Fig. 2: Two-color broadband nonlinear spectroscopy of hot carriers.

a Sketch of the temporal evolution of the energy distribution of the hot carriers in the picocavity as a function of the pump-probe delay. Left panel: electron distribution when the probe pulse (green curve) precedes (τ < 0) the pump pulse (orange curve). Middle-panel: hot carrier generation on excitation by the pump pulse, followed by anti-Stokes emission (vertical purple curves) upon interaction of the hot carriers with the probe pulse (vertical green curve, τ = 0). Four-wave mixing (FWM) process (vertical blue curve) occurs when the pump (orange) and probe pulses (green) temporally overlap. Right panel: relaxation of the hot carriers after the pump pulse excitation (τ > 0), which is tracked in real-time by the evolving anti-Stokes signal generated by the probe pulses. b Spectra of the pump pulse (orange curve), probe pulse (green curve), and the anti-Stokes spectrum measured at the temporal overlap (zero delay) between the pump and the probe pulses. The purple and blue shaded regions in the anti-Stokes spectrum depict the contributions of the hot carriers and the FWM signals, respectively. c, d Variation in the intensity of the hot carrier (red dots) and FWM (blue dots) contributions in the anti-Stokes spectra as a function of the increasing fluence of the pump (c) and probe (d) pulses, plotted in a dual-logarithmic scale, respectively. Green curves show the numerically calculated variation in the intensity of the hot carrier contribution (at 680 nm) in the simulated anti-Stokes spectra upon change of the fluence of the pulses. e Variation in the intensity of the hot carrier (red dots) and FWM (blue dots) contributions in the anti-Stokes spectra as a function of increasing tip height (Δz), with the corresponding decreasing tunneling current (top x-axis). Δz = 0 Å represents the height of the nanotip from the Au(111) surface at the tunneling condition of 8 nA at 100 mV. Probe and pump laser powers were fixed at 3.1 mW and 7.35 mW, respectively, in (c, d, and e). The delay between pump and probe pulses was set to be zero fs in (c, d, and e). Error bars in (c, d, and e) represent the standard deviation from the integrated spectral region.