Ultracompact biochemical sensor built with two-dimensional photonic crystal microcavity

We report an experimental demonstration of an ultracompact biochemical sensor based on a two-dimensional photonic crystal microcavity. The microcavity, fabricated on a silicon-on-insulator substrate, is designed to have a resonant wavelength ($\lambda$) near $1.5 \mathit{\mu }\mathrm{m}$. The transmission spectrum of the sensor is measured with different ambient refractive indices ranging from $n=1.0$ to $n=1.5$. From observation of the shift in resonant wavelength, a change in ambient refractive index of $\mathrm{\Delta }n=0.002$ is readily apparent. The correspondence between absolute refractive index and resonant wavelength agrees with numerical calculation to within 4% accuracy. The evaporation of water in a 5% glycerol mixture is also used to demonstrate the capability for in situ time-resolved sensing.