Picometer Scale Photothermal Tuning of Plasmonic Nanocavities\nand Interfacial Processes

The ability to precisely tune plasmon\nresonances is critical for\nadvancing nanophotonic and sensing technologies. In this work, we\nexploit the photothermal effect to achieve picometer-level tunability\nof plasmon resonances in nanorod-on-mirror nanocavities, using polyelectrolyte\n(PE) layers as dielectric spacers. The plasmon-induced thermal response\nof these soft materials allows real-time adjustment of the nanocavity,\nunlike stable inorganic spacers like aluminum oxide. Under continuous\nlaser illumination, the PE spacers undergo thickness reduction and\nphase transitions, leading to significant shifts in plasmon resonances.\nThese shifts are influenced by laser power and initial spacer thickness,\nwhich govern near-field enhancement and photothermal effects. It is\nshown that this precise tuning capability enables the exploration\nof various photophysical processes, including the excitation of higher-order\nplasmon modes, optomechanical enhancement of surface-enhanced Raman\nscattering signals, electron tunneling, molecular diffusion from the\nnanocavities, and the transition to charge transfer plasmons.