Fig. 4: Atomic-scale microscopy of hot carriers and nonlinear susceptibility (χ⁽³⁾).

a STM image of a single seven-atom-wide graphene nanoribbon (7-AGNR) on the Au(111) surface. The color bar represents the relative change in the vertical position of the Au nanotip (in Å) while scanning the GNR in the constant current mode of the STM with a tunneling current of 1 nA at 1 V. Bottom-right inset in (a) shows the chemical structure of the GNR. b, c Spatial variation of the intensity of the hot carrier (b) and the FWM (c) contributions in the anti-Stokes spectra recorded simultaneously with the topography shown in (a). The delay between the pump and the probe pulses was set to be zero. The hot carrier and the FWM contributions in the anti-Stokes spectra were evaluated by spectral integration in the wavelength range of 665–675 nm and 635–645 nm, respectively. Dashed black rectangles in (a, b and c) indicate the location of the GNR. d Variations of the hot carrier (red) and FWM (blue) signals intensities along the annotated lines in (b and c), respectively. Probe and pump laser powers were fixed at 2.5 mW and 5.5 mW, respectively.