  The behavior of the TE and TM electromagnetic waves in graphene at the
interface between two semi-infinite dielectric media is studied. The dramatic
influence on the TE waves propagation even at very small changes in the optical
contrast between the two dielectric media is predicted. Frequencies of the TE
waves are found to lie only in the window determined by the contrast. We
consider this effect in connection with the design of graphene-based optical
gas sensor. Near the frequency, where the imaginary part of the conductivity of
graphene becomes zero, ultrahigh refractive index sensitivity and very low
detection limit are revealed. The considered graphene-based optical gas sensor
outperforms characteristics of modern volume refractive index sensors by
several orders of magnitude.
