**2. Experimental process**

Tetragonal phase BiFeO<sup>3</sup> (T-BFO) films of ~30 nm thickness were fabricated epitaxially by pulsed laser deposition (PLD) technique on (001)-oriented LaAlO<sup>3</sup> . In this experimental process, KrF excimer laser with the wavelength of 248 nm was used for deposition, the deposition frequency is 3 Hz with an energy of about 240 mJ. The substrate was kept at 650°C with 11 Pa of oxygen atmosphere. In the course of deposition, the substrate holder was still rotated with the speed of 360°/min so that the thickness variation of the film can be reduced and the uniform composition of the film can be obtained as much as possible. Followed the deposition, oxygen stoichiometric T-BFO films were in situ annealed in 500 Pa oxygen pressure and cooled slowly down at 5°C/min to room temperatures to avoid the effect of deficient oxygen. Structural characterization was performed using X-ray diffraction (XRD), using M/s Bruker make D8-Discover system. Room temperature transmittance and reflectance spectra were collected by using a Perkin-Elmer Lambda-900 spectrometer (with the energy of 0.41–6.53 eV). For the conductive characteristics measurements, platinum electrodes of 200 μm length and interelectrode distances of 20 μm were fabricated by sputter deposition using conventional photolithography and lift-off technique. The photoelectric effect was measured by illuminating the gap between the electrodes with a *λ* ≈ 405 nm (*E* ≈ 3.06 eV) laser (Newport LQA405-85E) with a maximum power of 80 mW for the illumination, yielding the incident light power density on the sample surface up to 0.8 W/cm2 and simultaneously measuring the photocurrent using a high-input impedance electrometer (Keithley 6517).
