Low-loss spatial phase modulation of mm-waves via optically-tunable coupling

Efficient phase modulation at mm-wave and THz frequencies is an ongoing challenge. Here, we introduce a method for high-resolution dynamic spatial phase modulation of mm-waves, via selective photo-activation of a silicon-based resonant metasurface. In contrast to conventional methods of regulating the phase of reflected waves by shifting the resonance frequency of a modulator, we control the resonant coupling strength to realise low-loss binary phase modulation. We demonstrate a system operating at a wavelength of 6 mm, realising dynamically encoded holograms enabling focussing, beam steering, multi-casting and orbital angular momentum mode generation. Moreover, we show that optical losses scale favourably with frequency, making this approach highly promising for higher frequency applications.