一維含液晶層之準電漿子晶體

Abstract

由於表面模態的固有性質、外部特性與展望性的應用，使得近年來相關研究日趨熱絡。本研究提出一維準電漿子晶體，因具非對稱型含液晶層結構，本質上具備了電控可調與吸收光譜頻率選擇的特性；其組成係由一四分之一波長的金屬膜夾在兩八分之一波長的介電層間構成一個單位晶格，且在液晶層的左右分別設計一個週期與五個週期的單位晶格，以形成本研究準電漿子晶體的結構。在這樣的設計下，於可見光與近紅外波段的光譜上將出現數根高強度的吸收波峰，根據時域有限差分（FDTD）模擬軟體的分析，數根高強度的吸收波峰源自於液晶層中的共振腔模態（resonant modes）與介電金屬膜間的表面模態（surface modes）。此外，當施予一外加電壓於液晶層時，吸收波峰呈現藍移趨勢；特別的是，在特定電壓下藍移趨勢消失取而代為紅移，此現象稱之為交叉過度區（crossing avoidance）。盼本研究的拋磚能將相關結構應用在電漿子共振的生醫感測元件和光濾波器上。

Surface modes are a fascinating type of modes that are confined at a single boundary between two different media. Due to their intrinsic properties and extrinsic effects as well as their prospects for important applications, continuously renewed interest in surface modes has been sparked in recent years. In this study, we propose an electrically tunable and spectral absorption-frequency-selective device on the basis of one-dimensional quasi-plasmonic crystals containing a liquid-crystal (LC) defect layer. The structure is composed of two distinct metallo (M)-dielectric (D) multilayers sandwiching a 4.65-m-thick and planar-alignment nematic LC (HDN) layer. While one multiplayer has a (DMD)1 configuration, the other is configured as (DMD)5, where each transparent dielectric layer is Al2O3 of eighth wavelength in thickness and each metallic layer is quarter-wavelength-thick Ag. Sharp reflection peaks resembling a comb are present in the visible and near-infrared ranges in the spectrum which are originated from cavity modes and surface modes that are tentatively identified to be Dyakonov surface modes. The wavelengths of the reflection and absorption peaks can be tuned by applying an electric field to the LC defect layer, yielding a blueshift of up to ~70 nm in simulation and 35 nm in experiment as the applied voltage increases along with the spectacular phenomenon-anticrossing. This research is expected to bring about prospective applications in resonant cavities, optical filters, and plasmon–resonance sensing for physical, chemical and biological agents.