Designer matter: Fascinating interactions of light and sound with metamaterials
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troduction In the past 15 years, parallel advances in nanotechnology and in the understanding of complex wave–matter interactions have enabled a new designer matter paradigm based on metamaterials, which allows the control of electromagnetic and acoustic waves in unprecedented ways. Metamaterials are artificial materials with a bulk response—electromagnetic, acoustic, or mechanical—that is drastically different from that of their constituents used to synthesize them, and they have been explored by several groups worldwide for exciting opportunities in various areas of technology. Figure 1 shows some basic examples of optical metamaterials formed by nanoparticles that can provide strong interactions with light, despite being subwavelength.1–3 In this case, they exploit plasmonic resonances, which are collective excitations of electrons arising at the interface between metal and dielectrics or free space.4 We can engineer the collective response of these resonant particles by tailoring their geometry, orientation, alignment, and density throughout the different layers composing the metamaterial, in order to realize a bulk material with strong, localized light–matter interactions that can be homogenized and whose emergent properties are distinct from those of the constituent
nanoparticles. Based on this paradigm, we can change the way in which light, sound, and other waves interact with materials, propagate through them, diffract, scatter, reflect, or refract. Early research on metamaterials5 was driven by the opportunity to engineer unusual values of their constitutive parameters, such as a bulk index of refraction with a negative real part, which has been shown to enable the possibility of bending waves in unusual directions and realize flat lenses with no aberrations and superior resolutions,6 or cloaking devices that reroute the impinging waves to hide a given region of space from the background.7,8 These ideas have been inspiring researchers worldwide, but, so far, we have not yet fully seen the expected major impact on practical devices and technology. There are a few reasons behind the slow transition of these exciting basic research advances into practical technology. Importantly, metamaterials exploit enhanced light–matter interactions in subwavelength resonant structures, also enhancing losses and complying with stringent limits on the overall bandwidth of operation. Metamaterials are typically time-invariant, and therefore lack reconfigurability. Their response is mostly limited to linear operations, since small volumes do not allow the buildup of significant nonlinear responses for reasonable
Andrea Alù, The University of Texas at Austin, USA; [email protected] doi:10.1557/mrs.2017.188
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