Metasurfaces, Then and Now
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Metasurfaces, Then and Now book
The general function of most optical devices can be described as the modification of the wavefront of light by altering its phase, amplitude, and polarization in a desired manner. Conventional optical elements, such as lenses, liquid crystal, planar phased arrays, and vortex elements fabricated using grayscale lithography, are modifying the phase of the wavefront using “propagation effect,” φ = k r. This limits the longitudinal dimension of the optical elements to be several orders thicker than the wavelength, l > λ. The class of optical components that alter the phase of light waves includes lenses, prisms, spiral phase plates, axicons, and more generally spatial light modulators (SLMs), which are able to behave such many of these components by means of a dynamically tunable spatial phase response. A second class of optical components such as waveplates utilizes bulk birefringent crystals with optical 2anisotropy to change the polarization of light. A third class of optical components such as gratings and holograms is based on diffractive optics, where diffracted in-phase spherical waves departing from different parts of the components interfere in the far-field to produce the desired optical patterns. All of these components shape optical wavefronts using the propagation effect: the change in phase and polarization is gradually accumulated during light propagation. This approach is generalized in transformation optics  which utilizes metamaterials to engineer the spatial distribution of refractive indices and therefore bend light in unusual ways, achieving remarkable phenomena such as negative refraction, subwavelength-focusing, and cloaking [2–7].