Effect of the Pore Structure of Glass on the Formation of Birefringent Regions under the Effect of Femtosecond Laser Pulses

Nanoporous glass is a promising material for application in integrated optics and archival optical storage. Femtosecond laser pulses incident on porous glass can produce microstructures exhibiting form birefringence whose retardance and slow axis orientation depend on writing conditions. In this work, nanoporous glass was heat-treated at temperatures from 700 to 775°C to obtain samples differing in pore size and specific pore volume with the aim of further laser modification of their structure. The results demonstrate that, as the heat treatment temperature is raised, the retardance decreases and the range of femtosecond laser pulse energies where birefringent microregions can be produced narrows down. Analysis of the retardance and pore structure parameters has made it possible to demonstrate a critical effect of specific pore volume on the feasibility of local form birefringence writing. We have proposed heat treatment conditions that ensure protection of porous glass from the influence of adsorption of contaminants from the ambient atmosphere and allow the possibility of producing birefringent structures to be retained, which will extend the application area of nanoporous glass structured by a femtosecond laser beam.