| Summary: | 碩士 === 中原大學 === 物理學系 === 87 === Photorefractive crystals have emerged as the best candidate for storing large amount of optical image information. According to the Band Transport model, photorefraction results from absorption at impurity level inside crystals. Thus an understanding of impurity levels within a photorefractive crystal will facilitate further optimization of imputity doping as well as further understanding of photorefraction. However, at room temperature most impurity levels are broadband and may overlap with another impurity level spectrum. Thus it requires lowering he temperature in order to separate each individual impurity absorption spectrum. The purpose of this investigation is to study absorption spectroscopy and photorefractive effect for BaTiO3 at low temperature. As we will show later that lowest temperature that we were able to acquire absorption spectra was too high to produce sufficient narrow-band spectra. However, we did discover accidentally another temperature-dependent effect for extraordinary refractive index in BaTiO3.
The main obstacle for obtaining low temperature absorption spectra was that we were unable to prevent the crystal to become multiple domains during the cooling down process. This in turn was due to the fact that there is a phase transition, which should take place at 278K. Based on our observation(using its optical transmission as a criteria) the transition tool place at about 255K. This reduction of phase transition temperature has provided us with sufficient temperature window to investigate temperature-dependent two-wave mixing phenomenon. The results reveal that the lower the temperature was the longer it took to remove gratings. We suggest that this fact may be due to the reduction of thermal excitation of carriers. Its exact mechanism, however, requires further investigation.
A very interesting result in this study is that the extraordinary index in BaTiO3 is highly sensitive to the temperature variation. Although the temperature-induced grating may eventually disappear as temperature become stable, the fact that the incident beam often become a heating source by itself suggests that we may have to include the impact of temperature-induced grating in considering the effect for photorefractive gratings in BaTiO3 , especially, in the transient regime.
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