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Photonic/plasmonic metamaterials and metasurfaces
Photonic/plasmonic metamaterials and metasurfaces
This research topic includes
This research topic includes
Design, fabricate, and characterize THz anti-reflection coatings
Design, fabricate, and characterize waveguide mode converters
Design broadband and low-loss all-dielectric mirrors made by
Bragg structures
High-contrast subwavelength gratings
Dielectric post-array/hole-array metamaterials
Design, fabricate, and characterize all-dielectric and hollow-core THz fibers/waveguides, including
Bragg fibers (with Bragg structures)
Ultra-lightweight fiber made by metamaterials (i.e., metasurface fiber with high-contrast subwavelength gratings)
Parallel-plate waveguides (with Bragg structures)
Analyze characteristics of fiber modes and modal coupling techniques
Design functional lenses and reflective antenna (with post-array metasurfaces)
Design and analyze cavity-enhanced photodetectors (with post-array metasurfaces + Bragg structures)
Design and analyze metamaterial perfect absorbers (with post-/hole-array metasurfaces)
From 2013, we cooperated with Prof. Tsing-Hua Her (University of North Carolina at Charlotte, USA) to study a variety of THz/optical photonic devices. In 2013-2014, we designed a low-loss small-hollow-core THz TE01 Bragg fibers whose wall is composed of alternating relatively high-index HDPE layers and low-index air layers. The lateral Bragg photonic bandgap strongly confines the field inside the hollow core, allowing low-loss propagation even at near-cutoff operation. The high-efficiency coupling method of Bragg TE01 mode was also proposed and analyzed by the mode matching method. This idea has been published in “Optics Express” (Opt. Express 23, 27266 (2015)). Besides, combing the ideas of all-dielectric Bragg fiber and functional nanocomposites we developed in NTHU during 2015-2022, I have successfully applied the assistant research scholar from Ministry of Science and Technology (MOST) of Taiwan and got a 3-year research project (111-2112-M-007 for 2023-2025).
Since Bragg photonic bandgap usually requires multiple periods arranging along the propagation direction of wave, the system is usually bulky and hard to fabricate. In 2016, We and Prof. Tsing-Hua Her initialize the studies of subwavelength high-contrast gratings (HCGs), which is a surface-relief structure with high-index and low-index bars periodically rearranged in the lateral direction. Alternatively, a HCG can be regarded as a waveguide array that can support a set of waveguide-array modes. By properly controlling the interference between the excited grating modes, multiple extraordinary scattering phenomena can be realized, such as broadband total reflection, total transmission or high-Q Fano resonance. Through the dual-mode destructive interference during their multiple-reflection process withing the grating, a very lightweight HCG mirror can be realized. On the other hand, the constructive interference of two on-resonance waveguide-array modes can generate Fano resonance, whose ultra-high-Q feature can be employed for ultra-sensitive modulation and biomedical sensing. In 2020-2021, we and Prof. Tsing-Hua Her derived an analytical formula based on multimode Airy analysis to evaluate the tuning sensitivity of a high-Q HCG. The result has been published in “Optics Letters” (Opt. Lett. 46, 721 (2020)).
During 2017-2021, we also cooperated with Prof. Tsing-Hua Her to develop a series of high-performance photodetectors based on metamaterials/metasurfaces. Firstly, in 2017-2018, a half-confocal meta-cavity composed of a focusing mirror and a flat Bragg reflector was proposed. The ultra-thin focusing mirror can be realized by the aforementioned high-contrast grating (for 1D focusing) or silicon post array on silica substrate (for 2D focusing). The position of the cavity standing wave with maximum field strength was designed to coincide with the focus of the mirror. The absorber (mercury cadmium telluride, MCT) of the photodetector was placed at such focus, so that the local field and the corresponding absorption can be drastically enhanced by both the FP resonance and the focusing effect. Furthermore, the tight focusing can make a great reduction of total absorption volume (spot size ultra-thin MCT thickness), which can significantly reduce the dark noise (by 110 times) and increase the signal-to-noise ratio (by 40 times) as compared to the bulk devices. In 2018-2021, we kept developing a variety of perfect absorbers based on different metasurfaces for compact photodetectors with ultra-low dark noise, such as (1) cadmium telluride (CT) post array (for field enhancement) + thin MCT absorption layer on CT substrate, (2) germanium hole array (for field enhancement) + thin AlN absorption layer on TiN metallic substrate, and (3) silicon hole array + SiC absorption substrate. Those researching works were supported by USA National Science Foundation (NSF) I/U CRC Center for Materials (NSF awards 1624572).
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