Abstract
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.
Highlights
Photothermal sensing based on nanomechanical silicon nitride (SiN) resonators has demonstrated exceptional sensitivity for the detection of radiation [1] and single molecules [2]
We 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
In addition to the band gap, the resonance frequency, fr, of the fundamental defect mode is shown as well, in order to highlight a similar relation to the initial stress and that it remains in the band gap for all σinit values
Summary
Photothermal sensing based on nanomechanical silicon nitride (SiN) resonators has demonstrated exceptional sensitivity for the detection of radiation [1] and single molecules [2]. The direct measurement of photothermally absorbed power via the detuning of the resonance frequency enables thermomechanically limited low background noise and high responsivity. The possibilities of qualitative and quantitative analysis on a great variety of analytes ranging from complex chemical compounds [3,4] to nanoparticles [5,6] indicate the applicability of this technique to different sample requirements. Surrounding SiN mechanical resonators with phononic crystals (PnCs) has been shown to reduce acoustic radiation losses into the supporting silicon (Si) frame [7,8]. Patterning the PnC directly into the resonator leads to so-called “soft-clamping” in addition to reduced radiation losses [9], which results in enhanced damping dilution
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