Real-time measurement of micrometer-order amplitude transverse vibrations using the photo-emf effect in photoconductive materials

We report the use of the photo-emf effect in BTO photorefractive crystals to measure sub-micrometer-order amplitude transverse vibrations. The method is based on the illumination of the surface under analysis by a direct laser beam of wavelength λ = 532 nm and the collection of the back-scattered speckled patterns of light onto the photoconductor. A pattern of space-charge electric field is built-up in the photoconductive material volume in a time-scale corresponding to the response time of the material that is essentially controlled by charge-transport phenomena. A pattern of free electrons in the conduction band is simultaneously built-up with a much faster time-scale that depends on the excitation of electrons from photoactive centers inside the material band gap into the conduction band. If the illuminated target surface is static, the pattern of space-charge field and free electrons are in mutual equilibrium and no electric signal is detected. However, if the target is laterally vibrating, the speckle pattern of light is simultaneously moving and the fast pattern of free electrons follows. The pattern of space-charge field instead is comparatively much slower and is not able to follow it. In this way the free charge distribution and the pattern of electric field are mutually displaced proportionally to the amplitude of the target vibration and are not any more in equilibrium. An alternating current is therefore produced that can be detected to find out the size of the target vibration amplitude. We report experiments carried out with Bi₁₂TiO₂₀ crystals.