Compact diffractive optics for THz imaging

Abstract

we presented a compact diffractive silicon-based multilevel phase Fresnel lens (MPFL) with up to 50 mm in diameter and numerical aperture (NA) up to 0.86 designed and fabricated for compact terahertz imaging systems. The laser direct writing (LDW) technology based on picosecond laser was used to fabricate the diffractive optics on silicon with different number of phase quantization levels P reaching 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 lens. The influence of phase quantization number to the focused beam amplitude was estimated, and power transmission efficiency reaching more than 90 % was demonstrated. THz imaging resolution less than 1 mm using robust 50 mm diameter multilevel THz lens was achieved and demonstrated at frequency 580 GHz.