Fig. 2.

Nano-photocurrent experiment and modeling based on the SR theorem. (A) Nano-photocurrent map of Sample 1 at $\lambda =4.5 \hspace{0.17em}\mu$m, showing similar direction-switching behavior as in Fig. 1E. (B) Tip-mediated nonlinear photocurrent generation in the interior of the sample. Color plots are numerical calculation of the tip electric fields in the sample. (C) Magnitude and direction of the ${\mathbf{j}}_{\mathbf{l}\mathbf{o}\mathbf{c}}$ (red arrows) on a 100-nm-diameter circle centered at the tip position according to ${\mathbf{j}}_{\mathbf{l}\mathbf{o}\mathbf{c}}(\mathbf{r})=({\sigma }_{\mathit{aac}}{E}_a\widehat{a}+{\sigma }_{\mathit{bbc}}{E}_b\widehat{b})E_c$. Green arrows are schematics of the auxiliary field distribution $\nabla \psi$. (D) Model simulation of the nano-photocurrent pattern with the ${\mathbf{j}}_{\mathbf{l}\mathbf{o}\mathbf{c}}(\mathbf{r})$ profile in C, showing good agreement with the experiment. (E) Model simulation using $j_b={\sigma }_{\mathit{baa}}{E}_a{E}_a$, showing no direction-switching pattern near the contact. (Inset) The simulated $E_a{E}_a$ distribution and the corresponding photocurrent (red arrows). (F) Model simulation using $j_a={\sigma }_{\mathit{aab}}{E}_a{E}_b$, showing more sign changes near the contacts compared to the experiment in A. (Inset) The simulated $E_a{E}_b$ distribution and the corresponding photocurrent (red arrows). The magnitudes of the simulated photocurrent in E and F are scaled by 0.05 and 100 times, respectively. a.u., arbitrary units.