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Our recent exploration of pulsed molecular optomechanics in plasmonic nanocavities shows unexpected and unusual nonlinear effects.
Extreme plasmonic nanocavities are created by placing spherical Au nanoparticles above a gold planar mirror, forming Nanoparticle-on-Mirror (NPoM) constructs. Depositing a self-assembled molecular layer under the Au nanoparticles ensures placement of these molecules inside the high-field mode.
We perform power-dependent ps SERS measurements with on- and off- resonant pump conditions for several molecular systems including biphenyl-4-thiol (BPT) and p-terphenyl thiol (TPDT). Key results are the reversible nonlinear saturation of emission from the anharmonicity of this optomechanical molecular system. Our earlier work showed the superlinear antiStokes emission [1], and more recently superlinear Stokes emission is also observed [2], arising from the driven vibrational dynamics.
In our new data we identify several new power-dependent dynamics. One is the irreversible reconfiguration of the molecular and atomic-scale Au morphologies, which in the ultrasmall volume nanocavities here is inevitable. We correlate these effects with the instantaneous effective temperature of the molecules and compare this to that of the electrons in the surrounding NPoM structure.
The second new effect is a reversible saturation which comes from the anharmonicity of the vibrations, and again is only seen in such tightly coupled nanocavities. These experiments reveal the complexity of molecular-light interactions in extreme nano-optics and open up new ways to treat molecules as optomechanical systems that can be used in device configurations.
[1] Single-molecule optomechanics in picocavities, Science 354, 726 (2016)
[2] Pulsed molecular optomechanics in plasmonic nanocavities, PRX 8, 011016 (2018)
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