When two sub-wavelength metallic nanoparticles, each of them supporting a resonant plasmon, are brought within few nanometers or less from each other, the two plasmonic resonances are strongly coupled. The new eigenmodes of the system include in particular a dipolar mode, for which the maximum electric field is localized in the nanoscale gap between the particles. The local field enhancement compared to the incoming far field can be several hundred folds.
We present the design and fabrication of such plasmonic gap cavities, created by depositing gold nanospheres on an atomically flat gold surface, which has been functionalized with a self-assembled monolayer of thiol molecules. This system enables extremely large and reproducible enhancement of the Raman signal from the molecules.
Although these nanogap cavities have been used in SERS studies for some time already, a detailed understanding of the out-of-equilibrium physics under laser irradiation is missing. What is the local temperature of the electrons in the metal? Is the molecular vibration in equilibrium with the surrounding thermal bath?
We will present our latest results in the spectroscopy of these nanocavities under broadly tunable excitation. In particular, we want to clarify if a suitable detuning of the laser from the plasmonic resonance can lead to amplification of molecular vibrations  well above the thermal occupancy.
 P. Roelli et al, Nature Nanotechnology 11, 164–169 (2016)
Christophe Galland, Aqeel Ahmed, Philippe Roelli, Tianqi Zhu, and Santiago Tarrago, "Nanoscale thermometry and optically pumped phonons in plasmonic nanocavities (Conference Presentation)," Proc. SPIE 10672, Nanophotonics VII, 106720H (Presented at SPIE Photonics Europe: April 23, 2018; Published: 23 May 2018); https://doi.org/10.1117/12.2307187.5788813807001.
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