Metamaterials are artificially structured materials designed to control and manipulate physical phenomena such as light and other electromagnetic waves, sound waves and seismic waves in unconventional ways, resulting in exotic behavior that’s not found in nature. This class of micro- and nano-structured artificial media are predicted to be able to protect the building from earthquakes by bending seismic waves around it. Similarly, tsunami waves could be bent around towns, and sound waves could be bent around a room to make it soundproof.
Although metamaterials already have revolutionized optics, their performance has been limited by their inability to function over broad bandwidths of light. Designing a metamaterial that works across the entire visible spectrum remains a considerable challenge.
The fascinating functionalities of metamaterials typically require multiple stacks of material layers, which not only leads to extensive losses but also brings a lot of challenges in nanofabrication. Many metamaterials consist of complex metallic wires and other structures that require sophisticated fabrication technology and are difficult to assemble. The unusual optical effects do not necessarily imply the use of volumetric (3D) metamaterials. However, due to the high resonance loss and their 3D nature, the fabrication and practical applications of metamaterials are limited.
Metasurfaces are the two-dimensional version of metamaterials: extremely thin surfaces made up of numerous subwavelength optical nanoantennas, each designed to serve a specific function upon the interaction with light. The metasurface elements act as subwavelength phase or amplitude modulators, which can be static or dynamic. Arrays of these elements act in transmission or reflection to encode arbitrary optical functionality such as focusing, steering, and other kinds of wavefront manipulation.
The metasurfaces contain regularly spaced nanoparticles that can modulate electromagnetic waves over sub-micrometer wavelength scales. They allow or inhibit the propagation of electromagnetic waves in desired directions, can concentrate the waves and guide or can control the scattering of light with exceptionally high precision. These devices enable efficient beam steering, local control of optical polarization, and enhancement of emission and detection of light.
The main advantages of metasurfaces with respect to the existing conventional technology include their low cost, low level of absorption in comparison with bulky metamaterials, and easy integration due to their thin profile. Part of the promise of these devices lies in their ability to perform these complex optical functions using metasurfaces that are manufactured using standard lithographic techniques common in the semiconductor industry.
Progressive research on metasurfaces has enormously expanded from plasmonic metasurfaces composed of metallic meta-atoms to all-dielectric metasurfaces based on Mie resonance of high refractive index nanostructures. Nowadays, metasurfaces are of great interest as a promising platform for holography, beam steering, structural color, and other applications.
However, such passive metasurfaces have significant limitations in terms of device functionalities because the profiles of scattered wavefront are fixed once metasurfaces are designed and fabricated. For instance, meta-holograms can only generate a limited number of images under specific polarized incidences, and ultrathin beam deflectors with metasurfaces can only operate at a certain planned angle. To overcome these constraints of functionalities, new types of nanophotonic devices called active metasurfaces, the combination of passive metasurfaces and active optical materials, have been demonstrated in recent years.
Despite of many advances, limited working bandwidths and fixed wave-manipulation functionalities emerge as the main drawbacks in metasurfaces. In the case, many efforts have been devoted to achieving multifunctional and tunable metasurfaces that can dynamically control light upon external tuning.
The schemes for multifunctional metasurface design can be roughly divided into two categories. One is to simply merge the individually designed metasurfaces with different functions to form a multifunctional metasurface in segmented or interleaved configurations .
For example, one of the simplest multifunctionality strategies is to merge multiple monofunctional metasurfaces together by referring to the shared-aperture phase antenna. Polarization, wavelength, and angle of incidence are part of a non-exhaustive list of parameters of a given light source that are usually taken advantage of as multiplexing channels for multifunctional metasurfaces, which realize holography, structural color, and beam steering at the same time. This kind of multifunctional metasurface encompasses the benefits of multi-wavelength and angle-multiplexed holograms.
Another one is implemented by the integration of many similar or distinct functions into a single metasurface structure, which is similar to multiplexing in the field of telecommunication.
Nevertheless, the multifunctional meta devices based on segmented and interleaved metasurfaces will still be severely affected by functional crosstalk. Besides, the extremely low operational efficiency (approximately limited to 1/N, where N is the number of functions) will seriously restrict their practical application.
