A gold nanoparticle sitting on a gold mirror, separated by a molecular-scale gap. The gap is small enough that quantum effects matter — electron tunneling across the junction modifies the plasmonic response. At equilibrium, these quantum corrections are set by the gap geometry and the Fermi distribution.
The paper drives the system out of equilibrium with ultrafast laser pulses. The laser creates ballistic hot electrons — electrons with energy far above the Fermi level, not yet thermalized with the lattice. These hot electrons change the tunneling probability across the gap, modulating the quantum plasmonic response on sub-picosecond timescales.
The trick is indirect modulation. Rather than driving the plasmonic mode directly (which would require enormous field intensities in the gap and risk optical damage), the laser excites hot electrons in the bulk metal, which then modify the gap's quantum properties. The plasmonic response changes because the electrons crossing the gap are no longer in thermal equilibrium — their distribution function is transiently non-Fermi.
The experimental approach solves a longstanding problem: previous ultrafast plasmonic experiments couldn't reach the quantum regime without damaging the nanoscale structure. By decoupling the excitation (bulk absorption) from the modulation (gap tunneling), the system accesses quantum corrections that would be destroyed by direct illumination.
Control through the back door. The gap's quantum properties are modified not by what happens in the gap but by what happens in the metal around it. The equilibrium defines the baseline; the departure from equilibrium defines the control.