Efficient unidirectional nanoslit couplers for surface plasmons

Abstract

The emerging field of plasmonics is based on exploiting the coupling between light and collective electronic excitations within conducting materials known as surface plasmons. Because the so-called surface plasmon polariton (SPP) modes that arise from this coupling are not constrained by the optical diffraction limit, it is hoped that they could enable the construction of ultracompact optical components 1,2 . But in order that such potential can be realized, it is vital that the relatively poor light-SPP coupling be improved. This is made worse by the fact that the incident light that is conventionally used to launch SPPs in a metal film 3-6 is a significant source of noise, unless directed away from a region of interest, which then decreases the signal and increases the system's size. Back-side illumination of subwavelength apertures in optically thick metal films 7-13 eliminates this problem but does not ensure a unique propagation direction for the SPP. We propose a novel back-side slit-illumination method that incorporates a periodic array of grooves carved into the front side of a thick metal film. Bragg reflection enhances the propagation of SPPs away from the array, enabling them to be unidirectionally launched from, and focused to, a localized point.A picture of the proposed surface plasmon polariton (SPP) launcher is shown in Fig. 1. A periodic array of one-dimensional indentations is fabricated at the (output) metal surface close and parallel to the illuminated slit. The design of this device is based on two facts. The first one is that the reflection of SPPs by a periodic array of indentations presents maxima at the low-l edges of the plasmonic bandgaps [14][15][16] . For subwavelength indentations, the spectral locations of these edges can be obtained by folding the dispersion relation of SPPs for a flat metal surface into the first Brillouin zone, satisfying the following expression:where P is the period of the array, k p holds for the in-plane plasmon wavevector and m is the band index. Remarkably, although the reflectance maxima depend on the groove geometry (width and depth) and the number of grooves, their spectral locations do not. The second fact is that the phase picked up by the SPP on reflection is just mπ, precisely at the condition given by

Surface Plasmon Polariton Graphene Photodetectors

“…Interference between SPP waves was reported in pure metal systems [57,58,65,66]. To confirm that what shown in Fig.1b is due to interference of SPPs with the incident wave, we examine a series of d values, and plot the absorption in the exposed SLG within the first 100nm from the contact edge.…”

“…One way to overcome this is by diffraction, whereby the continuity of the component of momentum parallel to the surface is broken and incident light can scatter into SPPs [45]. This can be achieved by a nano-slit [57,58], a diffracting element [59,60], or a grating coupler [59,61]. In the latter case, in particular, a periodic array of metal ridges and grooves delivers the additional momentum according to [45]:…”

Compact Antenna for Efficient and Unidirectional Launching and Decoupling of Surface Plasmons

“…For the present device, the extinction ratio defined for δX = 0 is measured to be E R = 47 (η c − = 1.1% and η c + = 52%). The two figures-of-merit are remarkably high with respect to previous results [1][2][6][7][8][9][10]13,14]. In addition, they are achieved for normal incidence; this is really convenient in most applications.…”

Femtosecond Nanofocusing with Full Optical Waveform Control

“…The fan-shaped gratings developed for this work enable a high coupling bandwidth with FWHM of up to ∼ 100 nm (see supplementary 9 material). Improvements in the form of grating structures with optimized groove depth and holographic geometries, chirped gratings including Bragg reflectors [27], broadband coupling via fabricated micro-prism onto the tip shaft [28], spatial pulse shaping in the farfield excitation focus [29], and tip fabrication with reduced surface roughness could further improve the tip performance for nanofocus optical waveform control. Without the need for resonance behavior in order for spatial localization to occur, the process can be extended to a wide range of wavelengths, limited in principle only by material damping at short wavelengths, and taper angle and reduced spatial field confinement at long wavelengths.…”