We describe theoretically a new long-wave infrared optical modulator based on the characteristics of TM surface plasmons in graphene. Calculations made using a finite-τ random-phase approximation model, of relevant surface plasmon propagation parameters, are presented. We show that the plasmon losses vary as a function of carrier density; for large carrier densities, the interband absorption of the plasmon energy is blocked due to filling of the conduction band states, and for small carrier densities, the plasmon energy is absorbed by interband optical transitions. The carrier density versus plasmon loss curve exhibits a kink at the boundary between these two qualitatively dissimilar absorption mechanisms, corresponding to the intersection between the plasmon dispersion curve and the onset threshold for the interband absorption. The modulator device can be switched between high and low transmission states by varying the carrier density with an applied gate bias voltage. The device is limited in optical frequency to plasmons with photon energies less than the optical phonon energy (200 meV in graphene). An example modulator design for light with vacuum wavelength is presented. This modulator exhibits a contrast ratio in the transmitted optical power between ON and OFF states of . A simple circuit model indicates that the switching speed of the modulator should be limited by the carrier relaxation time .
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