Polarization sensitive optical elements by ultrafast laser nanostructuring of glass

Abstract

In this theses I will concentrate on femtosecond laser induced modification in silica glass. One type of modification in fused silica is subwavelength nanogratings. This modification exhibits form birefringence and therefore optical elements can be fabricated based on nanogratings. The main goal of my work was development and fabrication of practical optical elements based on femtosecond laser induced nanogratings.

In order to be able to fabricate optical elements, laser induced modification had to be uniform and exhibit strong birefringence. Also fabrication speed has to be high in order to write reasonable size optical element in short time. Optimal fabrication parameters were found by studying physical and optical properties of nanogratings.

The first polarization sensitive optical element to be demonstrated was polarization converter, which transforms Gaussian beam with uniform polarization distribution into a cylindrical vector beam. Later on, polarization converters for different types of laser beams and spectral ranges were demonstrated. Properties of generated beams were measured and compared with theoretical modelling. Generated beams were also successfully exploited for material modification and results are compared with theoretical simulations.

Same converters can be also used for optical vortices generation. Unique property of such converters is that they allow tuning orbital angular momentum of photons in the light beam and therefore adjusting the torque of the vortex beam. The important feature of this vortex generation technique is that optical angular momentum is changed without affecting intensity distribution of the laser beam. The benefits of tunable optical angular momentum were demonstrated in optical trapping experiment where silica micro spheres were trapped into a rotating ring is performed. The ability to tune the torque of the beam is proved by changing rotation speed change of the beads’ ring.

Birefringence can be exploited not only for polarization control but also for phase. Half-wave retarder can induce phase delay to circularly polarized light depending on the azimuth of slow axis of the retarder. Based on that, converter for “diffraction-free” Airy and Bessel beams are fabricated. Properties of generated beams are measured and compared with theoretical simulations. Airy beam converter based on laser induced nanogratings has unique ability to generate dual Airy beam which is superposition two Airy beams accelerating in opposite direction. Generated Airy beam is also used in material modification. Self-detachment of fibre-like structure is observed from laser modified substrate which is speculated to be caused by properties of Airy beam.

Finally, an array of micro waveplates is fabricated which together with a linear polarizer and a CCD camera allows real time polarization imaging. Full state of polarization can be measured. For the demonstration, polarization distribution of cylindrical vector beams was measured.

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