Fig. 3.

THz near-field photocurrent maps. a, b Sketches of the applied fields on the FET channel of NW1 with an applied V_DS = 0 V (a) and of NW2 with V_DS = 1 mV (b) together with the topographic maps (in greyscale) of the two FETs. c Near-field second-order photovoltage ∆V₂ maps of NW1 with V_DS = 0 V and at different gate voltages in the range V_G = 0–7 V. d Near-field second-order photocurrent maps ∆I₂ of NW2 with V_DS = 1 mV and V_G in the range 0–6 V. e ∆V₂ profiles of NW1 at different V_G values as obtained by averaging the horizontal cuts of the maps in panel c over a 50 nm range around the NW1 centre. f ∆I₂ profiles of NW2 at different V_G values obtained by averaging horizontal cuts of the maps in panel d over a 50 nm range around the NW2 centre. The flat, nearly zero, photocurrent region in panel e reflects the gate-induced screening of the THz field at the centre of the NW