Evanescent field optical readout of graphene mechanical motion at room temperature

Graphene mechanical resonators have recently attracted considerable attention for use in precision force-and mass-sensing applications. To date, readout of their oscillatory motion typically requires cryogenic conditions to achieve high sensitivity, restricting their range of applications. Here we report the demonstration of an evanescent optical readout of graphene motion, using a scheme which does not require cryogenic conditions and exhibits enhanced sensitivity and bandwidth at room temperature. We utilize a high-Q microsphere to enable the evanescent readout of a 70-mu m-diameter graphene drum resonator with a signal-to-noise ratio of greater than 25 dB, corresponding to a transduction sensitivity of S-N(1/2) =2.6 x 10(-13) mHz(-1/2). The sensitivity of force measurements using this resonator is limited by the thermal noise driving the resonator, corresponding to a force sensitivity of F-min = 1.5 x 10(-16) NHz(-1/2) with a bandwidth of 35 kHz at room temperature (T = 300 K). Measurements on a 30-mu m graphene drum have sufficient sensitivity to resolve the lowest three thermally driven mechanical resonances. The graphene drums couple both dispersively and dissipatively to the optical field with coupling coefficients of G/2 pi=0.21 MHz/nm and Gamma(dp)/2 pi = 0.1 MHz/nm, respectively.