Generation of an Array of Birefringent Nanogratings in the Bulk of Fluorite Irradiated by Ultrashort Laser Pulses with Different Durations

Birefringent microstructures recorded at a fixed depth under the action of laser pulses with a wavelength of 1030 nm, different durations in the range of 0.3–3.8 ps, and different energies have been obtained for the first time in the bulk of a fluorite plate. The dependences of the retardance in microstructures for orthogonal polarizations on the energy/intensity of radiation are monotonically increasing at any directions of laser pulses. A mechanism has been proposed for the formation of birefringent microstructures at the reflection of laser pulses from the near-critical bulk plasma near the focal region with the formation of a standing electromagnetic wave in front of the plasma along the optical axis. This wave is fixed in the material in the form of arrays of nanostructures planes (nanogratings) with the orientation of grooves of local modification of the material and its refractive index perpendicular to the laser polarization. The observed sublinear increase in the retardance in birefringent microstructures with an increase in the energy/intensity of laser radiation is described within this mechanism.