  Nanomechanical resonators made from van der Waals materials (vdW NMRs)
provide a new tool for sensing absorbed laser power. The photothermal response
of vdW NMRs, quantified from the resonant frequency shifts induced by optical
absorption, is enhanced when incorporated in a Fabry-Perot (FP) interferometer.
Along with the enhancement comes the dependence of the photothermal response on
NMR displacement, which lacks investigation. Here, we address the knowledge gap
by studying electromotively driven niobium diselenide drumheads fabricated on
highly reflective substrates. We use a FP-mediated absorptive heating model to
explain the measured variations of the photothermal response. The model
predicts a higher magnitude and tuning range of photothermal responses on
few-layer and monolayer NbSe$_{2}$ drumheads, which outperform other clamped
vdW drum-type NMRs at a laser wavelength of $532\,$nm. Further analysis of the
model shows that both the magnitude and tuning range of NbSe$_{2}$ drumheads
scale with thickness, establishing a displacement-based framework for building
bolometers using FP-mediated vdW NMRs.
