Browsing by Author "John, Sajeev"

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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
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Chen, Shu-Yu
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Kaiser, Alex
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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).
Achieving an Accurate Surface Profile of a Photonic Crystal for Near-Unity Solar Absorption in a Super Thin-Film Architecture
(2016-06)
Kuang, Ping
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Eyderman, Sergey
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Hsieh, Mei-Li
;
Post, Anthony
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John, Sajeev
In this work, a teepee-like photonic crystal (PC) structure on crystalline silicon (c-Si) is experimentally demonstrated, which fulfills two critical criteria in solar energy harvesting by (i) its Gaussian-type gradient-index profile for excellent antireflection and (ii) near-orthogonal energy flow and vortex-like field concentration via the parallel-to-interface refraction effect inside the structure for enhanced light trapping. For the PC structure on 500-mu m-thick c-Si, the average reflection is only similar to 0.7% for lambda = 400-1000 nm. For the same structure on a much thinner c-Si ( t = 10 mu m), the absorption is near unity (A similar to 99%) for visible wavelengths, while the absorption in the weakly absorbing range (lambda similar to 1000 nm) is significantly increased to 79%, comparing to only 6% absorption for a 10-mu m-thick planar c-Si. In addition, the average absorption (similar to 94.7%) of the PC structure on 10 mu m c-Si for lambda = 400-1000 nm is only similar to 3.8% less than the average absorption (similar to 98.5%) of the PC structure on 500 mu m c-Si, while the equivalent silicon solid content is reduced by 50 times. Furthermore, the angular dependence measurements show that the high absorption is sustained over a wide angle range (theta(inc) = 0-60 degrees) for teepee-like PC structure on both 500 and 10-mu m-thick c-Si.
An In-situ and Direct Confirmation of Super-Planckian Thermal Radiation Emitted From a Metallic Photonic-Crystal at Optical Wavelengths
(2020-03-23)
Lin, Shawn-Yu
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Hsieh, Mei-Li
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John, Sajeev
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Frey, B.
;
Bur, James A.
Planck's law predicts the distribution of radiation energy, color and intensity, emitted from a hot object at thermal equilibrium. The Law also sets the upper limit of radiation intensity, the blackbody limit. Recent experiments reveal that micro-structured tungsten can exhibit significant deviation from the blackbody spectrum. However, whether thermal radiation with weak non-equilibrium pumping can exceed the blackbody limit in the far field remains un-answered experimentally. Here, we compare thermal radiation from a micro-cavity/tungsten photonic crystal (W-PC) and a blackbody, which are both measured from the same sample and also in-situ. We show that thermal radiation can exceed the blackbody limit by >8 times at lambda = 1.7 mu m resonant wavelength in the far-field. Our observation is consistent with a recent calculation by Wang and John performed for a 2D W-PC filament. This finding is attributed to non-equilibrium excitation of localized surface plasmon resonances coupled to nonlinear oscillators and the propagation of the electromagnetic waves through non-linear Bloch waves of the W-PC structure. This discovery could help create super-intense narrow band thermal light sources and even an infrared emitter with a laser-like input-output characteristic.
Effectively infinite optical path-length created using a simple cubic photonic crystal for extreme light trapping
(2017-06-23)
Frey, Brian J.
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Kuang, Ping
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Hsieh, Mei-Li
;
Jiang, Jian-Hua
;
John, Sajeev
A 900 nm thick TiO2 simple cubic photonic crystal with lattice constant 450 nm was fabricated and used to experimentally validate a newly-discovered mechanism for extreme light-bending. Absorption enhancement was observed extending 1-2 orders of magnitude over that of a reference TiO2 film. Several enhancement peaks in the region from 600-950 nm were identified, which far exceed both the ergodic fundamental limit and the limit based on surface-gratings, with some peaks exceeding 100 times enhancement. These results are attributed to radically sharp refraction where the optical path length approaches infinity due to the Poynting vector lying nearly parallel to the photonic crystal interface. The observed phenomena follow directly from the simple cubic symmetry of the photonic crystal, and can be achieved by integrating the light-trapping architecture into the absorbing volume. These results are not dependent on the material used, and can be applied to any future light trapping applications such as phosphor-converted white light generation, water-splitting, or thin-film solar cells, where increased response in areas of weak absorption is desired.
Experimental demonstration of broadband solar absorption beyond the lambertian limit in certain thin silicon photonic crystals
(2020-07-16)
Hsieh, Mei-Li
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Kaiser, Alex
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Bhattacharya, Sayak
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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.
Light trapping and near-unity solar absorption in a three-dimensional photonic-crystal
(2013-10-15)
Kuang, Ping
;
Deinega, Alexei
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Hsieh, Mei-Li
;
John, Sajeev
;
Lin, Shawn-Yu
We report what is to our knowledge the first observation of the effect of parallel-to-interface-refraction (PIR) in a three-dimensional, simple-cubic photonic-crystal. PIR is an acutely negative refraction of light inside a photonic-crystal, leading to light-bending by nearly 90 deg over broad wavelengths (.). The consequence is a longer path length of light in the medium and an improved light absorption beyond the Lambertian limit. As an illustration of the effect, we show near-unity total absorption (>= 98%) in lambda 520-620 nm and an average absorption of similar to 94% over lambda = 400-700 nm for our alpha-Si: H photonic-crystal sample of an equivalent bulk thickness of (t) over tilde = 450 nm. Furthermore, we have achieved an ultra-wide angular acceptance of light over theta = 0 degrees-80 degrees. This demonstration opens up a new door for light trapping and near-unity solar absorption over broad.s and wide angles. (C) 2013 Optical Society of America
Photonic crystals with a continuous, Gaussian-type surface profile for near-perfect light trapping
(2018-04-01)
Kuang, Ping
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Bhattacharya, Sayak
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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)
Probing the intrinsic optical Bloch-mode emission from a 3D photonic crystal
(2016-10-14)
Hsieh, Mei-Li
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Bur, James A.
;
Du, Qingguo
;
John, Sajeev
;
Lin, Shawn-Yu
We report experimental observation of intrinsic Bloch-mode emission from a 3D tungsten photonic crystal at low thermal excitation. After the successful removal of conventional metallic emission (normal emission), it is possible to make an accurate comparison of the Bloch-mode and the normal emission. For all biases, we found that the emission intensity of the Bloch-mode is higher than that of the normal emission. The Bloch-mode emission also exhibits a slower dependence on (h(omega)/k(b)T) than that of the normal emission. The observed higher emission intensity and a different T-dependence is attributed to Bloch-mode assisted emission where emitters have been located into a medium having local density of states different than the isotropic case. Furthermore, our finite-difference time-domain (FDTD) simulation shows the presence of localized spots at metal-air boundaries and corners, having intense electric field. The enhanced plasmonic field and local non-equilibrium could induce a strong thermally stimulated emission and may be the cause of our unusual observation.
Review on Recent Progress of Three-dimensional Optical Photonic Crystal
(2014)
Hsieh, Mei-Li
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Kuang, Ping
;
Bur, James A.
;
John, Sajeev
;
Lin, Shawn-Yu
Over the past two decades, the field of photonic-crystals has become one of the most influential realms of contemporary optics. In this paper; we will review two recent experimental progresses in three-dimensional photonic-crystal operating in optical wavelengths. The first is the observation of anomalous light-refraction, an acutely negative refraction, in a 3D photonic-crystal for light trapping, guiding and near-unity absorption. The second is the observation of quasi-coherent thermal emission from an all-metallic 3D photonic-crystal at elevated temperatures.