Browsing by Author "Ju, Nyan Ping"

Now showing 1 - 7 of 7

Broadband Polarization-Insensitive Metamaterial Perfect Absorbers Using Topology Optimization
(2016-10-01)
Fu, Sze Ming
;
Zhong, Yan Kai
;
Ju, Nyan Ping
;
Tu, Ming-Hsiang
;
Chen, Bo-Ruei
A novel scheme for a perfect hyperbolic metamaterial (HMM) absorber is proposed, and experimental verification is provided. It has been shown previously that tapered HMM stacks can provide adiabatic waveguiding over a wide spectral range and thus are an ideal opaque absorber. Here, nontapered shape-optimized HMM absorbers are proposed, which facilitates the fabrication and promotes the large-area applications such as thermophotovoltaics (TPV). In the synthesis of the optimal patterns, we use 5-harmonic rigorously coupled wave analysis (RCWA) and experimental trials to shorten the trial-and-error time. The best pattern provides an averaged broadband experimental absorption of 88.38% over lambda = 1 mu m to lambda = 2 mu m, which is comparable to the state-of-the-art experimental effort using tapered HMM. The nontapered nature can be easier to fabricate from the semiconductor processing viewpoint. The physics behind the pattern-optimized HMM cavity is the broadband light coupling by the air-cavity and the unbounded photonic density of the states (PDOS) associated with the HMM. The topology optimized air cavity effectively couples the incident photons into the metal-dielectric stacking, eliminating the need of sidewall tapers. We believe the proposed topology-optimization methodology benefits the future design of compact metamaterial perfect absorbers (MPA), sensors, antenna, and thermophotovoltaic emitters, and absorbers.
Experimentally-implemented genetic algorithm (Exp-GA): toward fully optimal photovoltaics
(2015-09-21)
Zhong, Yan Kai
;
Fu, Sze Ming
;
Ju, Nyan Ping
;
Chen, Po Yu
;
Lin, Albert
The geometry and dimension design is the most critical part for the success in nano-photonic devices. The choices of the geometrical parameters dramatically affect the device performance. Most of the time, simulation is conducted to locate the suitable geometry, but in many cases simulation can be ineffective. The most pronounced examples are large-area randomized patterns for solar cells, light emitting diode (LED), and thermophtovoltaics (TPV). The large random pattern is nearly impossible to calculate and optimize due to the extended CPU runtime and the memory limitation. Other scenarios that numerical simulations become ineffective include three-dimensional complex structures with anisotropic dielectric response. This leads to extended simulation time especially for the repeated runs during its geometry optimization. In this paper, we show that by incorporating genetic algorithm (GA) into real-world experiments, shortened trial-and-error time can be achieved. More importantly, this scheme can be used for many photonic design problems that are unsuitable for simulation-based optimizations. Moreover, the experimentally implemented genetic algorithm (Exp-GA) has the additional advantage that the resultant objective value is a real one rather than a theoretical one. This prevents the gaps between the modeling and the fabrication due to the process variation or inaccurate numerical models. Using TPV emitters as an example, 22% enhancement in the mean objective value is achieved. (C) 2015 Optical Society of America
Fully Planarized Perfect Metamaterial Absorbers With No Photonic Nanostructures
(2016-02-01)
Zhong, Yan Kai
;
Fu, Sze Ming
;
Ju, Nyan Ping
;
Tu, Ming-Hsiang
;
Chen, Bo-Ruei
In the past, perfect metamaterial absorbers (PMAs) have required nanolitho-graphy patterning to boost broadband absoprtion. Tapered structures, in particular, are shown to achieve close-to-unity absorption over broadband using adiabatic light coupling. A nontapered PMA is desirable due to the fact that it is easier to fabricate using regular lithography techniques. This facilitates the scalability to large-area photonic applications such as thermophotovoltaics. In this work, we propose a fully planarized design with ultrathin metallic films for broadband PMAs. The design provides close-to-unity absorbance over a wide spectral range and is wavelength scalable from middle ultraviolet to long wavelength infrared. The planarized design is extremely easy to fabricate, and it requires no lithography nor etching. The design can be used with different moderate-extinction metals such as tungsten, titanium, tantalum, and nickel. The physics is that the thin layer of the moderate-extinction metal allows photons to penetrate through itself. The insertion of the dielectric between thin metal layers is necessary to spatially separate the ultrathin metallic thin film to boost the effect of thin-film absorption. As far as the bandwidth normalized to center wavelength is concerned, we believe that the experimental result demonstrated here shows the broadest bandwidth to date.
