Süperhidrofobik yüzey üzerinde duran sıvı mikrodamlaların inceltilmiş optik fiber ile ultrayüksek çözünürlükte optik spekstroskopisi

Optofluidics is a new research field that exploits unique properties of fluids for creating optical components and systems. Optofluidic devices combined with accurate control of liquids on small spatial scale via microfluidic technologies pave way for improved chemical and biological functionality in lab-on-a-chip analytical and preparative systems. Main advantages of such integrated systems are their compactness and direct compatibility with biological specimens. Examples of applications of integrated optofluidics include optical communication components, organic dye-based laser light sources, or biological sensing. Microdroplets, with their spherical gemetry and exceptionally smooth surfaces are ideally suited 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 also been demonstrated at low pump threshold powers. Apart from their significance for the fundamental physics research, microdroplets are well suited to another important line of applications that benefit from the sensitivity of the WGMs to the microdroplet size as well as 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-space coupling of light into the surface-supported droplets. Fiber tapers allow selective phasematched excitation of individual WGMs with a spectral resolution limited only by the linewidth 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. Single tapered optical fibers were used to couple the light from independent heating and probe lasers into individual microdroplets that were kept on a superhydrophobic surface in a highhumidity chamber. Size stabilization of microdroplets resulted from competition between resonant absorption of the infrared heating laser by a microdroplet whispering gallery mode and water condensation in the sample chamber. Microdroplet size was continuously monitored using the tunable red probe laser. Thanks to the narrow linewidth of the heating laser, 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.