Birefringence of 3 × 10-3 is demonstrated inside cross-sectional regions of 100 µm, inscribed by axially stretched Bessel-beam-like fs-laser pulses along the c-axis inside sapphire. A high birefringence and retardance of λ/4 at mid-visible spectral range (green) can be achieved utilizing stretched beams with an axial extension of 30-40 µm. Conditions of laser writing chosen ensure that there are no formations of self-organised nano-gratings. This method can be adopted for the creation of polarisation optical elements and fabrication of spatially varying birefringent patterns for optical vortex generation.
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