Applied Physics Seminar

Abstract:

In our lab, we have specialized in sensing temperature-induced changes of the spring constant of nanomechanical resonators by observing the resulting detuning of their resonance frequency. We are particularly interested in resonators, such as strings and drums, whose resonance frequency is determined by their intrinsic tensile stress. Since tensile stress is highly temperature dependent due to thermal expansion of the resonator material, nanomechanical strings and drums are excellent thermometers.

We have shown that such string and drum resonators are sensitive enough to detect the photothermal heating created by individual nanoparticles [1,2] and even single-molecules [3] that are sitting on top of the resonator. When a sample is illuminated with a probing laser, it absorbs part of the light and increases the temperature of the resonator in its vicinity. This local heating causes a measurable change in the resonance frequency. With a stress optimized silicon nitride drum [4], we have achieved a sensitivity of 16 pW/rtHz, which is an improvement of two orders of magnitude over state-of-the-art absorption-based microscopy techniques. Such a sensitivity not only has the potential for single- molecule spectroscopy, but it's also an interesting approach for typing of single samples such as bacteria, viruses, and biomolecules. Furthermore, the intrinsic sensitivity in the femtowatt range allows for the design of an "ideal" thermal infrared detector [5], which in the future could operate at the fundamental photon noise limit.

[1] T. Larsen, et al., "Photothermal analysis of individual nanoparticulate samples using micromechanical resonators." ACS Nano 7(7), 6188-6193, 2013.

[2] S. Schmid, et al., "Low-power photothermal probing of single plasmonic nanostructures with nanomechanical string resonators." Nano Letters 14(5), 2318-2321, 2014.

[3] M.-H. Chien, et al., S. Single-molecule optical absorption imaging by nanomechanical photothermal sensing. PNAS 115(44), 11150–11155, 2018.

[4] N. Luhmann, et al. "Effect of oxygen plasma on nanomechanical silicon nitride resonators." Applied Physics Letters 111(6), 063103, 2017.

[5] M. Piller, et al. "Nanoelectromechanical infrared detector", Proceedings of SPIE, 2019.