Low-loss plasmon-triggered switching between reflected free-space diffraction orders

Abstract

Surface plasmon coupling of a TM polarized free space incident beam by means of the + 1st or the -2nd order of a smooth corrugation grating at a metal surface causes the cancellation of the diffracted -1st order free space beam and a maximum of the 0th order Fresnel reflection whereas the converse occurs midway between these two conditions. This implies that angular tilting of the element or wavelength scanning provokes the switching between the -1st and 0th reflected orders. This plasmon-mediated effect on propagating free-space beams exhibits remarkably low absorption losses.

Guided-mode triggered switching between TE orders of a metal-based grating-waveguide

“…Whereas the sinusoidal metallic grating generating the propagating -1 st reflected order inherently also couples the incident wave to the co-and contra-propagating plasmon modes upon an angular tilt in the incidence plane [2], there is no such resonance in the same structure for the TE polarization. Triggering such switching effect for the TE polarization requires the existence of a TE-resonance.…”

“…In analogy to the plasmon-triggered TM switching [2], the plasmon mode is now replaced by the fundamental TE 0 mode which the grating couples either in the backward direction via its -2 nd order when the incidence angle θ −2 is smaller than the -1 st order Littrow angle or via its +1 st order when θ 1 is larger than the Littrow angle θ L . The corresponding phase matching conditions are, respectively:…”

“…The design process starts by finding numerically the depth of a sinusoidal grating at a silver surface at which the TE 0 th order cancels and the -1 st order is maximum under the Littrow condition. The considered optogeometrical parameters are the same as those of the plasmonic structure [2] to permit a comparison of the optical function: 22 degree incidence and λ/Λ = 633/844 nm/nm = 0.75 with air overlay in the absence of dielectric layer; the metal is silver too. Curves C and D in Figure 2 represent the 0 th and -1 st order spectra obtained with a grating depth d = 320 nm; the two kinks at 14.5 and 30 degrees on these two smooth curves represent the cutoffs of the +1 st and -2 nd diffraction orders respectively.…”

“…The angular range of maximum -1 st order around the Littrow angle θ L = sin −1 (λ/(2Λ)) (λ is the wavelength in vacuum, Λ the grating period) is usually very broad and is limited by the higher reflected orders starting to propagate. It was shown recently [2] that the situation changes radically for the TM polarization if the -2 nd or +1 st order of the grating couples to the forward, resp. backward surface plasmon in which case the efficiency of the propagating -1 st order experiences a sharp drop to 0 at either side of the Littrow angle where the 0 th order Fresnel reflection angular spectrum peaks at a close to 100% maximum [2].…”

“…It was shown recently [2] that the situation changes radically for the TM polarization if the -2 nd or +1 st order of the grating couples to the forward, resp. backward surface plasmon in which case the efficiency of the propagating -1 st order experiences a sharp drop to 0 at either side of the Littrow angle where the 0 th order Fresnel reflection angular spectrum peaks at a close to 100% maximum [2]. The condition for this high contrast plasmon-mediated switching to take place within a definite angular band around the Littrow angle is 3 K g ( = 3(2π/Λ)) only slightly larger than twice the plasmon propagation constant 2β = 2n e k 0 (n e is the surface plasmon effective index and k 0 = 2π/λ); this prevents higher order diffracted waves to propagate.…”