  Metasurfaces, composed of subwavelength electromagnetic microstructures,
known as meta-atoms, are capable of reshaping the wavefronts of incident beams
in desired manners, making them great candidates for revolutionizing
conventional optics. However, the requirement for external light excitation and
the resonant nature of meta-atoms make it difficult to fully integrate
metasurfaces on-chip or to control wavefronts at deep-subwavelength scales.
Here, we introduce the concept and design of a new class of metasurfaces,
driven by unidirectional guided waves, and being capable of arbitrary wavefront
control based on the unique dispersion properties of unidirectional guided
waves rather than resonant meta-atoms. Upon experimentally demonstrating the
feasibility and practicality of the unidirectional nature of our designs in the
microwave regime, we numerically validate this new principle through the design
of several microwave meta-devices using metal-air-gyromagnetic unidirectional
surface magnetoplamons, agilely converting unidirectional guided modes into the
wavefronts of 3D Bessel beams, focused waves, and controllable vortex beams. We
also numerically demonstrate sub-diffraction focusing, which is currently
beyond the capability of conventional metasurfaces. Furthermore, we directly
show how these concepts can be transferred to the terahertz regime, and discuss
their feasibility in the optical domain, too. Based on this nonresonant (that
is, broadband) mechanism and on standard plasmonic platforms, our metasurfaces
can be integrated on-chip, enabling the manipulation of electromagnetic waves
on deep subwavelength scales and over wide frequency ranges, thereby opening up
new opportunities for applications in communications, remote sensing, displays,
and so forth.
