Space-time photonic metamaterials

• Online Access: http://hdl.handle.net/2047/D20385570

Photonic metamaterials are artificially engineered structures composed of meta-atoms which exhibit unusual behaviors in interaction with light. They provide access to a wide range of electromagnetic constitutive parameters well beyond the reach of natural materials, which are determined by the geometry, composition and lattice arrangement of meta-atoms. Arranging subwavelength meta-atoms in a two dimensional space can form metasurfaces, capable of unprecedented modulation of light wavefront through imparting spatially variant local phase shifts to the incoming wave. The small footprint and ultrathin thickness of metasurfaces make them ideal candidates to replace conventional bulky optical components for applications where size and weight are highly constrained such as on-chip photonic networks and smallsat communication systems. Recently, there has been an immense effort to harness different physical mechanisms for post-fabrication control and real-time tuning of the optical response of metamaterials. Despite the fruitful progress in this ongoing effort, the response of such photonic metamaterials has been limited to the static or quasi-static regimes where the temporal variations are disregarded. Introducing time-modulation into metamaterials can enable a myriad of novel physical phenomena by giving rise to parametric frequency conversion processes and lifting some of the fundamental constraints such as Lorentz reciprocity. Moreover, it renders space-time as a four-dimensional design manifold in a holistic manner which can be harnessed to overcome several limitations of the static and quasi-static metamaterials. The objective of this dissertation is to investigate the less explored space-time photonic metamaterials, i.e., metamaterials with both space and time structure. We will systematically establish the fundamental concepts and operating principles of space-time metamaterials for light manipulation. In particular, we will introduce a new geometric phase shift occurring when the light undergoes frequency conversion in a time-modulated metasurface, complementing previously established resonant and Pancharatnam-Berry phase shifts widely used in metasurfaces. This dispersionless modulation-induced geometric phase shift elevates time-modulated metasurfaces over their quasi-static counterparts in terms of tunable wavefront engineering capabilities as it provides access to $2\pi$ phase span with uniform amplitude over a broad bandwidth. In the next step, we will develop several semi-analytical and numerical methods for first-principle simulation of space-time photonic metamaterials possessing multiscale features in both space and time. We will then propose several realizations of such structures at optical frequencies by incorporating high-speed electro-optical effects based on free-carrier tuning mechanisms into plasmonic and all-dielectric metamaterials. Several novel and unique applications of space-time photonic metamaterials are presented including dispersionless dynamic wavefront engineering, all-angle beam scanning with minimal sidelobe level, wavelength-multiplexed functionality, spatiotemporal manipulation of light, adaptive full-duplex and multiple access free-space optical communication, and free-space optical power isolation.