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Physical Review A

Measurement of angular distribution of radiation from dye molecules coupled to evanescent wave

Hyun Ho Choi, Hyoung Joo Kim, Jaewoo Noh, Chang‐Woo Lee, Wonho Jhe

Published Nov 8, 2002• 6 citations
Publisher

Abstract

We measured the angular distribution of spontaneous emission radiation from dye molecules near a plane boundary. We found a systematic change of the angular distribution depending on the pump beam angle. By comparing the experimental results obtained by normal pumping and evanescent pumping, we could discern between the characteristics of radiation from the molecules within subwavelength distance from the boundary and that from the molecules relatively far from the boundary. It is found that the exact shape of the angular distribution of radiation is determined by the local geometrical factor related to the pump beam angle and the radiation detection angle. The characteristics of the spontaneous emission beyond critical angle and the measured polarization dependence of the radiation are in good agreement with the triplet mode quantum theory, as well as semiclassical theory.

DOI
10.1103/physreva.66.053803

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We measured the angular distribution of spontaneous emission radiation from dye molecules near a plane boundary. We found a systematic change of the angular distribution depending on the pump beam angle. By comparing the experimental results obtained by normal pumping and evanescent pumping, we could discern between the characteristics of radiation from the molecules within subwavelength distance from the boundary and that from the molecules relatively far from the boundary. It is found that the exact shape of the angular distribution of radiation is determined by the local geometrical factor related to the pump beam angle and the radiation detection angle. The characteristics of the spontaneous emission beyond critical angle and the measured polarization dependence of the radiation are in good agreement with the triplet mode quantum theory, as well as semiclassical theory.
The angular distribution of the radiation emitted from a dye molecular solution in a planar symmetrical microcavity terminated by two high-reflectivity dielectric mirrors has been investigated. A quantum-mechanical analysis of this process was applied to a microcavity terminated by mirrors bearing dielectric multilayered coatings, whose optical parameters are calculated by means of the Lissberger–Wilcock algorithm. The experiment was performed for different polarizations of the pump beam and of the emitted radiation. The angular distribution of the spontaneous emission is strictly dependent on the behavior of the reflectivity and the phase of the mirrors relative to the angle of incidence. This restriction limits the microcavity confinement to relatively small values of the internal angle of emission.
In a previous study a general semiclassical formalism has been developed for lasers with arbitrary orientational distributions of the active molecules. Here that formalism is applied to the important case of an isotropic distribution of molecules, and this limit is especially relevant to liquid dye lasers. Extensive analytical and numerical results are presented for the polarization properties of such dye lasers. It is found that in general the polarization states of the pump and signal fields are coupled together and both change with distance in a laser amplifier.
Emission spectra of concentrated (~10 18 –10 19 cm –3 ) ethanol solutions of Rhodamine C excited by high-power (~10 25 –10 26 cm –2 s –1 ) laser radiation were measured. The emission spectrum consists of two narrow bands (with half-widths of ~200–300 cm –1 ). The long-wavelength band was interpreted as cooperative spontaneous emission, and the short-wavelength band was assigned to the amplified spontaneous emission. The ratio of their intensities depends on the dye concentration and the pump power density. The ratio of band intensities is assumed to be determined by the dephasing rate of excited molecules responsible for cooperative spontaneous emission.
The threshold pump power density for lasing in dye solutions is found to depend on the photon energy of pumping radiation. An increase in the pumping photon energy can significantly lower the threshold pump power of dye lasers. For an ethanol solution of rhodamine 6G with a concentration of 4×10 18 cm −3 , the threshold power density for pumping radiation with a wavelength of 532 nm is 20-fold higher than for pumping radiation with a wavelength of 347 nm. This phenomenon is associated with the competition of collective spontaneous emission, which can lead to the efficient deactivation of excited molecules in femtosecond times, and the dephasing of excited molecules due to the intramolecular nonradiative processes of absorbed-energy conversion. An increase in the dephasing rate with the increasing energy of exciting photons lowers the efficiency of collective spontaneous emission and increases the concentration of dephased excited molecules responsible for lasing.
Spontaneous emission of photons by an atom placed near the dielectric waveguide is analyzed using global free electromagnetic modes, satisfying necessary continuity conditions at the waveguide boundaries 1. These modes include modes extended over the whole space (travelling) and modes trapped to the waveguide. To describe angular properties of the spontaneous emission, a parameterization of the travelling modes by the outgoing waves has been proposed. Otherwise, using a more common parameterization of these modes by the incoming waves, one has to take into account the interference of different reflected and transmitted waves accompanying photons incoming from both sides of the waveguide 2, what was overlooked in 3. Angular characteristics of the travelling photon emission (a), distributions of the trapped photon emission (b), as well as global decay rates (c), have been calculated using the quantum approach and standard perturbation theory. Enhancement and suppression of radiative decay rates in various configurations have been shown. A certain calculational error, in the treatment of the density of trapped states, in an earlier paper by Khosravi and Loudon 4, affecting emission rates for thin waveguides, has been corrected.
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.