Enhanced energy transfer in single glycerol/water microdroplets standing on a superhydrophobic surface (original) (raw)
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
A self-stabilization mechanism locking the size of single inorganic salt (NaCl)–water microdroplets that are standing on a superhydrophobic surface and kept in a humidity-controlled chamber is demonstrated. The effect is based on the hysteretic behavior of a photothermal tuning cycle caused by the whispering gallery mode (WGM) absorption resonances that are observed when scanning the power of an infrared laser focused at the rim of a microdroplet. When locked, the microdroplet size and WGM spectrum are resilient to environmental perturbations and can be maintained for hours as the mechanism does not rely on a photobleachable dye. The bistable nature of the system is also demonstrated, enabling reversible switching between two sizes. A rate equation-based thermodynamical model of the hysteretic behavior is provided, giving good agreement with the experimental results. Our results may be used to establish stable experimental conditions for ultrahigh resolution spectroscopy of microdroplets. Other optical and biological applications that require exactly size-matched microdroplets can also benefit from the demonstrated self-stabilization mechanism.
Journal of Photochemistry and Photobiology A: Chemistry, 1996
Time-resolved fluorescence microscopy was used to study the fluorescence quenching of rh~lamine 6G (R6G) in single microdroplets with diameters equal to or larger than 2/zm. The mierodroplets were prepared by dispersing ethylene glycol solutions of R6G tn silicone oil. The fluorescence lifetime of R6G was found to be independent of the size of the droplet at low concentrations ( i .0 × 10-'* M or less); a value of 3.60 ± 0.02 ns was observed, which was measured for the bulk solution in a thin glass capillary, indicating that neither the radiative process nor the intramolecular radiationless processes of R6G were influenced by the size of the droplet for droplets greater than 2 #m in diameter. However, a significant size effect on the fluorescence decay was observed at higher concentrations of R6G. At the highest concentration of R6G (5.2 × 10-3 M) examined in this work, the apparent fluorescence lifetime decreased from 3.60 :t: 0.02 ns for droplets larger than 20/zm in diameter to approximately 0.4 + 0.07 ns for the smallest droplet of 2/.tin in diameter. Despite such a significant decrease in the fluorescence lifetime, the decay curves could be analysed satisfactorily by fitting to single exponential functions. When malachite green (MG) was added as an acceptor to a solution of R6G ( t 0-4 M), a large enhancement of F0rster-type energy transfer from R6G to MG, as compared with the bulk solution, was observed for droplets with diameters of less than I 0/zm. In this case, all the decay curves obtained were well described by the function derived from the FOrster-type energy transfer, but the effective critical distance increased by approximately a factor of two on going from a 10/zm droplet to a 2/~m droplet. On the other hand, the fluorescence quenching of R6G by potassium iodide (KI) showed only a small size effect on the quenching rate constant. The significant size effect observed for droplets of concentrated R6G solution can be interpreted in terms of enhanced FOrster-type ¢'~ergy migration, followed by energy trapping by dimers of R6G which act as quenching centres.
Optofluidic FRET microlasers based on surface-supported liquid microdroplets
Laser Physics Letters, 2014
We demonstrate optofluidic microlasers using highly efficient non-radiative Förster resonance energy transfer (FRET) for pumping of gain medium placed within liquid microdroplets situated on a superhydrophobic surface. Microdroplets generated from a mixture of ethylene glycol, glycerol, and water and stained with the FRET donor-acceptor dye pair Rhodamine 6G-Rhodamine 700 serve as active optical resonant cavities hosting high-quality whispering gallery modes. Upon direct optical pumping of the donor with a pulsed laser, lasing is observed in the emission band of the acceptor as a result of efficient FRET coupling between the acceptor and donor molecules. FRET lasing is characterized for different acceptor and donor concentrations, and threshold pump fluences of acceptor lasing as low as 6.3 mJ cm −2 are demonstrated. We also verify the dominance of the non-radiative FRET over cavity-assisted radiative energy transfer for the range of parameters studied in the experiments.
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.
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.
Raman lasing near 650 nm from pure water microdroplets on a superhydrophobic surface
Photonics and …, 2009
We demonstrate Raman lasing near 650 nm in pure water microdroplets located on a superhydrophobic surface. In the experiments, stationary, pure water microdroplets were prepared on a superhydrophobic surface and excited by a pulsed, frequencydoubled Nd:YAG laser at 532 nm. Intense laser emission was observed at frequencies corresponding to the whispering gallery mode resonances of the water microdroplets near 650 nm where Raman resonances due to OH-stretching bonds of water are located. Onoff behavior was observed during lasing and the average temporal inter-burst separation was determined from the time-dependent intensity traces. Our results can find applications in the development of novel organic light emitters for short-haul communication systems, and in the spectroscopic characterization of water microdroplets on a surface. #
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.
Absorption effects on the microdroplet resonant emission structure
Optics Letters, 1991
The effect of absorption on microdroplet resonance emission line intensities was studied in 15-,um-diameter Rhodamine 6G/ethanol solution droplets. Absorption was controlled by varying the concentration of the additive nigrosin. Spectrally integrated intensities of resonant features are found to be proportional to a droplet cavity mode efficiency Qa/(Qa + Q 0 ) expressed in terms of cavity output coupling and absorption factors Q 0 and Qa, respectively. These Q's are determined from linewidths calculated from Lorenz-Mie theory by using combinations of the real and complex indices of refraction. was determined to be 108 from the data.
Stimulated low-frequency emission from anisotropic molecules in microdroplets
Chemical Physics Letters, 1993
Spectral broadening of the input-laser line and the stimulated Raman line have been observed in microdroplets of carbon disultide, benzyl alcohol, toluene, chloroform and a binary mixture of carbon disultide in ethanol. The broadening is asymmetrical and extends more than 500 cm-' towards the low-frequency side of the input-laser line and the stimulated Raman lines. The broadening is proposed to arise from multiple-order stimulated low-frequency scattering associated with librational and dipoleinduced-dipole translational motions.