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.
The results of experimental studies of conditions for THz emission generation in laser filament created by focused ultrashort laser pulse of Ti: Sapphire complex are presented. The influence of magnitude and direction of applied external electric field, energy and polarization of pump radiation in one- and two-color optical schemes on the intensity, divergence, and polarization of THz radiation are investigated. THz spectra is estimated and optimal generation conditions are discussed.
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.
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.
THz generation experimental results in a filament plasma produced by focusing ultrashort Ti:Sapphire laser pulses at a wavelength of 950 nm are reported. A study of structure and time-resolved dynamics of forming the laser filament is carried out. THz radiation dependence from filament length and pump energy is also given. Optimal conditions for efficient THz generating were found.
This paper presents model study and experimental results on frequency conversion of Ti:Sapphire laser operating at 950 nm into the long-wavelength THz range in a β-BBO crystal. The generating spectrum in the range of 0.2 - 0.8 THz is studied in detail.
The effect of astigmatism on laser air filamentation was studied numerically and experimentally. Limited supply of energy to the filament is shown to be the main feature of aberrational focusing, which facilitated the organization of quasi-solitons in the post-filament region. Quasi-solitons are realized due to the balance between linear diffraction and Kerr nonlinearity in the presence of background environment around zones with increased intensity. A highly directional SC of visible light is formed outside the visible filament zone in the resulting spatial quasi-solitons.
The results of formation conditions studies of a highly directional supercontinuum (SC) in a visible spectrum range obtained upon aberration spherical-mirror focusing of a radiation pulse with a wavelength of 940 nm, duration of 70 fs, and energy of 8–15 mJ are presented. It is shown that after visible filament there are two directed white light beams diverging relative to each other at an angle of 1.40 . Formation every light beam occurs through a gradual conversion of the spectral composition from long wavelength to short wavelength (to 350 nm) in a spatially stable structure similar to a soliton with a transverse dimension ≤ 300 μm. The nature of the appearance these beams is due to formation of two zones with higher intensity before meridional plate owing to the distortion of the wavefront of the laser beam in conditions of the astigmatism and the Kerr effect. In result two minima in the phase distribution located outside the beam axis are realized, which lead to the appearance of two off-axis areas with higher radiation intensity and as a consequence of this the formation of two highly directional laser beams.
Ab-initio study on modification of commerce terahertz spectrometer with time resolution Z-3 (Zomega, USA) by substitution of ZnTe and GaP detectors and LT-GaAs generator for homemade of pure and S-doped GaSe is carried out. It was established that in spite of not optimized parameters pure and doped GaSe:S(0.3 mass%) crystal are comparable, relatively, in generation efficiency and detection sensitivity to commerce units due to lower nonlinear optical loss and much higher damage threshold. The advantages are in force from pump fluences of below 5 mJ/cm2 for pure GaSe. The closer S-doping to optimal concentration, the lover fluences resulting in the advantages. Pure and S-doped GaSe demonstrate higher reliability and larger dynamic range of operation. Recorded absorption spectra well match known spectra.
Model study of not phase matched and phase matched optical rectification or down-conversion of Ti:Sapphire laser pulses at 950 nm into THz and far-IRrange in pure and S-doped GaSe single crystals is carried out. First, the ordinary and extraordinary wave dispersions of the GaSe refractive indices were measured by terahertz time-domain spectroscopy (THz-TDS). Measured data were approximated in the form of Sellmeier dispersion equations for 0.62 – 2000 μm range with using available shorter wave data.
Detailed model study of THz generation by CO2 laser down-conversion in pure and solid solution crystals GaSe1-xSx is carried out for the first time. Both forward and backward collinear interactions of common (eo-e, oe-e, oe-o, oo-e, ee-o) and original (ee-e, oo-o) types are considered. Possibility of realization, phase matching angles and figure of merits are estimated for line mixing within 9 μm and 10 μm emission bands, as well between them. Dispersion properties of o- and e-wave refractive indices and absorption coefficients for GaSe, GaS and GaSe1-xSx crystals were preliminary measured by THz-TDS, approximated in the equation form and then used in the study. Estimated results are presented in the form of 3-D figures that are suitable for rapid analyses of DFG parameters. The most efficient type of interaction is eo-o type. Optimally doped (x = 0.09-0.13) GaSe1-xSx crystals are from 4 to 5 times more efficient at limit pump intensity than not doped GaSe crystals.
The spectral transparency method has been tested experimentally in the terahertz spectral region. The aerosol particle size distribution spectrum has been reconstructed from measurements of the spectral transparency coefficient by the method of Time-Domain THz spectroscopy. The particle size distribution spectra obtained by the spectral transparency method in the terahertz region and by the method of optical microscopy have been compared.
Damage threshold of non-linear GaSe crystals under IR fs (Ti:Sapphiere 800 nm laser and 1.1-2.9μm OPG) and ns (2. 79
Er3+:YSGG and 10.6μm CO2 laser) pulse pumping is studded in details. Local micro defects and field induced effects (GaSe dissociation, multiphoton absorptions and transient transparency origin effects) are identified as responsible for damage threshold in this case. Local (including nano scaled) defects and thermal effects are identified as reason of damage threshold under ns pulse pumping.
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