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.
The core concept of metamaterials is to craft materials by using artificially designed and fabricated structural units to achieve the desired properties and functionalities. These structural units – the constituent artificial ‘atoms’ and ‘molecules’ of the metamaterial – can be tailored in shape and size, the lattice constant and interatomic interaction can be artificially tuned, and ‘defects’ can be designed and placed at desired locations.
By engineering the arrangement of these nanoscale unit cells into a desired architecture or geometry, one can tune the refractive index of the metamaterial to positive, near-zero or negative values. Thus, metamaterials can be endowed with properties and functionalities unattainable in natural materials. 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.
You can also manipulate the light with the help of two-dimensional (2D) structures – so-called metasurfaces (or flat optics). Metasurfaces are thin films composed of individual elements that have initially been developed to overcome the obstacles that metamaterials are confronted with.
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.
Engineering the morphology and/or dielectric environment of these resonators allows controlling the phase, amplitude and polarization of light along the surface and yields properties that are not found in nature such as negative refraction. The possibility to generate arbitrary wavefronts has enabled a large number of exciting applications such as beam deflection, vortex beam, hologram generation and frequency conversion. Metasurfaces are an attractive alternative to conventional bulk optical components which offer the possibility to integrate complex optical functions in lightweight and cheap miniaturized components9. They hold great promise for applications such as portable or wearable devices, automotive, aeronautical and space applications and augmented/virtual reality
Planar metamaterials with subwavelength thickness, or metasurfaces, consisting of single-layer or few-layer stacks of planar structures can be readily fabricated using lithography and nanoprinting methods, and the ultrathin thickness in the wave propagation direction can greatly suppress the undesirable losses. For example, a lens can be created by spatially varying the width of dielectric Mie resonators composed of silicon (Si) pillars, thus encoding a parabolic phase profile.
Although many of the present demonstrators operate in the microwave regime, due either to the reduced cost of manufacturing and testing or to satisfy the interest of the communications or aerospace industries, part of the potential use of metasurfaces is found in the optical regime. Optical metasurfaces have been designed mimicking holograms, lenses or diffraction gratings, aiming at creating unique field structures that are not attainable with conventional optical elements.
Optical Metasurfaces applications
Moreover, metasurfaces enable new physics and a range of phenomena that are distinctly different from what can be achieved in bulk optics or three-dimensional metamaterials. One such example is the generalized law of reflection and refraction, where metasurfaces can be utilised for the redirection of an incident beam by employing antenna arrays with prescribed phase gradients, while ensuring unprecedented design flexibility with complete control of both amplitude and phase. Metasurfaces can also tailor near-field response, which is crucial when dealing with optical sources and detectors, enabling perfect absorption, emission enhancement, and detailed design of light-matter interaction properties. Metasurfaces enable a spatially varying optical response (e.g., scattering amplitude, designed at will, and facilitate the integration of functional materials to accomplish active control and greatly enhanced nonlinear response.
These materials could enable engineers to make flexible photonic circuits and ultra-thin optics for a host of applications, ranging from flexible tablet computers to solar panels with enhanced light-absorption characteristics. They could also be used to create flexible sensors to be placed directly on a patient’s skin, for example, in order to measure things like pulse and blood pressure or to detect specific chemical compounds.
1. Dispersionless flat lenses.
Flat lenses able to correct chromatic aberration over broad wavelength range, and to some degree reduce spherical aberration, coma, and other monochromatic aberrations, could revolutionize optical instrumentation. The prospect of largely shrinking the complexity and size of optical instruments (e.g., replacing the entire set of compound lenses in a camera lens with a few or even a single dispersionless and aberration-corrected flat lens) seems feasible in view of recent developments of metasurface lenses.
2. Optical modulators and spatial light modulators (SLMs) in the mid-infrared and THz spectral range.
The lack of compact and fast modulators and SLMs has been a big challenge that prevents the wide application of mid-infrared and THz technology in free-space communications, imaging, LIDAR (light detection and ranging), and homeland security (e.g., remote sensing, surveillance, and navigation in severe environments, such as foggy and dusty weather). Metasurfaces provide an ideal platform to create modulators in the mid-infrared and THz regimes and the subject has been under very active research investigation in the recent years.
Metasurfaces enable a strong interaction between light and materials with tunable electronic and optical properties, and allow for introducing spatially-varying optical response. They are therefore uniquely positioned to address the challenge of creating flat devices for substantially controlling the wavefront of light.
3. Radiative cooling metasurfaces.
Meta-surfaces that possess exceptional thermoregulatory properties have been an emerging field of research and have the potential to make an important technological impact. Fan and colleagues are pioneering the research on radiative cooling metasurfaces, which have strong reflectivity in the solar radiation spectrum and enhanced emissivity in the thermal radiation spectrum. Metasurfaces based on multilayered thin films have demonstrated in experiments passive cooling of objects to a few degrees below the ambient air temperature under direct sunlight. Chen and colleagues recently proposed a fabric that blocks sunlight and provides passive cooling via the transmission of thermal radiation emitted by the human body.
