設計連續譜中的準束縛態應用於超穎介面之三倍頻訊號增益

Abstract

連續譜中的準束縛態(quasi-Bound States in the Continuum, quasi-BIC)是一種特殊的共振態，具備極高的品質因子與強近場等特徵。就目前已被開發於產生quasi-BIC共振的超穎介面而言，大部分需在製程上精細控制結構的不對稱性或需斜向入射才能激發quasi-BIC共振。因此，本研究旨在設計相對容易製作且在正向入射下即可產生quasi-BIC的介電質超穎介面，並藉由模擬及實驗探討其特性並應用於三倍頻訊號之增益。本文設計之結構由週期性的非晶矽(amorphous silicon, a-Si)箏形奈米柱陣列組成，我們透過破壞箏形結構的對稱性以激發quasi-BIC共振，此共振發生在波長1300 nm附近且具Fano共振特徵，主要受磁偶極與電四極模態主導，另有一寬頻磁偶極模態存在於quasi-BIC共振附近；我們探討了改變結構不對稱參數α，對於Fano共振特徵的變化與共振品質因子的關係，以及不同品質因子之quasi-BIC共振對於三倍頻訊號增益波長及強度的影響等。在模擬結果中，該quasi-BIC共振之品質因子可高達四個數量級，且在箏形結構內部可以激發超過160倍的電場強度，並於共振處得到將近四個數量級的三倍頻訊號增益。實驗上，我們使用電子束微影製程製作樣品並量測樣品光譜，發現quasi-BIC共振頻寬相較模擬結果增加很多，這是由於該共振的頻寬對於入射光的角度非常敏感。我們探討一系列具不同結構不對稱參數之樣品的光譜特徵與其三倍頻訊號的波長及強度，其中不對稱參數α最小和最大的兩組樣品，其三倍頻訊號強度差異可達46.4倍，而α最小的樣品在雷射功率P_in約為4.23 mW時，三倍頻的轉換效率可達約1.75 × 10−6，顯示具高共振品質因子之quasi-BIC模態，其強近場分布能有效應用於增益非線性訊號的轉換。

Quasi-Bound States in the Continuum (quasi-BIC) is a resonant mode characterized by ultrahigh quality factor (Q-factor) along with strong near-field distributions. Nanostructures that sustain quasi-BIC resonances usually require precisely control of geometrical asymmetry in sample fabrication or oblique incidence in order to induce the quasi-BIC resonances. Therefore, the objective of this thesis is to design dielectric metasurfaces that are relatively easy to fabricate and can produce quasi-BIC resonances under normal incidence. The properties of quasi-BIC resonances are explored through simulations and experiments and applied for the enhancement of third-harmonic generation (THG).The designed nanostructures are composed of periodic amorphous silicon (a-Si) kite-shaped nanopillar arrays. By breaking the symmetry of the kite-shaped structure, the quasi-BIC resonance is excited in the wavelength range of 1300 nm with a Fano resonant line-shape. The quasi-BIC resonance exhibits a dominant magnetic dipole and electric quadrupole resonant features. In addition, a broadband magnetic dipole (MD) mode exists near the quasi-BIC resonance. We investigated the effect of structural asymmetry to the variations of q factors in Fano profile, the Q-factors of the resonances, as well as the influence of quasi-BIC resonances with different Q-factors on the THG wavelength and intensity. In the simulation results, the Q-factor of the quasi-BIC resonance can be as high as four orders of magnitude, and the electric field inside the kite-shaped structure can be enhanced by more than 160 times. In addition, the TH signal reaches more than four orders of magnitude enhancement at the resonance frequency. The samples were then fabricated by electron-beam lithography process, and the measured spectra shows a severe broadening of quasi-BIC modes compared to simulation results. This is attributed to the resonance bandwidth is highly sensitive to the oblique incident angle. We investigated the optical properties for a series of samples with different structural asymmetry parameters and the effects on the THG wavelength and intensity. Among the five structures we mainly explored, the sample with minimal structural asymmetry shows 46.4-fold THG enhancement compared to the largest one, showing the THG conversion efficiency of 1.75 × 10−6 at the average pump power around 4.23 mW. This demonstrates that the high Q-factor quasi-BIC modes with their strong near-field confinement can effectively achieve more highly efficient nonlinear conversion.