  Higher harmonic modes in nanoscale silicon cantilevers and microscale quartz
tuning forks are detected and characterized using a custom scanning optical
homodyne interferometer. Capable of both mass and force sensing, these
resonators exhibit high-frequency harmonic motion content with picometer-scale
amplitudes detected in a 2.5 MHz bandwidth, driven by ambient thermal
radiation. Quartz tuning forks additionally display both in-plane and
out-of-plane harmonics. The first six electronically detected resonances are
matched to optically detected and mapped fork eigenmodes. Mass sensing
experiments utilizing higher tuning fork modes indicate >6x sensitivity
enhancement over fundamental mode operation.
