Signal Amplification by Two-Wave Mixing in Dye-Doped Films

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The present work is concerned with observation of self-diffraction when two beams, signal beam and pump beam, are introduced into optical nonlinear media (dye-doped polymer films). In two-wave mixing. Two coherent beams intersect in the film and the one of them is enhanced at the expense of the other. The power transfer is performed by introducing a phase shift between the interference pattern and the phase-and/or absorption-grating. In an organic-dye-doped film, frequency shifting of one of two beams creates a moving grating and thus leads to the phase shift because of the nonzero response time of the nonlinear optical material.(1) For saturable dyes, the normalized self-diffracted intensity has a product of two components : the antisymmetric component with respect to the frequency difference OMEGA of the two waves, which is due to the dispersion of refractive index has a positive or negative exponential gain for OMEGA > 0 or OMEGA<0 for positive detuning parameter, respectively, and the symmetric one arising from absorption of the pump wave has a negative exponential gain for all OMEGA.(2) For xanthene dyes and azine dyes the intensity of the self-diffracted beam increased at most by several percent compared with that of the transmitted beam for OMEGA = 0.(3) For azo dyes, self diffraction was observed even when two beams were cross-polarized.(4) Amplification will be observed with a high power laser whose wavelength is off resonance.