It is interesting (but probably not surprising) to note that radiative cooling has always been essential for the survival of animals living in harsh environmental conditions. Yu and co-workers recently reported the thermoregulatory strategies that enable Saharan silver ants to forage in the midday sun on the desert surface where temperatures can reach 70◦C (which is not survivable by their primary predators). It was found that a monolayer of densely packed hairs with peculiar triangular cross-sections, in some sense a biological “metasurface”, enhances not only the ant body’s reflectivity in the visible and near-infrared, where solar radiation culminates, but also its emissivity in the mid-infrared.
The combined effect enables the ants to minimize absorption from solar radiation, and to efficiently dissipate heat back to the surroundings via blackbody radiation. Animals and plants living in extreme environments could provide us valuable scientific and engineering lessons on optical design and thermal management towards creating functional biomimetic metasurfaces. In general, meta-surfaces provide us a platform, so that by designing the structural hierarchy, compositional heterogeneity, and local anisotropy, we will be able to create coatings that are optically thin and have desired spectral properties (reflectivity, absorptivity, transmissivity, and emissivity) over an extremely broad electromagnetic spectral range. Such ultra-thin and ultra-broadband metasurfaces will open doors to a variety of new applications, including management of radiative heat transfer and structural coloration.
4. New material platforms for metasurfaces.
Investigations of materials with low losses, tunability, high melting point, and CMOS compatibility for metamaterials and metasurfaces have been very active in recent years. Transition-metal nitrides such as TiN show comparable optical properties as gold in the visible and infrared but have much higher melting points, a property that can be explored for metasurface applications involving high optical intensity. Transparent conducting oxides (TCOs) such as indium-tin-oxide enable one to control the spectral location of the epsilon-near-zero point, which is associated with enhanced optical near-fields; the resulting strong interaction between light and TCOs can be exploited for optical modulation and nonlinear optics.
Phase-change materials such as chalcogenide alloys that have been used in rewritable CDs, DVDs, and Blu-ray discs, can be switched between the amorphous and crystalline states by laser or electrical current pulses with controlled duration and intensity. This material system has recently been used to demonstrate all-optical, non-volatile, metasurface switch, and high-resolution solid-state displays.
SmNiO3, a prototypical phase-change perovskite nickelate, exhibits reversible large re-fractive index changes over an ultra-broad spectral range, from the visible to the long-wavelength mid-infrared. The super broad-band performance is due to strong electron correlation effects, and this new mechanism can be exploited to create a variety of active photonic devices.
Metasurfaces Enable Low-Loss Integrated Photonics Platform in Sep 2019
A University of Delaware (UD) research team has designed an integrated photonics platform using metasurfaces. To limit the loss of data, the team fabricated a high-contrast transmitarray (HCTA)-based metalens on a silicon-based chip programmed with hundreds of tiny air slots to enable on-chip parallel optical signal processing.
The 1D metalens has a numerical aperture up to 2.14, which can focus light to within 10 µm with less than 1 dB loss. Computational tasks based on Fourier transformation (FT) can be performed by cascading multiple layers of the HCTA-based metalenses. To demonstrate FT and spatial differentiation, the researchers layered three metasurfaces together.
The platform is foundry fabrication-compatible, ultracompact, and designed to achieve on-chip wavefront transformation with low insertion loss and broadband operation. “This is the first paper to use low-loss metasurfaces on the integrated photonics platform,” professor Tingyi Gu said. “Our structure is broadband and low loss, which is critical for energy-efficient optical communications.”
The UD researchers said their device is smaller and lighter than conventional devices of its type. It doesn’t require the manual alignment of lenses, so it is more scalable compared to the traditional free-space optics platforms.
The team’s experimental demonstration of the functional HCTA-based metalens could open new directions for on-chip diffractive optical systems. The on-chip metasurface could be integrated with multimode waveguides to perform mode transformation in mode division multiplexing systems. The 1D metasurface design could facilitate novel on-chip systems with low power consumption and ultracompact dimensions, including on-chip spectrometers, light detection and ranging devices, and diffractive optical computational circuits.
“It’s just much faster than conventional structures,” said Gu. “There are still a lot of technical challenges when you try to actively control them, but this is a new platform we are starting with and working on.”
New photonic integrated chip could enable developments for many optical technologies
A new technology that can allow for better light control without requiring large, difficult-to-integrate materials and structures has been developed by Penn State researchers. The new photonic integrated chip could allow for many advances in the optical field and industry, ranging from improvements in virtual-reality glasses to optical remote sensing, according to the researchers.Led by Xingjie Ni, assistant professor of electrical engineering, the research was recently published in Science Advances. Penn State electrical engineering doctoral candidates Xuexue Guo, Yimin Ding, Xi Chen and Yao Duan were co-authors on the paper.
Traditionally, scientists have had two options when it comes to controlling light for use in various optical devices. The first is a photonic integrated circuit (PIC) that can be incorporated onto small chips but has limited ability to control free-space light — light propagating in air, outer space or a vacuum, as opposed to being guided in fibers or other waveguides. The second is a newly emergent metasurface — an artificially engineered thin layer that allows for light manipulation at subwavelength scale but cannot be integrated on a chip.
