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IEEE Journal of Quantum Electronics

Polarization of fluorescence and bleaching of dyes in a high-viscosity solvent

Gerard A. Mourou, Marguerite-Marie Denariez

Published Aug 1, 1973• 18 citations
Publisher

Abstract

The behavior of the degree of polarization of the fluorescence and transmission versus intensity of complex molecules in high-viscosity solvents has been studied. We show that, in the usual case, when the electronic transition can be represented as a linear oscillator, molecules excited at low field have an anisotropic distribution; then, at high field, the distribution becomes isotropic. So, the relation between the transmission of the solution with intensity differs significantly from the transmission versus intensity relation derived by using the mean value of the cross section.

DOI
10.1109/jqe.1973.1077734

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Physical Review A•Oct 1, 1987•28 citations
The behavior of the degree of polarization of the fluorescence and transmission versus intensity of complex molecules in high-viscosity solvents has been studied. We show that, in the usual case, when the electronic transition can be represented as a linear oscillator, molecules excited at low field have an anisotropic distribution; then, at high field, the distribution becomes isotropic. So, the relation between the transmission of the solution with intensity differs significantly from the transmission versus intensity relation derived by using the mean value of the cross section.
We study the influence of the solvent viscosity in the local irregular intensity fluctuations characteristic of high Fresnel number dye lasers. The relative amplitude of the fluctuations in a flashlamp-pumped dye laser is analyzed, We complete our previous work by using several solvents and different excitation energies above threshold. A decrease of the relative fluctuation amplitude is found as the solvent viscosity or the pumping energy increases. This effect is theoretically analyzed in the framework of the semiclassical Maxwell-Bloch equations, with a model based on the orientation of the dye molecules driven by the laser field in the solvent. These results point out the importance of the orientation of the dye molecules in the intensity fluctuation phenomenon.
A calculation is made of the stimulated emission threshold of organic dye solutions as a function of the diffusion coefficient, intensity of induced singlet–singlet or triplet–triplet absorption at the stimulated emission wavelength, steepness of the leading edge of pump pulses, and other parameters. It is shown that, if the dipole moment of a fluorescence transition is perpendicular to the dipole moments of associated induced singlet–singlet or triplet–triplet absorption transitions, the laser threshold of high–viscosity solutions is considerably lower than that of low-viscosity systems. The maximum value of the ratio of the cross sections of the singlet-singlet absorption and stimulated emission, at which a given molecule ceases to emit stimulated radiation, increases from 1 to 3 when the rotational diffusion coefficient decreases from ∞ to 0.
Experimental investigations of the change of polarization state of probing radiation on passing through anisotropically excited solutions of rhodamine 6G and 3,6-tetramethyldiamino- N -methylphthalimide in glycerol were carried out. These investigations allowed us to find the phase anisotropy in these molecular systems. The parameters of the polarization ellipse (the ratio of axes , orientation, direction of rotation) were determined. The main factors (the intensity and orientation of the polarization plane of the probing radiation) influencing these parameters were established. The differences between the refractive indices were designed with the help of the obtained parameters of the polarization ellipse. The spectral behaviour of the amplitude and phase anisotropy for the long-wavelength band of absorption and fluorescence was studied and described qualitatively. The features of the spectral behaviour of for the indicated solutions, which are modelled by different systems of energy levels of the active molecules, were established.
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.
Energy transfer and coherence transfer between two atoms of different species are studied theoretically. The electric dipole-dipole interaction is the mechanism considered for the excitation transfer. A generalized perturbation method for a damping system is used. A general formula is derived for the polarization of the sensitized fluorescence resulting from the excitation transfer. This polarization is shown to be dependent on the angular momenta of the two atoms, and to be independent of their relative velocity as well as of their difference in the energy of excitation. The polarizations are found to be nonzero in many cases which demonstrate the transfer of coherence through collision processes. A general and compact formula is also derived for the polarization of pure fluorescence where excitation exchange has not taken place. The polarization of the sensitized fluorescence is in general smaller than that of the pure fluorescence. This means that the coherence is transferred only partially. Finally, the magnetic field effect has been included. This field depolarization depends on the gj{g}_{j}gj​ values and the lifetimes of the excited states of both atoms. The general feature of the field dependence also differs considerably from that of the Hanle effect for resonance fluorescence.
We model the response of a molecular medium to an intense pulse of light. We begin by reducing the Maxwell-Bloch equations for a multilevel molecular model system to a set of simplified propagation equations. We thereby obtain expressions for the frequency, temperature, and field intensity dependence of the ground and excited electronic state absorption cross sections, and the emission cross section, as well as the temperature dependence of the decay rate of the excited state. The phenomena of saturation, excited-state absorption, optically induced dichroism and birefringence, vibronic energy transfer, induced molecular alignment, decay of the matter-field interaction energy, and rotational diffusion are considered within the general formalism for describing the propagation of an intense pulse of light in the molecular medium. We find that the cross section for absorption (emission) of a photon is not identical to the cross section for molecular excitation (deexcitation) via the matter-field interaction. This is explained in terms of an energy balance between the matter, radiation, the matter-radiation interaction, and the bath responsible for the broadening of the molecular transitions. The inequality of these cross sections determines whether the system supplies or draws energy from the thermal bath. The rise in the vibrational temperature of molecules subjected to an intense short pulse is shown to significantly affect propagation of the pulse. The alignment of the molecular system resulting from saturation of the ground-state transition by a polarized laser pulse and the effects of the orientational dependence of the matter-field interaction including effects of rotational diffusion are determined. As an example of the formalism we consider pulse propagation in a Rhodamine 6G solution. We fit the parameters of the model to determine the frequency dependence of the ground- and excited-state absorption cross sections and the emission cross section and compare with experimentally determined cross sections. We describe the propagation of a pulse whose temporal width is comparable to the rotational diffusion time of the molecules in the dye solution and calculate polarization pulse probe signals.