  It was demonstrated that optical modulation together with simultaneous
terahertz (THz) imaging application enables an increase in contrast by an order
of magnitude illustrating hence the technique as convenient contactless tool
for characterization of graphene deposited on high-resistivity silicon
substrates. It was shown that the single- and double-layer graphene can be
discriminated and characterized via variation of THz image contrast using a
discrete frequency in a continuous wave mode. Modulation depth of 45 % has been
reached, the contrast variation from 0.16 up to 0.23 is exposed under laser
illumination for the single and double layer graphene, respectively. The
technique was applied in the development and investigation of graphene-based
optical diffractive elements for THz imaging systems.
