The Institute for Lasers, Photonics and Biophotonics, State University of New York at Buffalo, Buffalo, New York 14260-3000, USA.
Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/RX, Wright-Patterson AFB, Ohio 45433-7750, USA.
J Chem Phys. 2019 Sep 14;151(10):104202. doi: 10.1063/1.5119237.
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.
我们报告了在水相中观察到的后向受激米氏散射（SMS），这是由于双光子共振有机发色团的自组装纳米球在约 816nm 波长的约 10-ns 激光脉冲的泵浦下，光场诱导的空间再分布所致。在这样的系统中产生后向受激散射的泵能阈值明显低于纯水中的阈值。反向散射的增益源于感应布拉格光栅，它将部分能量从泵浦光束反射到反向米氏散射光束中。基于 SMS 脉冲起始的时间延迟取决于泵浦水平和溶剂粘度这一实验事实，提出了 SMS 产生的物理模型。我们的实验结果表明，在液体中悬浮的纳米粒子的空间再分布是感应布拉格光栅形成的主要贡献。这些纳米粒子受到的力与前向泵浦光束和反向米氏散射光束之间干涉产生的驻波场强度梯度成正比。当纳米粒子在凝胶状介质中冻结时，不会观察到 SMS，这在实验上支持了所提出的物理模型的有效性。
