Ultra-broadband reflectors covering the entire visible regime based on cascaded high-index-contrast gratings
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Ultra‑broadband reflectors covering the entire visible regime based on cascaded high‑index‑contrast gratings Xingzhe Shi1,2 · Yuanfu Lu2 · Changshui Chen1 · Songhao Liu1 · Guangyuan Li2 Received: 6 May 2020 / Accepted: 12 October 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract We report the design of ultra-broadband, highly reflective all-dielectric reflectors covering the entire visible regime based on two cascaded subwavelength high-index-contrast gratings (HCGs). We find that the spectral distance between the two gratings’ reflective bandwidths, which should be appropriately designed in order to extend the overall bandwidth for high reflectivity, is analogous to the well-known Rayleigh, Abbe and Sparrow criteria for resolution limits. Results illustrated with TM-polarized normal incidence show that high reflectivity above 98.5% covering 400–800 nm can be achieved for two cascaded HCGs with an appropriate spectral distance. The effects of key structural parameters on the bandwidth extension are discussed with physical insights. We expect this work will advance the engineering and applications of HCGs as ultrathin, ultra-broadband and all-dielectric reflectors.
1 Introduction Broadband reflectors are essential components in diverse optical devices such as tunable Fabry-Pérot etalon filters [1, 2] and wavelength tunable vertical-cavity surface-emitting lasers (VCSELs) [3–5]. Although metallic mirrors are broadband and have simplicity in fabrication (only one-layer deposition), these suffer from much higher loss compared with dielectric reflectors. Distributed Bragg reflectors (DBRs) composed of multiple alternating high- and low-index dielectric layers are lossless but require usually more than 15 layers to be effective in a wideband [6]. This complicate fabrication procedure adds time and cost. Alternatively, highindex contrast gratings (HCGs), which consist of periodic high-index bars surrounded by a low-index environment, are attractive due to the ultra-small thickness (less than one-third
* Guangyuan Li [email protected] 1
Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices, School of Information Optoelectronic Science and Engineering, South China Normal University, Guangzhou 510006, China
CAS Key Laboratory of Human‑Machine Intelligence‑Synergy Systems, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China
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of the wavelength), high reflectivity over a broad bandwidth, and local phase and polarization control [7–9]. Since Mateus et al. proposed [10] and demonstrated [11] HCG-based broadband reflectors in 2004, great progress has been achieved on clarifying the underlying physics [12–19], novel designs using new techniques [20–22], and expanding the promising applications [3–5, 23–28]. However, to date most of HCG-based broadband reflectors have been designed for the near-infrared regime, whereas those designed for the visible regime have showed limited bandwidth. For example, Hashemi et al. [29] d
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