Rainbow-trapping by adiabatic tuning of intragroove plasmon coupling

Abstract

Trapping broadband electromagnetic radiation over a subwavelength grating, provides new opportunities for hyperspectral light-matter interaction on a nanometer scale. Previous efforts have shown rainbow-trapping is possible on functionally graded structures. Here, we propose groove width as a new gradient parameter for designing rainbow-trapping gratings and define the range of its validity. We articulate the correlation between the width of narrow grooves and the overlap or the coupling of the evanescent surface plasmon fields within the grooves. In the suitable range (≲150 nm), this width parameter becomes as important as other known parameters such as groove depth and materials composition, but tailoring groove widths is remarkably more feasible in practice. Using groove width as a design parameter, we investigate rainbow-trapping gratings and derive an analytical formula by treating each nano-groove as a plasmonic waveguide resonator. These results closely agree with numerical simulations.

Adiabatic mode transformation in width-graded nano-gratings enabling multiwavelength light localization

“…4 indicates a counterintuitive inverse relationship between w and λ resonant , in contrast to that in a Fabry-Perot cavity where larger grooves support longer plasmonic wavelengths; that is, the plasmonic resonant wavelength within the groove increases with decreasing groove width (w). The underlying reason for this behaviour is the fact that the effective index of the groove increases as it narrows 11 . Hence, for a fixed groove depth the wavelength of the Fabry-Perot mode as supported by a narrower groove increases in accordance to Eq.…”

Tunable rainbow light trapping in ultrathin resonator arrays

“…Typically, these designs consist of arrays of resonators, with resonant wavelengths determined by the size, shape and composition of the individual units 3 . Combining nanostructures with different resonant wavelengths, for example multiresonant nanoparticles 29 and nanocavities [30][31][32][33] , into a single device provides tunable, position-dependent rainbow field enhancement. However, given the lack of analytical solutions for these resonators, it is not feasible to accurately predict the resonant properties of the nanostructures at the design stage without running a large number of iterative detailed simulations.…”

Transmission and absorption in a waveguide with a metamaterial cavity

“…In undamped periodic arrays of scatterers, stop bands are defined as the ranges of frequencies over which unattenuated wave propagation is prohibited within the array; these generally depend on scattering geometry and spacing. Rainbow trapping occurs when arrays are designed with a slow modulation of geometry and/or spacing along their length [8][9][10] and waves of different frequencies encounter stop bands at different positions along the array.…”

“…Rainbow trapping can be achieved by passive structures or micro-resonators in 2D or 3D 12,13 . One such device is to use a comb-like grating consisting of an array of grooves of tapered length or width which act as micro-resonators [8][9][10]14 .…”

“…The solution to the problem of discrete micro-channels is hard to solve by exact analytical methods and it is typical to use Finite Element Method simulations [8][9][10]14 , or asymptotic approximations 16 . Instead, here we take advantage of the contrast in lengthscales between the microstructure and the other lengthscales in the problem and use a homogenisation approach to replace the microstructured cavity by an effective medium/continuum.…”