, simply by adjusting its dipole phase. This modulation is a result of the interaction between the PTM's dipole and THz wave, which can be accurately reproduced by the spectral analysis method. Our technique offers promising opportunities to explore the plasma-based THz optics and potential applications across different disciplines, such as THz-sensing and near-field THz technology. (c) 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

We have studied the processes of electron acceleration and THz emission, undergoing simultaneously during laser excitation of the gas-cluster jet. The properties of the electron beam and THz radiation power were measured under various conditions of the cluster excitation and parameters of the laser radiation.