Recently, we have tried to develop a continuous wave (CW), tunable, and ultraviolet (UV) coherent light source through
sum-frequency generation (SFG) using a BBO nonlinear crystal with a two-stage frequency-conversion system using two
different external cavities for the enhancement of CW lights. In the first stage, we obtained the 532-nm light with the
second harmonic generation (SHG) of the 1064-nm light. A bow-tie external cavity incorporating four mirrors, whose
cavity length was controlled by the frequency stabilization method proposed by Hänsch and Couillaud, was employed
there. In the second stage, to generate the 312-nm light, we demonstrated doubly resonant sum frequency generation of
the 532-nm light from the first-stage and the 754-nm light from a single-frequency CW Ti:Sapphire laser. Considering a
nonlinear coefficient, it should be preferable to use a BiBO crystal for high-efficient SFG, but the 312-nm light might be
absorbed by the BiBO crystal. Therefore, we chose a BBO as a nonlinear crystal to avoid the absorption of the 312-nm
light.
Periodic inversion of spontaneous polarization in a ferroelectric substrate has realized quasi phase matching (QPM) and
thereby revolutionized nonlinear optics. In this paper, we report on the heat influence on the frequency conversion in
birefringence phase matching (BPM) by use of BaB2O4 (BBO) crystals as preparatory for efficient generation of the
second harmonics (SHs) by QPM we suggest. Indeed, QPM is achieved normally by polarization inversion, but we
suggest the periodic step structure to achieve QPM. Polarization inversion is generally formed by superimposed voltage.
However, the shorter wavelength region is, the shorter inversion cycle is. Therefore, if the vacuum ultraviolet (VUV)
region is treated, it becomes more difficult to form periodic inversion and the accuracy is more necessary. Accordingly,
it is necessary to consider the influence of crystal's heat, caused by absorption of laser, which affects frequency
conversion. We discuss validity of analytical approach about crystal's heat and frequency conversion by comparing
between both results of experiment and simulation with BBO crystals.
Quartz has been attracted attention as a nonlinear optical material without spontaneous polarization. Supposing that
twinning of quartz crystal could be achieved effectively rather by impressing bending stress, we studied on making a
periodically inverted structure to generate VUV lights by quasi-phase matching. We fabricated experimentally the
inverted structure locally by impressing the stress for the AT-cut quartz substrate that was heated. Moreover, the stress
distribution was calculated when the bending stress was impressed to the substrate with the rectangle structure, and then
the condition for the polarization inversion was also calculated.
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