  We report a high performance relative humidity (RH) microsensor by coating a
few-layer graphene oxide (GO) flake over a photonic crystal (PC) cavity. Since
the GO layer has a high water-activity and interacts with the evanescent cavity
mode strongly, the exposure of GO-PC cavity in varied humidity levels results
in significant resonant wavelength shifts, showing a slope of 0.68 nm/%RH in
the range of 60%-85% RH. By interrogating the power variation of the cavity
reflection, the microsensor presents an ultrahigh sensitivity exceeding 3.9
dB/%RH. Relying on the unimpeded permeation of water molecules through the GO
interlayers and microscale distribution of the cavity mode, the integrated
sensor has a response time less than 100 ms, which promises successful
measurements of human breathing. Combining with the ease of fabrication, this
high performance RH sensor has potential applications requiring optical access,
device compactness, and fast dynamic response.