Non-reciprocal meta-surfaces for nanophtonic light trapping: optical isolators versus solar cells
(2015)
Fu, Sze Ming
;
Zhong, Yan Kai
;
Yan, Sheng Lun
;
Ju, Nyan Ping
;
Chen, Po Yu
Nano-photonic concept has been shown to be very effective for boosting solar cell conversion efficiency. In this work, the novel concepts of non-reciprocality and chiral meta-surface are demonstrated to be very useful for photovoltaics. The asymmetric chiral patterns are designed and optimized using genetic algorithm. The enhanced optical absorption is observed and it is attributed to the non-reciprocal nature of the solar cell front meta-surface. The asymmetric solar cell front surface enhances the anti-reflection and, at the same time, forbids the photon escape once it enters the devices. This concept is very similar to optical isolators where one-way transmission is intentionally designed for enhanced optical diode effect. Preliminary experimental results are supplied to confirm the superiority of asymmetric chiral design. The ongoing experimental effort is adjusting the PECVD deposition conditions to boost the efficiency on patterned substrates.
Photonic Density of State (PDOS) Enhancement by Brillouin Zone Engineering
(2015)
Fu, Sze Ming
;
Zhong, Yan Kai
;
Yan, Sheng Lun
;
Ju, Nyan Ping
;
Chen, Po Yu
The material absorption can be enhanced by increased photonic density of state(PDOS). Here we demonstrate that through the design in the Brillouin zone of nanophotonic light trapping structures, the PDOS can be effectively increased. The theoretical work using rigorously coupled wave calculation shows the increased spectral absorption and the optimal quasi-guided mode excitations in terms of their quality factors and the number of excited modes. The experimental work is currently ongoing and the preliminary result confirms enhanced short circuit current if the PDOS is properly adjusted. Unlike random structures, the proposed Brillouin zone engineered structures are more feasible from modeling and device process viewpoints.
The rigorous wave optics design of diffuse medium reflectors for photovoltaics
(2014-04-21)
Lin, Albert
;
Fu, Sze Ming
;
Zhong, Yan Kai
;
Tseng, Chi Wei
;
Chen, Po Yu
Recently, diffuse reflectors are being incorporated into solar cells, due to the advantage of no metallic absorption loss, higher reflectance, decent light scattering property by embedded TiO2 scatterers, and the ease of fabrication. Different methods have been employed to analyze diffuse reflectors, including Monte Carlo method, N-flux method, and a one-dimensional approximation based on semi-coherent optics, and the calculated reflectance is around 80% by these methods. In this work, rigorous wave optics solution is used, and it is shown that the reflectance for diffuse medium mirrors can actually be as high as >99% over a broad spectral range, provided the TiO2 scatterer geometry is properly optimized. The bandwidth of diffuse reflectors is un-achievable by other dielectric mirrors such as distributed Bragg reflectors or high index contrast grating mirror, using the same index contrast. Finally, it is promisingly found that even if the distribution of TiO2 is random, the wide-band reflection can still be achieved for the optimized TiO2 geometry. Initial experimental result is included in the supplementary material which shows the high feasibility of diffuse medium mirrors for solar cells. (C) 2014 AIP Publishing LLC.
Toward ultimate nanophotonic light trapping using pattern-designed quasi-guided mode excitations
(2015-06-01)
Zhong, Yan-Kai
;
Fu, Sze-Ming
;
Ju, Nyan Ping
;
Lin, Albert
;
電子工程學系及電子研究所
In this work, a shape-optimized periodic pattern design is employed to boost the short circuit current of solar cells. A decent result of an additional 16.1% enhancement in short circuit current is achieved by solely pattern-wise optimization, compared to the baseline structure that is already under full parameter optimization. The underlying physics is that the shape-optimized pattern leads to optimal quasi-guided mode excitations. As a result of the pattern design, a single strongly confined quasi-guided mode is replaced with several weakly confined modes, to cover a broader spectral range. Previous works of optimized periodic gratings result in gradually varied grating heights and require grayscale lithography leading to high process complexity. Using randomized pattern for isotropic Lambertian light trapping, on the other hand, leads to an overly large simulation domain. The proposed pattern design methodology achieves the optimal balance between the slow-light enhancement strength and the enhancement spectral range for nanophotonic light trapping using quasi-guided modes. (C) 2015 Optical Society of America