In this work, we present the first experimental observation of the enhanced THz radiation in plasmonic photoconductive antenna (PCA) based on $\mathrm{Bi}_{2-\mathrm{x}} \mathrm{Sb}_{\mathbf{x}} \mathrm{Se}_{\mathbf{y}} \mathrm{Te}_{3 \text {-y }}$ topological insulator (TI). By using numerical simulations, we optimized the plasmonic grating geometry to maximize optical light transmission, stimulating efficient surface plasmon-polariton (SPP) excitations along the TI/grating interface. We fabricated two equal PCAs, the one of which contains high-aspect ratio plasmonic grating. The THz spectral measurements showed a $\sim 9$-fold enhancement of the emitted THz power in the plasmonic TI-based PCA. The results of our study can open a pathway toward time-domain plasmonics-enhanced THz setups utilizing innovative photoconductive materials.

A number of methods have been demonstrated to increase the sensitivity of fiber refractometers to changes in the surrounding media refractive index (SMRI). The sensor system is based on the effect of lossy mode resonance (LMR) in a thin multimode fiber with a thin film coating. A chemically resistant thin film of aluminum oxide (Al2O3) was applied to the silica surface of an optical fiber by metalorganic chemical vapor deposition (MOCVD). For the first time, the possibility of obtaining aluminum oxide from a vapor-gas mixture of trimethylaluminium - isopropyl alcohol in a hydrogen reducing medium to reduce the carbon content in the sediment has been demonstrated. Using in situ control during the deposition of the coating, the movement of resonances in the transmission spectrum of the optical path was observed. The optical parameters of the deposited coating under various synthesis conditions in the temperature range from 470 to 550 degrees C were estimated by the movement, shape and depth of several orders of resonances. When creating test sensors, deposition was stopped when the resonance was in the spectral range of 400-1700 nm and their sensitivity in aqueous salt solutions (NaCl), as well as chemical resistance in acids (HCl, HNO3, H2SO4, H3PO4) were evaluated. The maximum sensitivity of the implemented sensors for media with refractive indices of 1.34-1.37 was 2950 nm/RIU, which is one of the highest values found in the literature for sensors with Al(2)O(3 )coating. The dependence between the temperature of the synthesis of coatings and the rate of their dissolution in orthophosphoric acid is demonstrated. It is shown that an increase in sensitivity of up to 30 % can be realized by changing the roughness of the surface of the coating or silica glass.

A method for controlled annealing of thin-film vanadium oxide coatings on the surface of quartz optical fibers is demonstrated. Thin-film coatings were obtained on a chemically thinned optical fiber surface using the method of chemical vapor deposition from organometallic compounds using vanadyl isopropoxide. It is shown that in an inert atmosphere, thin-film amorphous coatings consist mainly of a mixture of V2O5 and V6O13, which is confirmed by Raman spectra. Coatings annealed in a reducing environment demonstrate a second-order phase transition near room temperature (~65 ℃). Annealing was carried out at temperatures from 225 to 420 ℃ with process control using the transmission spectrum of the optical path.

The presented work shows an influence of geometry passive Q-switch based on a nanopowder-polymer on the characteristics of an erbium ring fiber laser. The saturable absorber Bi2Te3 was applied to two types of specially prepared samples of thinned sections of optical fiber, and the resulting structures were built into the ring circuit of an erbium fiber laser. In the first case, the surface of the light guide segment was thinned by chemical etching, and in the second, by mechanical grinding. For the first time, the time-domain optical reflectometry technique was used to control the quality of the resulting sample surface and the resulting losses. For a more complete comparison, a set of different optical splitters was used, providing a change in the feedback coefficient in the resonator from 5 to 95 %. A study in the mode of passive Q-switching using polarized light showed that for both types of modulators there are optimal ratios of the amount of energy output from the resonator to obtain the highest values of the average generation power. The d-shape fiber modulator allows mode-locking to be observed without any additional changes to the laser circuit. The pulse duration in this case was 5 ns with a repetition rate of 4 MHz. A Q-switch based on a chemically thinned fiber segment made it possible to obtain narrower pulses in Q-switching mode (up to 700 ns at 38 kHz), a stable peak position in the transmission spectrum, and an average power of 34 mW.

A technique for obtaining vanadium oxides of various stoichiometries from vanadyl isopropoxide by chemical vapor deposition is demonstrated. Various vanadium oxides from the Magnelli series have been obtained. Thin-film coatings are applied to the surface of silicon and sapphire. Coatings deposited in an argon flow at temperatures from 220 ˚C to 290 ˚C were evaluated by transmission spectra, Raman scattering and X-ray diffraction. Four oxides (V3O5, VO2, V6O13, V2O5) with different ratios were found in the synthesized amorphous structures. In a number of coatings on silicon substrates, a phase transition of the second kind with a jump in electrical resistance up to 10 times at a temperature of 69 ˚C, was recorded.

