Study of low temperature matrix crystals as non-linear optical elements

利用高功率脉冲激光和谐波、和差、四波混频、参量振荡等非线性光学效应，研制出宽频带的相干光源。例如，在SoHrt波长范围内，像BBO晶体这样的非线性材料以良好的效率产生低至210 nm的辐射。然而，固态材料在210 nm以下的波长范围内存在不可避免的自吸收现象。因此，气相中的稀有气体原子或金属蒸气交替产生较短波长的辐射。但由于气相密度低，不具备位相匹配条件，其转换效率很低(最高可达10^~(-6)~(-6))，为了克服这一问题，提出了利用稀有气体原子的低温矩阵作为非线性光学器件来产生较短波长的辐射，并进行了实验研究。我们已经成功地在低温下制备了一些稀有气体原子和分子的透明晶体。利用Xe的大的非线性光学性质，将其纯晶体本身用作非线性光学器件。通过检测单模激光产生的532 nm和355 nm脉冲激光的三次谐波，直接研究了晶体的非线性。用单色仪和日盲光倍增管检测真空紫外区的谐波。晶体表面产生了355 nm的二次谐波和532 nm的三次谐波。另一方面，在晶体中弱均匀地产生了355 nm的三级峰。前者的发射可能与Xe_2分子有关。从而证实了稀有气体晶体作为真空紫外区非线性材料的可能性。

Coherent light sources in the wide frequency region are developed by using of high power pulse laser and non-linear optical effects like harmonics-, sum-, difference-generation, four waves mixing and parametric oscillation. In the sohrt wave length region, for example, non-linear materials like BBO crystal generates the radiation down to 210 nm in good efficiency. However, solid state materials have an inevitable self-absorption in shorter wave lenght than 210 nm. Therefore, rare gas atoms or metal vapor in the gas phase are alternated to generate the shorter wave length radiation. However their conversion efficiencies are quite small (at most 10^<-6>) because of low density and lack of phase matching condition in the gas phase.In order to remove the problem of low density, a low temperature matrix of the rare gas atom was proposed to use as a non-linear optical device for generating the shorter wave length radiation and tried in this research. We have succeeded in making transparent crystal of some rare gas atoms and molecules at low temperature. Using the large non-linear optical property of Xe, its pure crystal were used itself as a non-linear optical device. Non-linearity of the crystal was directly investigated by detecting the third order harmonic generation of pulse laser of 532 and 355 nm from a single mode YAG laser. The harmonics in the vacuum ultra-violet region were detected by a monochromater and solar-blind photomultiplier. The second order harmonics of 355 nm and the third harmonics of 532 nm were strongly generated from the surfaces of the crystal. On the other hand, the third order of 355 nm was weakly generated homogeneously in the crystal. The former emission may be related to the Xe_2 exicimer molecule. Thus we could confirm the possibility of the rare gas crystal as non-linear materials in the VUV region.