Moreover, several judiciously designed non-interleaved metasurfaces have been proposed to further improve the operation efficiency and the crosstalk. The axial and lateral multifocus metalens can be created by interleaving three distinct metalenses that have a shared focal length and laterally separated focal spots at different operating wavelengths.
In addition, the metasurface configurations based on supercell and multilayer share excellent performance in specific application fields, such as wide-angle optics and achromatic metalens. Inspired by the concept of meta-grating, the metaatoms in a metasurface are replaced by the supercells in a number of recent studies in order to explore the efficient large-angle scattering.
The other is to use linear property of the Fourier transform to add the complex transmission profiles and meet the requirement for multiple functions in one time. Recently, by introducing the concept of supercell and considering the non-local effects, the second scheme has been extended and realizes series of wide-angle optical elements
As for the tunability, it can be commonly addressed by combining static metasurfaces with active materials, therefore leading to that the optical response of such metasurfaces can be controlled by altering the effective permittivity of the active material via external stimuli. Obviously, studies on the characteristics of material platforms will directly drive the development of tunable metasurfaces.
The phase, amplitude, or polarization can be dynamically controlled by external physical stimuli to active materials. With enhanced device performance and functionalities, a wide range of applications of active metamaterials such as sensors, data storages, and tunable lenses have been demonstrated.
The most common external stimuli include electrical, optical, and thermal stimulations. Different stimuli to the active material often induce different responses. Therefore, the selection of the most effective stimuli according to the characteristics of the active material itself is the basic principle of designing tunable metasurfaces. For example, vanadium dioxide (VO2) can easily switch between insulator and metal by controlling the temperature, so thermal-controlled VO2-based metasurfaces are the most prevalent.
For most TCOs, PCMs and 2-DMs, external electrical and optical stimuli ease the control of their optical
responses by changing the free carriers inside the materials.
There have been many studies on the demonstration of dynamically tunable optical responses using active electro-optic materials such as transparent conducting oxides (TCOs), graphene, and heavily doped semiconductors. In particular, TCOs are well known as near-infrared (NIR) plasmonic materials due to their large bandgaps and quasi-parabolic conduction bands. Thus, modulation of amplitude, phase or polarization in NIR regime deeply have been achieved by TCO-based metasurfaces.
Now, as a further step toward the ultimate goal of dynamic nanophotonics, realization of ultracompact all-optical devices and systems, the implementation of multiple electromagnetic functions in one design has been highly desired since such active metasurfaces that integrate diversified functionalities allow not only for efficient use of limited space but also for ease of optical system design. Many of the reported studies about multifunctional devices were either amplitude- or phase-based applications including optical memory, beam deflecting, splitting, focusing and polarization switching
More effective laser control possible via new metasurface system reported in August 2021
A team at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS; Cambridge, MA) has developed a single metasurface that tunes various properties of laser light, including wavelength. It does so without additional optical components.
Specifically, the metasurface splits light into multiple beams and “controls their shape and intensity in an independent, precise, power-efficient way.” According to the researchers, this breakthrough paves the way toward lightweight, more efficient optical systems for applications from quantum sensing to VR/AR.
A tunable laser, it comprises two components: a laser diode and a reflective metasurface. Traditional systems rely instead on a light-controlling network of individual pillars. The new system utilizes supercells—“groups of pillars which work together to control different aspects of light.” According to the researchers, when light from the diode hits the supercells on the metasurface, part of that light is reflected back. This in turn creates a laser cavity between the diode and the metasurface, while the remaining light reflects to a second beam independent from the first. To change the wavelength, the team “simply moves the metasurface with respect to the laser diode.”
The researchers note that precisely controlling properties of laser light is critical to current technologies and applications such as commercial VR headsets and microscopic imaging for biomedical research.
“Our approach paves the way to new methods to engineer the emission of optical sources and control multiple functions … in parallel in a single metasurface,” says Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS and senior author of the paper. “In addition to controlling any type of laser, this ability to generate multiple beams in parallel and directed at arbitrary angles, each implementing a different function, will enable many applications from scientific instrumentation to augmented or virtual reality and holography.” Reference: C. Spägele et al., Nat. Commun., 12, 3787 (2021); https://doi.org/10.1038/s41467-021-24071-2.
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