The possibility of assessing the influence of external conditions, such as glucose content in solution and external lighting, on life activity has been shownblue-green algae with control mode real time. Using fabricated fiber sensors, the time dependence, lasting more than 40 hours, and changes in the refractive index of a suspension containing microalgae Chlorella Vulgaris during their life activity were studied.

Thin-film coatings of tin oxide on the surface of a chemically thinned section of a single-mode quartz fiber have been obtained and experimentally characterized. The materials were synthesized on the fiber surface by metal-organic chemical vapor deposition (MOCVD). To change the surface morphology, we used different amounts of tetramethyltin (SnMe 4 ) supplied by a carrier gas (dried air) to the deposition zone by varying the temperature of the bubbler with the reagent. During deposition, the transmission spectrum of the optical path was recorded in real time, and the temperature of the bubbler in the experiments varied from –20 to +20°С. After studying the surface on a scanning electron microscope, the deposited films were tested for chemical resistance to an aqueous solution of sulfuric acid and the sensitivity of the lossy mode resonance (LMR) to changes in the refractive index of the environment in the range from 1.35 to 1.41 was evaluated. Samples produced at higher reagent flow rates exhibited a greater resonance sensitivity of 3800 nm/refractive index unit (RIU) for the first-order TM component of the resonance, but such coatings dissolve noticeably in concentrated sulfuric acid solutions, in contrast to coatings obtained with low reagent consumption.

Экспериментально исследована генерация третьей гармоники терагерцового излучения в топологических изоляторах на основе халькогенидов висмута и сурьмы. Обнаружено, что эффективность преобразования в третью гармонику обратно пропорциональна энергии Ферми электронов, при этом лучшим из исследованных материалов по эффективности преобразования является теллурид сурьмы.

Terahertz third-harmonic generation in topological insulators based on bismuth and antimony chalcogenides has been experimentally investigated. It has been found that the third-harmonic conversion efficiency is inversely proportional to the electron Fermi energy; among the materials under study, antimony telluride has the highest conversion efficiency.

Using the technology of metalorganic chemical vapor deposition (MOCVD), fully fiber refractometers based on the lossy mode resonance (LMR) were obtained and investigated. The sensors are made on the basis of a section of optical fiber etched to the core with films of titanium dioxide (TiO2) and tin (TiO2/SnO2) deposited on the side surface. The sensitivity of the obtained sensors to the refractive index of the surrounding liquid medium is compared depending on the composition and thickness of the film coating. The sensitivity of the fiber sensor with a two-layer coating in the wavelength range of 1.33–1.35 microns was 4850 nm/RIU. Before measurements, cells suspended in liquid were concentrated on the surface of the sensor due to the effect of optical tweezer. Particles were attracted from the suspension when launching into the fiber radiation from a diode laser with a power of 10 mW. With the help of fabricated fiber sensors, the change in the refractive index of a suspension containing living cells of the microalgae Chlorella Vulgaris was monitored. The possibility of assessing the influence of external conditions on the life process of algae with real-time control using fabricated fiber sensors is shown.

A comparative study of figure-of-merit fiber sensors of the mass concentration of NaCl solutions based on single-mode and multi-mode fibers was carried out. Lossy mode resonance is realized on chemically thinned sections of optical fibers to various diameters (from 26 to 100 μm) coated with ZnTe. Thin-film coatings were applied using the method of metalorganic chemical vapor deposition (MOCVD). Samples of single-mode and multi-mode fiber sensors were created in such a way that the depth and spectral position of resonances in aqueous NaCl solutions coincided. Sensors implemented on a single-mode fiber have a higher sensitivity (5930 nm/refractive index unit (RIU)) compared to those on a multi-mode fiber (4860 nm/RIU) and a smaller half-width of the resonance in the transmission spectrum. According to the results of experiments, figure-of-merit sensors are in the range of refractive indices of 1.33–1.35 for: multi-mode fiber—25 RIU−1, single-mode fiber—75 RIU−1. The sensitivity of the resulting sensors depends on the surface roughness of the ZnTe coating. The roughness of films synthesized on a single-mode fiber is four times higher than this parameter for a coating on a multi-mode fiber. For the first time, in the transmission spectrum during the synthesis of a thin-film coating on a multi-mode fiber, the possibility of separating the first nine orders of resonances into electric and magnetic transverse components has been demonstrated. The characteristics of sensors with the operating wavelength range in the visible (500–750 nm) and infrared (1350–1550 nm) regions of the spectrum are compared. The characteristics of multi-mode lossy mode resonance sensors are demonstrated, which make them more promising for use in applied devices than for laboratory research.

Topological insulators made of bismuth chalcogenides are promising materials for sources and converters of terahertz radiation. The operation of a topological insulator nanofilm photoconductive antenna is demonstrated. The generation of the third and fifth harmonics of the terahertz frequency radiation in topological insulators of various chemical compositions is observed.

