We present for the first time cavity-controlled fluorescence spectra and decay curves of single dipole emitters interacting at room temperature with the first longitudinal mode of a Fabry-Perot microcavity offering a lambda/2-spacing between its silver mirrors. The spontaneous emission rate of individual dye molecules was found to be enhanced by the Purcell effect by up to three times compared to the rate in free space, in agreement with theoretical predictions. Moreover, our new microcavity design was found to provide long-term stability and single-molecule sensitivity under ambient conditions for several months without noticeable reduction of the cavity-Q value. We consider this as a significant advance for single-photon sources operating at room temperature.
We present for the first time cavity-controlled fluorescence spectra and decay curves of single dipole emitters interacting at room temperature with the first longitudinal mode of a Fabry-Perot microcavity offering a lambda/2-spacing between its silver mirrors. The spontaneous emission rate of individual dye molecules was found to be enhanced by the Purcell effect by up to three times compared to the rate in free space, in agreement with theoretical predictions. Moreover, our new microcavity design was found to provide long-term stability and single-molecule sensitivity under ambient conditions for several months without noticeable reduction of the cavity-Q value. We consider this as a significant advance for single-photon sources operating at room temperature.
Optical microcavities are structures which confine light to a small region in the range of one wavelength. The radiation of a quantum emitter is coupled to cavity resonances which leads to an optical confinement of the broadband fluorescence. A practical design for this single-mode microcavity is formed by two silver mirrors enclosing a transparent dielectric medium with single quantum emitters. Steiner et al. have shown that the fluorescence spectra and decay lifetimes of single molecules in this Fabry-Perot type microcavity are strongly dependent on the resonator length.
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.
Summary form only given. Organic dye molecules have been used in great many scientific and technological applications. Among these, the polycyclic aromatic hydrocarbons (PAH) behave very favourably as quantum emitters when embedded in organic crystals at temperatures below 4K. PAHs can serve as solid-state emitters with remarkable optical properties such as near-unity quantum efficiency and high photostability. However, transitions to short-lived molecular vibrational levels reduce the scattering cross section by a branching ratio of about 30-50%. In order to overcome this shortcoming and render the molecule as a two-level system, one can enhance its zero-phonon emission by using the Purcell effect, e.g., via coupling to an optical cavity, to obtain a near-unity branching ratio. Here we report the first coherent coupling of a single molecule to a scannable, tunable and broadband microcavity. The cavity consists of a planar distributed Bragg reflector and a micromirror with a radius of curvature of 5 μm fabricated with focused ion-beam milling [1] and coated with silver. By integrating an organic matrix in the microcavity, we are able to coherently couple individual molecules to the cavity mode at liquid helium temperature.
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.
This thesis investigates the coupling of the fluorescence of nitrogen-vacancy (NV) centers in diamond to tunable optical microresonators at ambient conditions, in particular in the regime of Purcell enhancement. We use fiber-based, open-access Fabry-Pérot cavities optimized for high finesse and ultra-small mode volume. Different regimes of cavity enhancement are studied that are complementary to each other: A first experiment relies on a high-finesse cavity with dielectric mirrors. The scaling laws of Purcell enhancement are explicitly demonstrated by a large-range variation of both the cavity mode volume (V = 16 − 600 µm^3 ) and the quality factor (Q = 6 · 10^3 − 2 · 10^6). We detect an enhancement of the emission spectral density by up to a factor of 300. The full potential of this resonator can be exploited with emitters having a linewidth which is narrower than the resonance linewidth of the cavity. This concept holds promise for the implementation of wavelength-tunable, narrow-band single-photon sources as well as the generation of indistinguishable single-photons at ambient conditions. However, for broad-band emitters like the NV center at room temperature, the emission lifetime is not affected noticeably in this configuration. In order to directly observe lifetime changes and Purcell-enhanced single-photon emission, we manufacture fiber-based cavities with silver-coated mirrors having ultra-small mode volumes, as small as V = 1.0 λ^3 = 0.34 µm^3. We demonstrate cavity-enhanced fluorescence imaging, which allows to locate and analyze several single NV centers with one cavity. The Purcell effect is evidenced by an enhanced fluorescence collection of up to 1.6 · 10^6 photons per second from single-NV centers and a tunable variation of the emission lifetime corresponding to an effective Purcell factor of up to 2. We furthermore investigate a benefcial regime of optical confinement where the Fabry-Pérot cavity mode is combined with additional mode confinement by the diamond nanocrystal itself, enabling sub-λ^3 mode volumes. We perform simulations that predict effective Purcell factors of up to 11 for NV centers and of up to 63 for silicon-vacancy centers, revealing a great potential for bright single-photon sources and effcient spin readout at ambient conditions.
Solid-state quantum emitters such as the N-V center in diamond have applications ranging from single-photon sources to quantum sensors, with some quantum properties preserved even under ambient conditions. A central challenge is to access these quantum properties efficiently, by catching as much emitted light as possible. In this study a tunable optical microcavity, with a mode volume as small as one wavelength cubed, is used to enhance spontaneous emission of single N-V centers in diamond, via the Purcell effect. This allows the authors to extract large amounts of light from several individually addressable emitters, and to control their spontaneous-emission rate.