Metamaterials and Metasurfaces—Historical Context, Recent Advances, and Future Directions

The trajectory of technological progress is ultimately guided by constraints at the physical level. In building a better device or system, we are bound 1) by the properties of the materials available to us and 2) by our understanding of physical phenomena. The physical laws of the universe, immutable as they are, lead us naturally to question whether we may be able to “engineer” raw materials to better allow us to achieve, control, and manipulate natural phenomena for useful purposes. In order to do this, we must first define what we mean by the term “material.” The perception that a material must appear homogeneous to the naked eye (i.e., “a uniform goop, with no discontinuous bits and pieces” [1]), natural though it may be, is flawed: surely, all materials may be considered heterogeneous on some level of scale, but more importantly, this perspective is tied specifically to the electromagnetic response of these materials to wavelengths of light that are visible to the human eye. For example, although a diamond displays familiar macroscopic properties such as color, luster, and dispersion when viewed under visible light, illumination using X-rays results in a diffraction pattern that reveals its crystalline structure. Thus, the macroscopic properties of a material, e.g. polarizabilities, permittivity, permeability, refractive index, intrinsic bulk or surface impedance, and so on, are revealed only under illumination by wavelengths of light much longer than the size of its scatterers (i.e., its atoms and molecules) and their spacing (e.g., the lattice constants of a crystal). Therefore, it would seem that engineering such macroscopic properties of materials would require control of scattering at length scales of fractions—say several hundredths or even just tenths—of a wavelength, a prohibitive task if dealing in the nanometers or Angstroms. Fortunately, the reach of the electromagnetic spectrum permits us to examine the long-wavelength condition at frequencies where such length scales become much more accessible, such as the microwave and terahertz. Advances in nanoscale fabrication have extended this reach even further to infrared and visible light. At such scales, it becomes possible to synthesize scatterers to exhibit electric and magnetic responses that may then, in analogy to their natural counterparts, be homogenized to describe effective macroscopic electromagnetic properties apparent under illumination by correspondingly long wavelengths.