355 nm脉冲激光在甲烷中的高效多波长拉曼转换

 研究了脉冲Nd:YAG激光（355 nm）泵浦的甲烷中多级Stokes光的产生和惰性气体对其转换效率的影响，其中一级和二级Stokes光的最大能量转换效率分别可达71%和38%（对应量子效率为79%和48%），大大高于已往文献报道的20%。在0.5 MPa下，可同时获得322 nm（3.6%），355 nm（24.5%），396 nm（24.3%），448 nm（22.3%）和515 nm（9.3%）的多波长输出。甲烷压力对多级Stokes转换有显著影响：高气压利于产生高效的一级Stokes光，而低气压则适合于高级Stokes光的产生。根据级联受激拉曼散射(SRS)和四波混频(FWM)理论对实验结果进行了分析，结果表明甲烷中高级Stokes光的产生是SRS和FWM协同作用的结果。加入的氦气增强了甲烷中Stokes光的转换效率，而氩气的作用恰恰相反，利用热透镜效应可以很好地解释这些现象。

Efficient multi-wavelength Raman conversion in methane pumped by pulsed laser at 355 nm

Multi-order Stokes generation in pure CH_4 and its He/Ar mixture pumped by a pulsed Nd:YAG laser at 355 nm were investigated. Maximum energy conversion efficiencies as high as 71% and 38% (i.e. quantum efficiency of 79% and 48%), much higher than those reported in previous literatures, were attained under 2.5 MPa and 1 MPa for the first and second Stokes respectively. Under 0.5 MPa CH_4, significant multi-wavelength outputs could be obtained with energy distribution of 3.6% at 322 nm, 24.5% at 355 nm, 24.3% at 396 nm, 22.3% at 448 nm, and 9.3% at 515 nm. High-pressure CH_4 was good for highly efficient first Stokes conversion, while low-pressure CH_4 was suitable for higher order Stokes generation. The experimental results were analyzed in detail in terms of cascade stimulated Raman scattering and four-wave mixing, indicating that such efficient high-order Stokes generation in CH_4 should be the result of their cooperative effect. The addition of He enhanced the Stokes conversion efficiencies, while Ar acted adversely, which could be explained by the thermal lens effect.