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 2 pi. 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 degrees while maintaining an average transmission amplitude of 0.77. The performance manifests a substantially enhanced tunability afforded by a weak electro-refraction of Delta n = 4 x 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.