Optomechanical nanoresonator readout with optical downmixing

We report an optomechanical downmixing measurement method for nanomechanical resonators, which enables the readout of resonance frequencies larger than the bandwidth of the photodetector. We show this method can easily readout the Brownian noise of the device, in which case the performance of the system is not degraded with respect to direct measurement. In the case when the performance is limited by the photodetector, this technique allows employing low-bandwidth/low-noise photodetectors, leading to higher resolution for sensing applications. This is key in particular for mass spectrometry applications where both high displacement sensitivity and multi-mode operation are required.