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Quantum Optoelectronics

Fluorescence lifetimes of Oriented Molecules in Microdroplets

Michael D. Barnes, N. Lermer, William Barclay Whitten, J. Michael Ramsey, S. Arnold

Published Jan 1, 1997• 0 citations

Abstract

Over the last several years there has been considerable interest in the properties of atomic resonance fluorescence in an optical cavity with a primary dimension comparable to the relevant transition wavelength.1 In particular, there has been great interest in the realization of strong atom-cavity coupling2 and the suppression of spontaneous emission into "free-space" modes. However, an important but poorly understood issue relevant to low- or zero-threshold condensed phase optical devices is the nonradiative coupling of the emitting species to a thermal bath. Unlike experiments involving dilute atomic beams where the transition is well defined and broadening is negligible, coupling to a thermal bath induces spectral broadening which is usually much larger than the cavity resonance width and may often exceed the cavity mode spacing. Interesting examples of such systems are solvated dyes whose condensed phase dynamics are well known and characterized. To date, several studies have been made on fluorescence properties of solvated dyes in microcavities,3,4,5 however additional complexities such as spatial and orientational averaging have obscured to some extent the connection between radiative and nonradiative processes in such systems. We discuss the observation of spontaneous emission rate modification (both enhancement and apparent suppression) for molecular species in a microcavity where both the molecular position and transition moment orientation are well defined.6 These experiments serve as an interesting test case of molecule-cavity systems in which the emitting species is nonradiatively coupled to a thermal bath.

DOI: 10.1364/qo.1997.qthb.5

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Over the last several years there has been considerable interest in the properties of atomic resonance fluorescence in an optical cavity with a primary dimension comparable to the relevant transition wavelength.1 In particular, there has been great interest in the realization of strong atom-cavity coupling2 and the suppression of spontaneous emission into "free-space" modes. However, an important but poorly understood issue relevant to low- or zero-threshold condensed phase optical devices is the nonradiative coupling of the emitting species to a thermal bath. Unlike experiments involving dilute atomic beams where the transition is well defined and broadening is negligible, coupling to a thermal bath induces spectral broadening which is usually much larger than the cavity resonance width and may often exceed the cavity mode spacing. Interesting examples of such systems are solvated dyes whose condensed phase dynamics are well known and characterized. To date, several studies have been made on fluorescence properties of solvated dyes in microcavities,3,4,5 however additional complexities such as spatial and orientational averaging have obscured to some extent the connection between radiative and nonradiative processes in such systems. We discuss the observation of spontaneous emission rate modification (both enhancement and apparent suppression) for molecular species in a microcavity where both the molecular position and transition moment orientation are well defined.6 These experiments serve as an interesting test case of molecule-cavity systems in which the emitting species is nonradiatively coupled to a thermal bath.

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.

Fluorescence decay rate modification has been observed for molecules in liquid microspheres (3.5 to 25 \ensuremath{\mu}m in diameter) where the molecular position and transition moment orientation are well defined. For small sizes (

<6

\ensuremath{\mu}m), the decay rates scale roughly as ${r}^{\ensuremath{-}1}$ and agree quantitatively with semiclassical calculations. For larger sizes (

>15

\ensuremath{\mu}m) where the cavity storage time and fluorescence lifetime are roughly equal, an anomalous decrease in the decay rate of $\ensuremath{\approx}30%$ is observed which is interpreted as a consequence of intermediate molecule-cavity coupling.

We investigate experimentally and theoretically the fluorescence emitted by molecular ensembles as well as spatially isolated, single molecules of an organic dye immobilized in a quasi-planar optical microresonator at room temperature. The optically excited dipole emitters couple simultaneously to on- and off-axis cavity resonances of the microresonator. The multi-spectral radiative contributions are strongly modified with respect to free (non-confined) space due to enhancement and inhibition of the molecular spontaneous emission (SpE) rate. By varying the mirror spacing of the microresonator on the nanometer-scale, the SpE rate of the cavity-confined molecules and, consequently, the spectral line width of the microresonator-controlled broadband fluorescence can be tuned by up to one order of magnitude. Stepwise reducing the optical confinement, we observe that the microresonator-controlled molecular fluorescence line shape converges towards the measured fluorescence line shape in free space. Our results are important for research on and application of broadband emitters in nano-optics and -photonics as well as microcavity-enhanced (single molecule) spectroscopy.

In recent papers we have reported the first proposal and realization of the “microscopic cavity” (or “microcavity”) i.e. with relevant dimension d~d = (λ/2) where λ is the wavelength of the cavity-confined radiation (1). By this apparatus the Spontaneous-Emission (SE) inhibition-enhancement process in molecular physics (“Purcell effect”) has been demonstrated at optical frequencies (1). In the present work we report the demonstration that the phase of (SE) “enhancement” (i.e. d=d) is actually nonexistent in a microcavity as (SE) is overcome and merges into Stimulated-Emission (StE) at exceedingly low levels of molecular excitation, thus realizing, for the first time in Optics, a “Zero-Threshold-Laser” action. This is a direct consequence of the peculiar and novel condition of Quantutm-Statistical-Electrodynamics offered by the microcavity and consisiting of a resonant single-mode field-atom interaction. Assume that an atom or molecule with an homogeneous-broadened resonance at λ and linewidth Δλ = (λ·Δv/v), placed in a microcavity, emits a fluorescence photon at time t=to, and suppose that this is the only photon present in the cavity at that time.

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.