In this work, a laser module (quantron) with transverse pulse diode pumping of a cylindrical Nd3+:YAG active element by the method of non-sequential ray tracing in the Zemax software environment is modeled. Numerically obtained distributions of the absorbed pump radiation power over the cross section of the active element and calculated the pumping efficiency of the quantron. A methodology for optimizing the quantron design is proposed, which results in an increase in the pumping efficiency of the active element.

In this paper, we first present an experimental demonstration of terahertz radiation pulse generation with energy up to 5 pJ under the electron emission during ultrafast optical discharge of a vacuum photodiode. We use a femtosecond optical excitation of metallic copper photocathode for the generation of ultrashort electron bunch and up to 45 kV/cm external electric field for the photo-emitted electron acceleration. Measurements of terahertz pulses energy as a function of emitted charge density, incidence angle of optical radiation and applied electric field have been provided. Spectral and polarization characteristics of generated terahertz pulses have also been studied. The proposed semi-analytical model and simulations in COMSOL Multiphysics prove the experimental data and allow for the optimization of experimental conditions aimed at flexible control of radiation parameters.

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.

The paper presents the current state of research on the generation of terahertz radiation in the optical breakdown of gases by laser radiation, consisting of the fundamental and second harmonics of femtosecond laser radiation. We consider the main approaches to describing the generation of terahertz radiation. The influence of the focusing mode, spatial and temporal modulation, changes in the wavelength, and polarization state of the pump radiation is shown. Also considered is the influence of the properties of the medium into which laser radiation is focused.

A method for generation of ultra-wideband electromagnetic pulses with a nanosecond length and a picosecond rise time has been proposed and studied. A horn antenna with a photoconductive switch irradiated by laser pulses has been used as an emitter. It is shown that the length of ultra-wideband electromagnetic pulses is determined by the antenna length and the semiconductor material and the rise front is determined by the front of laser pulses used to initiate a photoconductive switch. Typical pulse lengths of 1 ns with a rise front of up to 34 ps are reported.

The propagation of graphitization wave through the diamond bulk under multipulse laser irradiation is a largely self-guided process. This fact assists the production of graphitized wires oriented along a laser beam and greatly complicates formation of the structures oriented differently. Here, we develop new approaches to control laser graphitization that should empower the potential of 3D laser microstructuring inside a diamond crystal. Two techniques are investigated: (i) a laser seed damage of crystal with subsequent exposure at a lower laser fluence, thus restricting the propagation of the graphitization wave toward the beam and (ii) formation of a dominant microfracture perpendicular to the laser beam, thus guiding growth of the graphitized thread.

A relatively simple fiber sensor system for high-power electromagnetic field measurements is presented. The system utilizes an all-dielectric fiber tip sensor head with CdTe electro optic (EO) crystal as an electric field transducer. The fiber sensor of small size converts electric field strength into modulation of light power. The rise time of the EO sensor is measured and evaluated theoretically. The system can be used for ultra-wideband high-power electric field measurements.

It has been shown recently that a photoconductive antenna (PCA) based on a nitrogen-doped diamond can be effectively excited by the second harmonic of a Ti:sapphire laser (λ = 400 nm). The THz emission performance of the PCA can be significantly increased if a much stronger electric field is created between the close-located electrodes. To produce a homogeneous electric field over the entire excited diamond volume, the laser fabrication of deep-buried graphite electrodes inside the diamond crystal was proposed. Several electrodes consisting of the arrays of buried pillars connected by the surface graphite stripes were produced inside an HPHT diamond crystal using femtosecond and nanosecond laser pulses. Combining different pairs of the electrodes, a series of PCAs with various electrode interspaces was formed. The THz emission of the PCAs equipped with the buried electrodes was measured at different values of excitation fluence and bias voltage (DC and pulsed) and compared with the emission of the same diamond crystal when the bias voltage was applied to the surface electrodes on the opposite faces. All examined PCAs have demonstrated the square-law dependencies of the THz fluence on the field strength, while the saturation fluence fluctuated in the range of 1200–1600 µJ/cm2. The THz emission performance was found to be approximately the same for the PCAs with the surface electrodes and with the buried electrodes spaced at a distance of 1.4–3.5 mm. However, it noticeably decreased when the distance between the buried electrodes was reduced to 0.5 mm.

Field experiments were carried out to measure ultrawideband subnanosecond radiation pulses in the time domain, taking into account the influence of reflection from the Earth’s surface. The results of these experiments make it possible to prepare experiments in the free atmosphere on real paths of 10 km or more in length. A technical solution for the use of a single-channel ultrawideband emitter with a pulse duration of about 50 ps, which is optimal in terms of weight and size characteristics and lifting to heights of up to 1000 m, is substantiated. A specially designed measuring antenna in the form of a passive antenna array with high sensitivity is used as a receiving measuring channel.

For the first time, the ability of semiconducting diamond to convert near-IR laser radiation into terahertz radiation has been demonstrated. A set of photoconductive antennas based on single-crystal diamonds doped with boron (~1 ppm) was assembled and tested under conditions of pumping with ultrashort (τ opt ≈ 150 fs) radiation pulses with a wavelength of 800 nm and a pulsed voltage (τ E ≈ 10 ns, E bias ≈ 10 kV/cm). The characteristics of the boron-doped emitters were compared with recently implemented nitrogen-doped diamond antennas pumped by 400-nm-wavelength radiation pulses, since substituting nitrogen requires a much higher quantum energy for single-photon excitation of carriers. The results obtained are another step towards the use of diamond as a material for high-performance photoconductive antennas.

