Development of the crystal growth and application of the novel, wide-bandgap, non-oxide nonlinear optical crystal BaGa4S7 in optical parametric amplifiers

该项目致力于一种新发现的非线性光学晶体BaGa4S7或简称BGS的晶体生长、光学表征和应用的发展，该晶体在中红外波段透明至约12微米。采用定向晶种和提高温度梯度的垂直Bridgman-Stockbarger方法生长晶体，可以保证晶体在所需的晶体取向上尺寸较大，以实现相匹配，而水平梯度冻结技术的实施将减少缺陷浓度和残余吸收，提高BGS的抗损伤能力。在这两种情况下，都将进行生长后处理(退火)，剩余损失将通过吸收测量来量化。新生长的晶体的光学损伤阈值和克尔系数将用1.03微米的超短脉冲(ps和fS)在与设想的非线性频率下转换实验类似的条件下测量。BGS的非凡性能，例如硫化物(非氧化物)材料的极宽带隙(3.54 eV，即在UV中)以及由此产生的高破坏阈值和低群速度失配和色散，使一些具有短和超短激光脉冲的独特应用能够将工作在1微米光谱范围内的广泛分布和功率可扩展的二极管泵浦激光光源的波长转换到5微米以上的中红外波段。光参量放大(OPA)，特别是啁啾脉冲OPA的研究将在1ps长脉冲泵浦的BGS和飞秒白光连续谱的种子下产生，以100 kHz的重复频率产生能量、峰值和平均功率(约1W量级)接近8.7微米的闲置脉冲，这对强场物理和阿特金斯科学很有意义。阿普尔。100ps长、短周期的中红外脉冲本质上是载波包络相位稳定的。虽然这样的短脉冲将使用拉伸-压缩概念来实现，但也将在亚ps脉冲持续时间下研究纯OPA，因为这种特定的魔术空转波长对应于所有三个相互作用的波，即泵浦波、信号波和空转波的相等群速度。具有低剩余损耗和均匀质量的大尺寸BGS光学元件的可用性是降低泵浦阈值和提高光学参量振荡器(OPO)转换效率的先决条件，这些OPO在近1微米处再次被ns脉冲泵浦，并产生更高(在MJ水平上)能量的空闲脉冲。这种装置在100赫兹的重复频率下无损伤长期运行的演示将为它们的商业化和在6.5微米附近的微创激光手术中的应用开辟道路。

This project is devoted to the development of the crystal growth, optical characterization and applications of one recently discovered nonlinear optical crystal, BaGa4S7 or shortly BGS, transparent in the mid-IR up to about 12 micron. Crystal growth by the vertical Bridgman-Stockbarger method with oriented seeds and increased temperature gradient will ensure larger sizes in the desired crystallographic orientation for phase-matching while implementation of the Horizontal Gradient Freeze technique will reduce the concentration of defects and the residual absorption, improving the damage resistivity of BGS. Post-growth treatment (annealing) will be carried out in both cases and the residual losses will be quantified by absorption measurements. The optical damage threshold of the newly grown crystals and the Kerr coefficient will be measured with ultrashort (ps and fs) pulses at 1.03 micron under similar conditions as in the nonlinear frequency down-conversion experiments envisaged. Extraordinary properties of BGS, such as the extremely wide band-gap (3.54 eV, i.e. in the UV) for a chalcogenide (non-oxide) material and the resulting high damage threshold and low group velocity mismatch and dispersion enable some unique applications with short and ultrashort laser pulses to convert the wavelength of widely spread and power-scalable diode-pumped laser sources operating in the 1 micron spectral range to the mid-IR above 5 micron. Optical parametric amplification (OPA) and in particular chirped-pulse OPA will be studied with BGS pumped by 1 ps long pulses and seeded by femtosecond white light continuum to generate idler pulses with unprecedented energy, peak and average (of the order of 1 W) powers near 8.7 micron at a repetition rate of 100 kHz, interesting for strong-field physics and attoscience. The appr. 100 fs long, few-cycle mid-IR pulses will be intrinsically carrier-envelope-phase stabilized. While such short pulses will be achieved employing the stretching-compression concept, also pure OPA will be investigated at sub-ps pulse durations because this particular magic idler wavelength corresponds to equal group velocities of all the three, pump, signal, and idler interacting waves. The availability of large size optical elements of BGS with low residual losses and uniform quality is a prerequisite for reduction of the pump threshold and increase of the conversion efficiency in optical parametric oscillators (OPOs) pumped again near 1 micron by ns pulses and generating idler pulses of yet higher (on the mJ level) energy. The demonstration of damage-free long-term operation of such devices at a repetition rate of 100 Hz will open the way for their commercialization and application in minimally invasive laser surgery near 6.5 micron.