Browsing by Author "Chen, Bo-Ruei"

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A Multimetal Broadband Metamaterial Perfect Absorber With Compact Dimension
(2016-04-01)
Zhong, Yan Kai
;
Fu, Sze Ming
;
Tu, Ming-Hsiang
;
Chen, Bo-Ruei
;
Lin, Albert
We propose an extremely simple multiple-metal metamaterial perfect absorber (MPA). The dimension of our proposed design is only 221 nm for the visible wavelength range from 400 to 700 nm. This is comparable with past efforts on MPAs using plasmonics at the same wavelength range, whereas the plasmonic excitation is absent in our proposal. A unity broadband absorption can be achieved with ultrathin metallic films. In addition, the wavelength scalability is possible using our design, and the fully planar simple configuration facilitates large-area photonic design without the need for lithography and etching. The physics is the field penetration and the field absorption for the photons at different wavelength ranges using different metallic layers. We also show that the adjustment of the individual layer thickness is critical to attaining a perfect wave impedance matching to vacuum. The titanium (Ti), nickel (Ni), and aluminum (Al) triple-metal configuration is used to demonstrate the concept experimentally, and a close match to the theoretical result is observed. The absorption band can be further widened with more stacking layers with various metals. We believe that the proposed design is very promising in the aspects of simple processing and scalable for large-area broadband unity absorption. It thus improves the future implementation of MPAs and facilitates a wide range of relevant applications.
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.
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.
Omnidirectional, polarization-independent, ultra-broadband metamaterial perfect absorber using field-penetration and reflected-wave-cancellation
(2016-05-16)
Zhong, Yan Kai
;
Lai, Yi-Chun
;
Tu, Ming-Hsiang
;
Chen, Bo-Ruei
;
Fu, Sze Ming
In this work, we present the result of nickel (Ni)-based metamaterial perfect absorbers (MPA) with ultra-broadband close-to-one absorbance. The experimental broadband characteristic is significantly improved over the past effort on metamaterial perfect absorbers. An indepth physical picture and quantitative analysis is presented to reveal the physical origin of its ultrabroadband nature. The key constituent is the cancellation of the reflected wave using ultra-thin, moderate-extinction metallic films. The ultra-thin metal thickness can reduce the reflection as the optical field penetrates through the metallic films. This leads to minimal reflection at each ultra-thin metal layer, and light is penetrating into the Ni/SiO2 stacking. More intuitively, when the layer thickness is much smaller than the photon wavelength, the layer is essentially invisible to the photons. This results in absorption in the metal thin-film through penetration while there is minimal reflection by the metal film. More importantly, the experimental evidence for omni-directionality and polarization-insensitivity are established for the proposed design. Detailed measurement is conducted. Due to the ultrathin metal layers and the satisfactory tolerance in dielectric thickness, the broadband absorption has minimal degradation at oblique incidence. Such a wide angle, polarization-insensitive, ultra-broadband MPA can be very promising in the future, and the optical physics using sub-skin-depth metal film can also facilitate miniaturized high-performance nano-photonic devices. (C) 2016 Optical Society of America.
The External Light Trapping for Perovskite Solar Cells Using Nanoimprinted Polymer Metamaterial Patterns
(2016-01-01)
Lin, Albert
;
Fu, Sze Ming
;
Chen, Bo-Ruei
;
Yan, Sheng-Lun
;
Zhong, Yan Kai
In this work, we develop the process flow for nano-imprinted metamaterial patterns for perovskite solar cell light trapping. Nanoimprint can be scaled for large-area photovoltaics at low cost, compared to conventional lithography techniques. While conventional grating is normally underneath the solar cell active layers, the degraded electrical characteristic can be a problem. On the other hand, the external light trapping separates the electrical and optical designs and, therefore, it is more promising for complex photonic management without sacrificing the diode characteristics. We demonstrate the concept by perovskite solar cells with metamaterial patterns. Imprinting a deeper metamaterial pattern is suggested to enhance the far field effect for further efficiency improvement.
The External Light Trapping Using Down-Conversion Polymer and Diffuse Trench Reflectors
(2016-01-01)
Lin, Albert
;
Han, Hau-Vei
;
Huang, Chien-Yao
;
Chen, Bo-Ruei
;
Fu, Sze Ming
External light trapping has been proposed as an alternative to internal light trapping. The advantage is that the electrical and optical characteristics can be designed separately. This enhances the degree of improvement that nano-photonics can contribute to the solar cell efficiency and J(SC). In this work, we use diffuse trench reflectors with down-conversion polymer to demonstrate the concepts of a low-cost, widely applicable scheme for the external light trap. A >50% enhancement of the J(SC) can be observed with proper designs and configurations. The proposed external light trap can be applied to nearly all thin-film solar cell technologies since the external optical components do not affect the electrical diode characteristic of the solar cells. The efficient external light trap is attributed to the high reflectance of the disuse mirror and its wide-angle diffractions, optical confinement due to the trench reflector, and the additional short-wavelength spectral enhancement by the down-conversion mechanism.