Tunable All-Dielectric Metasurfaces for Phase-Only Modulation of Transmitted Light Based on Quasi-bound States in the Continuum

Phase-only modulators are of great importance for dynamic control over the wavefront of light in a wide range of applications where high efficiency and uniform amplitude are required. Electro-optical tuning approaches based on electro-refraction induced by free carrier effects are of particular interest for developing phase-only modulators due to offering high speed and low power consumption. Here, an electro-optically tunable all-dielectric metasurface is proposed operating in the near-infrared frequency regime for dynamic control over the phase retardation of transmitted light while maintaining a high amplitude with minimal variations over the phase modulation range. The metasurface consists of a zigzag array of elliptical silicon nanodisks connected in each column via silicon nanobars serving as biasing lines. The constituent elements of the metasurface are configured as multijunction p–n structures with moderate doping levels whose multigate biasing enables modulation of carrier concentrations. Due to broken symmetry in the zigzag arrangement, the symmetry-protected bound states in the continuum supported by the metasurface collapse into Fano resonances with extremely high quality-factors under normal incidence. The spectral overlap of excited electric and magnetic quasi-bound states in the continuum is exploited to establish a Huygens' regime with maximal transmission and highly steep spectral phase agility of 2π. The electro-optical shift of the Huygens mode via the electro-refraction induced by carrier accumulation in multijunction p–n structures under applied bias voltage yields a wide dynamic phase span of 240° while maintaining an average transmission amplitude of 0.77. The performance manifests a substantially enhanced tunability afforded by a weak electro-refraction of Δn = 4 × 10–3 which is attributed to the ultrahigh Q-factors of the quasi-bound states in the continuum leading to the significant increase in the lifetime of photons and field confinement within the active regions of resonators. The potential application of such a multifunctional transmittive metasurface is numerically demonstrated in two different areas, namely dynamic polarization control and tunable pulse compansion.