Abstract
Reflection loss on the optical component surface is detrimental to performance. Several researchers have discovered that the eyes of moths are covered with micro- and nanostructured films that reduce broadband and wide-angle light reflection. This research proposes a new type of moth-eye subwavelength structure with a waist, which is equivalent to a gradient refractive index film layer with high–low–high hyperbolic-type fill factor distribution. The diffraction order characteristics of a moth-eye subwavelength structure are first analyzed using a rigorous coupled wave analysis. The moth-eye structural parameters are optimized within the spectral range of 2–5 μm using the finite-difference time-domain method. The experimental fabrication of the moth-eye structure with a waist array upon a silicon substrate is demonstrated by using three-beam laser interferometric lithography and an inductively coupled plasma process. The experimental and simulation results show good agreement. The experimental results show that the reflectivity of the moth-eye structure with a waist is less than 1.3% when the incidence angle is less than 30°, and less than 4% when the incidence angle is less than 60°. This research can guide the development of AR broadband optical components and wide-angle applications.
Highlights
Fresnel reflection occurs when light travels from one medium into another owing to the large refractive index discontinuity at different medium interfaces, which can have harmful effects on the transmission and utilization of light energy [1,2,3,4]
We derive the Rayleigh expansion of a diffraction grating from the rigorous coupled wave analysis (RCWA) results
− jk0 (k xi /k0 )2 − nl 2 nl k0 < k xi where kxi and kl,yi are the components of the normal tangent to k in the reflecting and transmission regions; λ is the incident wavelength; k0 = 2π/λ is the incident wave vector; θ is the angle of incidence; Λ is the structural period; and n1 is the refractive index of different media (l = 1, 2)
Summary
Fresnel reflection occurs when light travels from one medium into another owing to the large refractive index discontinuity at different medium interfaces, which can have harmful effects on the transmission and utilization of light energy [1,2,3,4]. Reflection in optical systems can lead to stray light such as ghost images. Slight reflections may produce cold reflection effects, causing serious deterioration in the overall image uniformity [5,6]. Almost 30% of the energy incident on the surface of a silicon substrate is reflected, which constitutes a great loss [7]. The moth-eye structure has been developed as a new anti-reflection method with the discovery of moth-eye anti-reflection (AR) ability in bionics [8,9]. By analyzing the structure of a moth eye, scientists discovered that the outer surface of the moth eye is covered by a regular array of periodic protuberances, wherein the micro-nano structure is smaller in scale than the wavelengths of light; Coatings 2018, 8, 360; doi:10.3390/coatings8100360 www.mdpi.com/journal/coatings
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