Wavelength dependence of photoinduced deformation in BiFeO${}_3$

Optomechanical effects in polar solids result from the combination of two main processes, electric field-induced strain and photon-induced voltages. Whereas the former depends on the electrostrictive ability of the sample to convert electric energy into mechanical energy, the latter is caused by the capacity of photons with appropriate energy to generate charges and, therefore, can depend on wavelength. We report here on mechanical deformation of BiFeO${}_3$ and its response time to discrete wavelengths of incident light ranging from 365 to 940 nm. The mechanical response of BiFeO${}_3$ is found to have two maxima in near-UV and green spectral wavelength regions.

Figure 1

Optical microscope picture of a 90-μm-thick crystal of BFO. The longest edge is along [101].

Figure 2

Photoinduced deformation of the BiFeO${}_3$ crystal as a function of time. The inset shows the wavelength dependence of the average strain value taken during 15 s illumination time. The horizontal line represents the average strain level induced by white spectrum light (measured before all other wavelengths).

Figure 3

Response time of the photoelastic effect in the BiFeO${}_3$ crystal as a function of wavelength. The horizontal dash line represents the BFO deformation response time induced by white spectrum light. The inset shows the response time determination approach using fits [red (dark gray) lines] for 530-nm wavelength.

Figure 4

Current as a function of electric field in darkness and under 530 nm.

Figure 5

(a) Time dependence of the photostriction (right scale) and sample temperature change (left scale) due to light 365 nm. (b) Thermal expansion measured along [101] direction in BiFeO${}_3$ crystal in darkness.