Analysis of the infrared sensitivity of a quartz resonator application as a thermal sensor

It is well-known that the quartz is able to absorb infrared radiation, especially between 5 μm and 7.7 μm. We take advantage of this feature to study the resonant frequency behavior of the conventional quartz crystal resonator (a doubly rotated SC-cut, vibrating on its 3rd overtone-10 MHz thickness shear mode) exposed to infrared radiation. The goal of this work is not only to design a first IR prototype sensor but also to provide information on the influence of radiative exchanges in temperature controlled ovens used in quartz frequency standards. The detailed steps of preliminary experiments are described. In order to preserve a high quality vibration and to reduce acoustic losses, the quartz resonator must be kept under vacuum: all the experiments had been performed in this condition, keeping in mind that in its final version the sensor will be set under vacuum into a sealed enclosure. In addition, the specific geometry of the deposited electrode combines a good IR absorption of the sensitive quartz with its vibration capability. Moreover, a frequency beat technique is used to cancel the ambient temperature drift. Two complementary models are compared with experimental results: the first one is an electrical analogy model, set up in order to study the global behavior of the temperature in the quartz crystal resonator and the second one is a finite elements model which allows us to understand the temperature gradient effect on the resonator frequency