Subwavelength moth eye structures are the nanostructures arranged uniformly whose feature size is less than the incident optical wavelength. These structures are promising to reduce the reflection of any material by creating a refractive index gradient profile at the interface surface. Mid-wave infrared (MWIR) is an important wavelength to investigate the moth eye structures for various applications like photovoltaic, solar cells and display technologies. In this paper, we fabricated two different moth eye structures Nano pillars and Nano holes using the simple and robust lithography technique. Using silicon dioxide as a hard mask, structures are transferred onto gallium arsenide substrate using different etching conditions. We compared the transmission of nanoholes and nanopillars structures and find out that nanoholes structures shows better transmittance in MWIR. We also obtained theoretical transmission data using rigorous coupled wave analysis (RCWA) which agrees with our experimental data. Moreover, Nano holes structures has an advantage over nanopillars structure as the former are resistant against contamination which therefore will not lead to decrease in transmission performance. The characterization results of the structures are obtained from SEM which shows the morphologies of the structures. Our approach is reproducible and can be easily applied to any optical devices which require antireflective property.
Nano-arrays are an important structure for building chemical filters, photonic crystal waveguides, antireflection, or transmission devices. There are different methods of lithography to produce these nano-arrays, which include contact and projection photolithography, E-beam direct writing, and X-ray lithography. Contact photolithography is the most widely used method due to its simplicity and good for time and cost-saving. However, there are penalties that come with these benefits which include problems of generating Newton rings and difficulties of transferring patterns faithfully for situations at and beyond the diffraction limit.
In this work, we fabricated nano-arrays for high power antireflection applications using contact photolithography. Fortunately for the antireflection application, pattern periodicity is more important than obtaining the exact shape of the nanostructure. The fabricated structure, even though not the same as the original pattern, can still produce promising antireflection results. We have studied how the range of the distance between the mask and the photoresist affects the shapes of the produced patterns including holes, posts, and cones. The experimental results with different shapes and periodic patterns produced by different diffraction distances are explained with simulation results involving Fourier transformation and Fresnel diffraction of the mask patterns.
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