Figure 2. Measurements performed under ambient conditions.

(a) Transmission spectra of device at different stages of fabrication. An intrinsic absorption of 0.01 dB/μm in lower graphene layer was estimated from the reduction in extinction ratio. (b) Applied voltages between two graphene layers cause a reproducible shift of transmission minimum, as is clear in inset. The shift is observable for all minima, as shown in the Supplementary Information. Only two distinct voltages have been plotted for clarity. (c) Wavelength shift Δλ as a function of applied voltage in steps of 5 V. The corresponding values of Δn, using eq. 1, are also plotted. (d) Change in absorption Δα as a function of applied voltage for the device. The hysteresis is mainly attributed to oxide grown with water process.