Following the generation of tunable terahertz waves from GaP in the 0.5- to 3-THz region, we extended the frequency region up to 7 THz, using an optical parametric oscillator and a YAG laser (1.064 μm). The tuning angle θin increased superlinearly in the 3- to 7-THz region, so that the total reflection took place at 5 THz, which was avoided by rotating the crystal relative to the incident optic axis. As a result, terahertz output peak powers of 100 mW at up to 5.6 THz and 3 mW at 7 THz were obtained, at pump and signal energies of 3 mJ, respectively.
The phonon modes of molecular crystals in the terahertz frequency region often feature delicately coupled inter- and intra-molecular vibrations. Recent advances in density functional theory such as DFT-D(*) have enabled accurate frequency calculation. However, the nature of normal modes has not been quantitatively discussed against experimental criteria such as isotope shift (IS) and correlation field splitting (CFS). Here, we report an analytical mode-decoupling method that allows for the decomposition of a normal mode of interest into intermolecular translation, libration, and intramolecular vibrational motions. We show an application of this method using the crystalline anthracene system as an example. The relationship between the experimentally obtained IS and the IS obtained by PBE-D(*) simulation indicates that two distinctive regions exist. Region I is associated with a pure intermolecular translation, whereas region II features coupled intramolecular vibrations that are further coupled by a weak intermolecular translation. We find that the PBE-D(*) data show excellent agreement with the experimental data in terms of IS and CFS in region II; however, PBE-D(*) produces significant deviations in IS in region I where strong coupling between inter- and intra-molecular vibrations contributes to normal modes. The result of this analysis is expected to facilitate future improvement of DFT-D(*).
We report, for the first time, the success of
normalGaAs
molecular layer epitaxy (MLE) using
AsH3
as an As containing gas and trimethyl‐gallium (TMG) as a Ga containing gas. Growth conditions for a single layer by layer deposition process were investigated as a function of the substrate temperature, the pressure in the growth chamber, the admittance quantity per cycle, and photoirradiation. The substrate temperature of 500°C fulfilled the conditions for a monolayer growth, where the film thickness per one cycle is saturated with the admittance quantity, at the higher temperature that increased with the TMG admittance quantity. The electrical properties of the film grown by MLE method strongly depended on the gas admittance rate, i.e., pressure in the growth chamber. The carrier density of the film proportionally decreased with increasing
AsH3
pressure and decreasing TMG pressure. However, all growth films show p‐type behavior with a carrier density of
1×1018–1020 cm−3
. Photoirradiation during the growth by a high pressure Hg lamp, Ar laser (514.5 nm), and with a doubler (257.3 nm) largely improved the surface morphology and electrical properties of the films by MLE.
Articles you may be interested inComment on "Collinear phase-matched terahertz-wave generation in GaP crystal using a dual-wavelength optical parametric oscillator" [J. Appl. Phys. 95, 7588 (2004)
]A frequency-tunable terahertz wave was generated from GaP crystals using an optical parametric oscillator as the pump source and a YAG laser ͑1.064 m͒ as the signal source. By tuning the very small angle, in , between the pump and signal light beam directions, tunable terahertz waves over the frequency range from 0.5 to 3 THz were obtained. The THz frequency changed almost linearly with the angle in . The precise noncollinear phase matching condition is discussed. The pulsed peak power of the THz wave was as high as 480 mW at 1.3 THz.
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