Geometric phase circular gratings fabricated by ultrafast laser nanostructuring for symmetric simultaneous spatio-temporal focusing

The recent advances in flat optics have challenged the limitations of conventional optics by implementing planar elements that instead of rely on dynamic phase manipulate light waves via subwavelength-spaced phase shifters with spatially varying phase response [1]. One of the approaches for designing geometric (Pancharatnam-Berry phase [2]) phase optical elements (GPOE) is to exploit the transparent dielectrics which originate form birefringence. The desired phase pattern of the wave is directly encoded in the optical axis orientation, and is equal to twice the rotation angle of the local retarder. A decade ago, the formation of self-organized subwavelength periodicity structures, referred to as nanogratings, in the bulk of silica glass after irradiation with ultrashort light pulses was observed [3]. Such a periodic assembly behaves as a uniaxial birefringent material with optical axis oriented parallel to the polarization of incident laser beam, and serves as a perfect candidate for designing geometric phase profiles of nearly any optical components. The performance of fabricated GPOE varies depending on the processing conditions reaching the efficiency and transmittance higher than 90%, as well as the phase gradient higher than π rad/μm [4]. The silica glass based optics with a damage threshold of 22.8 J/cm 2 demonstrate the potential of high-power applications.