A Microlens Super-Surface Film with Regular Graded Circular Hole-Like Subwavelength Structures for Highly Focusing Strength

Jinshuang Wu,Yuegang Fu,Bowei Yang,Leimengting Zhang,Yixin Zhang,Yahong Li

doi:10.3390/coatings9120776

Jinshuang Wu, Yuegang Fu + Show 4 more

Open Access

https://doi.org/10.3390/coatings9120776

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Abstract

Using the fact that a sub-wavelength structure exhibits the same thermal expansion coefficient as a micro-lens array, we design a micro-lens super-surface film with regular circular hole-like subwavelength structures to realize the high performances of central highly focusing strength and short focal length. In addition, based on the Fresnel—Kirchhoff diffraction theory, the influences of subwavelength structural period and height on the focusing performance of a micro-lens are analyzed. Furthermore, the finite-difference time-domain method is utilized to optimize the structural parameters. Via direct laser writing and an inductively coupled plasma process, we fabricated a square micro-lens array consisting of a 1000 × 1000 micro-lens unit with a sub-wavelength structure, and the optical focusing performance was measured in the visible light band. Finally, the experimental results indicate that the focal length is decreased to 15 μm, the focal spot central energy is increased by 7.3%, and the light transmission, enhanced via inserting sub-wavelength structures, corresponds to 3%. This proves that the designed micro-lens array with a regular-graded circular hole-like subwavelength structure can achieve central high focusing and a short focal length. This has applications in several fields of wavefront detection and light field imaging systems.

Highlights

Micro-lens arrays are widely applied in homogenizing systems [1,2], light-field imaging [3,4], and Hartmann sensors [5]
The energy efficiency of the micro-lens array is improved via designing a circular hole-like subwavelength structure on the surface of the micro-lens
A micro-lens array that is composed of micro-lens with a 7 μm calibre and 230 nm vector height is selected

Summary

Introduction

Micro-lens arrays are widely applied in homogenizing systems [1,2], light-field imaging [3,4], and Hartmann sensors [5]. The efficiency of energy utilization is an important parameter of a micro-lens array. An anti-reflection coating is typically deposited on the surface of a micro-lens to decrease reflection and improve the efficiency of energy utilization [6,7]. Conventional anti-reflection coatings are generally affected by the fact that the material with the desired refractive index is usually either expensive or scare. AR coatings usually have a multilayer-structure, the coefficients of thermal expansion (CTE) can be different between layers, including the substrate. The difference of CTEs may induce stress if the process temperature is different from the environmental temperature

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