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We report the observation of backward stimulated Mie scattering (SMS) due to light-field induced spatial redistribution of self-assembled nanospheres of a two-photon resonant organic chromophore in water, pumped by ∼10-ns laser pulses of ∼816-nm wavelength. The pump-energy threshold for generating backward stimulated scattering in such a system is remarkably lower than that in pure water. The gain of backscattering originates from an induced Bragg grating that reflects partial energy from the pump beam into the backward Mie scattering beam. Based on the experimental fact that the time-delay of the SMS pulse onset depends on both the pump level and the viscosity of the solvent, a physical model of SMS generation is proposed. Our experimental results have shown that the major contribution to the formation of an induced Bragg grating is spatial redistribution of nanoparticles suspended in the liquid. These nanoparticles are driven by a force that is proportional to the intensity gradient of the standing-wave field resulting from interference between the forward pump beam and the backward Mie scattering beam. When the nanoparticle motion is frozen in a gel-like medium, no SMS is observed, which experimentally supports the validity of the proposed physical model.

The nonlinear optical properties of four metallic (Au-, Au/Ag-, Ag-, and Pt-) nanoparticle suspensions in toluene have been studied in both femtosecond and nanosecond regimes. Nonlinear transmission measurements in the femtosecond laser regime revealed two-photon absorption (2PA) induced nonlinear attenuation, while in the nanosecond laser regime a stronger nonlinear attenuation is due to both 2PA and 2PA-induced excited-state absorption. In the nanosecond regime, at input pump laser intensities above a certain threshold value, a new type of stimulated (Mie) scattering has been observed. Being essentially different from all other well known molecular (Raman, Brillouin) stimulated scattering effects, the newly observed stimulated Mie scattering from the metallic nanoparticles exhibits the features of no frequency shift and low pump threshold requirement. A physical model of induced Bragg grating initiated by the backward Mie scattering from metallic nanoparticles is proposed to explain the gain mechanism of the observed stimulated scattering effect.

The backward stimulated Bragg scattering (SBgS) of CdTe(x)Se(1-x) quantum dots in chloroform is investigated at three pump laser wavelengths (532, 816, and 1064 nm) in nanosecond regime. The spectral and temporal structures of the backward stimulated scattering and pump threshold dependence on the concentration are presented in this paper. The energy conversion efficiency from input pump pulse to SBgS pulse was measured to be >or=14%. In addition, the samples exhibit multi- (two-, three-)photon absorption capability over the spectral range we investigated. More importantly, both mechanisms of SBgS and multiphoton absorption provided an enhanced optical limiting performance. The measured nonlinear transmissivity was changed from approximately 0.73 to approximately 0.17 for 532 nm laser pulses and from approximately 0.9 to approximately 0.35 for 816 nm laser pulses when the input pulse energy was changed from 10 to approximately 1500 microJ.

Degenerate two-photon absorption (TPA) spectral properties of five AFX chromophore solutions have been studied using a single and spectrally dispersed sub-picosecond white-light continuum beam. In a specially designed optical configuration, optical pathways inside the sample solution for different spectral components of the focused continuum beam were spatially separated from each other. Thus, the nondegenerate TPA processes coming from different spectral components can be eliminated, and the direct nonlinear absorption spectrum attributed to degenerate TPA processes can be readily obtained. Using this new technique, the complete TPA spectra for these five highly two-photon-active compounds (AF-380, AF-350, AF-295, AF-270, and AF-50) were obtained in the spectral range from 600 to 950 nm on an absolute scale of TPA cross section. The relationship between the molecular structures and their TPA spectral behaviors are discussed. In general the measured TPA spectra are not identical with the linear absorption spectra on the scale of absorbed photon(s) energy. Moreover, for some sample (such as AF-380), the TPA spectrum is totally different from the linear spectrum, which implies the difference of molecular transition pathways and selection rules for one- and two-photon excitation processes. At high excitation intensity levels (>or=15 GW/cm(2)), the saturation behavior of TPA transition can be observed obviously in AF-350 and AF-380 solutions that exhibit much higher nonlinear absorptivity than the other chromophores investigated.