Femtosecond laser modulated into Bessel beam by spatial light modulator for Fresnel zone plate processing

Abstract

Femtosecond laser processing is an important machining method for micro-optical components such as Fresnel zone plate (FZP). However, the low processing efficiency of the femtosecond laser restricts its application. Here, a femtosecond laser Bessel beam is proposed to process micro-FZP, which is modulated from a Gaussian beam to a Bessel annular beam. The processing time for FZP with an outer diameter of 60 μm is reduced from 30 min to 1.5 min on an important semiconductor material gallium arsenide (GaAs), which significantly improves the processing efficiency. In the modulation process, a central ablation hole that has an adverse effect on the diffraction performance is produced, and the adverse effect is eliminated by superimposing the blazed grating hologram. Meanwhile, the FZP machined by spatial light modulator (SLM) has good morphology and higher diffraction efficiency, which provides a strong guarantee for the application of micro-FZP in computed tomography and solar photovoltaic cells.

1. Introduction

Fresnel zone plate (FZP) is a critical diffraction optical element, which has been widely used in aircraft carrier optical landing aid, space telescope flexible lens, computed tomography, solar photovoltaic and other aspects [1−3]. With the development of the integration of opto-electro-mechanical systems, the size of FZPs have reached the level of micron and nanometer [4−6].

Femtosecond laser machining has become a vital machining method for micro FZPs due to its high transient energy density, ultra-short pulse width, wide controllable pulse repetition rate and non-contact processing. Femtosecond laser processing adopts point-by-point scanning mode without a mask and can realize the processing of arbitrary three-dimensional complex structures. The processing precision is high, and the component topography is compatible with most structures [7, 8]. However, compared to other methods, such as nanoimprint copying and lithography, point-by-point scanning processing has low efficiency, which limits the industrial application of femtosecond laser machining for FZP [9−11]. Therefore, improving the processing efficiency will broaden its application in the field of micro FZPs processing.

The methods of multi-point machining to improve the machining efficiency are put forward, among which micro-lens array is more widely used. MATSUO et al [12] applied micro-lens array for femtosecond laser microfabrication, and two-dimensional periodic patterns were recorded by ablation on a glass surface. KATO et al [13] utilized a micro-lens array to produce multiple spots, and more than 200 spots were simultaneously fabricated by optimizing the exposure condition for the photopolymerizable resin. In addition to the micro-lens array, the researchers are also investigating other methods. DONG et al [14] designed configuration and geometry of multiple beams by changing the set parameters of the lens and aperture masks. STANKEVICIUS et al [15] demonstrated femtosecond laser fabrication using direct laser writing, optical vortex beam and holographic lithography. The processing time for a single micro-tube was 1/400 for the holographic lithography technique and 1/500 for the optical vortex method compared to the direct laser writing. However, the number and distribution of focal points in these processing methods are fixed and cannot be adjusted.

Spatial light modulator (SLM) can flexibly shape the laser beam, which is a prospective method to improve the processing efficiency. SLM consists of several independent units arranged spatially into two-dimensional arrays. Each unit can receive optical or electrical signals independently and change its optical properties according to the signals, to modulate the light waves irradiated on it. For example, through the rotation of the polarization plane, the polarization state and amplitude are modulated, or the conversion of incoherent light to coherent light is achieved [16−20].