  The use of electric fields for signalling and control in liquids is
widespread, spanning bioelectric activity in cells to electrical manipulation
of microstructures in lab-on-a-chip devices. However, an appropriate tool to
resolve the spatio-temporal distribution of electric fields over a large
dynamic range has yet to be developed. Here we present a label-free method to
image local electric fields in real time and under ambient conditions. Our
technique combines the unique gate-variable optical transitions of graphene
with a critically coupled planar waveguide platform that enables highly
sensitive detection of local electric fields with a voltage sensitivity of a
few microvolts, a spatial resolution of tens of micrometres and a frequency
response over tens of kilohertz. Our imaging platform enables parallel
detection of electric fields over a large field of view and can be tailored to
broad applications spanning lab-on-a-chip device engineering to analysis of
bioelectric phenomena.
