Fig. 2.

Sample and photocurrent imaging. a SEM picture (functional layers false color coded) of a fabricated device. Bias and transimpedance amplifier (TIA) connections are marked. Inset: Zoom-in of the etched optical opening. Scale bar: 100 µm. b Photocurrent map at −10 V bias. Approximate position of optical access opening marked in red. Scale bar: 5 µm. c Laser power dependence of the photocurrent at the position of maximum two-photon contribution corresponding to a bright spot in panel d. Fit $f\left(x\right)=a{x}^2+bx$ separates linear and quadratic contributions, where $x$ stands for optical power. Fit parameters are: $a$ = 117.77 $\pm$ 6.43 fA mW⁻², $b$ = 10.96$\pm$ 0.93 pA mW⁻¹. Black crosses denote measurement points, yellow solid line fit $f\left(x\right)$. Purple dashed-dotted line shows linear and green dotted line quadratic contribution. d Map of two-photon excitation contribution to photocurrent at maximum laser power obtained via fit parameter $a$. Scale bar: 5 µm.