Stimulated Rayleigh-Bragg scattering in two-photon absorbing media

Abstract

The origin and mechanism of backward stimulated Rayleigh scattering in two-photon absorbing media are studied theoretically and experimentally. This type of stimulated scattering has the unusual features of no frequency shift and low pump threshold requirement compared to all other known stimulated scattering effects. This frequency-unshifted stimulated Rayleigh scattering effect can be well explained by a two-photonexcitation-enhanced Bragg grating reflection model. The reflection of the forward pump beam from this stationary Bragg grating may substantially enhance the backward Rayleigh scattering beam, providing a positive feedback mechanism without causing any frequency shift. A two-counterpropagating-beam-formed grating experiment in a two-photon absorbing dye solution is conducted. The measured dynamic behavior of Bragg grating formation and reflectivity properties are basically consistent with the predictions from the proposed model.

Combined nonlinear effects in two-photon absorption chromophores at high intensities

“…17 shows good agreement with the results of this model. To get an order of magnitude of the numbers involved, consider the experiment of He et al 16,17 and the results shown in Figure 6. For a laser wavelength of 532 nm and T e ~ 1 ns, 5,26 I sat ~ 0.7 GW/cm 2 .…”

“…20,21 It is possible however, under TPA resonant conditions, that n 2 could have a finite response time (i.e., χ (3) is complex with a finite damping coefficient 21 ). However, if this is included in the development of the coupled-wave intensity equations, it would lead to a term in the gain G that is proportional to I L , not 2 L I , which would be inconsistent with the experimental results of He et al 17 It is also quite likely that the Kerr refractive term would be much smaller than the term proportional to…”

“…He et al 17 have observed this kind of dependence and have verified in their case that this is due to a stimulated scattering phenomenon. We consider this next.…”

“…16,17 They considered a stimulated thermal Rayleigh scattering model based on TPAenhanced temperature and density fluctuations, but ruled this out due to (a) the broad linewidth of their pump laser, which would severely reduce the gain of the stimulated wave, and (b) the fact that the peak gain in this theory is for an anti-Stokes-shifted wave, contrary to their experimental results. They concluded that the stimulated wave was due to a Bragg grating formed by the superposition of the incident laser beam and an elastically (Rayleigh) backscattered wave, creating an index grating via a TPA-resonance enhanced nonlinear index coefficient n 2 .…”

“…Parameters have been adjusted to yield results close to the experimental data of He et al 16,17 The coefficients used are β = 9.46 cm/GW (the value quoted in Ref. 17), η = 0.04, g = 1800 cm 3 /GW 2 , and γ eff = 120 cm 3 /GW 2 , with d = 1 cm. Here the transmittance is the ratio of output power to input power.…”