Browsing by Author "Bhattacharya, Sayak"

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A low cost and large-scale synthesis of 3D photonic crystal with SP2 lattice symmetry
(2019-08-01)
Hsieh, Mei-Li
;
Chen, Shu-Yu
;
Kaiser, Alex
;
Yan, Yang-Jhe
;
Frey, B.
In this work, a novel lithographic method is proposed to prepare three-dimensional (3D) photonic crystal (PC) that is different from conventional top-down and bottom-up approaches. The method combines a 2D optical mask and off-the-axis double optical exposures to create a desirable 3D PC structure. Since the method uses only two optical exposures of a photo-resist layer, it is inherently a low-cost, high throughput and wafer-scale lithographic method. The method is implemented to make a slanted post 3D PC having the SP2 lattice symmetry. Three types of SP2 3D PC structures were successfully fabricated with a minimum feature size of d=1.5 mu m over a large scale of 8x10 mm(2), without any observable fabrication defects. The SP2 PCs are: (i) SU8 posts in air background, (ii) air pores in CdS background and (iii) Pt coated on SU8 SP2 templates. A spectroscopic study of the SP2 PCs shows select spectral regions of high reflectance, indicating the existence of a photonic stop band. This low-cost and large-scale method could enable broader technological impacts of 3D PC materials in areas such as thermo-photovoltaics and above room-temperature Bose-Einstein Condensation. Furthermore, this off-axis method could lead to the creation of an entirely new class of slanted-rod based photonic crystals, such as topological photonic crystal in 3D. (C) 2019 Author(s).
Experimental demonstration of broadband solar absorption beyond the lambertian limit in certain thin silicon photonic crystals
(2020-07-16)
Hsieh, Mei-Li
;
Kaiser, Alex
;
Bhattacharya, Sayak
;
John, Sajeev
;
Lin, Shawn-Yu
The tantalizing possibility of 31% solar-to-electric power conversion efficiency in thin film crystalline silicon solar cell architectures relies essentially on solar absorption well beyond the Lambertian light trapping limit (Bhattacharya and John in Nat Sci Rep 9:12482, 2019). Up to now, no solar cell architecture has exhibited above-Lambertian solar absorption, integrated over the broad solar spectrum. In this work, we experimentally demonstrate two types of photonic crystal (PhC) solar cells architectures that exceed Lambertian light absorption, integrated over the entire 300-1,200 nm wavelength band. These measurements confirm theoretically predicted wave-interference-based optical resonances associated with long lifetime, slow-light modes and parallel-to-interface refraction. These phenomena are beyond the realm of ray optics. Using two types of 10-mu m thick PhC's, first an Inverted Pyramid PhC with lattice constant a=2,500 nm and second a Teepee PhC with a=1,200 nm, we observe solar absorption well beyond the Lambertian limit over lambda =950-1,200 nm. Our absorption measurements correspond to the maximum-achievable-photocurrent-density (MAPD), under AM1.5G illumination at 4-degree incident angle, 41.29 and 41.52 mA/cm(2) for the Inverted Pyramid and Teepee PhC, respectively, in agreement with wave-optics, numerical simulations. Both of these values exceed the MAPD (=39.63 mA/cm(2)) corresponding to the Lambertian limit for a 10-mu m thick silicon for solar absorption over the 300-1,200 nm band.
Photonic crystals with a continuous, Gaussian-type surface profile for near-perfect light trapping
(2018-04-01)
Kuang, Ping
;
Bhattacharya, Sayak
;
Hsieh, Mei-Li
;
John, Sajeev
;
Lin, Shawn-Yu
We show a unique design of teepee-like photonic crystal (TP-PC) structure that possesses a true gradient, Gaussian-type surface profile for smooth and accurate index matching between air and silicon for near-perfect light trapping. Such funnel-like, inverse-conical topography is capable of achieving near-zero optical reflection and near-unity solar absorption with excellent angular response over the entire visible light wavelength range. The fabrication only requires standard microelectronics reactive-ion etching (RIE) process. We demonstrate how various process parameters, such as RIE gas mixture ratio, RIE power, thickness of silicon dioxide (SiO2) coatings, and lattice constant of the photonic crystal, can impact the details of the "Gaussian" profile and further improve the optical performance of the TP-PC structure at broad-lambda, broad-theta, especially in the ultraviolet (UV) wavelength range. Our finite-difference time-domain (FDTD) simulation of the TP-PC structure reveals existence of multiple absorption resonances in the 800- to1100-nm wavelength range. Poynting vector plots show that such strong absorption enhancements at the resonant frequencies are due to long-lifetime photonic modes arising from parallel-to-interface refraction of the incoming sunlight and formation of vortex-like energy flow pattern inside the TP-PC structure. Our design will lead the way for future development of ultrathin, high-efficiency c-Si solar photovoltaics. (C) 2018 Society of Photo-Optical Instrumentation Engineers (SPIE)