Multi-Parametric Birefringence Control in Ultrashort-Pulse Laser-Inscribed Nanolattices in Fluorite

An ultrashort-pulse laser inscription of embedded birefringent microelements was performed inside bulk fluorite in pre-filamentation (geometrical focusing) and filamentation regimes as a function of laser wavelength, pulsewidth and energy. The resulting elements composed of anisotropic nanolattices were characterized by retardance (Ret) and thickness (T) quantities, using polarimetric and 3D-scanning confocal photoluminescence microscopy, respectively. Both parameters exhibit a monotonous increase versus pulse energy, going over a maximum at 1-ps pulsewidth at 515 nm, but decrease versus laser pulsewidth at 1030 nm. The resulting refractive-index difference (RID) Delta n = Ret/T similar to 1 x 10(-3) remains almost constant versus pulse energy and slightly decreases at a higher pulsewidth, generally being higher at 515 nm. The birefringent microelements were visualized using scanning electron microscopy and chemically characterized using energy-dispersion X-ray spectroscopy, indicating the increase of calcium and the contrary decrease of fluorine inside them due to the non-ablative inscription character. Dynamic far-field optical diffraction of the inscribing ultrashort laser pulses also demonstrated the accumulative inscription character, depending on the pulse energy and the laser exposure. Our findings revealed the underlying optical and material inscription processes and demonstrated the robust longitudinal homogeneity of the inscribed birefringent microstructures and the facile scalability of their thickness-dependent retardance.

Automated summary

In this study, ultrashort-pulse laser inscription was applied to embed birefringent microelements in bulk fluorite, and the effects of laser wavelength, pulsewidth, and energy were investigated. The resulting elements were characterized in terms of retardance and thickness using polarimetric and 3D-scanning confocal photoluminescence microscopy. The refractive-index difference remained almost constant versus pulse energy, with a slight decrease at higher pulsewidths. The birefringent microelements exhibited accumulative inscription and a facile scalability of thickness-dependent retardance.