Films of zinc telluride (ZnTe) were deposited on the surface of a chemically thinned section of an optical fiber by metalorganic chemical vapor deposition. The boundary values of temperatures and the concentration ratios of the initial tellurium and zinc precursors at which the synthesis of ZnTe coatings is possible are determined. The influence of the position of the thinned part of the optical fiber in the reactor on the growth rate of films on the side surface of the fiber was studied, on the basis of which, the parameters of the deposition zone were determined. By placing a section of an optical fiber with an etched cladding in the center of this zone, sensitive elements for refractometers were created. The principle of their operation is based on the dependence of the spectral position of the lossy mode resonance (LMR) maximum on the refractive index (RI) of the external medium. It has been found that even thin films deposited on a light guide in a continuous process have cracks. It is shown that the interruption of the deposition process makes it possible to avoid the appearance of defects in the zinc telluride layers even with the repeated deposition of the sensor. The sensitivity of the spectral position of the LMR to changes in the RI of aqueous sodium chloride solutions in the range from 1.33 to 1.35 for the first transverse electric and transverse magnetic LMRs was 6656 and 6240 nm per refractive index unit, respectively.

We study photoconductive antennas based on thin film of topological insulator and semiconductor heterostructure. A shift of the spectrum of a dipole antenna on a topological insulator to the long-wavelength region was found in comparison with a similar semiconductor antenna. Ways to increase the efficiency due to the deposition of plasmon gratings, as well as controlling the frequency response by changing the antenna impedance will be discussed.

We discuss experimental and theoretical studies of the generation of the third terahertz (THz) frequency harmonic in thin films of Bi2Se3 and Bi2-xSbxTe3-ySey (BSTS) topological insulators (TIs) and the generation of THz radiation in photoconductive antennas based on the TI films. The experimental results, supported by the developed kinetic theory of third harmonic generation, show that the frequency conversion in TIs is highly efficient because of the linear energy spectrum of the surface carriers and fast energy dissipation. In particular, the dependence of the third harmonic field on the pump field remains cubic up to the pump fields of 100 kV/cm. The generation of THz radiation in TI-based antennas is obtained and described for the pump, with the energy of photons corresponding to the electron transitions to higher conduction bands. Our findings open up possibilities for advancing TI-based films into THz photonics as efficient THz wave generators and frequency converters.

In this study, the efficient generation of terahertz radiation by a dipole photoconductive antenna, based on a thin island film of a topological insulator, was experimentally demonstrated. The performance of the Bi1.9Sb0.1Te2Se antenna was shown to be no worse than those of a semiconductor photoconductive antenna, which is an order of magnitude thicker. The current–voltage characteristics were studied for the photo and dark currents in Bi1.9Sb0.1Te2Se. The possible mechanisms for generating terahertz waves were analyzed by comparing the characteristics of terahertz radiation of an electrically biased and unbiased topological insulator.

In this work, we fabricated and investigated lossy mode resonance (LMR) based fiber-optic refractometers, using a niobium pentoxide coated optical fiber as a sensitive element. In order to do that, thin Nb2O5 films were deposited on the surface of chemically thinned optical fibers by metalorganic chemical vapor deposition (MOCVD). The sensitivities of the first transverse electric (TE) and transverse magnetic (TM) LMRs to the surrounding medium refractive index (SMRI) were measured and compared. Aqueous solutions of glucose and sodium chloride were used as test liquids. The sensor sensitivity to a change in the SMRI enhanced with an increase in the dissolved substance concentration and was greater for glucose solution. The maximum response of the 1-st TE and TM LMRs was 6580 and 6120 nm per refractive index unity (RIU), respectively.

The oscillation of erbium doped fiber lasers (EDFL) in the Q-switch mode is investigated. Etched out fiber tapers coated with two types of saturable absorbers (SA) are tested as passive Q-switches. SA of the first type comprises two-component silicone with nano-flakes of a Bi2Te3 single crystal dissolved in it. A thin film of Bi2Te3 synthesized by the Metalorganic Chemical Vapor Deposition (MOCVD) method on the surface of the fiber taper and coated with the same silicone served as SA of the second type. Housed within a refrigerator the fabricated sample of fiber taper with SA was in series plug into the fiber ring of the laser. A maximum is observed in the temperature dependence of the EDFL pulse repetition rate with both types of Q-switches. A decrease in temperature to -4 degrees C (for the first type) and to 9.5 degrees C (for the second one) led to the oscillation wavelength change from 1560 to 1530 nm. Cooling of Q-switches of the first and second types was accompanied by a decrease in the duration of laser pulses by a factor of 2.5 and 3.5, respectively, at a pump power corresponded to the conversion of laser oscillation from CW to repetitively-pulsed mode. At a given laser pump power, a 20% and 40% increment in the pulse repetition rate is observed upon cooling of passive Q-switches based on Bi2Te3 nanopowder-in-polymer and Bi2Te3 thin-film, up to temperatures of -14.8 degrees C and 4.4 degrees C, respectively, as compared to that one at room temperature. A diminution in the temperature of a passive Q-switch made it possible to obtain pulsed laser oscillation in a circuit that did not support the pulsed mode at room temperature.