Ultrashort-pulse laser surface and bulk nano- and micromachining of dielectrics have multiple promising applications in micro-optics, microfluidics, and memory storage. The fundamental principles relate intrinsic inter-band multi-photon (MPA) and laser-induced intra-band free-carrier absorption (FCA) to particular ablation mechanisms and features. These principles are yet to be quantified into a complete set of basic experimental laser-matter interaction parameters, describing photoexcitation, relaxation, and final ablation. In this study, we considered the characteristic double-crater structure of single-shot ablation spots on dielectric surfaces and single-shot transmission spectra to extract crucial information about the underlying basic processes of ultrafast photoexcitation and laser energy deposition. Specifically, energy-dependent crater profiles and accompanying prompt self-phase modulation (SPM) spectral broadening were studied in single-shot surface ablation experiments on fluorite (CaF2) surface photo-excited by tightly focused 515- or 1030-nm, 300-fs laser pulses. Crater size dependence demonstrated two slopes, scaling proportionally to the squared focal 1/e-radius at higher energies (intensities) for larger ablated spots, and a much smaller squared 1/e-radius at lower energies (intensities) for (sub) micron-wide ablated spots, indicating a transition from 1D to 3D-ablation. As a result, these slopes were related to lower-intensity wavelength-dependent multi-photon inter-band transitions and wavelength-independent higher-intensity linear absorption in the emerging near-critical electron-hole plasma (EHP), respectively. Crater depth dependences on the local laser intensity fitted in the corresponding ranges by multi- and one-photon absorption provided the corresponding absorption coefficients. Spectral broadening measurements indicated even values for the red and blue shoulders of the laser pulse spectrum, representing the SPM effect in the weakly excited fluorite at the leading pulse front and providing the corresponding Kerr coefficient. In the second regime, the blue-shoulder broadening value saturated, indicating the appearance of near-critical plasma screening at the trailing pulse front, which is consistent with our calculations. These complementary experiments and related analysis provided an important set of key basic parameters, characterizing not only surface ablation, but also propagation of high-intensity ultrashort laser pulses in bulk fluorite, and enabling precise forecasting of optimal energy deposition for high-efficiency ultrashort-laser micro-structuring of this dielectric material.

中文翻译：

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在高数值孔径聚焦下萤石从 3D 到 1D 飞秒激光烧蚀的转变中揭示的能量沉积参数

超短脉冲激光表面和电介质的体纳米和微加工在微光学、微流体和存储器存储方面具有多种有前景的应用。基本原理将固有的带间多光子 (MPA) 和激光诱导的带内自由载流子吸收 (FCA) 与特定的烧蚀机制和特征联系起来。这些原理尚未量化为一套完整的基本实验激光-物质相互作用参数，描述光激发、弛豫和最终消融。在这项研究中，我们考虑了电介质表面单次烧蚀点的特征双坑结构和单次透射光谱，以提取有关超快光激发和激光能量沉积的基本过程的关键信息。具体来说，在由紧密聚焦的 515 或 1030 纳米、300 飞秒激光脉冲光激发的萤石 (CaF2) 表面的单次表面烧蚀实验中，研究了能量依赖的陨石坑轮廓和伴随的快速自相位调制 (SPM) 光谱展宽. 陨石坑大小的相关性表现出两个斜率，对于较大的烧蚀点，在较高能量（强度）下与焦点 1/e 半径的平方成正比，对于（亚）微米，在较低能量（强度）下与 1/e 半径平方成正比-宽烧蚀点，表明从 1D 到 3D 烧蚀的过渡。因此，这些斜率分别与新兴的近临界电子空穴等离子体 (EHP) 中的低强度波长相关多光子带间跃迁和波长无关的高强度线性吸收有关。陨石坑深度依赖于通过多光子吸收和单光子吸收在相应范围内拟合的局部激光强度，提供了相应的吸收系数。光谱展宽测量表明激光脉冲光谱的红肩和蓝肩的值是均匀的，代表了前导脉冲前沿弱激发萤石中的 SPM 效应，并提供了相应的克尔系数。在第二个区域中，蓝肩展宽值饱和，表明在拖尾脉冲前沿出现近临界等离子体屏蔽，这与我们的计算一致。这些补充实验和相关分析提供了一组重要的关键基本参数，不仅表征表面烧蚀，而且表征高强度超短激光脉冲在块状萤石中的传播，