  In this work we present a new mechanism for designing phase-gradient
metasurfaces (PGMs) to control an electromagnetic wavefront with high
efficiency. Specifically, we design a transmission-type PGM formed by a
periodic subwavelength metallic slit array filled with identical dielectrics of
different heights. It is found that when Fabry-Perot (FP) resonances occur
locally inside the dielectric regions, in addition to the common phenomenon of
complete transmission, the transmitted phase differences between two adjacent
slits are exactly the same, being a non-zero constant. These local FP
resonances ensure total phase shift across a supercell that can fully cover the
range of 0 to 2Pi, satisfying the design requirements of PGMs. More studies
reveal that due to local FP resonances, there is a one-to-one correspondence
between the phase difference and the permittivity of the filled dielectric. A
similar approach can be extended to the reflection-type case and other
wavefront transformation, creating new opportunities for wave manipulation.
