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The emergence of photonic metasurfaces has enabled a new paradigm of light control through the introduction of surface discontinuities. Space-gradient metasurfaces consist of planar arrays of nano-structured antennas which induce spatially varying phase and/or polarization to propagating light. As a consequence, photons propagating through space-gradient metasurfaces can be engineered to undergo a change to their momentum, angular momentum and/or spin state. This has led to a relaxation of Snell’s law, a pivotal relation in optical engineering, and has enabled a whole new family of flat optical devices. We have utilized the engineered control over photonic spin and momentum to develop a set of ultra-compact metasurface based devices including a chiroptical spectrometer that can be used for biochemical sensing, a polarization rotator with possible applications for secure quantum communication, and nano-cavities to enhance photonic spontaneous emission using the Purcell effect.
It has been recently demonstrated that the field of flat photonics is further empowered by utilizing time-gradient metasurfaces with dynamic responses to propagating light. A new genus of optical devices and physical effects can be realized provided one can overcome some fundamental limitations of metasurfaces with space-gradient alone. Photons experience inelastic interactions with time-varying metasurfaces resulting in a Doppler-like wavelength shift. Furthermore, Snell’s relations are modified to a more universal form not limited by Lorentz reciprocity, hence meeting all the requirements to build magnetic-free optical isolators. Consequently, metasurfaces with both space- and time-gradients can have a strong impact on a plethora of photonic applications and provide versatile control over the physical properties of light.
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