Overcoming Barriers to Dynamic Phase-only Modulation in Transmissive Metasurfaces via Diffraction Control

Achieving dynamic phase-only modulation in transmissive metasurfaces—where the phase of transmitted light is changed without altering its amplitude—is essential for wavefront shaping and optical information processing. However, transmissive metasurfaces face intrinsic challenges due to interference between continuum and resonance modes. This interference induces a transmission zero in systems utilizing a resonance with an input port and a transmission port, limiting both the phase-only controllability and the achievable phase range. In this work, we present a comprehensive, step-by-step diffraction-based methodology to overcome such constraints. We formulate the underlying theory, establish a theoretical upper bound, and demonstrate a proof-of-concept metasurface leveraging the Pockels effect in lithium niobate. We explore two specific metasurface configurations utilizing germanium and silicon. The germanium-based metasurface demonstrates a phase shift of ~250° with a uniform transmission amplitude of ~0.45 at a wavelength of 3 µm. The silicon-based design achieves a ~300° phase shift with an amplitude of ~0.4, operating at telecommunication wavelengths.