Controlling the Polarization State of Light with a Dispersion-Free Metastructure

By combining the advantages of both a metallic metamaterial and a dielectric interlayer, we demonstrate the general mechanism to construct the dispersion-free metastructure, in which the intrinsic dispersion of the metallic structures is perfectly cancelled out by the thickness-dependent dispersion of the dielectric spacing layer. As examples to apply this concept, a broadband quarter-wave plate and a half-wave plate are demonstrated. By selecting the structural parameters, the polarization state of light can be freely tuned across a broad frequency range, and all of the polarization states on the Poincaré sphere can be realized dispersion free.

Popular Summary

Metamaterials possess unique optical properties not found in nature, such as negative refractive indices. The components of metamaterials are characteristically smaller than the wavelengths of optical light, thereby allowing metamaterials to affect incident light waves. Lorentz resonance properties of metamaterials limit the spectral window over which light can be manipulated, however, making broadband applications of metamaterials difficult. We find that integrating a metallic metamaterial with a dielectric interlayer creates a dispersion-free device capable of controlling broadband light.

We assemble a layer of metallic metastructure with a perfect electric conductor (a 100-nm-thick silver mirror) separated by a dielectric layer of ${\mathrm{SiO}}_2$. The intrinsic dispersion of the metallic metamaterial can be perfectly canceled out by the thickness-dependent dispersion of the dielectric spacing layer, and we verify this concept experimentally by designing and fabricating dispersion-free quarter-wave and half-wave plates. We can tune the polarization state by properly selecting the parameters of the metamaterial structure, and we recover bandwidths of about 30%–70% of the central frequency of 100 THz, a significant improvement over the narrow spectral window allowed by Lorentz resonance. We also find that the amplitude ratio between the reflected-light components can be controlled by modulating the length and width of the arms of the L-shaped gold metastructure.

We expect that the ability to control broadband light without dispersive losses will have broad applications in nanophotonics and the construction of miniaturized nanoscale optical devices.

Figure 1

(a) Schematics to show the interaction of light with each interface of the system made of an array of L patterns ($z=d$) in front of a perfect electric conductor ($z=0$). The mirror image of the array is located at $z=-d$. (b) Structural parameters of the L pattern. The thickness of the silver mirror layer is $h=100\mathrm{nm}$, ${\mathrm{SiO}}_2$ layer thickness is $d=510\mathrm{nm}$. The length $l$, width $w$, and thickness $b$ of each arm of the L pattern are 1220, 120, and 120 nm, respectively. The lattice constant is 1550 nm. (c) Simulated transmission of a layer of gold L-pattern array only, without the silver mirror layer underneath. The black and red lines are the normalized amplitude of the electric component of the transmitted light of 45°-and 135°-polarized incidence. (d) Simulated amplitude ratio of the reflected light, which becomes unity in the shaded frequency band. The inset shows the total reflectance. (e) Simulated phase difference of the reflected light, which is either 90° or $-90°$.

Figure 2

(a) Field-emission scanning electron micrograph of the array of gold L patterns fabricated by electron beam lithography. The bar represents $5\mu \mathrm{m}$. (b) Experimentally measured reflectance for the $x$- and $y$-polarized incident light. (c),(d) Experimentally measured amplitude ratio and phase difference of the sample under the illumination of $x$- and $y$-polarized light, respectively.

Figure 3

(a) Relationship of the interlayer thickness $d$, the frequency and the amplitude ratio of the reflected light. The color represents the amplitude ratio of the reflected light. (b) Calculated amplitude ratio and phase difference of the reflected light under the $x$-polarized incident light, which is similar to that shown in Figs. 1 and 1. (c) Amplitude of the $x$ and $y$ components of the irradiation of the structure and the amplitude of $|-e^{ikd}+e^{-ikd}|$, as a function of frequency. (d) Calculated amplitude of the $x$ and $y$ components of the reflected light.

Figure 4

(a) Simulated reflectance for a broadband half-wave plate. (b) Experimentally measured reflectance of the fabricated sample, which is in good agreement with simulations. (c) Measured intensity ratio between the $x$ and $y$ components of reflectance. (d) The wave plate to generate the elliptical polarized light has been fabricated. The measurements show that the amplitude ratio between the two components of the reflected light deviates from 1 in the shaded region, whereas the phase difference (inset) between the two orthogonal components remains 90°.