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Physical Review A

Microcavity effect of dyes adsorbed on a levitated droplet

H. Yukawa, S. Arnold, Kenjiro Miyano

Published Sep 1, 1999• 16 citations
Publisher

Abstract

The fluorescence spectra and lasing behavior of dye molecules adsorbed on a droplet levitated in an electrodynamic trap have been studied. A simple classical model describes the dye--cavity-mode interaction quantitatively well when the homogeneous linewidth of the dye is wider than the free spectral range of the cavity. When the free spectral range becomes comparable with the homogeneous linewidth, the prediction of the model systematically deviates, indicating that stochastic fluctuations of the radiating dipoles need to be taken into account.

DOI
10.1103/physreva.60.2491

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The fluorescence spectra and lasing behavior of dye molecules adsorbed on a droplet levitated in an electrodynamic trap have been studied. A simple classical model describes the dye--cavity-mode interaction quantitatively well when the homogeneous linewidth of the dye is wider than the free spectral range of the cavity. When the free spectral range becomes comparable with the homogeneous linewidth, the prediction of the model systematically deviates, indicating that stochastic fluctuations of the radiating dipoles need to be taken into account.
A significant limitation in the detection and spectroscopy of individual dye molecules in liquid room temperature samples is the loss of fluorescence signal through chromophore photobleaching. Although often considered a fundamental restriction on sensitivity, it has been shown that the probability per excitation cycle of photobleaching may be reduced by enhancing the spontaneous emission rate of the chromophore located inside a microdroplet, which acts as a optical cavity. Studies of glycerol microdroplets containing R6G dye molecules, and levitated in a three-dimensional electrodynamic trap, have demonstrated a significant enhancement of both the fluorescence decay rate [1] and the total fluorescence yield [2] of the chromophore. Modification of the emission rate is greatest for molecules located near the surface of small (< 10 μm) microdroplets, where the influence of the droplet cavity modes of the droplet is most significant. To exploit this surface sensitivity, surfactant forms of rhodamine dyes were recently studied [3]. Fluorescence emission of the dye molecules, constrained to the droplet surface and with fixed transition moments perpendicular to the surface normal, were shown to be significantly enhanced at small droplet sizes. Additionally, at larger droplet sizes corresponding to higher droplet Q’s, preferential emission into the cavity modes was observed, however the detection of fluorescence photons was delayed by the storage time of the cavity, which may be on the same order as the fluorescence lifetime. These measurements, performed on relatively concentrated dye solutions, suggest that microcavity effects may be exploited to greatly enhance single molecule detection sensitivity in glycerol microdroplets. Furthermore, the large cavity storage time of the high-Q droplets opens the possibility for re-excitation of the molecule following emission into the droplet resonance, and subsequent stimulated emission, that is, single-molecule lasing.
Micrometer-sized transparent droplets act as natural high-Q cavities to visible light, and this paper shows them to exhibit strongly enhanced dye emission owing to cavity quantum-electrodynamic effects. A vibrating-orifice aerosol generator1 was used to produce a linear stream of virtually identical (less than 2 parts in 105 diameter variation) falling droplets 14 μm diameter, 10-5 M solution of Rhodamine 6G in ethanol; the droplets were excited by a cw argon-ion laser. Droplet emission was examined with a double 1 m Spex spectrograph and a photomultiplier. Recent development of elastic-scattering-size spectroscopy1,2 allowed identification of the various cavity modes contributing to the emission. This was necessary because of the different Q values and spatial properties of the resonant modes. Q values were calculated from Lorenz-Mie theory and were found to be consistent with observed fluorescence linewidths for the experimentally resolvable fourth-order modes. It was experimentally determined that Q's of only 103 were sufficient to observe sizable enhancements in the A coefficient. Lasing was observed to occur at reduced thresholds, e.g., 400 W/cm2 for fourth order modes having Q's of 2 × 103. This corresponds to a cavity induced enhancement in the B coefficient of 103.
This work explores the feasibility of inducing lasing action in a single dye molecule using an optical microcavity. A single-molecule light source is proposed, in which a microcavity-coupled dye molecule is pumped by multiple energy donors via an intermolecular resonance energy transfer mechanism. A quantitative model based on realistic parameters for microcavities and for organic dyes shows that stimulated emission may become dominant over spontaneous fluorescence under optimal yet technically reachable conditions, suggesting the likelihood of achieving a single-molecule lasing photon source with the proposed device. Lasing of a single dye molecule generates signals much brighter than spontaneous fluorescence, and may find applications in a wide range of fields from physics and chemistry to biology.
Fluorescence decay kinetics of Rhodamine 6-G molecules in levitated glycerol microdroplets (4--20 microns in diameter) have been investigated to determine the effects of spherical cavity resonances on spontaneous emission rates. For droplet diameters greater than 10 microns, the fluorescence lifetime is essentially the same as in bulk glycerol. As the droplet diameter is decreased below 10 microns, bi-exponential decay behavior is observed with a slow component whose rate is similar to bulk glycerol, and a fast component whose rate is as much as a factor of 10 larger than the bulk decay rate. This fast component is attributed to cavity enhancement of the spontaneous emission rate and, within the weak coupling approximation, a value for the homogeneous linewidth at room temperature can be estimated from the fluorescence lifetime data.
Measurements of the integrated fluorescence yield per molecule of Rhodamine 6G (R6G) in 4–16-μm-diameter levitated microdroplets show a size dependence that is attributed to a net increase in the fluorescence decay rate for the smaller (4–5-μm) droplets. The average fluorescence yield in 4-μm droplets (for which we have previously observed emission-rate enhancement) is approximately a factor of 2 larger than the yield measured for larger droplets for which emission-rate enhancement does not occur. These results suggest that there is little emission-rate inhibition in this system, even though the fluorescence emission spectrum overlaps several cavity resonances. A mechanism based on spectral diffusion is postulated for the apparent absence of cavity-inhibited emission and is illustrated by Monte Carlo calculations using time-dependent line-shape functions.
We have measured spectrally resolved fluorescence lifetimes of dye incorporated in high-quality photonic crystals, made of colloidal spheres. The emission spectrum shows a pronounced Bragg notch. In contrast, the fluorescence lifetime does not depend on the interaction between light and the photonic crystal. The results are explained with a simple model of an atom in a cavity. The effects of homogeneous and inhomogeneous broadening of the emission spectrum of dye inside photonic crystals are discussed.