Birefringent Elements for Optical Microscopy by Ultrafast Laser Writing

Differential interference contrast (DIC) microscopy is one of the most powerful label-free optical imaging techniques available for imaging biological and transparent media with high spatial resolution. Wollaston or Nomarsky prisms that shift the reference beam are key elements in this imaging technology [1]. These prisms are made from birefringent crystals (quartz) and are difficult to manufacture for the small shear angles required at high magnifications. Femtosecond laser writing has emerged as a technologically attractive method for birefringence patterning in silica glass by forming anisotropic nanopores with ultrahigh transmittance (>99%) [2] creating negative form birefringence (−0.0016), comparable to the positive birefringence of a quartz crystal (+0.009). Here we demonstrate that high performance birefringent elements for DIC microscopy, geometric phase prism (GP prism) and retardance gradient prism (X prism), can be fabricated by modulating slow axis or retardance of the birefringent modification with ultrafast laser writing.