Fluorescence and Multiwave Mixing Induced by Photon Absorption of Excited Molecules

Omar A. Ormachea, Alexei L. Tolstik

This work presents theoretical and experimental studies of the energy efficiency of multiwave mixing in complex molecular media (dye solutions) exhibiting higher order nonlinearities. There is a great variety of such media, including the resonance ones revealing nonlinearities of higher orders due to the absorption saturation effect and transitions between different excited states of the molecules. However, the majority of previous studies of multiwave mixing have been performed in the resonant medium approximation disregarding the induced anisotropy effect. A new method for enhancement of multiwave mixing in solutions of laser dyes has been proposed and substantiated with the use of lasing as an additional light beam, whose absorption by the molecules in the excited states leads to the formation of thermal dynamic gratings resultant in the improved efficiency of the energy exchange. The theoretical models for the formation of nonlinear dynamic holograms adequately describing the control process over multiwave mixing by an independent light beam, the frequency of which is tuned into the induced absorption band from the molecules in the excited state, have been developed. Polarization multiwave mixing has been realized in Rhodamine-6G dye solution using nonlinearities up to the ninth order inclusive. It has been demonstrated that on orthogonal polarization of the hologram recording waves the diffraction efficiency is lowered by one-two orders, the diffracted wave polarization being dependent on the diffraction order (the polarization plane alignment is identical within the orders of the same evenness).

H.P. Grieneisen, Norman A. Kurnit, A. Szöke

José Luis Arce Diego, Félix Fanjul-Vélez, David Pereda Cubián +3 more

The potentialities of diffraction methods for wavefront transformations may be considerably widened due to the use of nonlinear recording of dynamic holograms enabling multiwave mixing in media with the fifth and higher order nonlinearities. There is a great variety of such media including the resonant ones for which the presence of higher order nonlinearities is conditioned by the absorption saturation effect and transitions between different excited states of the molecules. However, in the majority of previous studies of multiwave mixing the resonant medium approximation has been used disregarding the induced anisotropy effect. This work presents a theoretical model and experimental studies of the energy efficiency of multiwave mixing in complex molecular media exhibiting higher order nonlinearities, in two cases: when the nonlinear cavity is introduced in the Fabry-Perot interferometer (IFP), and without it. Owing to the use of different combinations of mutually aligning polarizations for interacting waves, one is enabled to determine the contribution into the interaction efficiency by various dynamic gratings and nonlinearity mechanisms. The role of polarization gratings resultant from spatial modulation of the light field polarization state at the orthogonal polarization of the hologram recording waves has been established. Comparisons between the contributions of the above gratings and “normal” gratings recorded by identically polarized light beams into the process of multiwave mixing have demonstrated that the relation of these contributions is dependent on the intensity of interacting waves, and also it has been found that polarization of the diffracted wave is dependent on the diffraction order.

Pedro F. González-Dı́az

N. Bloembergen

J. Supronowicz, J. B. Atkinson, L. Krause

Cross sections for ${7}^{2}D$ and ${8}^{2}D$ fine-structure mixing in Rb, induced by collisions with various ground-state atoms and ${\mathrm{N}}_{2}$ molecules, have been determined using methods of atomic fluorescence. Rb vapor, pure or mixed with a buffer gas, was irradiated in a glass fluorescence cell with pulses of radiation from a ${\mathrm{N}}_{2}$ laser-pumped dye laser, populating a $^{2}D$ fine-structure state by two-photon absorption. The resulting fluorescence included a direct component arising from the optically excited state and a sensitized component due to the collisionally populated fine-structure state. Measurements of relative intensities of the two components in relation to buffer-gas pressure yielded the following cross sections (in units of ${10}^{\ensuremath{-}14}$ ${\mathrm{cm}}^{2}$): $Q({7}^{2}{D}_{\frac{3}{2}}\ensuremath{\rightarrow}{7}^{2}{D}_{\frac{5}{2}})=14.1,15.5,\mathrm{and}10.0$; $Q({7}^{2}{D}_{\frac{3}{2}}\ensuremath{\leftarrow}{7}^{2}{D}_{\frac{5}{2}})=9.0,10.5,\mathrm{and}6.9$, for Kr, Xe, and ${\mathrm{N}}_{2}$, respectively; $Q({8}^{2}{D}_{\frac{3}{2}}\ensuremath{\rightarrow}{8}^{2}{D}_{\frac{5}{2}})=8.9,4.9,12.4,24.9,25.8,12.1,\mathrm{and}43.1$; $Q({8}^{2}{D}_{\frac{3}{2}}\ensuremath{\leftarrow}{8}^{2}{D}_{\frac{5}{2}})=5.8,3.2,9.4,15.1,17.6,8.3,\mathrm{and}28.5$, for He, Ne, Ar, Kr, Xe, ${\mathrm{N}}_{2}$, and Rb, respectively. Cross sections for the effective depopulation of the $^{2}D$ states were also determined.