Mr. Zhongyang Li Profile

Zhongyang Li

PhD student

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Proceedings Article | 5 October 2015 Presentation

Large-area lithography-free perfect absorbers, color filters, and photodetectors at visible frequencies using ultra-thin silver or amorphous silicon films (Presentation Recording)

Zhongyang Li, Serkan Butun, Koray Aydin

Proceedings Volume 9546, 95461P (2015) https://doi.org/10.1117/12.2187139

KEYWORDS: Optical filters, Photodetectors, Plasmonics, Silver, Amorphous silicon, Visible radiation, Silicon films, Nanofabrication, Fabry–Perot interferometers, Optoelectronic devices

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Plasmonic materials and metamaterials have been widely utilized to achieve spectral transmission, reflection and absorption filters based on localized or delocalized resonances arising from the interaction of photons with nanoscale patterns. However, the realization of visible-frequency, high-performance, large-area, optical filters based on nanoplasmonic materials is rather challenging due to nanofabrication related problems (cost, fabrication imperfection, surface roughness) and optical losses of metals. Here, we propose and demonstrate large-area perfect absorbers and transmission color filters and photodectors that could overcome the difficulties associated with nanofabrication using a lithography-free approach. Our resonant flat optical design is based on a modified, asymmetric metal-insulator/semiconductor-metal (MI/SM) based Fabry-Perot cavity incorporated with plasmonic, lossy ultra-thin (~ 30 nm) Ag or (~ 5-15 nm) amorphous Si films. We demonstrated a narrow bandwidth (~17 nm) super absorber with 97% maximum absorption with a performance comparable to nanostructure/nanoparticle-based super absorbers. We also investigated transmission filters in which different colors can be obtained by controlling the spacer thickness of silicon dioxide or amorphous silicon. With measured performance of transmission peak intensity reaching 60% and a narrow-band of ~ 40 nm, our color filters exceed the performance of widely studied plasmonic nanohole array based color filters and make a good candidate for large-area narrow-band photodetection devices. Such plasmonic loss incorporated Fabry-Perot cavities using ultra-thin metallic or semiconductor films could suggest active and practical applications in spectrally selective optical (color and absorber) filters, optoelectronic devices with controlled bandwidth such as narrow-band photodetectors, and light-emitting devices.

Proceedings Article | 5 October 2015 Presentation

Visible-frequency metasurfaces for broadband anomalous reflection and high-efficiency spectrum splitting (Presentation Recording)

Zhongyang Li, Edgar Palacios, Serkan Bütün, Koray Aydin

Proceedings Volume 9544, 954428 (2015) https://doi.org/10.1117/12.2187115

KEYWORDS: Polarization, Optical resonators, Visible radiation, Plasmonics, Wavefronts, Electromagnetic radiation, Holograms, Resonators, Antennas, Diffraction

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Metasurfaces offer new degrees of freedom in moulding the optical wavefronts by introducing abrupt and drastic changes in the amplitude, phase and/or polarization of electromagnetic radiation at the wavelength scale. By carefully arranging multiple subwavelength anisotropic or gradient optical resonators, metasurfaces have been shown to enable anomalous transmission, anomalous reflection, optical holograms and spin-orbit interaction. However, experimental realization of high-performance metasurfaces that can operate at visible frequency range has been a significant challenge due to high optical losses of plasmonic materials and difficulties in fabricating several subwavelength plasmonic resonators with high uniformity. Here, we propose a highly-efficient yet a simple metasurface design comprising of a single, anisotropic trapezoid-shape antenna in its unit cell. We demonstrate broadband (450 - 850 nm) anomalous reflection and spectrum splitting at visible and near-IR frequencies with 85% conversion efficiency. Average power ratio of anomalous reflection to the strongest diffraction mode was calculated to be on the order of 1000 and measured to be on the order of 10. The anomalous reflected photons have been visualized using a CCD camera, and broadband spectrum splitting performance has been confirmed experimentally using a free space, angle-resolved reflection measurement setup. Metasurface design proposed in this study is a clear departure from conventional metasurfaces utilizing multiple, anisotropic and/or gradient optical resonators, and could enable high-efficiency, broadband metasurfaces for achieving flat high SNR optical spectrometers, polarization beam splitters, directional emitters and spectrum splitting surfaces for photovoltaics.

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