Agiotis Leonidas, Meunier Michel
Laser Processing and Plasmonics Laboratory, Department of Engineering Physics, Polytechnique Montréal, C.P. 6079, succ. Centre-ville, Montréal, QC, H3C 3A7, Canada.
Nanophotonics. 2022 Jan 18;11(5):1051-1062. doi: 10.1515/nanoph-2021-0775. eCollection 2022 Feb 1.
We report on experimental observations of phenomenological self-trapping in plasmonic colloids of varying plasmon peaks in the visible/near infrared. A femtosecond (fs) oscillator is used in both pulsed (35 fs, 76 MHz) and continuous wave (cw) operation for comparison. We show that for both modes and for all examined colloids (and under typically applied external focusing conditions in self-trapping studies in colloidal media) nonlinear propagation is governed by thermal defocusing of the focused beam, which precedes the steady-state regime reached by particle diffusion, even far from the plasmon resonance (or equivalently for non-plasmonic colloids, even for low absorption coefficients). A strategy for the utilization of high repetition fs pulses to mitigate thermal lensing and promote gradient force-induced self-trapping is discussed. Notably, nonlinear thermal lensing is further accompanied by natural convection due to the horizontal configuration of the setup. Under resonant illumination, for both fs and cw cases, we observe mode break-up of the beam profile, most likely due to azimuthal modulation instability. Importantly, time-resolved observations of the break-up indicate that in the fs case, thermal convection heat transfer is reduced in magnitude and significantly decoupled in time from thermal conduction, presumably due to temperature increase confinement near the particles. We anticipate that our findings will trigger interest toward the use of high repetition fs pulses for self-channeling applications in nano-colloids.
我们报告了在可见光/近红外波段具有不同等离子体峰的等离子体胶体中现象学自陷的实验观察结果。为了进行比较，使用了飞秒（fs）振荡器的脉冲（35 fs，76 MHz）和连续波（cw）操作。我们表明，对于两种模式以及所有研究的胶体（并且在胶体介质自陷研究中通常应用的外部聚焦条件下），非线性传播受聚焦光束的热散焦控制，这在粒子扩散达到稳态之前就已发生，即使远离等离子体共振（或者等效地对于非等离子体胶体，即使吸收系数很低）。讨论了利用高重复频率飞秒脉冲减轻热透镜效应并促进梯度力诱导的自陷的策略。值得注意的是，由于装置的水平配置，非线性热透镜效应还伴随着自然对流。在共振照明下，对于飞秒和连续波情况，我们都观察到光束轮廓的模式破裂，最有可能是由于方位调制不稳定性。重要的是，对破裂的时间分辨观察表明，在飞秒情况下，热对流热传递的幅度减小，并且在时间上与热传导明显解耦，这可能是由于粒子附近的温度升高受到限制。我们预计我们的发现将引发人们对在纳米胶体中使用高重复频率飞秒脉冲进行自通道应用的兴趣。
