Fabrication of an Anti-Reflective Microstructure on ZnS by Femtosecond Laser Bessel Beams.

Xun Li,Ming Li,Yan Guo,Hongjun Liu

doi:10.3390/molecules26144278

Abstract

As an important mid-infrared to far-infrared optical window, ZnS is extremely important to improve spectral transmission performance, especially in the military field. However, on account of the Fresnel reflection at the interface between the air and the high-strength substrate, surface optical loss occurs in the ZnS optical window. In this study, the concave antireflective sub-wavelength structures (ASS) on ZnS have been experimentally investigated to obtain high transmittance in the far-infrared spectral range from 6 μm to 10 μm. We proposed a simple method to fabricate microhole array ASS by femtosecond Bessel beam, which further increased the depth of the microholes and suppressed the thermal effects effectively, including the crack and recast layer of the microhole. The influence of different Gaussian and Bessel beam parameters on the microhole morphology were explored, and three ASS structures with different periods were prepared by the optimized Bessel parameters. Ultimately, the average transmittance of the sample with the ASS microhole array period of 2.6 μm increased by 4.1% in the 6 μm to 10 μm waveband, and the transmittance was increased by 5.7% at wavelength of 7.2 μm.

Highlights

IntroductionIn view of its excellent optical transmission in mid- and far-infrared, ZnS has critical applications in many optic components (lens, optical fibers, windows, etc.) and optoelectronic devices (solar cells, photodetectors, light-emitting diodes, etc.) [1]
In view of its excellent optical transmission in mid- and far-infrared, ZnS has critical applications in many optic components and optoelectronic devices [1]
Explained from the characteristics of penetration geometry, nonlinear robustness and interaction phenomenology [24,25], the zero-order Bessel beam shows its decisive advantage over the Gaussian beam in making deeper microholes in transparent materials by femtosecond laser pulses

Summary

Introduction

In view of its excellent optical transmission in mid- and far-infrared, ZnS has critical applications in many optic components (lens, optical fibers, windows, etc.) and optoelectronic devices (solar cells, photodetectors, light-emitting diodes, etc.) [1]. Antireflective subwavelength structures (ASS) may be used instead of thin film coating to reduce Fresnel scattering and increase spectral transmittance [7,8,9], providing a continuously and linearly graded refractive index profile at the interface of substrate and air [10,11]. It has the advantages of wide bandwidth, large angular response, high mechanical strength, and surface hydrophobicity [12,13,14]. The current ASS manufacturing technologies, such as machining [15], electron beam lithography [16], lithography [17], reactive ion lithography [18], interference lithography [19]

Methods

Results

Conclusion