High-$Q$ Quasibound States in the Continuum for Nonlinear Metasurfaces

Open Access

High- $Q$ Quasibound States in the Continuum for Nonlinear Metasurfaces

Zhuojun Liu, Yi Xu, Ye Lin, Jin Xiang, Tianhua Feng, Qitao Cao, Juntao Li, Sheng Lan, and Jin Liu

Phys. Rev. Lett. 123, 253901 – Published 17 December 2019

Abstract

Sharp electromagnetic resonances play an essential role in physics in general and optics in particular. The last decades have witnessed the successful developments of high-quality ( $Q$ ) resonances in microcavities operating below the light line, which however is fundamentally challenging to access from free space. Alternatively, metasurface-based bound states in the continuum (BICs) offer a complementary solution of creating high- $Q$ resonances in devices operating above the light line, yet the experimentally demonstrated $Q$ factors under normal excitations are still limited. Here, we present the realizations of quasi-BIC under normal excitation with a record $Q$ factor up to 18 511 by engineering the symmetry properties and the number of the unit cells in all-dielectric metasurface platforms. The high- $Q$ quasi-BICs exhibit exceptionally high conversion efficiency for the third harmonic generation and even enable the second harmonic generation in Si metasurfaces. Such ultrasharp resonances achieved in this work may immediately boost the performances of BICs in a plethora of fundamental research and device applications, e.g., cavity QED, biosensing, nanolasing, and quantum light generations.

Received 25 July 2019
Revised 5 September 2019

DOI:https://doi.org/10.1103/PhysRevLett.123.253901

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Light-matter interaction Nonlinear optics

Semiconductor microcavities

Optical second-harmonic generation Polarized optical microscopy

Atomic, Molecular & Optical

Authors & Affiliations

Zhuojun Liu¹, Yi Xu^2,*, Ye Lin², Jin Xiang³, Tianhua Feng², Qitao Cao⁴, Juntao Li¹, Sheng Lan³, and Jin Liu^1,†

¹State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics, Sun Yat-sen University, Guangzhou 510275, China
²Department of Electronic Engineering, College of Information Science and Technology, Jinan University, Guangzhou 510632, China
³School of Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou 510006, China
⁴State Key Laboratory for Artificial Microstructures and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China

^*yi.xu@osamember.org
^†liujin23@mail.sysu.edu.cn

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Issue

Vol. 123, Iss. 25 — 20 December 2019

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Images

Figure 1
Transition from BICs to quasi-BIC. (a) Periodic arrangement of Mie resonators supporting BICs ( $W = 400$ , $H = 500$ , and $P = 720 nm$ ). (b) Band structure related to the MD BIC mode. Inset: the electromagnetic field distribution of the BIC mode in a single unit, where the color code presents the amplitude of the magnetic field, the blue arrows indicate the in-plane electric field vectors, and the white square outlines the structure. (c) $N \times N$ Si blocks with symmetric defects that supports quasi-BICs. (d) Dependence of the $Q_{r}$ of quasi-BIC eigenmodes (infinite size) on the degrees of in-plane symmetry breaking for two types of symmetry breaking methods. (e) The transmission spectra of $9 \times 9$ Si blocks with symmetric defects excited by normally incident $x − / y$ -polarized plane waves from the $z$ direction ( $S_{1} = 60$ and $S_{2} = 160 nm$ ). The electromagnetic field distributions of the quasi-BIC and the one in the center unit cell at the resonance are shown by the insets.
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Figure 2
Engineering the $Q_{r}$ and resonance wavelengths of the quasi-BIC in all-dielectric metasurfaces. (a) The calculated transmission spectra of $21 \times 21$ Si blocks with different defect sizes. (b) $Q_{r}$ extracted from (a) as a function of the defect size. (c) The calculated transmission spectra for Si metasurfaces ( $S_{1} = 60$ and $S_{2} = 160 nm$ ) with different array sizes. (d) $Q_{r}$ extracted from (c) as a function of the array size.
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Figure 3
Experimental demonstration of high- $Q$ quasi-BICs in Si metasurfaces. (a) SEM image of the fabricated device. (b) Transmission spectra of metasurfaces ( $21 \times 21$ Si blocks array) with different defect sizes in the unit cell. (c) The extracted $Q$ factors from (b) as a function of the defect size. Inset: the calculated $Q_{nr}$ for the devices. (d) Representative transmission spectra of Si metasurfaces with different array sizes. Here the targeted defect size is $S_{1} = 65 nm$ . (e) Statistics of measured $Q$ factors for all the fabricated Si metasurfaces with different array sizes.
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Figure 4
THG/SHG generations in all-dielectric metasurfaces. Comparison of THG (a) and SHG (b) in all-dielectric metasurfacs and a reference Si thin film with the same thickness as the metasurfaces. (c) Dependence of THG/SHG on the polarization of the pumping laser. (d) The polar plots of the THG/SHG signals (fixing the pumping at $y$ polarization). (e) Power dependences of SHG/THG in logarithmic scale, showing quadratic and cubic power scalings, respectively. (f) Dependences of THG/SHG on the pumping wavelength.
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