Gui Guan, Adak Amitava, Dandapat Manika, Carlson Daniel, Morrill Drew, Guggenmos Alexander, Kapteyn Henry, Murnane Margaret, Pervak Vladimir, Liao Chen-Ting
Opt Express. 2021 Feb 15;29(4):4947-4957. doi: 10.1364/OE.409216.
Dispersive dielectric multilayer mirrors, high-dispersion chirped mirrors in particular, are widely used in modern ultrafast optics to manipulate spectral chirps of ultrashort laser pulses. Dispersive mirrors are routinely designed for dispersion compensation in ultrafast lasers and are assumed to be linear optical components. In this work, we report the experimental characterization of an unexpectedly strong nonlinear response in these chirped mirrors. At modest peak intensities <2 TW/cm-well below the known laser-induced damage threshold of these dielectric structures-we observed a strong reflectivity decrease, local heating, transient spectral modifications, and time-dependent absorption of the incident pulse. Through computational analysis, we found that the incident laser field can be enhanced by an order of magnitude in the dielectric layers of the structure. The field enhancement leads to a wavelength-dependent nonlinear absorption, that shows no signs of cumulative damage before catastrophic failure. The nonlinear absorption is not a simply two-photon process but instead is likely mediated by defects that facilitate two-photon absorption. To mitigate this issue, we designed and fabricated a dispersive multilayer design that strategically suppresses the field enhancement in the high-index layers, shifting the high-field regions to the larger-bandgap, low-index layers. This strategy significantly increases the maximum peak intensity that the mirror can sustain. However, our finding of an onset of nonlinear absorption even at 'modest' fluence and peak intensity has significant implications for numerous past published experimental works employing dispersive mirrors. Additionally, our results will guide future ultrafast experimental work and ultrafast laser design.
色散介质多层反射镜，特别是高色散啁啾反射镜，在现代超快光学中被广泛用于控制超短激光脉冲的光谱啁啾。色散反射镜通常用于超快激光器中的色散补偿，并被认为是线性光学元件。在这项工作中，我们报告了这些啁啾反射镜中意外强烈的非线性响应的实验表征。在适度的峰值强度<2 TW/cm²（远低于这些介电结构已知的激光诱导损伤阈值）下，我们观察到反射率显著下降、局部加热、瞬态光谱变化以及入射脉冲的时间依赖性吸收。通过计算分析，我们发现结构的介电层中入射激光场可增强一个数量级。场增强导致与波长相关的非线性吸收，在灾难性失效之前没有累积损伤的迹象。非线性吸收不是简单的双光子过程，而是可能由促进双光子吸收的缺陷介导。为了缓解这个问题，我们设计并制造了一种色散多层结构，策略性地抑制高折射率层中的场增强，将高场区域转移到带隙更大的低折射率层。这种策略显著提高了反射镜能够承受的最大峰值强度。然而，我们发现在“适度”的能量密度和峰值强度下就出现非线性吸收，这对许多过去发表的使用色散反射镜的实验工作具有重要意义。此外，我们的结果将指导未来的超快实验工作和超快激光设计。
