Abstract
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.
Highlights
We report on experimental observations of phenomenological self-trapping in plasmonic colloids of varying plasmon peaks in the visible/near infrared
We show that for both modes and for all examined colloids 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
In the case of plasmonic nanocolloids, several studies have reported that self-trapping of laser light is possible by virtue of particle concentration gradients arising from the enhanced particle polarizabilities and exerted on them optical forces [7, 8, 10, 14, 15]
Summary
Nonlinear self-trapping of laser light in soft-matter systems, such as dielectric [1,2,3,4,5] or plasmonic colloids [6,7,8,9,10,11,12,13,14,15] as well as biological media [16,17,18,19,20], has attracted increased attention over the past decade. Others have demonstrated in the same context that the beam is not self-trapped; a self-channeling effect (a phenomenological self-trapping) is observed because of nonlinear thermal lensing, giving the impression of a self-trapped beam, when the laser field is tuned near the plasmon resonance [6, 9, 13]. In this case, the medium acts as a laser-induced (due to optical absorption by the particles) thermal lens, which tends to collimate the externally focused beam, much like an optical telescope. The conditions that demarcate the dominance of either thermal or particle diffusion (due to optical forces) effects, especially far from the plasmon resonance, in the context of self-channeling in plasmonic colloids remain unclear
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