Ni and his fellow researchers solved this problem by incorporating the best qualities of the two previous options into a new, hybrid photonic architecture that has metasurfaces integrated onto a PIC chip while maintaining high light controllability. “This incorporation of the PICs and metasurfaces makes it possible to drive the metasurfaces using guided waves inside the PICs,” Ni said. “It enables routing light among different metasurfaces, performing multiple complex functions on a single chip.”
This new development could have applications in optical communications, optical remote sensing — LiDAR — free-space optical interconnects for data centers and virtual reality and augmented reality displays. “The developed technology will pave exciting ways for building multifunctional PIC devices with flexible access to free space as well as guided, wave-driven metasurfaces with full on-chip integration capability,” Ni said.
According to Ni, the most intriguing aspects of his research are the implications for future developments and the success of combining the best traits of existing technology. “I think the most exciting part of the research is that we married two powerful technologies with complementary capabilities — integrated photonics and metasurfaces,” he said. “Our hybrid system has the advantages from both the metasurfaces and the PICs. In addition, our design is highly flexible and modular. A library of the building blocks can be established for reusing and creating consistent functional components across various devices or systems.”
Funding for this research came from the Gordon and Betty Moore Foundation, the National Aeronautics and Space Administration Early Career Faculty Award, the Office of Naval Research and the Penn State Materials Research Science and Engineering Center.
Nanostructures Allow High Harmonic Generation with Pulsed Lasers, reported in July 2021
Cornell University researchers have developed nanostructures that enable record-breaking conversion of laser pulses into high-harmonic generation. The work paves the way for new scientific tools for high-resolution imaging as well as the study of physical processes that occur at the scale of an attosecond.
High harmonic generation is used to merge photons from a pulsed laser into one ultrashort photon with higher energy, producing extreme ultraviolet (EUV) light and x-rays used for a variety of scientific purposes. Gases have traditionally been used as sources of harmonics, but a research team led by Gennady Shvets in the College of Engineering found that engineered nanostructures constitute an easier solution.
The nanostructures that the team created make up an ultrathin resonant gallium phosphide metasurface that overcomes many of the usual problems associated with high harmonic generation in gases and other solids. The gallium phosphide materials permit harmonics of all orders without re-absorbing them, and the specialized structure is able to interact with the laser pulse’s full spectrum.
“Achieving this required engineering of the metasurface’s structure using full-wave simulations,” said Maxim Shcherbakov, who was a postdoctoral associate at Cornell. “We carefully selected the parameters of the gallium-phosphide particles to fulfill this condition, and then it took a custom nanofabrication flow to bring it to light.”
Those efforts yielded nanostructures capable of generating both even and odd harmonics — a limitation of most other harmonic materials — covering a wide range of photon energies between 1.3 and 3 electron volts. The level of conversion efficiency enables scientists to observe the molecular and electronic dynamics within a material with just one laser shot, which helps preserve samples that may otherwise be degraded by multiple high-powered shots.
To the researchers’ knowledge, the study is the first to observe high-harmonic generated radiation from a single laser pulse, which allowed the metasurface to withstand high powers — five to 10 times higher than previously shown in other metasurfaces. “It opens up new opportunities to study matter at ultrahigh fields, a regime not readily accessible before,” said Shcherbakov. “With our method, we envision that people can study materials beyond metasurfaces, including but not limited to crystals, 2D materials, single atoms, artificial atomic lattices, and other quantum systems.”
With the preliminary work of demonstrating the advantages of nanostructures for high harmonic generation, the team now seeks to improve high-harmonic devices and facilities by stacking the nanostructures together to replace a solid-state source, such as crystals.
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.
Machine learning enhances light-matter interactions in dielectric nanostructures
A paper published in Advanced Photonics “Enhanced light-matter interactions in dielectric nanostructures via machine-learning approach,” suggests that machine-learning techniques can be used to enhance metasurfaces, optimizing them for nonlinear optics and optomechanics. The discovery has promising possibilities for the development of a wide range of photonic devices and applications including those involved in optical sensing, optoacoustic vibrations, and narrowband filtering.
Metasurfaces are versatile platforms used to manipulate the scattering, color, phase, or intensity of light that can be used for light emission, detection, modulation, control and/or amplification at the nanoscale. In recent years, metasurfaces have been a subject of undergoing intense study as their optical properties can be adapted to a diverse set of applications, including superlenses, tunable images, and holograms. According to Advanced Photonics Co-Editor-in-Chief, SPIE Fellow, and Head of Photonics & Nanotechnology Group at King’s College London Anatoly Zayats, this work marks an exciting advancement in nanophotonics.
“Optimization of metasurfaces and metamaterials for particular applications is an important and time-consuming problem,” said Zayats. “With traditional approaches, only few parameters can be optimised, so that the resulting performance is better than for some other designs but not necessarily the best. Using machine learning, one can search for the best design and cover the space of parameters not possible with traditional approaches.”
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