This paper deals with the mathematical model of a new opto-thermal sensor based on resonant opto-thermo-acoustic detection (ROTADE). In ROTADE sensing, a thermal diffusion wave excites a resonant acoustic vibration in a quartz tuning fork (QTF) detector. The model is solved using the finite element method and couples heat transfer and thermoelastic deformation to determine the strength of the generated signal. Moreover, the authors solve the optimal design problem which requires the maximization of the signal as a function of the geometry of the QTF. The optimal design found without imposing any constraint on the resonance frequency produces a signal that is 24 times greater than that obtained with a current sensor.