Nanoimprinted antireflective surface nanostructures for optical applications in the mid-infrared: Reducing optical losses resulting from Fresnel reflection in chalcogenide glass

Mikkel Berri Lotz

Research output: Book/ReportPh.D. thesisResearch

Abstract

A breakthrough in nonlinear fiber-optics and fiber based supercontinuum lasers has recently led to the development of a new breed of mid-infrared supercontinuum light sources. Unfortunately, the intensity of these light sources are severely dampened by Fresnel reflection loss due to the relatively high refractive index of the fiber-optic material, known as chalcogenide glass.
This thesis concerns itself with the development of a direct thermal nanoimprinting technique, aimed at patterning the surface of chalcogenide glass based optical components with broadband antireflective surface nanostructures, known as motheye structures, in the pursuit of reducing Fresnel reflection loss. The objectives of the thesis is to design and fabricate highly efficient moth-eye structures with antireflective properties in the mid-infrared spectral bands: 3.5–4.5 µm, 6–8 µm and 3–8 µm. First, an analytical model for calculating the transmittance of a blank chalcogenide glass window is established and subsequently used to calculate a maximum transmittance improvement of ∼14% and ∼22% for the first and second imprint on an As2Se3 window. The diffraction grating behavior of imprinted surface textures is also analysed as the grating equation is used to derive the zero-order grating condition, defining the upper limit for the period of the antireflective patterns. Then, using a simulation-based approach, we conduct a study of the design and optimization of different antireflective moth-eye structures, where a parabolic shaped moth-eye structure is suggested to be the most promising moth-eye design. With pattern periods and structure heights between p = 0.5–1.675 µm and h = 0.5–1.675 µm, the parabolic shaped moth-eye structure is predicted to produce excellent antireflection in the 3.5–4.5 µm band. A similar region is also identified for the 6–8 µm band.
By conventional deep-UV lithography and subsequent dry-etching process, a silicon master is fabricated containing hexagonal patterns of densely packed moth-eye structures, each pattern with a different sized period and height. Once inverted to a nickel mold and used to directly pattern the surface of As2Se3 windows, broadband antireflective properties are observed, with peak transmittance improvements between 12.2% and 13.28%. The method therefore demonstrates a fast and cost-effective way of transferring tailor-made antireflective patterns onto chalcogenide glass based components.
Utilizing the lithography post processing step thermal reflow during mold fabrication, a second iteration of moth-eye structures are fabricated with secant ogive-like profiles and higher packing densities. Once imprinted on the surfaces of As2Se3 windows, the surfaces demonstrate vastly improved antireflective properties, achieving an average transmittance improvement of 12.36% from 3.3–12% µm. 
Applying the same method to imprint chalcogenide glass fiber end-facets, we see a 30% improvement in the transmitted power of a mid-infrared supercontinuum light source with a spectrum from 2.1–4.2 µm. Analysing the beam profile output by an imprinted photonic crystal fiber, there is no evidence that the imprint prevents coupling to the fiber core nor that it changes the shape of the beam profile, demonstrating that this nanoimprinting technique can also be used to deliver antireflective properties to fibers.
Original languageEnglish
PublisherDTU Nanolab
Number of pages169
Publication statusPublished - 2019

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