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 dependent. This physical property may be exploited to measure small overheating of the material induced by a weak optical absorption. A sample of congruent LiNbO₃ is exposed to a laser beam, with a wavelength λ chosen inside the transparency window. One of the piezoelectric resonances is selected 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 temperature is used to determine k accurately, the capacitance-rather than frequency-being the temperature-sensitive electrical parameter of the resonance