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

该项目致力于最近发现的非线性光学晶体BaGa 4S 7或简称BGS的晶体生长，光学特性和应用的发展，在中红外透明高达约12微米。通过具有定向种子和增加的温度梯度的垂直Bridgman-Stockbarger方法的晶体生长将确保在用于相位匹配的所需晶体取向中的较大尺寸，而水平梯度冻结技术的实施将减少缺陷的浓度和残余吸收，提高BGS的抗损伤性。在这两种情况下都将进行生长后处理（退火），并通过吸收测量来量化残留损失。新生长的晶体的光学损伤阈值和克尔系数将在与设想的非线性频率下转换实验中类似的条件下用1.03微米的超短（ps和fs）脉冲测量。BGS的非凡特性，例如极宽的带隙（3.54eV，即在UV中）（非氧化物）材料和由此产生的高损伤阈值和低群速度失配和色散使得一些具有短和超短激光脉冲的独特应用能够转换宽分布和功率可缩放的二极管的波长。泵浦激光源工作在1微米光谱范围到5微米以上的中红外光谱范围内。利用飞秒白色连续光作为种子光，在100 kHz重复频率下，利用1 ps长脉冲抽运BGS，产生峰值功率和平均功率均在8.7 μ m附近的闲频脉冲，研究光参量放大（OPA），特别是啁啾脉冲OPA。的apr。100 fs长，几个周期的中红外脉冲将是固有的载波包络相位稳定。虽然这样的短脉冲将采用拉伸-压缩概念来实现，但是也将在亚ps脉冲持续时间处研究纯OPA，因为该特定的魔闲频波长对应于所有三个相互作用的泵浦波、信号波和闲频波的相等群速度。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增加光学参量振荡器（OPO）中的转换效率的先决条件，所述光学参量振荡器（OPO）再次以ns脉冲泵浦接近1微米并且产生更高（在mJ水平上）能量的闲频脉冲。此类设备以100 Hz重复率无损伤长期运行的演示将为其在6.5微米附近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.