Ultrahigh resolution optical spectroscopy of liquid microdroplets using tapered optical fiber waveguides

Abstract

Optofluidik, optik bileşenleri ve sistemleri oluşturmak için akışkanların eşsiz ozelliklerini kullanan yeni bir araştırma alanıdır. Optofluidik cihazlar analitik entegre sistemlerin kimyasal ve biyolojik özellikleri incelenebilir. Entegre sistemlerin en büyük özelliği karmaşık olmamaları ve biyolojik örneklerle uyumlu olmalarıdır. Optik iletişim bileşenleri, organik boya bazlı lazer ışık kaynakları ya da biyolojik algılama entegre opto-akışkan uygulamaların örneklerindendir. Küresel simetrileriye ve yüzey pürüzsüzlüğüne sahip mikrodamlalar optofluidik uygulamaları için uygundur. Mikrodamlacıkların optofluidik uygulamaları için pozisyonlarının kontrollü bir şekilde sabitlenmesi gerekmektedir. Bu tezde damlacıkların pozisyonlarının sabitlenmesi için su tutmayan yüzeyler kullanılmıştır. Tezin ilk kısmında, titreştirilen süperhidrofobik bir yüzey üzerinde bulunan mikrometre ölçekli bireysel sıvı damlalarının mekanik rezonanslarının analizine bağlı olarak mikroskopik kontak açılarının ölçümü için yeni bir metot bulunmuştur. Bu amaçla, bu çalışmada, damlacığın mekanik rezonanslarınınve büyüklüklerinin eşzamanlı olarak belirlenebilmesi için ışıyan boya içeren damlacıklarınlazer ışınıyla uyarılmasıyla elde edilen fısıldayan galeri kiplerinin (FGM) optik spektroskopisi kullanılmıştır. Damlacıklar, optik mikrokovuk rolü üstlenmişlerdir ve damlacıkların ışıma spektrumu, spektral genişlikleri damlacığın titreşim genliklerine bağlı olan fısıldayan galeri kiplerini karakterize eder. Fısıldayan galeri kipinin genişliğini, farklı büyüklüklü damlaların altlık titreşim frekanslarının bir fonksiyonu olarak izleyerek, damlacıkların maksimal fısıldayan galeri kiplerinin genişlemesinin olduğu frekanslardaki rezonans frekansları belirlenmiştir.Ardından, damlacık büyüklüklüğü, Lorenz-Mie saçılma teorisi kullanılmasıyla fısıldayan galeri kiplerin kip-eşleştirmesi ile belirlenmiştir. Verilen bir damlacık için, kontak açısı, yüzeyde duran damlacığın aksisimektrik salınımları için geliştirilen teori kullanılarak, damlacık mekanik rezonans frekansının ve büyüklüklüğün bağımsız ölçümlerinden elde edilmiştir. Tezin ikinci kısmında ise inceltilmiş optik fiber kullanılmıştır. İnceltilmiş optik fiber ile damlacıkların FGMlerinin kalite faktörleri ölçülmüştür. Daha sonra ikinci bir lazer kullanarak damlacıkların hacmi sabitlenmiş ve FGMleri kontrol edilebilmiştir.

