用於波長可切換雷射之電控共軛光子能隙結構

Abstract

本論文提出以共軛型光子能隙結構作為液晶雷射之應用。共軛型光子晶體是將膽固醇液晶與染料，注入於非完全週期性之一維光子晶體作為缺陷層的複合式結構。在能隙邊緣或光子能隙內之缺陷峰，因光子長時間駐留及光波局域化而易受激放射，進而產生雷射。實驗上刻意將膽固醇液晶之布拉格反射頻帶設計於光子晶體能隙的外側，觀察並操控其缺陷模態雷射與能隙邊緣型雷射之輻射輸出特性。根據研究結果顯示，藉由施加電壓改變膽固醇液晶分子排列狀態，可以在能隙邊緣型雷射與缺陷模態雷射間切換，達到個別或同時激發兩種模態雷射；且因膽固醇液晶具有雙穩態的特性，故可在非常節能的情況下運作。除此之外，本研究不僅呈現雷射波長在寬頻帶快速切換的方法，更利用此機制提出一個簡單的方式，示範出可同時激發紅、綠、藍雷射光而輸出一離散型白光雷射光源；這項新穎的複合裝置與超連續光相比，具有低成本與體積小之優勢，使得這項技術具備取代超連續白光雷射的潛力。

This thesis proposes a conjugated photonic bandgap structure as an application of liquid crystal laser. The conjugated photonic crystal (CPC) is a hybrid structure which made of a cholesteric liquid crystal (CLC) defect layer sandwiched by two non-periodic photonic crystals (PCs). At the Bragg reflection bandedges or the defect modes inside the photonic bandgap, stimulated emission, i.e. laser, inclines to occur because of the long dwell time of photons and the localization of light. The Bragg reflection band of the CLC was intentionally located beside the bandgap of the one-dimensional PC in order to observe and manipulate the defect- and the bandedge-mode lasing characteristics. Maneuvering the molecular orientation of the CLC through applying voltage allows the lasing mode to be switched between bandedge- and defect-mode, either of each mode or simultaneously dual-mode are applicable. Thanks to its bistable nature of CLCs, such devices can be operated in quite an energy-saving mode. Besides, these demonstrations not only show the methods of switching laser wavelength in a broad range but also provide a facile way to produce a discrete white-light laser source by stimulating red, green, and blue laser coincidently. Compared with the supercontinuum, such a novel hybrid device has advantage of low-cost and miniaturization generation, permitting this technology to hold promise in replacing the supercontinuum white light laser.