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Control of molecular Spontaneous Emission in an optical lambda/2-microresonator

Mathias Bernhard Steiner

Published Nov 9, 2006• 2 citations
OpenAlex

Abstract

Control of molecular Spontaneous Emission in an optical λ/2-microresonator\n \nIn this thesis, the influence of a planar optical λ/2-microresonator on the Spontaneous Emission rate of embedded molecules and the implications on the microresonator-controlled emission is investigated by means of scanning confocal optical microscopy and spectroscopy. The experimental results obtained for molecular ensembles as well as for single molecules are compared with calculations and computer simulations based on existing theoretical models.

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Control of molecular Spontaneous Emission in an optical λ/2-microresonator\n \nIn this thesis, the influence of a planar optical λ/2-microresonator on the Spontaneous Emission rate of embedded molecules and the implications on the microresonator-controlled emission is investigated by means of scanning confocal optical microscopy and spectroscopy. The experimental results obtained for molecular ensembles as well as for single molecules are compared with calculations and computer simulations based on existing theoretical models.
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.
The authors studied spatially isolated terrylene molecules immobilized in a quasiplanar optical λ∕2-microresonator using confocal microscopy and spectroscopy at variable temperatures. At T=1.8K, they observed individual molecules relaxing into microresonator-allowed vibronic levels of their electronic ground state by emission of single fluorescence photons. Coupling the purely electronic transition of embedded molecules to the longitudinal photonic mode of the microresonator resulted in an ultimate spectral narrowing and an increased collection efficiency of the emitted single photon wave trains.
The decay rate of an excited molecule (atom) embedded in a dispersive and absorbing planar cavity is derived by using a recently obtained compact form of the Green's function for a multilayer. As a by-product, a hint is provided for a straightforward extension of the results obtained for lossless cavities of other shapes to the corresponding absorbing cavities. The decay rate in an absorbing cavity consists of the spontaneous emission rate and of the nonradiative rates caused by the near-field interaction of the molecule with the cavity medium and, for nearby molecules, with the cavity mirrors. Only the spontaneous emission rate is satisfactorily described in the macroscopic approach adopted. The theory is applied to an analysis of the effects of the weak cavity absorption on the decay rate in a dielectric microcavity formed by two metallic mirrors. As expected, dissipation in the cavity medium spoils the conditions for controlled spontaneuos emission and strongly suppresses the intensity of spontaneous emission. However, its effect on the spontaneous emission rate is much less pronounced.
This thesis discusses the possibility to modify the spontaneous emission rate in molecules. The modification is achieved by preparing the molecule initially in a superposition of vibrational eigenstates of the first electronic state. Thus the spontaneous emission rate depends on the coefficients of the superposition. A set of coefficients that maximizes the spontaneous emission rate will be found. These coefficients as well as the corresponding spontaneous emission rate, however, will turn out to depend crucially on the Franck Condon factors of the molecule of concern. These Franck Condon factors can be quite different for different molecules. For this reason a part of the thesis is devoted to numeric calculations on the basis of data of specific real molecules. Finally, the results will be discussed in the greater context.
We present a quantum electrodynamical procedure to demonstrate the all-optical control of molecular fluorescence. The effect is achieved on passage of an off-resonant laser beam through an optically activated system; the presence of a surface is not required. Following the derivation and analysis of the all-optical control mechanism, calculations are given to quantify the significant modification of spontaneous fluorescent emission with input laser irradiance. Specific results are given for molecules whose electronic spectra are dominated by transitions between three electronic levels, and suitable laser experimental methods are proposed. It is also shown that the phenomenon is sensitive to the handedness of circularly polarized throughput, producing a conferred form of optical activity.
The quantum theory of the spontaneous emission (SpE) from an active microscopic cavity (microcavity) is given with emphasis on mirror separations of the order of the optical wavelength. The theory is based on a complete set of orthonormal-mode functions that include both transverse polarizations and span the infinite three dimensional space that pervades and surrounds the microcavity. SpE rates for different active-dipole orientations and cavity configurations are calculated. The SpE pulse shape detected outside the cavity is shown to be generally nonexponential. A detailed computer simulation of the process is presented on the basis of the given theory in the perspective of our experiment, for a cavity terminated by mirrors bearing either metal- or semiconductor-multilayered coatings. We then report an extensive experimental verification of the theory by adopting an Eu-dibenzoylmethane complex as active medium with SpE from the 5^{5}5${\mathit{D}}_{0\mathrm{\ensuremath{-}}}^{7}$F2{\mathit{F}}_{2}F2​ line at \ensuremath{\lambda}=6111 \AA{}, under coherent uv excitation at ${\ensuremath{\lambda}}_{\mathit{p}}$=3547 \AA{}. The results show evidence of ``SpE inhibition'' and ``enhancement,'' of nonexponential decay of SpE signals, and of competition with superradiance and stimulated emission. Finally we report the results of an experimental test of the algorithm adopted in all computer calculations of the optical parameters of the multilayered structures used for cavity confinement.