The novel design of a terahertz large aperture photoconductive antenna (LAPCA) is reported. It features a longitudinal orientation of the bias electric field within the photoconductive substrate, and has the advantage of a small interelectrode gap, resulting in a higher field for the same applied voltage. The proposed LAPCA configuration has been tested with a nitrogen-doped (∼10 ppm) synthetic monocrystalline diamond, which is a promising material for high-intensity and high-power terahertz sources. Two antennas with different high-voltage electrode realizations were assembled, pumped by a 400 nm femtosecond laser, and tested for THz emitter function. The experimental data are found to be in good correlation with the numerical simulation results. The performance of antennas with the conventional transverse E-field configuration and the novel longitudinal configuration is compared and discussed.

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.

The results of the first direct experiments on the passage of pulses of ultra-wideband radiation of subnanosecond duration in the Earth’s atmosphere at a distance of more than 10 km are presented. In contrast to the work calculated, the preservation of the amplitude–time shape of the pulses in the process of increasing the distance is shown. The establishment of this fact is of decisive importance in the practical application of ultra-wideband pulses in new technological developments.

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.

In this paper, we study the optical anisotropy induced by femtosecond laser radiation in air during an optical breakdown. Using a transverse pump-probe technique, we demonstrate that this anisotropy appears in a narrow range of pump intensities, which are close to the optical breakdown threshold in air and lead to a phase shift of probe radiation, polarized collinear to the pump. The intensity range where an induced intense anisotropy occurs makes it possible to estimate the magnitude of the 5th-order Kerr nonlinear refractive index component in air.

To address a challenging problem of super-resolution terahertz (THz) endoscopy, in this paper, an antiresonant hollow-core waveguide was coupled with a sapphire solid immersion lens (SIL), aimed at subwavelength confinement of guided mode. The waveguide is formed by a polytetrafluoroethylene (PTFE)-coated sapphire tube, the geometry of which was optimized to ensure high optical performance. SIL was judiciously designed, fabricated of bulk sapphire crystal, and then mounted at the output waveguide end. Study of the field intensity distributions at the shadow side of the waveguide-SIL system revealed the focal spot diameter of ≃0.2λ at the wavelength of λ = 500 μm. It agrees with numerical predictions, overcomes the Abbe diffraction limit, and justifies super-resolution capabilities of our endoscope.

Artificial opals fabricated by sedimentation and self-assembly of colloidal SiO2 nanoparticles and annealed at different temperatures were recently considered favorable terahertz (THz) optical materials with manageable optical properties. However, interactions between such a porous material and water vapour in a humid atmosphere can hamper their THz applications due to the related changes in the material parameters and additional power loss. To quantify such an effect, in this paper, moisture adsorption by artificial SiO2 opals is studied using THz pulsed spectroscopy. Particularly, opals of two kinds were sedimented from the colloidal suspension of 300-nm-diameter SiO2 nanoparticles with different intraglobular structures and porosity. They were annealed at temperatures of 200–800°C aimed at changing their internal structure, porosity, and THz optical properties. Opals were dehydrated in a vacuum and then exposed to a humid atmosphere with 82.0 ± 2.0% relative humidity, while their THz complex dielectric permittivity was evaluated in situ in the 0.5–2.5 THz range. The observed changes in the THz dielectric curves were analyzed using the sum rule and the adsorption kinetics models. Our findings reveal a strong dependence of the THz dielectric response, amount of adsorbed water, and adsorption time constant on the opal type and annealing conditions. This effect has a general character: it can hamper real-live applications of a variety of porous THz optical materials and, thus, should be taken into account during their synthesis.

The generation of terahertz radiation in a photoconductive emitter based on nitrogen-doped single-crystal diamond was realized for the first time. Under 400 nm femtosecond laser pumping, the performance of diamond antennas with different dopant levels was investigated and compared with a reference ZnSe antenna. Terahertz waveforms and corresponding spectra were measured. A low saturation level for high-nitrogen-containing diamond substrate was revealed. The results indicate the prospects of doped diamond as a material for high-efficiency large-aperture photoconductive antennas.

The main properties of halide perovskites useful for solar cells make them also attractive for terahertz (THz)applications. This class of materials, well studied in the optical range, remains much less studied in the THz range. Meanwhile, increasing the efficiency of pulsed terahertz sources and detectors due to more advanced designs or new materials is one of the main directions in the field of terahertz technologies. Here, room-temperature detection of THz pulses propagating in free space with unbiased halide perovskites is demonstrated. The ultrafast change of conductivity that occurs in single crystals and polycrystalline films of lead methylammonium halides excited by a femtosecond laser enables efficient coherent detection of THz radiation. The results demonstrate the viability of solution-processable halide perovskite for the fabrication of photoconductive THz detectors and the further development of scalable and cost-effective sensor manufacturing for THz time-domain spectroscopy, imaging, and other photonic THz devices.

A compact waveguide Tm:YAP laser with a pulse repetition rate of 8 GHz is developed. A controllable change in the intracavity loss provides continuous tuning of the central emission wavelength of the laser operating in the Q-switched mode-locking in the range from 1925 to 1950 nm, as well as makes it possible dual-wavelength lasing. The main approach of this study is the use of waveguide structures inside the Tm:YAP crystal and the saturable absorber based on graphene. This approach is universal for producing compact lasers with the gigahertz pulse repetition rate, operating in a wide spectral range.