介面梯度超穎材料之電磁波調控研究

Abstract

超穎材料(plasmonic metamaterials)由次波長(sub-wavelength)尺寸的侷域共振結構組成，這些共振結構隨電磁波激發產生電偶(electric dipole)及磁偶極矩(magnetic dipole)共振，並展現一些特殊的電磁波特性如負折射(negative refraction)、完美成像(perfect imaging)等。近年來藉由一種介面梯度超穎材料(graded metamaterial systems)，實現許多新穎的現象如影形斗篷(invisibility cloaking)及侷域彩虹(trapped rainbow)等。近年來N. Yu等人根據Generalized Snell’s law的理論基礎設計了尺寸漸變的奈米天線(optical antenna)振列，並在近紅外光(8微米)波段實現異常(anomalous)穿透與反射的調控效果。X. Ni等人以類似的結構將尺寸縮小使之於波長2微米工作，並具有寬頻(broadband)工作的特性。 Sun 等人透過一種反射式的結構於微波(microwave)波段將入射的傳遞波近乎100%轉為表面波。這系列研究主要是透過不同尺寸的超穎材料在介面上不同位置產生漸變的電磁波相位(phase)，並在遠場干涉(interference)後建構出特定指向的平面波前(wave front)。 在此研究中透過模擬設計介面結構，使結構將正向入射平面波高效率(78%)調控到異常反射角度，並在850nm光頻段工作下具有200nm的寬頻調控效果。此設計結構的調控特性可應用於分光元件(beam splitter)、表面電漿波耦合器(SPP coupler)及光吸收元件(light absorber)等。

Plasmonic metamaterials are artificial composites made by sub-wavelength local resonance structures of electric and/or magnetic type(s) exhibiting novel electromagnetic properties, such as negative refraction, perfect imaging, etc. In last several years, various graded metamaterial systems have brought us new fascinating phenomena such as invisibility cloaking [1], trapped rainbow [2], etc. Recently, N. Yu et. al. showed that a graded optical antenna array could realize anomalous reflection and refraction for light at infra-red (8 micrometer), following a generalized Snell’s law [3], and X. Ni et. al. soon pushed the idea to 2-micrometer wavelength with a relative broad operation bandwidth [4]. Sun et. al. further proved that a particular gradient-index meta-surface can convert a propagating wave to a surface wave with nearly 100% efficiency [5], and demonstrated the idea in microwave frequency regime. The key idea behind this set of works is to utilize the local reflection/refraction phase properties of a gradient metamaterial, so that coherent beams can be formed by constructive interference. In this work, we push the idea to visible frequencies. We designed and fabricated a graded meta-surface working around 850 nm, and demonstrated that an incident beam can be redirected to a non-specular channel after reflection by our system. The measured conversion efficiency from the incident beam to the anomalous reflection one is quite high (up to 78%), and the working bandwidth is very broad (about 200 nm). We believe that our systems can have broad applications including beam splitter, SPP coupler, light absorber, etc.