metasurface to achieve a certain functionality in reflection or transmission mode. Recent years have witnessed the development of phase-gradient metasurfaces with different configurations for a wide range of applications such as beam-steering, focusing, and holography. [1,2] These structures allow for integration of various functionalities into a flat surface thus leading to a dramatic reduction in the footprint of optical systems by replacing conventional bulky optical components.Spatiotemporal control over the phase of transmitted and reflected lights across the 2π span is the key to achieve the desired functionalities. The phase tuning mechanisms of metasurfaces have been mostly based on varying the size of nanoantennas within the resonant scattering regime or rotating half-wave plate elements to imprint a geometric phase shift on the circularly polarized light scattered with the opposite handedness. [3,4] For a nanoantenna supporting a single isolated electric or magnetic resonance, the maximal resonant phase agility is π which hinders full control over the light wavefront without employing a back mirror. Metasurfaces with both electric and magnetic responses can be used to overcome this limitation via spectrally overlapping [5] and interleaving [6] electric and magnetic resonances for operation in transmission and reflection modes, respectively. The spectral overlap of electric and magnetic dipole resonances in a Huygens' metasurface satisfies the first Kerker condition leading to suppressed backward scattering from the elements thus eliminating the reflection and giving rise to maximal transmission with 2π phase agility. Although Huygens' metasurfaces have been constructed using metallic elements in microwave regime, bringing plasmonic Huygens' metasurfaces into optical frequencies is challenging due to their complex geometries and arrangements, and is further hindered by the significant increase in the ohmic loss of plasmonic materials at higher frequencies. [7] All-dielectric metasurfaces consisted of high-index subwavelength elements embedded in a low-index environment can eliminate these limitations in that they can support both electric and magnetic resonances with simple geometries and do not suffer from significant dissipative losses. [8][9][10] Moreover, they are fully compatible with standard In this paper, an electrically tunable all-dielectric metasurface doublet is proposed operating at mid-infrared frequency regime with dynamic 2π phase span in transmission mode. Each layer of the metasurface consists of a periodic array of silicon nanobars configured into p-i-n junctions in which the double carrier injection into the intrinsic region under forward bias allows for tuning of the silicon refractive index. The physical mechanism is based on the spectral overlap of high quality factor guided mode resonances supported by each constituent layer establishing an extreme Huygens' operation regime of nearly reflectionless transmission with steep phase spectrum. The short response time of field-driven car...

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We introduce a new class of coherent light emitters in the mid-infrared (MIR) range using quasi-bound states in the continuum (Q-BICs). The emitter is fabricated on a phononic substrate and supports the Friedrich-Wintgen interfering resonances.