Plasmonic Metasurfaces Based on Phase Bifurcation

Chen Yan Nanophotonics and Metrology Laboratory, Swiss Federal Institute of Technology Lausanne (EPFL), Lausanne, Switzerland Thottungal Valapu Raziman Nanophotonics and Metrology Laboratory, Swiss Federal Institute of Technology Lausanne (EPFL), Lausanne, Switzerland Kuang-Yu Yang Nanophotonics and Metrology Laboratory, Swiss Federal Institute of Technology Lausanne (EPFL), Lausanne, Switzerland Olivier J.F. Martin Nanophotonics and Metrology Laboratory, Swiss Federal Institute of Technology Lausanne (EPFL), Lausanne, Switzerland

The field of plasmonics has been largely concerned about controlling both the near-field and far-field responses of plasmonic nanostructures that enables various practical applications in wave-front engineering with metasurfaces [1]. It is well known that harmonic oscillators can model the related physical quantities, including scattering, absorption and extinction, to describe not only the individual plasmonic resonances but also coupled responses such as the plasmonic analogue of electromagnetically induced transparency [2]. Although the intensity extracted from these models explains very well the experimental results, we discuss in this work the advantages of using phase information to provide deeper insights into the physics of the system.

We experimentally design and demonstrate a phase gradient metasurface for shaping the wave-front at a specific wavelength. Sufficient phase variation is achieved by tuning the geometric parameters of a Fano-resonant structure on a metallic backplane. A dual-color routing metasurface with high directionality is realized as a proof-of-concept. In order to explain the design rule, we construct a simple model for extracting the phase information from plasmonic resonances including the effect of substrates utilizing the classical oscillator model and the transfer matrix method. Analyzing the time-independent complex field on an Argand diagram allows a more comprehensive understanding and effective design of the structures. Based on the same principle, we show that the asymmetric spectral feature of reflected field from a Fano-resonant metasurface is more pronounced with incidence from a high index glass substrate. Furthermore, the spectrum can contain phase dislocation and bifurcation in the frequency domain with properly tuned coupling strength. The resulting 2π phase jump at this range of frequencies will be instrumental for increasing the sensitivity of phase-based measurements such as ellipsometric detection [3].

[1] N. Yu and F. Capasso, “Flat Optics with Designer Metasurfaces”, Nat. Mater. 13 (2014) 139-150

[2] Y. S Joe et al., “Classical Analogy of Fano Resonances”, Phys. Scr. 74 (2006) 259-266

[3] F. Abelès, “Surface Electromagnetic Waves Ellipsometry”, Surf. Sci. 56 (1976) 237-251

chen.yan@epfl.ch