This article focuses on the top-seeded solution growth (TSSG) of single crystals of the low-temperature non-centrosymmetric modification of barium borate, β-BaB2O4 (β-BBO), using a mixed solvent comprising NaBaBO3 and V2O5. We examine the linear absorption properties across a wide spectral range and nonlinear second harmonic generation (SHG) from 532 nm to 266 nm in β-BBO samples under Nd:YAG laser radiation pumping. The mean linear absorption for ordinary waves in vanadium-based crystals at wavelengths from 300 to 600 nm is found to be an order of magnitude higher than that in standard samples, measuring 0.0059 and 0.0007 cm-1, respectively. However, crystals grown from the proposed vanadium-based solution still exhibit similar SHG efficiency to standard samples. Deviations from the quadratic dependence are observed at pump power densities exceeding 20 MW/cm2, possibly attributed to temperature effects resulting from insufficient temperature stabilization.
The study of short-lived induced (singlet-singlet) and long-lived induced (in particular, triplet-triplet) absorption capacity of crystal violet (CV) in various solvents was carried out by the pump-probe method. Water, dimethyl sulfoxide, isopropyl alcohol, and ethyl alcohol were selected as solvents. The formation of triplet states in various CV solvents was revealed upon excitation by nanosecond radiation of the 4th harmonic of a Nd: YAG laser (wavelength 266 nm, average power – 25.5 mV, repetition frequency – 3 Hz, pulse duration – 10 ns, peak power – 10-12 MW/cm2). It is shown that the spectrum of the induced CV absorption in ethanol contains two closely spaced bands at 400 and 485 nm. Short-lived and long-lived induced CV absorption in isopropanol at room temperature (λmax=400 nm) was recorded. Keywords: crystal violet (CV), induced absorption spectra, spectroscopy, pump-probe method.
Optical properties of different commercial plastics for fused deposition modeling 3D printing are defined at room temperature in the spectral range 0.2˗1.2 THz. We compare absorption coefficients and refractive index of ABS, PETG, and SBS printed 1-4 mm plates. Different types of optical elements for controlling high-power THz radiation are studied. A comparison is made of the efficiency of attenuation of linearly polarized THz radiation with homemade band-pass polarizers obtained by etching copper from a flexible polyimide substrate. Filters and polarizers created using 3D printing or by deposition of polymer matrix with magnetic particles under external field are cost-effective and can be easily changed or replaced. Comparison between plastic insets, filters based on magnetic particles, and polyimide film filters are made.
We present the thorough studies of dielectric properties of BiB3O6 (BIBO) crystal in the subterahertz range. We observe a large birefringence Δn = nZ −nX = 1.5 and the values of absorption coefficients of all three axes to be less than 0.5 cm−1 at the frequency of 0.3 THz. The difference from visible range in angle Φ between the dielectric axis z and crystallophysical axis χ is found to be more than 6°. The simulated phase-matching curves in the xz plane of the crystal show the optimal value of the angle θ to be around 25.5°±1° for an efficient millimeter-wave generation under the pump of 1064 nm laser radiation.
Optical properties of single axes Li2B4O7 (LB4) crystal are defined at room temperature in the spectral range 0.03˗0.5 THz. Dispersion of the refractive index components are approximated in the form of Sellmeier equations. Dispersion properties were used to determine phase-matching conditions for THz wave generation by collinear difference frequency generation processes. To the damage threshold under the pump by train of hundreds of 60 fs pulses of Ti:sapphire laser operating at 950 nm is found to be 250 TW/cm2, as well as the coherence length and efficiency of all possible types of three wave interactions are defined.
Potential efficiency of the THz wave generation by down-conversion of the visible emission in nonlinear β-BBO crystal is estimated. Eight types of interactions are found possible. Four interaction types were realized in the carried-out experiments. Optical damage threshold under fs pumping was estimated.
We report on the fabrication of liquid automated filter capable of controlling the intensity of terahertz radiation in the range of 0.4-1.4 THz. The filter is a cell with a magnetic fluid (5% dispersion of 5BDSR particles in 80W-90 synthetic oil) placed on the path of THz radiation between a pair of crossed Helmholtz coils. The prototype created allows achieving an extremely high attenuation coefficient, up to 35 dB. Automatic start-up and stirring system make filter operation repeatable, even with multiple on/off filter state changes. All this allows us to speak about the operability of the presented technology and the possibility of further development of the prototype, the use of similar filters in any THz photonics systems, in which it is required to control the intensity of polarized radiation.
Optical properties of a Li2B4O7 (LB4) crystal are determined in the spectral range 0.2-1.6 THz. Dispersion of the refractive index components for o- and e-wave are approximated in the form of Sellmeier equations. They are subsequently used to determine the possible interaction types and to calculate the phase-matching angles to get THz waves by difference frequency generation. The damage threshold is determined as well as the coherence length for all possible types of three wave interactions under the pump by fs Ti: Sapphire laser pulses at 950 nm. The efficiency of the processes is estimated. Using trains of hundreds of pulses at 950 nm it was found to be 1.32 times of that for β-BBO crystal laser pump.
We report on the fabrication of free-standing fluoropolymer composite films anisotropic in terms of THz photonic transmission. We used metallic Fe nanoparticles (80-110 nm diameter) obtained by the electric explosion as well as submicron (5 μm average) 5BDSR magnetic alloy particles two-step ground by a ball mill. These particles were dispersed in an acetone solution of the F42-l fluoropolymer. Upon casting, they could be oriented and assembled in alignment with an external magnetic field during solvent evaporation. Oriented composite films of such kind are considered as flexible polarizers with an irregular linear structure. Optical properties in the THz range of spectra were examined using a commercial THz-TDS T-Spec 1000, as well as a custom-made THz-TDS setup with a precise linear polarization of THz emission using a bunch of polarizer and analyzer. In addition, the optical properties of fabricated fluoropolymer composites are compared with copper foiled photoetched polyimide gratings with regular spacing. Irregular structure of magnetophoretic orientated 5BDSR fluoropolymer composites allows using them as ultra-wideband THz polarizers. Single-layer 5BDSR polarizer possesses an extinction ratio within 3:1 to 16:1 in the range of 0.2–2.4 THz.
We report results on comparative study of SHG in powder of promising nonlinear γ-Ga2S3 crystal. Digallium trisulfide powders with particle size from 20 μm to 500 μm were tested in comparison with powders of well-known LBO, BBO, KABO, KDP, and LN crystals under the pumping by 7 ns 1064 Nd:YAG laser. Laser-induced damage threshold of different powder fractions were determined. The γ-Ga2S3 shown high damage threshold and large SHG intensity: 56 times to that in LBO powder, 15 in BBO, 50 in KABO, 67 in KDP, and 3 in LN (for particle size: 20–50 μm), that renders it amongst the most promising crystal for frequency conversion of high-intense nanosecond radiation of near-IR lasers by optical rectification technique.
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