Optofluidics is a new research field that exploits unique properties of fluids for creatingoptical components and systems. Optofluidic devices combined with accurate control ofliquids on small spatial scale via microfluidic technologies pave way for improved chemicaland biological functionality in lab-on-a-chip analytical and preparative systems. Mainadvantages of such integrated systems are their compactness and direct compatibility withbiological specimens. Examples of applications of integrated optofluidics include opticalcommunication components, organic dye-based laser light sources, or biological sensing.Microdroplets, with their spherical gemetry and exceptionally smooth surfaces are ideallysuited for applications in optofluidics. It is well known that spherical dielectric microobjects can act as optical resonators hosting so-called whispering gallery modes (WGMs) - optical resonances that possess very high quality factors while being confined to relatively small volumes near the microobject?s rim. Consequently, very high optical field density can be achieved in microdroplets that can have already found many applications in quantum and nonlinear optics. Fundamental studies in cavity quantum electrodynamics have led to the demonstration of manipulation of the spontaneous emission rate of dye molecules.Dye lasing, stimulated Raman scattering, and Raman lasing in microdroplets have alsobeen demonstrated at low pump threshold powers. Apart from their significance for thefundamental physics research, microdroplets are well suited to another important line ofapplications that benefit from the sensitivity of the WGMs to the microdroplet size as wellas the refractive indices of the inner liquid and the outer medium. These properties naturally make microdroplets very attractive in biological and chemical sensing, and in developing optical devices that necessitate a tunable optical microcavity, e.g. tunable optical switches or light sources. In order to utilize microdroplets in optofluidic applications, one must be able to stabilize their position in a controlled way. In this thesis, superhydrophobic surfaces were used to stabilize droplet position. Surface-supported microdroplets can host high-quality WGMs and thus, they represent an ideal model system for studying fundamental optical properties and applications of liquid-based optical resonant cavities. The thesis is organized as follows..In the first part, we introduce a novel method for the microscopic contact angle measurement that is based on the analysis of mechanical resonances of individual micrometer-sized liquid droplets supported by a vibrated superhydrophobic surface. Fluorescence spectra of the dye-doped droplets excited by laser light feature whispering gallery modes (WGMs) whose spectral widths depend on the droplet vibration amplitude, thus enabling precise measurements of the droplet mechanical resonant frequency. Following droplet size determination by WGM mode-matching, we calculate the contact angles from the dependence of the measured mechanical resonant frequency on the droplet size for two surfaces with different superhydrophobicity levels, and a good correlation with the values measured by direct imaging of millimeter-sized droplets is found. In addition to that oscillatory deformations of micrometer-sized NaCl-water droplets by an AC electric field are used for contact angle measurements on superhydrophobic surfaces.In the second part, tapered optical fiber waveguide coupling is used instead of free-spacecoupling of light into the surface-supported droplets. Fiber tapers allow selective phasematched excitation of individual WGMs with a spectral resolution limited only by thelinewidth of the laser used. We measured ultrahigh quality factors (Q-factors) of the optical whispering-gallery modes excited via a tapered optical-fiber waveguide in single glycerolwater microdroplets standing on a superhydrophobic surface in air. Owing to the high contact angle of glycerol-water mixture on the superhydrophobic surface (> 155?), droplets with the geometry of a truncated sphere minimally distorted by gravity and contact line pinning effects could be generated. Q-factors up to 2.3×106 were observed for such droplets with radii of 100?200 ?m exposed to the ambient atmosphere in a closed chamber with controlled relative humidity. Placement of microdroplets in a constant humidity environment permitted prolonged characterization of Q-factors of individual microdroplets. We found that the Q-factors of liquid droplets in air were stable over more than an hour and their measured values were limited mostly by the thermally-induced droplet shape fluctuations.In the last part, we demonstrate long-term stabilization of the size of liquid optical microcavities based on surface-supported salt-water microdroplets surrounded by air. Singletapered optical fibers were used to couple the light from independent heating and probelasers into individual microdroplets that were kept on a superhydrophobic surface in a highhumidity chamber. Size stabilization of microdroplets resulted from competition betweenresonant absorption of the infrared heating laser by a microdroplet whispering gallery modeand water condensation in the sample chamber. Microdroplet size was continuously monitoredusing the tunable red probe laser. Thanks to the narrow linewidth of the heatinglaser, stabilization of the 110 ?m radius of a microdroplet with a precision down to 0.54 nm was achieved for a period of 410 s. Additionally, we demonstrate controllable tuning of size of self-stabilized surface-supported liquid aerosols using a tunable IR heating laser which has a very narrow linewidth.