  Development of broadband thermal sensors for the detection of, among others,
radiation, single nanoparticles, or single molecules is of great interest. In
recent years, photothermal spectroscopy based on the shift of the resonance
frequency of stressed nanomechanical resonators has been successfully
demonstrated. Here, we show the application of soft-clamped phononic crystal
membranes made of silicon nitride as thermal sensors. It is experimentally
demonstrated how a quasi-bandgap remains even at very low tensile stress, in
agreement with finite element method simulations. An increase of the relative
responsivity of the fundamental defect mode is found when compared to that of
uniform square membranes of equal size, with enhancement factors as large as an
order of magnitude. We then show phononic crystals engineered inside
nanomechanical trampolines, which results in additional reduction of the
tensile stress and increased thermal isolation, resulting in further
enhancement of the responsivity. Finally, defect mode and bandgap tuning is
shown by laser heating of the defect to the point where the fundamental defect
mode completely leaves the bandgap.
