Narrow band perfect absorber for maximum localized magnetic and electric field enhancement and sensing applications

Abstract

Plasmonics offer an exciting way to mediate the interaction between light and matter, allowing strong field enhancement and confinement, large absorption and scattering at resonance. However, simultaneous realization of ultra-narrow band perfect absorption and electromagnetic field enhancement is challenging due to the intrinsic high optical losses and radiative damping in metals. Here, we propose an all-metal plasmonic absorber with an absorption bandwidth less than 8 nm and polarization insensitive absorptivity exceeding 99%. Unlike traditional Metal-Dielectric-Metal configurations, we demonstrate that the narrowband perfect absorption and field enhancement are ascribed to the vertical gap plasmonic mode in the deep subwavelength scale, which has a high quality factor of 120 and mode volume of about 10(-4) × (λres/n)(3). Based on the coupled mode theory, we verify that the diluted field enhancement is proportional to the absorption, and thus perfect absorption is critical to maximum field enhancement. In addition, the proposed perfect absorber can be operated as a refractive index sensor with a sensitivity of 885 nm/RIU and figure of merit as high as 110. It provides a new design strategy for narrow band perfect absorption and local field enhancement, and has potential applications in biosensors, filters and nonlinear optics.

Figure 1. A schematic diagram of the all-metal perfect absorber with a magnified unit cell (enclosed in dashed box) shown in the inset: periodic arrays of coupled thick silver disks are placed directly on the surface of a uniform silver film.

The closely spaced silver disks have a radius (r) of 80 nm, thickness (t) of 100 nm and gap distance (g) of 20 nm. The period constant (p) of the arrays is 470 nm and the bottom silver film has a thickness of 100 nm. In addition, the whole structure is placed on a glass substrate, and the surrounding material is assumed to be air.

Figure 2

(a) The absorption and reflection spectrum with magnified spectrum shown in the inset. (b) Distributions of the electric field |E_x| (color bar in the x-y plane), magnetic field |H_y| (color bar in the x-z plane) and current density (small arrow lines in the x-z plane) at resonance (top row), and mapping of the absorbed power density (bottom row) in both the x-y and x-z planes. Peak absorption and resonant wavelength as the radius (c) and thickness (d) of the structures varies. Absorbance as a function of the incident angle and wavelength for (e) TM-polarized and (d) TE-polarized light. (g) Simulated absorbance spectra when the damping rate of the silver film is two times that of bulk silver due to the surface scattering and grain boundary effects in a thin film.

Figure 3

(a) Electric (blue dash-dot curve) and magnetic field (red dash-dot curve) intensity enhancement at a point in the middle of the gap and height t/2 over the silver film, and electric field intensity enhancement at height t (blue curve). (b) Diluted electric field intensity enhancement (circles connected with solid lines) and the peak absorptivity (squares connected with dashed lines) as the period varies. Inset: resonant wavelength vs period with other parameters unchanged.

Figure 4

(a) Reflection spectrum of the all-metal perfect absorber with the refractive index varying from 1 to 1.05 with a step interval of 0.01. (b) Resonant wavelength as a function of the surrounding low refractive index. The red line is the linear fitting with the slope representing the sensitivity S.