中红外（5μm）周期量级飞秒光脉冲的产生与放大技术研究

Because of the wavelength-scaling effects in strong-field physics, mid-infrared few-cycle pulses have attracted more and more research interests in the field of ultrafast lasers. Nonlinear frequency-conversion is a widely used method to build up mid-infrared ultrafast laser sources, which can directly convert existing powerful near-infrared lasers into the mid-infrared waveband. However, due to the lack of mid-infrared materials and related technologies, such a method has not been used to generate a powerful ultrafast laser with a wavelength of >5 μm. In this project, the piezoelectric crystal of LGN will be used to generate ultrafast mid-IR lasers for the first time. We have theoretically proved that the LGN crystal can support broadband phase matching for the conversion from near-infrared lasers to mid-IR lasers. We will study the technique of intra-pulse difference-frequency-generation to generate the 5-μm few-cycle seed lasers from a typical Ti:sapphire femtosecond laser, and extend the technique of frequency-domain optical parametric amplification from conventional near-IR waveband to new mid-IR waveband. This distinctive technical route can solve the challenges in the generation, amplification and dispersion-control of the 5-μm few-cycle lasers. By this route, we will produce a five-cycle 5.3-μm ultrafast laser, with a peak power of 10 GW, higher than the current state-of-the-art results by one order of magnitude. After focused, its intensity will reach 10^15 W/cm^2, which can drive some high-field physical experiments, such as high-order harmonic generation.

由于强场物理呈现显著的波长效应，中红外周期量级脉冲是超快激光领域新的发展趋势。将现有高功率近红外激光非线性频率转换到中红外波段，是获得中红外超快激光的主要途径。然而受中红外材料与超快技术的制约，至今未能产生一定峰值功率的波长大于5μm的超快激光。材料方面，本项目计划将压电领域的LGN晶体首次应用于产生超快激光，我们已经在理论上探明了该晶体优越的中红外宽带位相匹配特性；技术方面，本项目将创新性地研究从飞秒钛宝石激光产生中红外新波段（5μm）周期脉冲的带内自差频技术，并将以往近红外波段的频率域光参量放大技术拓展应用到中红外新波段，解决5μm波长处周期脉冲种子光的产生、放大以及色散控制的困难，形成有特色的技术方案。本项目将产生5个光波振荡周期的5.3μm波长的飞秒激光，峰值功率（10GW）高于当前国际最好结果一个数量级，聚焦光强（10^15 W/cm^2）达到开展高次谐波产生等强场物理实验的要求

高能量密度物理的核心物理量电子振动能Eq取决于驱动激光光强与波长平方的乘积（Iλ^2），决定了强场的物理效应以及光与物质相互作用的状态。为提高Eq，科学家在不断提高驱动激光的聚焦光强I，波长λ集中在材料和技术比较成熟的800nm和1μm两个近红外波段。沿着这条路线，继续提升聚焦光强I变得越来越困难。于是，增加波长λ被视作进一步增加Eq的新发展方向。不同于当前国际普遍采用的“非氧化物半导体晶体+2μm泵浦光源”技术路线，本项目提出并发展了一种基于新型氧化物LGN晶体将现有成熟近红外超短超强激光直接转换至中红外波段的新路线，有望产生太瓦级中红外超快激光。本项目取得了以下研究成果：.（1）研究了一种新型LGN氧化物晶体，明确了其优异的中红外位相匹配性能及在产生中红外超短超强激光方面的应用前景；基于LGN晶体的带内差频过程，首次将800nm钛宝石超快光场直接转换至波长在3-7μm之间可调谐的中红外周期量级超快光场，转换效率优于千分之一。.（2）提出并模拟验证了基于LGN晶体利用800nm超宽带激光和1μm高功率碟片泵浦源直接研制太瓦级5.2μm中红外强激光的技术路线，演示了峰值功率0.1TW，脉冲宽度为7个光学周期、CEP稳定的输出性能，峰值功率水平比现有同波段激光系统高2个数量级。.（3）发明了支撑中红外参量放大的温度与波长同时不敏感的位相匹配技术、基于共振级联非线性的超快群速度调控技术和准参量啁啾脉冲放大技术，分别解决了高功率泵浦下中红外超快放大器的热效应、如何实现泵浦与信号的飞秒级精确同步以及如何实现接近量子极限放大效率的关键问题。.项目资助期内，共发表SCI论文8篇（项目负责人均为第一作者或通信作者，7篇为第一资助）；授权2项美国专利，新申请1项中国专利；在国内外学术会议和论坛做学术报告7次；协助培养出站博士后1名、毕业博士2名、毕业硕士1名。