Metamaterials are a class of material with widely tunable optical properties and show promise for exotic applications such as negative refractive index materials. They are composed of materials with contrasting permittivity (ε) and permeability (μ) arranged in discrete periods below the diffraction limit. The material period should be much smaller than the wavelength (or wavelength range) of interest such that the impingent radiation experiences the effective material properties, and can be periodic in one-dimension (1D), two-dimensions (2D), or three-dimensions (3D) to offer control over the effective index of refraction anisotropically.

Metamaterials are commonly characterized by their effective index of refraction, n, where n2ε.μ. Many examples of metamaterials employ plasmonic metal shapes taking advantage of their resonance behaviors to fabricate selective absorbers, emitters, and waveguides.

Due to the nanometer-scale probe size, electron-based excitation, and the recent advent of angle-resolved (AR) and wavelength- and angle-resolved (WAR) modes of operation, cathodoluminescence (CL) proves to be an ideal technique for investigating metamaterials. In the case of the Au-based asymmetric split ring (ASR) resonator metamaterial, CL has shown to be useful for observing their emission spectrum and directionality.

The spectral emission of an Au thin film asymmetric split ring resonator metamaterial across the material surface boundary from flat gold to several microns into the ASR region.

The difference between photonic crystals and metamaterials is period: To have the photonic bandgap the atoms and the lattice constant in the former have to be comparable in size with the wavelength, while for the latter the periodicity should be much smaller than the wavelength.

Determining photonic band structure by energy-momentum spectroscopy in an electron microscope