基于数值孔径（NA）、自旋/轨道角动量（OAM）及原位超连续谱（SC）诊断的聚甲基戊烯（PMP）非线性特性飞秒工程
Femtosecond Engineering of PolyMethylPentene(PMP) Nonlinearity via NA, Spin/orbital angular momentum (OAM), and In-Situ SC Diagnostics
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中文总结 AI 辅助
本研究明确OAM对PMP非线性特性的影响，揭示χ(3)张量分量的作用机制，实现OAM控制的亚微米手性结构制造，为光子器件确定性制造提供工程框架。
中文摘要 AI 辅助
飞秒涡旋光束在光子制造领域已得到广泛研究，但轨道角动量（OAM）对透明聚合物的三阶极化率、能量沉积及结构对称性的影响仍不明确。为填补这一空白，本研究探讨飞秒涡旋光束与聚甲基戊烯（PMP）的相互作用，以建立激光微加工中OAM对光束整形和能量沉积的控制机制。实验采用775 nm飞秒激光，经倍频得到387.5 nm激光，通过反射式液晶空间光调制器进行相位整形，借助计算机生成全息图 imprint OAM，同时通过四分之一波片独立控制自旋。结合张量非线性极化的理论模型，以及对低数值孔径（NA）成丝、采用先进光谱仪的超连续谱光谱分析、NA分别为0.4和0.7的高NA刻蚀的系统实验，结果表明所有 regime 均由相同的χ(3)张量分量主导。从微扰行为到耗散行为的转变仅由局部强度是否超过等离子体形成阈值决定。基于各向同性材料模型的分析框架，本研究的关键创新在于明确χ₁₁₂₂主导能量沉积和等离子体阈值，而χ₁₂₂₁介导横向非线性电流和与螺旋度相关的不对称性，从而将相位拓扑转化为永久性手性结构。这实现了OAM控制的亚微米特征尺寸结构化，特征尺寸约为0.43 μm直径、约200 nm²面积，局部强度达到10¹⁴--10¹⁵ W/cm²，为通过精确控制拓扑电荷和聚焦条件确定性制造手性波导及光子器件提供了工程框架。
英文摘要
While femtosecond vortex beams are widely studied for photonic fabrication, how OAM affects third-order susceptibility, energy deposition, and structural symmetry in transparent polymers remains unclear. To address this gap, we investigate femtosecond vortex beam interactions with polymethylpentene (PMP) to establish OAM control over beam shaping and energy deposition in laser micromachining. A 775 nm femtosecond laser is frequency-doubled to 387.5 nm and phase-shaped by a reflective liquid crystal-spatial light modulator, with OAM imprinted via computer-generated holograms and independently controlled spin via a quarter-wave plate. Theoretical modeling of the tensorial nonlinear polarization, combined with systematic experiments on low-NA filamentation, supercontinuum spectroscopy using an advanced spectrometer, and high-NA inscription with NA = 0.4 and 0.7, reveals that the same $χ^{(3)}$ tensor components govern all regimes. The transition from perturbative to dissipative behavior is determined solely by whether the local intensity exceeds the plasma formation threshold. Through an analytical framework based on the isotropic material model, the key innovation lies in identifying that $χ_{1122}$ governs the energy deposition and plasma threshold, whereas $χ_{1221}$ mediates the transverse nonlinear current and helicity-dependent asymmetry, thereby converting phase topology into permanent chiral structures. This enables OAM-controlled structuring with sub-micrometer feature sizes (approximately 0.43 $μ$m diameter and approximately 200 nm$^2$ area) and localized intensities reaching $10^{14}$--$10^{15}$ W/cm$^2$, providing an engineering framework for deterministic fabrication of chiral waveguides and photonic devices through precise control of topological charge and focusing conditions.
