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 $70\text{−}\mu \mathrm{m}$-diameter graphene drum resonator with a signal-to-noise ratio of greater than 25 dB, corresponding to a transduction sensitivity of $S_N^{1/2}=2.6\times {10}^{-13}\mathrm{m}{\mathrm{Hz}}^{-1/2}$. The sensitivity of force measurements using this resonator is limited by the thermal noise driving the resonator, corresponding to a force sensitivity of $F_{\min }=1.5\times 10^{-16}\mathrm{N}{\mathrm{Hz}}^{-1/2}$ with a bandwidth of 35 kHz at room temperature ($T=300\mathrm{K}$). Measurements on a $30\text{−}\mu \mathrm{m}$ 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 $G/2\pi =0.21\mathrm{MHz}/\mathrm{nm}$ and ${\mathrm{\Gamma }}_{\mathrm{dp}}/2\pi =0.1\mathrm{MHz}/\mathrm{nm}$, respectively.

Figure 1

(a) SEM image of a graphene drum resonators; a broken drum shows the contrast due to the graphene. (b) Illustration showing a microsphere with a graphene resonator in the evanescent field. (c) Illustration showing the optical coupling rates associated with the cavity measurement, discussed in the text. (d) Schematic of the measurement system.

Figure 2

Experimentally measured normalized transmission ($T_{12}$) spectra with the position of the graphene resonator $x$ within the evanescent field. At 0 nm, the graphene is far from the microsphere surface; at 100 nm, it is almost touching. Laser detuning is defined as $\mathrm{\Delta }=0$ at ${\omega }_c$ for the $x=0\mathrm{nm}$ spectra. (Inset) Extracted optical resonance frequency ${\omega }_c$ and loss due to the graphene ${\kappa }_g$.

Figure 3

(a) Measured spectrum showing the fundamental mechanical resonance of a $70\text{−}\mu \mathrm{m}$-diameter graphene drum, spectrum analyzer resolution bandwidth $(\mathrm{RBW})=100\mathrm{Hz}$. The traces correspond to drum displacements of $-30$ (purple line), $-15$ (green line), and 0 nm (blue line) from the final position which gave the maximum signal transduction. (b) Spectra showing the three lowest-order resonances of a $30\text{−}\mu \mathrm{m}$-diameter resonator, $\mathrm{RBW}=10\mathrm{kHz}$.

Figure 4

Mechanical resonance spectra of a $20\text{−}\mu \mathrm{m}$-diameter resonator with optical power dissipated by the graphene $P_g$, with (inset) extracted resonance peak ${\omega }_{\text{eff}}$. Spectra are offset by 2.5 dB m on the vertical scale.