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

该项目致力于开发一种最近发现的非线性光学晶体BaGa4S7（简称BGS）的晶体生长、光学特性和应用，该晶体在中红外波段透明，最高可达12微米。利用定向种子和增大温度梯度的垂直Bridgman-Stockbarger方法生长晶体，将确保在相匹配所需的晶体取向上具有更大的尺寸，而采用水平梯度冻结技术将减少缺陷的浓度和残余吸收，提高BGS的损伤电阻率。在这两种情况下都将进行生长后处理（退火），剩余损耗将通过吸收测量来量化。在与所设想的非线性频率降频实验相似的条件下，用超短脉冲（ps和fs）测量新生长晶体的光学损伤阈值和克尔系数。BGS的特殊特性，例如硫族（非氧化物）材料的极宽带隙（3.54 eV，即在UV中）以及由此产生的高损伤阈值和低群速度失配和色散，使得一些独特的短和超短激光脉冲应用能够将广泛分布和功率可扩展的二极管泵浦激光源的波长转换为在1微米光谱范围内工作到5微米以上的中红外波段。光学参数放大（OPA），特别是啁啾脉冲OPA将在BGS上进行研究，BGS被1ps长的脉冲泵浦，并由飞秒白光连续体种子产生前所未有的能量，峰值和平均功率（约1w）接近8.7微米，重复频率为100 kHz，对强场物理学和原子科学很感兴趣。从新加坡。100秒长，少周期的中红外脉冲本质上是载波包络相位稳定的。虽然这种短脉冲将采用拉伸压缩概念来实现，但纯OPA也将在亚ps脉冲持续时间下进行研究，因为这种特殊的神奇空闲波长对应于所有三种，泵浦，信号和空闲相互作用波的相等群速度。BGS中具有低残余损耗和均匀质量的大尺寸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（OPOs）转换效率的先决条件，OPOs在1微米附近再次被ns脉冲泵浦，并产生更高（mJ级）能量的空闲脉冲。该装置在100 Hz重复频率下无损伤长期运行的演示，将为其在近6.5微米的微创激光手术中的商业化和应用开辟道路。

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.