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Optical control of THz reflectivity with surface waves

We propose an experimental demonstration of a THz modulator with a visible optical command. The device is a n-doped GaAs grating with subwavelength dimensions. The principle of this modulator is the control of the THz resonant absorption by surface waves supported by the grating. This absorption is modulated with low power visible light, leading to a modulation of the reflected THz beam. From experimental polarized THz reflectivity measurement of the grating, we show that a depletion layer at the surface of the doped GaAs has to be taken into account to correctly describe the observed resonant absorption. From experimental observation and modeling we are able to ascribe this absorption to the coupling of incident THz light with surface plasmon-phonon polariton mode propagating along each wall of the grating. Thus, each wall acts as a nano-antenna that resonantly absorbs light. The grating can be viewed as a metamaterial composed of individual resonators. The theoretical model indicates that the reflectivity dip linked to the surface wave is sensible to the electronic density in the walls of the grating. We performed an experiment to measure the THz reflectivity while illuminating the grating with visible photons having energy higher than the bandgap of GaAs. The created photoelectrons change the effective mode index, leading to a shift of the resonant absorption frequency. This demonstrates the modulation of THz radiation around 8.5 THz with a visible optical command at room temperature.