Surface patterning by laser ablation and polymerisation

Fabrication of metasurfaces by subtractive and additive (ablation and polymerisation) techniques are demonstrated using ultra-short laser pulses. Stokes polarimetry was used to determine optical retardance from laser structured regions on the surface and in the bulk. 3D laser writing/printing of optical elements and devices has to deliver precision of surface definition as well as thickness control down to deep sub-wavelength, λ/10, in precision and accuracy. Optical elements are now made by laser patterning of glasses and crystals on the surface and in the bulk. The history of light-matter interaction reflected in chemical, thermal, and mechanical modifications are all very important for performance of optical elements and their longevity, especially for micro-elements which cannot be post-processed by lapping and polishing and have to be used as made. Density of light-induced defects, color centers in commercial optical elements recorded in silica using self-organized gratings was shown to cause a considerable absorption at deep-UV wavelengths where such optical elements are promising for direct excitation of protein luminescence at wavelengths λ <; 280 nm. Defects formed inside glasses and crystals using focused ultra-short (sub-1 ps) laser pulses are usually point defects and their pairings with different charge states (via electron trapping). Tailored thermal annealing can be used to reduce number of defects, e.g., an activation of interstitial oxygen to restore defects in oxides. Absorption of defects, charge distribution in photo-refractive materials, mechanical stress induced refractive index changes due to nano-gratings are all contributing to performance, especially for optical elements recorded inside the bulk of glass or crystal. We have demonstrated that Stokes polarimetry combined with electron spin resonance (EPR) allowed for the quantitative determination of optical retardance and defect concentration from regions structured by ultrashort laser pulses. The very same Stokes polarimetry is also useful to characterize surface patterns made by laser ablation and polymerisation. Light phase accumulated over propagation length, d, in a material of natural or form birefringence, Δn, is: δ = 2π/λ Δnd. In addition to this “dynamic phase”, a geometrical phase ±lφ, which is defined by a spatial in-plane variation of the orientation of the optical axis, φ is added with l being a number. Ablation ripples and polymerised gratings with different duty cycle provides an excellent geometrical phase control to engineer such metasurfaces with precisely controlled local orientation of the optical axis. Polymerisation of spatially varying patterns in SZ2080 resist was used to show potential of dielectric metasurfaces for at optics over vis-IR-THz spectral ranges.