Birefringent microstructures in bulk fluorite produced by ultrafast pulsewidth-dependent laser inscription

Abstract Three-segment microtracks were for the first time inscribed in bulk fluorite in the scanning mode via multishot exposure by tightly-focused 515-nm ultrashort laser pulses of variable pulse energy and pulsewidth (0.3–3.8 ps). The local micro/nanoscale material modification and related refractive-index changes within the microstructures, envisioned by optical and cross-sectional electron microscopy, result in their form birefringence acquired by polarimetric microscopy. Retardance and thickness of the microstructures were measured by optical polarimetry and confocal scanning Raman/photoluminescence microscopy, respectively, exhibiting the intensity- and pulsewidth-dependent trends, with the maximal effect for ≈1-ps laser pulses. These and complementary optical self-reflectance studies enabled for the first time to consider ultrafine, pulsewidth-dependent interferential longitudinal sub-structure of the birefringent microtracks. Such regular longitudinal sub-structure was proposed to emerge via back-reflection and interference of laser radiation prior near-critical laser-generated plasma in the focus, and the following transverse plasmon propagation in each nanosheet of ionized fluorite in the resulting sub-wavelength interference pattern. Our findings open the way for ultrashort-pulse laser inscription of embedded optical microstructures in non-oxygen fluoride materials and shed light onto the underlying laser-dielectric interactions.