Subwavelength grating wideband reflectors with tapered sidewall profile
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Subwavelength grating wideband reflectors with tapered sidewall profile W. Yu1, D. Wu1, X. Duan2, and Y. Yi1, 2* 1 University of Michigan, MI 2 Massachusetts Institute of Technology, Cambridge, MA *e-mail: [email protected]
Abstract One main difference between practical device and ideal design for subwavelength grating structure is the tapered sidewall profile of grating, which is normally obtained by the practical CMOS-compatible fabrication and etching process. Our work has investigated the impacts of tapered sidewall profile on the subwavelength grating wideband reflector characteristics. Both zero-contrast gratings (ZCG) and high- contrast gratings (HCG) are numerically investigated in detail and the results show a distinct differences of the impacts of tapered sidewall profile of grating. The simulation results reveal that this factor play a critical role in determining the reflection bandwidth, average reflectance, and the band edge. Our study has potential in guiding the utilization of subwavelength grating wideband reflector on application of a variety of nanophotonic devices and their integration, as well as to facilitate the design of the fabrication process on the control of tapered sidewall profile.
Introduction The periodic subwavelength grating structure has been widely studied in the field integrated nanophotonics. Zero-contrast gratings (ZCG) based on guided-mode resonance (GMR) structures [1,2] and high-contrast grating (HCG)[3,4], were investigated as wideband reflectors. These types of nano structures have potential in many applications including light filters [5], optical sensing [6,7], and photodetectors [8], offering flexible tunability of resonance wavelength and high quality factor. In recent years, nanorod arrays (NRAs) with high aspect ratio attracted much attention for its potential applications such as antireflection, self-cleaning and superhydrophobicity [9-12]. This structure could be fabricated by various template-assisted etching fabrication techniques. Dry etching technique is one most favorable method due to its excellent controllability and compatibility with the CMOS technology. Transverse etching in most dry etching processes normally gives tapered sidewall profile of gratings, instead of straightly vertical gratings. To the best of our knowledge, most of the previous works focus on a vertical sidewall profile. In this paper, we have performed numerical studies utilizing the subwavelength grating structures with tapered sidewall profile on the glass substrate to study how the wideband reflectance will be significantly impacted by the tapered grating sidewall profile. Finite difference time domain (FDTD) simulation is performed to investigate the effects on reflectivity, reflection bandwidth, and band edge due to the tapered sidewall profile. Tolerance to the degree of taper and future applications of these tapered subwavelength nanoscale grating structures are also discussed based on our simulation results.
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