Broadband Dispersion-Engineered Optical Metamaterials

Introduction to Dispersion Engineering

Metamaterials have been demonstrated to exhibit remarkable electromagnetic properties, including negative [81, 93], zero/low [27, 55, 102], and high [20, 82] indices of refraction. These properties, which also include independent control of the refractive index and the intrinsic impedance enable novel optical design strategies and have the potential to enhance the performance of existing devices or to introduce entirely new device functionalities. Recent examples of metamaterial devices that have emerged include artificial mirrors [16], structures with electromagnetic induced transparency [64, 105], flat wave collimating lenses [31], ultrathin absorbers [45, 58], and metamaterial emitters with customized angle- and polarization- dependent emissivity [14, 68]. The unique optical properties arise from the specific geometry and arrangement of nanoscale inclusions in the metamaterial nanostructure that are typically aligned in a periodic lattice [56].

The resonant properties resulting from the metamaterial nanostructure, including the effective refractive index and group delay, are generally strongly dependent on wavelength, which can cause signal distortion and narrow operational bandwidths [17, 32, 35, 80], limiting the widespread use of metamaterials in practical optical devices. However, broadband optical metamaterials can be realized by choosing to operate at wavelengths sufficiently far from the resonant band of the nanostructured inclusion to avoid strong dispersion. Such a strategy has been utilized in the microwave regime to design ground plane cloaks [66, 94] and Luneburg lenses [57, 69]. However, in choosing to avoid highly dispersive regions, this approach excludes negative and zero/low index values, which are among the most important and interesting regions associated with metamaterials. Another alternative strategy to avoiding resonant bands altogether is to exploit them by tailoring the dispersive properties of the metamaterial to specific device needs in order to improve existing components or to enable new optical functionalities [18, 29, 109]. This powerful dispersion engineering design technique has recently been applied at microwave frequencies to demonstrate novel broadband radiated-wave components [3, 44, 46, 61] and planar guided-wave devices [4, 36, 75].

In the following section, we show that the metamaterial dispersion of a metallodielectric fishnet structure can be controllably tailored across the negative, zero, and positive refractive index values to produce a specific broadband optical filtering function by adding deep-subwavelength inclusions into the structure. This general design strategy allows the structure to be optimized to produce an optically thin metamaterial band-pass filter with high transmission and nearly constant group delay throughout the

3.0 gm to 3.5 gm band in the mid-IR and high rejection outside of this band. The dispersion-engineered metamaterial filter is also extended to demonstrate a bifunctional metamaterial prism that both filters an incident wave and provides wavelength-dependent steering of the transmitted beam. These examples illustrate how this powerful dispersion engineering design approach overcomes the narrow bandwidth limitations of previous optical metamaterials and enables opportunities to create novel and practical metamaterial- enabled devices and components.

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