The spontaneous emission from a thin layer of a dilute solution of fluorescent dye molecules within an optical microcavity has been studied. Strong fluctuations in fluorescence intensity are observed when the average number of molecules in the measurement volume is small. We have performed experiments that simultaneously characterize these fluctuations over nine orders of magnitude of time, from ns to s. These measurements have identified photon antibunching characteristic of single-molecule emission, along with triplet-state shelving and diffusion driven number fluctuations. The results give fundamental information on the kinetics of dye molecules and allow one to speculate about the prospect of using single molecules as sources of single photons for quantum optics.
The spontaneous emission from a thin layer of a dilute solution of fluorescent dye molecules within an optical microcavity has been studied. Strong fluctuations in fluorescence intensity are observed when the average number of molecules in the measurement volume is small. We have performed experiments that simultaneously characterize these fluctuations over nine orders of magnitude of time, from ns to s. These measurements have identified photon antibunching characteristic of single-molecule emission, along with triplet-state shelving and diffusion driven number fluctuations. The results give fundamental information on the kinetics of dye molecules and allow one to speculate about the prospect of using single molecules as sources of single photons for quantum optics.
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.
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.
The lack of reliable quantitative methods in fluorescence microscopy prevents the detailed investigation of basic molecular processes. Single-molecule fluorescence spectroscopy offers several techniques to determine the stoichiometry of fluorescent probes. Among them, an approach using the single-photon emission of an individual quantum system, also known as photon antibunching, yields access to both the emitter number and their molecular brightness. Previous work demonstrated the feasibility of the method to estimate up to five dyes without further physical characterization. In this work, I investigated and improved the reliability and robustness of this method. I developed different fluorescent probes with a defined label stoichiometry to explore the performance of our approach \nin experiments. I could demonstrate, in combination with simulations, that it can provide reliable estimates of up to 18 fluorescent emitters at a time resolution of approximately 200 ms. Furthermore, I inspected the dependency of the estimation bias and precision on the laser intensity, the analysis time and the number of expected molecules in detail. My results designate the scope for future stoichiometry determinations. In initial experiments, I analyzed the labeling distributions of different fluorescent probes. In addition, I could combine our counting approach with super-resolution STED-microscopy and identify critical parameters for possible applications
We derive the moment generating function for photon emissions from a single molecule driven by laser excitation. The frequencies of the fluoresced photons are explicitly considered. Calculations are performed for the case of a two-level dye molecule, showing that measured photon statistics will display a strong and nonintuitive dependence on detector bandwidth. Moreover, it is demonstrated that the antibunching phenomenon, associated with negative values of Mandel's Q parameter, results from correlations between photons with well separated frequencies.
The extraordinarily short optical dephasing time of organic dye molecules in polymer hosts is known to be under 20 fs between 100 and 200 K. Short dephasing times suggest enormously broadened homogeneous linewidths on the order of 10 nm. Direct observation of single dye molecule fluorescence spectra shows a similarly broadened line, giving comparable dephasing times for room temperature measurements. While this single molecule linewidth agrees with that of the bulk sample, it is inaccurate to attribute this broadening to homogeneous mechanisms. Repeated single molecule fluorescence measurements permit determination of the homogeneous nature of this linewidth, as distinct from the inhomogeneous nature of dispersion in bulk samples. Spectral diffusion in single molecule spectra is evidenced by multiple peaks, visible despite broad spectra lines at room temperature. In the particular experiment discussed in this thesis, fluorescence from single organic dye molecules in thin polymethyl methacrylate films was studied with scanning confocal microscopy. Photon antibunching exhibited by the photon-pair correlation function g(2) (τ) at τ = 0 confirms that single molecule emission spectra were obtained.
Motivated by the recent progresses in one-atom laser and single-molecule manipulation, this work presents the model and simulations of a single dye molecule laser device, which consists of a single dye molecule as an acceptor in a microcavity pumped by multiple donors via the intermolecular resonance energy transfer mechanism. The photon intensity is calculated at the emission peaks of donor and acceptor dyes for different pump rates and donor numbers. Finally, it is demonstrated that stimulated emission gains a distinct advantage over spontaneous emission under appropriate conditions. This work would have many important applications in a wide range of fields from physics and chemistry to nanotechnology in future.