Development of Highly-Efficient Generation-Method for Vacuum-Ultraviolet Coherent Radiation

本项目的目的是发展一种基于气相原子非线性和频产生的新型高效真空-紫外相干光产生方法。选择气相原子氢作为非线性介质。在122 nm和103 nm处产生了相干的真空紫外光辐射(Lyman-α和β辐射)，没有发生共振吸收。该方法最重要的一点是利用原子的精确共振条件获得介质的共振增强的巨大非线性，同时利用强耦合引起的量子干涉消除共振吸收以获得所产生的辐射的透明度。作为一种基频激光辐射源，研制了一种新型的单纵模脉冲染料激光器，它可以产生时间平滑的空间轮廓光滑的傅里叶极限激光脉冲。时间和空间的平稳性是为实现理论预测而达到实验条件的关键问题。在温度为300K、氢气流量为0.1~1托的微波放电条件下，制备了原子密度最大为5×10~(-3)~(-3)的氢原子非线性介质。从理论和实验上研究了量子干涉非线性光学过程的基本物理特性。和频实验是在强耦合条件下进行的，激发强度为40 mW/cm2，相互作用长度为5 cm。在122 nm和103 nm的辐射下，获得了2×10^~(-4)的转换效率。考虑到原子密度和激发强度，得到的效率是一个很高的值。目前使用大气氢的产生方法可能为各种应用提供新的可能性，例如监测原子氢。

The aim of this project is to develop a new type of highly-efficient generation-method for vacuum-ultraviolet coherent radiation based on nonlinear sum-frequency generation in gas-phase atoms. Gas-phase atomic hydrogen was chosen as a nonlinear medium. Coherent VUV-radiation at 122 and 103 nm (Lyman-alpha and beta radiation) was generated without suffering resonant absorption. The most important point of this method is to use an exact resonance condition for atoms for obtaining a resonantly enhanced huge nonlinearities for the medium, and simultaneously to remove resonant absorption for obtaining a transparency for the generated radiation using quantum interferences induced by strong-coupling.As a fundamental laser radiation source, a new type of single-longitudinal-mode pulsed-dye-laser was developed, which generates temporally smooth Fourier limit laser-pulse with smooth spatial profile. The smoothness both in temoral and spatial aspects is an essential issue to achieve experimental conditions for realizing the theoretical predictions. The nonlinear medium ; atomic hydrogen system, was prepared with a maximum atom-density of 5x10^<14> cm^<-3> at a temperature of 300 K through a microwave discharge for H_2 gas of 0.1 to 1 Torr.Basic physical characteristics for nonlinear optical processes using quantum interferences were investigated theoretically and experimentally. The sum-frequency generation experiments were carried out under strong-coupling condition using 40 MW/cm^2 excitation intensity with an interaction length of 5 cm. A conversion efficiency of 2x10^<-4> was achieved for 122 and 103 nm radiation. The obtained efficiency is a very high value considering the atom-density and excitation intensity. The present generation method using atmic hydrogen may offer a new possibility for various applications, such as to monitor atomic hydrogen.