Tunable Nonreciprocal Fano Resonance Transmission

Plasmonic effects in metasurfaces have generated a great deal of interest in the materials and optics community and inspired many groundbreaking studies in the last quarter century, both in basic research and in pursuit of applications such as sensing of molecules, nonlinear optics for new light sources, and optoelectronics for electrical readout of optical (visible or infrared) signals. The effect of Extraordinary Optical Transmission (EOT) through a periodic arrays of holes in the metal films was discovered in 1998 [1] and is due to excitation of surface plasmon polaritons (SPPs) by the incident electromagnetic field. Another interesting metasurface effect under study is the Fano resonance in periodically patterned metal films due to coupled plasmonic modes. This can be understood as two oscillators with different linewidths; typically a ‘background’ process and a ‘resonance’ process, the latter with a much narrower resonance spectrum. The coupling between these two oscillators creates a resonance with an asymmetric line shape, so that photon scattering is almost suppressed at lower energies while it is enhanced at higher energies, off resonance. <br/> <br/>In this presentation, we describe our study (currently submitted for publication) of a Fano-resonant metasurface with EOT that is, interestingly, nonreciprocal; e.g., transmission in the top-to-bottom direction is higher than in the bottom-to-top direction. The metasurface consists of a gold film of variable thickness (50-300 nm) atop a 3 mm-thick Cr adhesion layer on glass with a periodic array of holes (cylinders) in the Au film (1200 nm period, square array, and 600 nm diameter), and is modeled as consisting of two plasmonic oscillators representing SPPs in the metasurface on the top and bottom surfaces of the gold. The bottom oscillator is more highly damped than the top oscillator, due to the proximity of the Cr film, which has large ohmic losses. The interesting nonreciprocity in transmission is due to the fact that the top oscillator, at the top surface of the plasmonic Au (the Au-air interface), is damped less strongly than the plasmonic oscillator at the Au-Cr interface. As a result, the evanescent field from the wave, incident on the top of the metasurface, penetrates deeper into the metasurface and transmission is higher. The Fano resonance for the top-to-bottom transmission is calculated to be at 756 nm, while the Fano resonance for the bottom-to-top transmission is calculated to be broader and have a slightly redshifted resonance wavelength due to the proximity of the Cr.<br/>For deeper holes, the plasmon modes of the top and bottom interfaces decouple due to the finite penetration depth of the plasmonic oscillation, and reciprocity is present due to the ohmic link between two weakly coupled oscillators. For shallower holes (~ 100 nm), the top-to-bottom coupling is strong enough to render the transmission nonreciprocal.<br/> <br/>Prof. Richard M. Osgood, Jr., who passed away unexpectedly this past fall, was a leader in optical materials, optical devices, and in plasmonics. The perspective of Osgood’s past efforts, related to this research [2], will be briefly mentioned.<br/> <br/>[1] Nature 391 667 1998. [2] Phys. Rev. Lett. 95 137404 2005.