Accurate determination of the weak optical absorption of piezoelectric crystals used as capacitive massive bolometers

The resonant frequency in a piezoelectric crystal is strongly temperature d ependent. This physical property may be exploited to measure small overheat ing of the material induced by a weak optical absorption. A sample of congr uent LiNbO3 is exposed to a laser beam, with a wavelength lambda chosen ins ide the transparency window. One of the piezoelectric resonances is selecte d and used as an internal thermal probe to record both dynamical and steady state overheating induced by the residual absorption k of the material. A bolometer-type equation describing the time dependence of the sample temper ature is used to determine k accurately, the capacitance-rather than freque ncy-being the temperature-sensitive electrical parameter of the resonance.