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Journal of the Optical Society of America B

Emission of R6G dye in Fabry–Perot cavities in weak and strong coupling regimes

Md Omar Faruk, Nelly Jerop, Mikhail A. Noginov

Published Sep 3, 2020• 5 citations

Abstract

We have studied spectra and angular distribution of emission in Fabry–Perot cavities formed by two silver mirrors separated by a layer of poly (methyl methacrylate) polymer doped with rhodamine 6G (R6G) dye in low ( 20 g / l ) and high ( 200 g / l ) concentrations. The frequency of emission radiated to a cavity mode was larger at large outcoupling angles—the “rainbow” effect. At the same time, the angle of the strongest emission was also determined by the cavity size: the larger the cavity, the larger the angle. The angular distribution of emission is commonly dominated by two symmetrical lobes (located at the intersection of the three-dimensional emission cone with a horizontal plane) pointing to the left and to the right of the normal to the sample. Despite the strong Stokes shift in R6G dye, the branch of the cavity dispersion curve obtained in the emission experiment is positioned above the one obtained in the reflection (extinction) experiment. Some dye molecules are poorly coupled to cavity modes. Their emission has very broad angular distribution with the maximum at θ = 0 ∘ . The signatures of strong cavity–exciton coupling were observed at high dye concentration ( 200 g / l ) but not at low concentration ( 20 g / l ). The evidence of the effect of strong coupling on emission is exemplified by a strong difference in the angular distribution of emission in two almost identical cavities, one with and another without strong coupling. Most importantly, we have demonstrated the possibility to control the ground state concentration, the coupling strength, and the dye emission spectra with Q-switched laser pulses.

DOI: 10.1364/josab.403612

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We have studied spectra and angular distribution of emission in Fabry–Perot cavities formed by two silver mirrors separated by a layer of poly (methyl methacrylate) polymer doped with rhodamine 6G (R6G) dye in low ( 20 g / l ) and high ( 200 g / l ) concentrations. The frequency of emission radiated to a cavity mode was larger at large outcoupling angles—the “rainbow” effect. At the same time, the angle of the strongest emission was also determined by the cavity size: the larger the cavity, the larger the angle. The angular distribution of emission is commonly dominated by two symmetrical lobes (located at the intersection of the three-dimensional emission cone with a horizontal plane) pointing to the left and to the right of the normal to the sample. Despite the strong Stokes shift in R6G dye, the branch of the cavity dispersion curve obtained in the emission experiment is positioned above the one obtained in the reflection (extinction) experiment. Some dye molecules are poorly coupled to cavity modes. Their emission has very broad angular distribution with the maximum at θ = 0 ∘ . The signatures of strong cavity–exciton coupling were observed at high dye concentration ( 200 g / l ) but not at low concentration ( 20 g / l ). The evidence of the effect of strong coupling on emission is exemplified by a strong difference in the angular distribution of emission in two almost identical cavities, one with and another without strong coupling. Most importantly, we have demonstrated the possibility to control the ground state concentration, the coupling strength, and the dye emission spectra with Q-switched laser pulses.

Abstract Light–matter interaction leads to excitation of molecules, which, in turn, can exchange energy with a localized electromagnetic field. This can be used for engineering of the electronic and vibrational energy levels of the molecules. This study considers the conditions for the emergence of the strong light–matter coupling regime for organic dye molecules in a tunable Fabry–Perot microcavity formed by a convex mirror and a flat reflecting surface. The sample studied was made of hexagonal boron nitride (hBN), polyvinylpyrrolidone polymer (PVP, 55 kDa), and rhodamine 6G fluorophore (R6G). Strong light–matter coupling was obtained in a sample with a low concentration of PVP. Adjustment of the optical path length in the microcavity by varying the thickness of the hBN–R6G–PVP film made it possible to obtain a high density of modes in the cavity (several tens of (λ/n)3) and, hence, to study the weak and strong light–matter coupling regimes. The results offer the possibility of studying the basic mechanisms of resonant light–matter interaction at room temperature, as well as developing new practical applications of the strong coupling effect.

Coupling of macroscopic ensembles of highly concentrated R6G molecules with aluminum and silver Fabry–Perot cavities of different sizes, in which one or two standing wave modes are in resonance with the absorption transition(s) of the dye molecules, have been studied. (i) It has been found that strong coupling of the cavity with one molecular eigenstate (S0→S1 transition) does not perturb the energies of the other molecular states. (ii) The S0→S2 absorption transition is too weak to strongly couple to the resonant cavity (“λ” standing wave mode). However, the resonant cavity causes a blue shift of the S0→S2 excitation band. (iii) The splitting of the dispersion curve is theoretically predicted and experimentally observed at strong coupling of the cavity resonance (“λ” standing wave mode) with the material resonances of Ag, including bound electron and “epsilon equal zero” transitions.

In this study, we demonstrate that the fluorescence of fluorescein dye molecules can be significantly modified in both the weak and strong coupling regimes by placing the molecules inside a perturbative or non-perturbative Fabry-Pérot cavity. By controlling the cavity thickness and dye concentration, we selectively altered the frequency and number of cavity transmission peaks, thereby establishing conditions for weak or strong coupling for fluorescein molecules. Using steady-state fluorescence spectroscopy, we confirmed that both the excitation and emission spectra were substantially tuned in these coupling regimes, enabling tunable and multicolor fluorescence from a single type of dye molecule, fluorescein.

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.

We have performed an experimental study of a high-Q Rhodamine 6G ring dye laser and have measured bichromatic emission with wavelength spacings as large as 110 A\r{} when the laser operated bidirectionally. The bichromatic emission vanished at all excitations when the laser was forced into unidirectional operation using a Faraday isolator. However, when a weak reflected beam was allowed to make a single pass in the direction opposite to that allowed by the Faraday device, bichromatic emission is recovered at the higher pump powers. The experiments show that no discontinuities occur in the intracavity power as a function of excitation and that the intracavity fields which are completely consistent with the measured thresholds are not capable of producing Rabi splittings larger than a few angstroms. In addition, we have observed no structure in the spontaneous emission spectrum of the dye when the bichromatic emission (80-A\r{} splitting) was present in the laser mode. We have been able to quantitatively fit the threshold and wavelengths of the bichromatic emission using a distributed feedback mode analysis with a cavity-mode-induced susceptibility grating.