垂直側壁孔洞狀寬頻超穎材料完美吸收器

Abstract

此篇論文提出新型結構的雙曲性超穎材料吸收器(hyperbolic metamaterial absorber)，在本論文中將介紹其結構的設計原理、運作機制以及實際樣本的製作流程與其結果，經量測實驗樣本在特定波段其吸收效率可達約90%之吸收效率。近幾年內，超穎材料完美吸收器進展十分迅速，其中梯形堆疊結構超穎吸收器可提供寬頻且高吸收的效果。然而，梯形堆疊結構超穎吸收器的製作流程複雜且相當耗時，較不適合應用於大面積吸收器製程的情況，例如太陽能熱光電電池(thermophotovoltaics)之接收器，故此篇論文著重於垂直側壁結構的超穎吸收器，製作流程簡單且耗時更短，較適用於大面積製程與量產的情況。除此之外，此結構設計可應用的波段很廣，從光學黑洞(optical black hole)的操作波段至微波完美吸收器(microwave perfect absorber)的操作波段皆可適用此類型的設計。雙曲性超穎材料吸收器吸收的物理機制在於垂直側壁孔洞結構，此結構經由共振將電磁波侷限於結構內，藉此達到寬頻且高吸收的效果。在大多數的情況下近導模態(quasi-guided mode)侷限能力較弱，較難以達成寬頻的條件，而超穎材料完美吸收器的孔洞狀結構可以達成寬頻共振效果且能維持合理震盪強度以維持高吸收效率，能達成此效果的原因是由於垂直側壁的鋁－二氧化矽(Al-SiO2)堆疊結構產生雙曲性的色散曲線(dispersion curve)，進而導致結構內的光子能量密度(photonic density of state)顯著增加而提升吸收效率。

A new structure for a nearly-perfect hyperbolic meta-material(HMM) absorber is proposed, and initial experimental verification is provided. To date, HMM PMAs are realized using tapered stacks that can provide adiabatic waveguiding over a wide spectral range. Nevertheless, the tapered nature can prevent its usage for large-area applications such as the emitters in thermophotovoltaics (TPV). The design proposed here has decent wavelength scalability and can be used from optical black holes to microwave perfect absorbers. The physics behind the HMM straight-sidewall cavity is the broadband highly confined resonance. While, in most of the cases, the broadband quasi-guided modes are weekly confined in nature, the HMM cavity can provide broadband resonances but still maintain reasonably strong oscillation strength for high absorption. This is because the photonic density of state (PDOS) is boosted dramatically by the hyperbolic dispersion of the straight-sidewall Al/SiO2 stacks.