3D laser printing by ultra-short laser pulses for micro-optical applications: towards telecom wavelengths

TL;DR

This paper demonstrates rapid 3D laser printing of micro-optical elements with sub-wavelength resolution suitable for telecom wavelengths, using femtosecond laser pulses to create birefringent vortex generators with high retardance.

Contribution

It introduces a fast, precise method for fabricating micro-optical vortex generators with sub-wavelength features for telecom applications using femtosecond laser polymerisation.

Findings

Fabricated polymerised vortex generators with ~150 nm width.

Achieved retardance up to 315 nm at visible wavelengths.

Demonstrated potential for high-purity vortex generators at telecom wavelengths.

Abstract

Three dimensional (3D) fast (< 0.5 hour) printing of micro-optical elements down to sub-wavelength resolution over 100 micrometers footprint areas using femtosecond (fs-)laser oscillator is presented. Using sub-1 nJ pulse energies, optical vortex generators made of polymerised grating segments with an azimuthally changing orientation have been fabricated in SZ2080 resist; width of polymerised rods was ~150 nm and period 0.6-1 micrometers. Detailed phase retardance analysis was carried out manually with Berek compensator (under a white light illumination) and using an equivalent principle by an automated Abrio implementation at 546 nm. Direct experimental measurements of retardance was required since the period of the grating was comparable (or larger) than the wavelength of visible light. By gold sputtering, transmission-type optical vortex generators were turned into reflective ones with augmented retardance, n.h defined by the form birefringence, n, and the height h = 2d where d is the thickness of the polymerised structure. Retardance reached 315 nm as measured with Berek compensator at visible wavelengths. Birefringent phase delays of 180 degrees (or half-wavelength) required for high purity vortex generators can be made based on the proposed approach. Optical vortex generators for telecom wavelengths with sub-wavelength patterns of azimuthally oriented gratings are amenable by direct laser polymerisation.