Since its discovery, the asymmetric Fano resonance has been a characteristic feature of interacting quantum systems. The shape of this resonance is distinctively different from that of conventional symmetric resonance curves. Recently, the Fano resonance has been found in plasmonic nanoparticles, photonic crystals, and electromagnetic metamaterials. The steep dispersion of the Fano resonance profile promises applications in sensors, lasing, switching, and nonlinear and slow-light devices.

Metamaterials, artificial electromagnetic media that are structured on the subwavelength scale, were initially suggested for the negative-index 'superlens'. Later metamaterials became a paradigm for engineering electromagnetic space and controlling propagation of waves: the field of transformation optics was born. The research agenda is now shifting towards achieving tunable, switchable, nonlinear and sensing functionalities. It is therefore timely to discuss the emerging field of metadevices where we define the devices as having unique and useful functionalities that are realized by structuring of functional matter on the subwavelength scale. In this Review we summarize research on photonic, terahertz and microwave electromagnetic metamaterials and metadevices with functionalities attained through the exploitation of phase-change media, semiconductors, graphene, carbon nanotubes and liquid crystals. The Review also encompasses microelectromechanical metadevices, metadevices engaging the nonlinear and quantum response of superconductors, electrostatic and optomechanical forces and nonlinear metadevices incorporating lumped nonlinear components.

Photonic components with adjustable parameters, such as variable-focal-length lenses or spectral filters, that can change functionality upon optical stimulation, could offer numerous useful applications. Tuning of such components is conventionally achieved by either micro-or nano-mechanical actuation of their constitutive parts, by stretching or heating. Here we report a new type of dielectric metasurface for making reconfigurable optical components that are created with light in a non-volatile and reversible fashion. Such components are written, erased and re-written as two-dimensional binary or grey-scale patterns into a nanoscale film of phase change material by inducing a refractive-index-changing phase-transition with tailored trains of femtosecond pulses. We combine germanium-antimony-tellurium-based films with a sub-wavelength-resolution optical writing process to demonstrate a variety of devices: visible-range reconfigurable bi-chromatic and multi-focus Fresnel zone-plates, a super-oscillatory lens with sub-wavelength focus, a grey-scale hologram and a dielectric metamaterial with on-demand reflection and transmission resonances.A metasurface made of carefully designed discrete metallic or dielectric elements can exhibit interesting abilities for directing the flow of electromagnetic radiation across the entire electromagnetic spectrum with similar capabilities to planar holograms in optics 1,2,3,4,5,6,7,8,9,10 . As substantial efforts are now focused on developing metamaterials with switchable 11, 12 and reconfigurable metadevices 13 driven by thermal 14,15 , electrostatic 16 and magnetic forces 17,18 and stretching 19 , and we are witnessing the emergence of concepts of randomly accessible reconfigurable metamaterials in the microwave 20,21,22 and optical regions of the spectrum 23, 24 thus making reconfigurable photonic devices 2 controllable by external signals a realistic possibility. Here we introduce and demonstrate dynamic photonic components written into a dielectric film that can be randomly and reversibly reconfigured with light. Recent work demonstrated control of a metamaterial with light 25 . In contrast, the technique reported here allows random and non-volatile two-dimensional control of optical properties of the film with diffraction-limited resolution and in a femtosecond (fs) time-frame. This provides much more flexibility and allows the creation of various photonic functions difficult or impossible with the above mentioned technologies. The randomly reconfigurable metasurface uses phase-change material and is written, erased and re-written as a two-dimensional binary or grey-scale pattern into a nanoscale thin film by inducing a refractive-index-changing phase-transition with tailored trains of femtosecond pulses.We use phase-change medium, the chalcogenide compound Ge 2 Sb 2 Te 5 (GST), which is widely exploited in rewritable optical disk storage technology and non-volatile electronic memories due to its good thermal stability, high switching speed and large number of achievable rewr...