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

Active photonic systems comprising arrays of active metasurfaces─arrays of tunable resonators─offer dynamic wavefront control at subwavelength scales. Transmissive metasurfaces are an essential requirement in cascaded arrays of metasurfaces and enable integration with chip-scale light sources and detectors. However, most existing active phase control metasurface designs are reflective due to fundamental limitations in single-resonance transmissive architectures, which typically exhibit a transmission null at resonance and restrict transmitted phase shifts to 0-180°. We report an approach to overcome these constraints by introducing additional diffraction ports in reflection while maintaining a single transmission port. This configuration enables continuous 0-360° phase tuning in transmission using a single resonance while avoiding the transmission zero. Moreover, we analytically demonstrate using temporal coupled-mode theory that this approach supports a spectrally flat transmission amplitude across the entire phase range─an effect previously observed only in multiresonant (Kerker-type) systems. Unlike those, our design allows dynamic phase control with a single resonance and a constant transmission. To validate our theory, we present proof-of-concept active metasurfaces using lithium niobate as the tunable material. Two designs are explored via full-wave simulations: one using high-Q germanium Mie resonators at 3 μm, achieving ∼250° tunable phase shift with constant transmission amplitude ∼0.45; and another using silicon resonators at telecom wavelengths, demonstrating ∼300° phase shift with amplitude ∼0.4. Both approach the theoretical transmission bound of 0.5. Our approach enables compact, dynamically tunable transmissive metasurfaces with near-ideal phase and amplitude characteristics, paving the way for integrated, reconfigurable metasurfaces.