This paper investigated an infrared (IR) sensitive Lateral Field Excitation (LFE) Film Bulk Acoustic Resonator (FBAR). The resonant frequency of the LFE FBAR decreased when there was IR (peak wavelength at 750nm) illumination on the device. A linear relationship between the resonant frequency and the IR intensity was obtained with a detection limit of 9 μW/mm2. The sensing mechanism is attributed to the fact that the Young's modulus of the resonator material (ZnO) depends on temperature. In general, for a resonator operating in a bulk mode, a change in the Young's modulus translates into a shift of the resonant frequency. Thus, the sensitivity of the FBAR relies on its temperature coefficient of resonant frequency (TCF). Thickness Field Excitation (TFE) FBAR possessed a larger TCF. However, it showed a lower sensitivity to IR compared with the LFE FBAR. This was due to the reflection of IR radiation from the top electrode on the TFE FBAR.