- 學位論文
利用光介電泳力操控聚苯乙烯微球作為可調式光學共振腔
Tunable Optical Resonators by Manipulating Polystyrene Microspheres through Image-Controlled Dielectrophoretic Force
摘要
光學微球型共振腔因具有高品質因子與小模場體積等特點，被廣泛地運用在光通訊、非線性光學、染料雷射和生物感測等領域。光在球型共振腔的共振模態WGM(Whispering-gallery Modes)一般是透過耦光元件消逝場耦合(Evanescent field coupling)而激發，如何調控共振腔與耦光元件之間的耦合距離是極為重要的課題。 本研究將微流體系統與光學系統加以整合，提出了一個可調式光學共振腔的構想，利用投影機的光誘發光介電泳力(optically-induced dielectrophoresis)操控微米球共振腔，微調共振腔與單模脊狀波導之間的距離。我們在光電泳晶片內滴入混有聚苯乙烯球(n=1.571)的蔗糖水溶液(n=1.399)，並以SU-8負光阻(n=1.569)製作波導結構於晶片上。經由實驗測試發現，我們可以操控聚苯乙烯球並控制與光波導維持一穩定的距離；經量測光波導的穿透頻譜，藉由頻譜上峰值的深度與半高寬，可將聚苯乙烯球與光波導操作在過耦合(over-coupling)，近臨界耦合(near critical-coupling)，和弱耦合(under-coupling condition)等三種情況。 本研究成功地利用光介電泳晶片操控聚苯乙烯微米球，作為一可調式的光學共振腔，此元件最顯著的特點為可藉由一低功率的投影機，控制光學共振腔的耦合條件。
並列摘要
Optical microsphere resonators are widely applied in various fields including, optical communications, researches on nonlinear optical effect, dye lasers, and label free detection for biosensing , due to the high quality factor and small mode volume. However, to effectively excite the microsphere resonator is challenging since the external light should be coupled to the resonant modes, or called whispering gallery modes (WGM), with a precisely controlled distance. In this study, a new integrated microsphere resonator optofluidic device is presented. The microsphere resonators are manipulated by optically-induced dielectrophoresis(ODEP) for precisely tuning the coupling distance between the resonator and a single mode rib waveguide. We fabricate single mode rib waveguide structure made by SU-8(n=1.569) on the ODEP device, with polystyrene beads(PSB, n=1.571) of 100-μm diameter were suspended in a liquid chamber with high-density sucrose solution (n=1.397). We have manipulated PSB which is pushed away and keeps a stable distance from the waveguide. Through measuring transmittance spectra at different coupling distance, PSB is operated at the over-coupling, near critical-coupling and under-coupling condition, respectively, by examining the transmission dips of the spectra. We have successfully realized a compact optofluidic platform for studying tunable microsphere optical resonators in an aqueous medium. Asalient feature of this platform is that the microsphere can be freely operated in any of the coupling conditions via a low-power image projector.