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.
In recent years, the terahertz sources have attracted much attention for its special use compared with so for UV-visible range. Crystal with high quality and special properties is their indispensable part. Optical properties such as the absorption coefficient and refractive index at three basic crystal orientations (x, y, and z-direction) of the BIBO crystal have been studied by the Terahertz Time-Domain Spectroscopy at room temperature. A large birefringence was observed. The measured refractive index components were approximated in the form of Sellmeier equations. Phase-matching curves for collinear down-conversion of IR laser frequencies into the THz domain were preliminarily estimated. A prospect of BIBO crystals as THz generators are discussed, and comparison with popular crystals is given.
In this paper, a high efficient terahertz source based on n-type gallium phosphide crystal via cryogenic process is investigated through collinear difference frequency generation pumper by 1064 nm Nd:YAG laser and its OPO system. Absorption coefficient of this crystal at THz range shows a dramatic decrease from ~ 50 cm-1 to 0.5 cm--1 as the temperature decreases from 300 k to 80 k. Four times enhancement of the terahertz emission power and much more broad spectra range (~ 0.2- 3.8 THz) has been achieved in this kind of 0.5 mm length gallium phosphide crystal during the whole varied temperature difference frequency generation from 300 k to 80 k. These results indicate that cooling down the crystal temperature is an effective way to improve the terahertz source property, such as terahertz output power and frequency range.
We launched into a development of a new stand-off sensing system that can detect atmospheric and hazardous gases in real atmosphere utilizing THz technology. Narrow line width, <0.1 cm-1, long-wave (mid-IR and THz) source based on difference frequency generation (DFG) in collinear configuration in GaSe0.91S0.09:Al(0.03 at. %) using seeded YAG laser and KTP OPO as pump sources was designed. The low optical loss coefficient and large hardness, together with the simplicity of the processing, make GaSe0.91S0.09:Al(0.03 at. %) as a high-reliable and effective THz-wave generator suitable for out-of-door application. We demonstrate incoherent terahertz wave detection by stand-off room temperature Schottky diodes located at over 110 m using open waveguide formed by multitude HPE lenses, mirrors and/or unpolished reflectors.
Coherent terahertz generation is achieved in CdTe crystal at room temperature under the collinear difference frequency generation mechanism. This coherent terahertz source is pumped by the ns pulsed Nd:YAG Laser (1064 nm) and the tunable optical parametric oscillation (OPO 1050-1080 nm ) with the terahertz wave range 107.4 -381.9 μm and peak power of ~ 0.5 W. The emitted THz wave is continously tuned from 109.6 μm to 373.7 μm when the signal wavelength from OPO is changed from 1067.6 nm to 1074.9 nm. When the OPO signal wavelength is tuned from 1054.0 - 1061.6 nm, a terahertz wavelength region (107.4 - 381.9 μm) is achieved in the experiment. Compare with other terahertz sources like GaSe THz sourec or terahertz parametric generation source, this terahertz source is extreamly suited for the high-resolutional THz imaging application due to the simple way of the THz wavelength tunability.
A THz/subTHz radiation detector based on MOCVD-grown modulation-doped InxGa1-xAs/InP structure is proposed. Devices have bow-tie metallic antennas to improve the couple efficiency about 5 dB and are fabricated with mesas of 3 μm depth by wet etching. Detection by hot electron effects under external electromagnetic radiation is explained. Measurements performed at electromagnetic wave frequency f=0.0375 THz show the detector having sensitivity about 6 V/W and noise equivalent power (NEP) about 1.6×10-9 W/Hz1/2 at room temperatures.
The terahertz radiation from the crystal of Cadmium Telluride (CdTe) can be achieved in difference frequency generation
(DFG) experiment of 1064 nm nanosecond laser for collinear configuration. For the isotropic crystal CdTe, the exact
phase matching could not be fulfilled by the two NIR lasers. However, if the interaction length is smaller than the
coherent length, quasi-phase matching could be achieved. In order to understand the property of the coherent length in
CdTe, the property of the refractive index and the absorption coefficient at the THz region is analyzed by two kinds of
transmission spectra: one from Terahertz Time-Domain Spectroscopy (TDS) and the other from Fourier Transform
Infrared Spectrometer (FTIRS). From the transmission spectra of FTIRS, four absorption lines are detected: 2.1 Thz, 4.2
Thz, 7.4 Thz, and 8.6 Thz. Also additional two little absorption peaks occurred at 1.6 Thz and 1.8 Thz. This maybe
explained by the phone mode LO-LA and LA-TA, respectively. Below 1.0 Thz, the absorption coefficient is small and
constant (about 5cm-1). Based on the refractive index spectra of THz-TDS, the coherent length calculated increases
linearly with the THz wavelength in the region of 200μm to 900μm, with its' value 3.6 mm at 300μm. Then
interaction length can be long enough to satisfy the quasi-phase matching condition. A high power and compactable
terahertz source can be obtained from CdTe under 1 Thz, which can be tuned continually and operated under room
temperature.
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