To suppress the Fresnel reflection loss and enhance light transmission, subwavelength structured surfaces are used as antireflector. In this work, rigorous coupled wave (RCWA) analysis-based design approach has been adopted to simulate nano-sized moth-eye like structures on silicon substrate for wide-angle, short-wave infrared (SWIR) antireflector. Large reflection loss due to high refractive index of silicon is detrimental to optical performance. The proposed surface relief moth-eye structures introduce gradient refractive index to the surface depending on substrate materials, structure geometry, height, periods. Optimum selection of these parameters during design and fabrication are essential steps for the effective quelling of undesirable reflection from air-surface boundary and enhance transmission. Due to subwavelength nature, maintaining accuracy of all design parameters during fabrication on silicon is challenging. Careful tradeoff is required to fix tolerance of each parameter depending on priority to overall performance. In this work, with help of Taguchi optimization techniques, optimum combination of the structure height, periods, and top surface area of moth-eye structure are selected for antireflector. Analysis of variance (ANOVA) approach has been opted to identify the contribution of individual design parameters to performance. This performance model based on RCWA design, Taguchi optimization techniques and ANOVA analysis acts as a tool to predict the performance trend and fix tolerance of design parameters. For wavelength range (700nm -3000nm), with optimized height 600 nm, period 200 nm, and flat top diameter 70 nm of tapered moth-eye structures, the obtained reflectance is less than 1 % for the incidence angle up-to 45°.
Hybrid solar system converts the incident solar radiation into electrical energy by utilizing complete solar spectrum. Such a system uses spectrum splitting solar concentrator for separation of solar spectrum into light and heat component and focusses on high-efficiency multi-junction solar cell and heat receiver. In this paper, chirped volume holographic grating (CVHG) is investigated as solar concentrator cum spectrum splitter. The grating is designed to separate the visible light of solar spectrum from the infrared wavelength band. The main advantage of CVHG over normal volume gratings are high diffraction efficiency, large bandwidth separation and lightweight. Chirp rate, period of grating, modulation index, thickness of grating, and grating profile are the critical parameters to be optimized. CVHG using photopolymer as a recording medium was designed with a dimension of 200x200x100µm3. The grating was simulated using rigorous coupled wave analysis for the incident solar spectrum wavelength ranging from 0.3 to 3.0 µm. CVHG designed with hyperbolic index profile showed concentration ratio of 15x and spectrum separation at 1.0µm.
The requirement of the complex surface is increasing significantly with growing needs to fulfill the demands of many challenging applications. The advancements in manufacturing technologies attract optical designers to think about more and more complex shapes. The demand for hybrid surfaces i.e. diffractive structures over spherical, aspheric and freeform surfaces is growing for many future applications e.g. hyperspectral imaging, spectroscopy, beam shaping, wavefront coding. The hybrid freeform surfaces are the most complicated and most demanding surfaces for the above applications due to the advantages of the freeform base profile and diffractive structures. The use of these components leads to significant improvement in the performance of these optical systems along with considerable miniaturization and cost reduction. Challenges in fabrication and characterization of diffracto-freeforms are a limiting factor for its realization. This work aims to explore the fabrication and characterization strategy for diffracto-freeform optics. The cubic freeform shape with diffractive structures has been chosen for the study. The slow tool servo machining is utilized for the fabrication of this hybrid surface. The tool path development strategy and the effect of process parameters are discussed. The base shape and diffractive structures are characterized separately by utilizing the optical profiler measurements. The average error in the fabricated diffractive structures height and there radial distance from center is found less than 10% and 15% respectively. The characterization of the fabricated surface and fabrication issues are discussed in this paper. The current study is helpful to understand the various issues involved in the development of such complicated surfaces.
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