  Over the past years, photonic metasurfaces have demonstrated their remarkable
and diverse capabilities for achieving advanced control over light propagation
by confining electromagnetic radiation within the deeply subwavelength
thickness of these artificial films. Here, we demonstrate that metasurfaces
also offer new unparalleled capabilities for decreasing the overall dimensions
of integrated optical components and systems. We propose an original approach
of embedding a metasurface inside one of the most fundamental optical elements
-an optical cavity- to drastically scale-down the thickness of the optical
cavity. We apply this methodology to design and implement a metasurfaces-based
nano-cavity where the Fabry-Perot interferometric principle has been modified
to reduce the cavity thickness below the conventional half-wavelength minimum.
In addition, the nano-cavities with embedded metasurfaces can support
independently tunable resonances at multiple bands. As a proof-of-concept,
using nano-structured metasurfaces within 100-nm nano-cavities, we
experimentally demonstrate high spatial resolution color filtering and spectral
imaging. The proposed approach can be extrapolated to compact integrated
optical systems on-a-chip such as VCSEL, high-resolution spatial light
modulators, imaging spectroscopy systems, and bio-sensors.
