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In this paper, we investigate the local oxidation of titanium thin films under the action of picosecond laser pulses. Periodical structures were recorded by the multi-beam interference scheme utilizing various numbers of laser beams, and the relationship between spatial resolution and the contrast of the structures was studied. The Raman spectra of the laser processing regions confirmed the oxidation even under the action of a single picosecond pulse. An analytical simulation of titanium film oxidation in the interference field was provided, and obtained results are correlated with the experimental data. The results of theoretical modeling show that the thermochemical effects of picosecond laser pulses allow recording periodic structures with a period of 0.65 lines per μm. The demonstrated results are important in the adaptation of technological laser systems for the manufacture of diffractive optical elements.

We present a compact diffractive silicon-based multilevel phase Fresnel lens (MPFL) with up to 50 mm in diameter and a numerical aperture up to 0.86 designed and fabricated for compact terahertz (THz) imaging systems. The laser direct writing technology based on a picosecond laser was used to fabricate diffractive optics on silicon with a different number of phase quantization levels P reaching an almost kinoform spherical surface needed for efficient THz beam focusing. Focusing performance was investigated by measuring Gaussian beam intensity distribution in the focal plane and along the optical axis of the lens. The beam waist and the focal depth for each MPFL were evaluated. The influence of the phase quantization number on the focused beam amplitude was estimated, and the power transmission efficiency reaching more than 90% was revealed. The THz imaging of less than 1 mm using a robust 50 mm diameter multilevel THz lens was achieved and demonstrated at 580 GHz frequency.