Spectral tuning of liquid microdroplets standing on a superhydrophobic surface using electrowetting (original) (raw)
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Applied Physics …, 2007
We demonstrate large spectral tuning of glycerol/water microdroplets standing on a superhydrophobic surface using the optical scattering force exerted by a 1064 nm Nd 3+ :YVO 4 solid-state laser. Spectral tuning up to 30 nm is presented in the whispering gallery modes as a result of the deformation of the microdroplets toward a truncated prolate spheroid geometry. Observed large spectral tuning is also reported to be highly reversible. This demonstration can inspire novel, largely tunable optical switches or filters based on liquid microdroplets kept in a sealed chamber.
2007
Abstract. Glycerol/water microdroplets take almost spherical shapes when standing on a superhydrophobic surface. Hence they are suitable to function as optical microcavities. Using Rhodamine B doped water microdroplets, large spectral tunability of the whispering gallery modes (WGMs)(> 5 nm) was observed. Tunability was achieved by evaporation/condensation in a current controlled mini humidity chamber. Experiments revealed a mechanism stabilizing the volume of these microdroplets with femtoliter resolution.
Controlled Deformation and Raman Lasing in Microdroplets Standing on a Superhydrophobic Surface
2009
Liquid microdroplets are ideally suited for applications that benefit from the properties of the whispering gallery modes (WGMs) because of their spherical shapes, smooth surfaces, biocompatibility, and flexible nature. In this thesis, we present nondegeneracy in WGMs as a result of deformation in the glycerol-water microdroplets using uniform electric field. Raman lasing from individual pure water microdroplets standing on a superhydrophobic surface is also presented in this thesis.
Controlled observation of nondegenerate cavity modes in a microdroplet on a superhydrophobic surface
We demonstrate controlled lifting of the azimuthal degeneracy of the whispering gallery modes (WGMs) of single glycerol–water microdroplets standing on a superhydrophobic surface by using a uniform electric field. A good agreement is observed between the measured spectral positions of the nondegenerate WGMs and predictions made for a prolate spheroid. Our results reveal fewer azimuthal modes than expected from an ideal spherical microdroplet due to the truncation by the surface. We use this difference to estimate the contact angles of the microdroplets.
Journal of Materials Chemistry, 2011
Emission spectra from microdroplets doped with CdSe/ZnS quantum dots (QDs) have been recorded on superhydrophobic coatings (water contact angle > 170 ). Whispering gallery modes (WGMs) with Q-factors as high as 4.0 Â 10 3 were discernible. Excitation parameters for optimal microdroplet WGM response illumination and acquisition are also presented. Fluorescent QDs provide a robust WGM reporting mechanism under extreme continuous wave microdroplet excitation (465.5 mW) for periods greater than 15 minutes. In this format droplets could be optically tuned on demand using combinations of QDs. Ionic liquid QD-doped droplet emission was found to be spectrally stable: a >75% improvement in WGM blue-shift was recorded (due to droplet evaporation) compared to QD-doped glycerol/water droplet emission. Theoretical analysis of emission spectra confirmed the observed emission response curves correspond to Mie theory suggesting the droplets are extremely close to spherical on the surface. This versatile liquid resonator system has direct implications in high performance room-temperature laser development, telecommunications research and lab-on-a-chip based diagnostics. † Electronic supplementary information (ESI) available: Time dependent WGM emission analysis of glycerol/water and ionic liquid/water microdroplets. See
Photothermal Tuning and Size Locking of Salt-Water Microdroplets on a Superhydrophobic Surface
Liquid microdroplets are attractive as optical microcavities with tunable resonances for applications in quantum optics and biological sensing, owing to their flexible nature and spherical shape. Salt-water microdroplets can be used in such experiments while standing on a superhydrophobic surface that preserves their spherical geometry. Here, we report how the photothermal effect enables continuous tuning or locking of the whispering gallery mode (WGM) spectrum and size of salt-water microdroplets on a superhydrophobic surface. Local heating by an infrared laser focused at the center of a microdroplet causes it to depart from its equilibrium size, shifting the WGM spectrum. This photothermal tuning effect is fully reversible and can be used to tune the microdroplet radius with a precision reaching 1 Å. We combine this effect with fluorescence excitation spectroscopy using a fixed wavelength laser to measure Q-factors of up to ∼10E5 . Conversely, focusing the heating laser to the microdroplet rim reveals absorption resonances, leading to a hysteretic behavior when cycling the laser power. We show that this behavior can be used to lock the size of a microdroplet and make it exhibit optical bistability. WGM resonances of locked microdroplets are probed using a tunable laser, showing a spectral locking precision reaching <0.01 nm over tens of minutes. These results indicate that the wavelength stability and positioning challenges inherent to liquid microdroplets in air can be overcome, providing an easily tunable and lockable alternative to solid optical microcavities and making them potential candidates for studies in cavity optomechanics.
JOSA B, 2012
We report measurements of ultrahigh quality factors (Q-factors) of the optical whispering-gallery modes excited via a tapered optical-fiber waveguide in single glycerol–water microdroplets standing on a superhydrophobic surface in air. Owing to the high contact angle of the glycerol-water mixture on the superhydrophobic surface (>155°), microdroplets 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 μm 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 for individual microdroplets. We found that the Q-factors in air were stable over more than 1 h and their measured values were limited mostly by the thermally induced droplet shape fluctuations.
Optics Communications, 2007
We report laser emission from single, stationary, Rhodamine B-doped glycerol/water microdroplets located on a superhydrophobic surface. In the experiments, a pulsed, frequency-doubled Nd:YAG laser operating at 532 nm was used as the excitation source. The microdroplets ranged in diameter from a few to 20 µm. Lasing was achieved in the red-shifted portion of the dye emission spectrum with threshold fluences as low as 750 J/cm 2 .
Journal of the Optical Society of America B, 2016
Optical whispering gallery modes (WGMs) were observed in elastic scattering spectra recorded from oil-in-water emulsion droplets in a microfluidic channel. Droplets with diameters ranging between 15 and 50 μm were trapped by optical tweezers near the tip of a single mode fiber that enabled the excitation of the WGMs using a tunable laser. Quality factors of the WGMs were observed to increase with droplet size. WGMs with quality factors of more than 10 4 were observed for droplets with diameters around 45 μm. In some cases, recorded WGMs drifted monotonically to the blue end of the spectrum due to droplet dissolution in the host liquid. Fluctuating spectral shifts to both blue and red ends of the spectrum were also observed. These were attributed to the presence of randomly diffusing particulate contaminants in the droplet liquid, indicating the potential of optically trapped droplet resonators for optical sensing applications.
2007
We demonstrate resonant enhancement of energy transfer from donor to acceptor molecules in single glycerol/water microdroplets standing on a superhydrophobic surface. At donor emission wavelengths resonant with a whispering gallery mode, we recorded energy transfer rates up to 10 times higher than the non-radiative (Förster) energy transfer rate, by using the gradual acceptor photobleaching technique.