A laser source of wavelength 532 nm and the diameter of ~315 µm was connected to a tilting stage through an optical fiber. The tilting stage enabled easy adjustment of the incident direction of the laser beam. Electrical measurements of the photodetecting device were performed using a semiconductor characterization system (Keithley 4200). The photoresponse was recorded in real time by using a data acquisition system (DAQ) (Keithley 3706A) and a source meter (Keithley 2612). The raw photoresponse data from the DAQ were input into a program (encoded in MATLAB) to allow visualization of the photocurrent mapping. The program represents the intensity of the photocurrent mapping by a color scale and calculates the direction based on Eq. (1).