Holographic imaging relies on controlling the phase of electromagnetic waves, but conventional phase modulators are often large and expensive. Here, we demonstrate wide-range THz phase modulation with metasurfaces integrating magnetic resonators and gate-controlled grapheme, whose thicknesses are roughly one-tenth the wavelength of the incident radiation. We use a one-port resonator model to explain the essential features of the proposed metasurface, and we show that graphene is a gate-tunable loss material that can be used to modulate the critical transition in the resonator and achieve an extremely large phase modulation.
Previous efforts to modulate the phase of electromagnetic waves largely relied on changing the refractive index of materials or using a metasurface-based phase modulator; a drawback of the latter was that it could not be tuned remotely. Furthermore, many previous studies were only able to achieve phase modulation over a narrow range, which limited the applications of this technique. Here, we modulate the optical conductivity of graphene to change the phase of waves reflected off of the graphene metasurface. We employ a five-layer metasurface, and we change the resistance of the graphene by applying a voltage. We use spectroscopy to study both the amplitude and the phase of the electromagnetic radiation that is reflected, and we employ simulations to explore the physics of our technique. We are able to achieve phase modulation of 180 degrees, but we note that our results are susceptible to a reduction in reflectance (i.e., losses) that is due to absorption and radiation leakage. Finally, we use simulations to reproduce and support our experimental findings.
We expect that our findings will pave the way for other photonic applications in the THz regime.
