Metasurfaces have become a rapidly growing field of research in recent years due to their exceptional abilities in light manipulation and versatility in ultrathin optical applications. They also significantly benefit from their simplified fabrication process compared to metamaterials and are promising for integration with on-chip nanophotonic devices owing to their planar profiles. The recent progress in metasurfaces is reviewed and they are classified into six categories according to their underlying physics for realizing full 2π phase manipulation. Starting from multi-resonance and gap-plasmon metasurfaces that rely on the geometric effect of plasmonic nanoantennas, Pancharatnam–Berry-phase metasurfaces, on the other hand, use identical nanoantennas with varying rotation angles. The recent development of Huygens' metasurfaces and all-dielectric metasurfaces especially benefit from highly efficient transmission applications. An overview of state-of-the-art fabrication technologies is introduced, ranging from the commonly used processes such as electron beam and focused-ion-beam lithography to some emerging techniques, such as self-assembly and nanoimprint lithography. A variety of functional materials incorporated to reconfigurable or tunable metasurfaces is also presented. Finally, a few of the current intriguing metasurface-based applications are discussed, and opinions on future prospects are provided.

https://www.onlinelibrary.wiley.com/doi/epdf/10.1002/smtd.201600064

Fundamentals and Applications of Metasurfaces

As nanofabrication technology progresses, the emerging metasurface has offered unique opportunities for holography, such as an increased data capacity and the realization of polarization-sensitive functionality. Multicolor three-dimensional (3D) meta-hologram imaging is one of the most pursued applications for meta-hologram not yet realized. How to reduce the cross-talk among different colors in broad bandwidth designs is a critical question. On the basis of the off-axis illumination method, we develop a novel way to overcome the cross-talk limitation and achieve multicolor meta-holography with a single type of plasmonic pixel. With this method, the usable data capacity can also be improved. It not only leads to a remarkable image quality, with a signal-to-noise ratio (SNR) five times better than that of the previous meta-hologram designs, but also paves the way to new meta-hologram devices, which mark an advance in the field of meta-holography. For example, a seven-color meta-hologram can be fabricated with a color gamut 1.39 times larger than that of the red, green, and blue (RGB) design. For the first time, a full-color meta-holographic image in the 3D space is also experimentally demonstrated. Our approach to expanding the information capacity of the meta-hologram is unique, which extends broad applications in data storage, security, and authentication.

Multicolor 3D meta-holography by broadband plasmonic modulation

Holography has been identified as a vital platform for three-dimensional displays, optical encryption, microscopy and artificial intelligence through different physical dimensions. However, unlike the wavelength and polarization divisions, orbital angular momentum (OAM) of light, despite its helical wavefront being an independent physical dimension, has not been implemented as an information carrier for holography due to the lack of helical mode index selectivity in the Bragg diffraction formula. Here, we demonstrate OAM holography by discovering strong OAM selectivity in the spatial-frequency domain without a theoretical helical mode index limit. As such, OAM holography allows the multiplexing of a wide range of OAM-dependent holographic images with a helical mode index spanning from −50 to 50, leading to a 10 bit OAM-encoded hologram for high-security optical encryption. Our results showing up to 210 OAM-dependent distinctive holographic images mark a new path to achieving ultrahigh-capacity holographic information systems harnessing the previously inaccessible OAM division. The orbital angular momentum degree of freedom is used to demonstrate 10 bit holographic images with a helical mode index spanning from −50 to 50.

Orbital angular momentum holography for high-security encryption

Metasurfaces, two-dimensional versions of metamaterials, retain the great capabilities of three-dimensional counterparts in manipulating electromagnetic wave behaviors, while reducing the challenges in fabrication. By judiciously engineering parameters of individual building blocks (such as geometry, size, and material) and selecting specific design algorithms, metasurfaces are promising to replace conventional electromagnetic elements in nanoplasmonic/photonic devices. Significantly, such concept can be readily promoted to other disciplines, such as acoustics, thermal physics, and seismology. In this article, the latest advances in full control of electromagnetic waves with metasurfaces are briefly reviewed from a functionality perspective. A broad avenue towards real-life applications of metamaterials has been opened up, although they are still at their infant stage. At the end, several promising approaches are suggested to extend the applications of metasurfaces.

Advances in Full Control of Electromagnetic Waves with Metasurfaces

The growing amount of data that is generated every year creates an urgent need for new and improved data storage methods. Nanomaterials, which have unique mechanical, electronic and optical properties owing to the strong confinement of electrons, photons and phonons at the nanoscale, are enabling the development of disruptive methods for optical data storage with ultra-high capacity, ultra-long lifetime and ultra-low energy consumption. In this Review, we survey recent advancements in nanomaterials technology towards the next generation of optical data storage systems, focusing on metallic nanoparticles, graphene and graphene oxide, semiconductor quantum dots and rare-earth-doped nanocrystals. We conclude by discussing the use of nanomaterials in data storage systems that do not rely on optical mechanisms and by surveying the future prospects for the field. New solutions are needed to meet the growing demand for data storage systems with ultra-high capacity, ultra-long lifetime and ultra-low energy consumption. Nanomaterials, including metal nanoparticles, graphene and graphene oxide, semiconductor quantum dots and rare-earth-doped nanocrystals, hold promise for the next generation of optical data storage methods.

Nanomaterials for optical data storage

The emerging graphene-based material, an atomic layer of aromatic carbon atoms with exceptional electronic and optical properties, has offered unprecedented prospects for developing flat two-dimensional displaying systems. Here, we show that reduced graphene oxide enabled write-once holograms for wide-angle and full-colour three-dimensional images. This is achieved through the discovery of subwavelength-scale multilevel optical index modulation of athermally reduced graphene oxides by a single femtosecond pulsed beam. This new feature allows for static three-dimensional holographic images with a wide viewing angle up to 52 degrees. In addition, the spectrally flat optical index modulation in reduced graphene oxides enables wavelength-multiplexed holograms for full-colour images. The large and polarization-insensitive phase modulation over π in reduced graphene oxide composites enables to restore vectorial wavefronts of polarization discernible images through the vectorial diffraction of a reconstruction beam. Therefore, our technique can be leveraged to achieve compact and versatile holographic components for controlling light.

/pdf/athermally-photoreduced-graphene-oxides-for-three-4pedaj3yu4.pdf

Athermally photoreduced graphene oxides for three-dimensional holographic images

In this manuscript, we report the refractive-index (RI) modulation of various concentrations of nitrogen-doped carbon dots (N@C-dots) embedded in poly(vinyl alcohol) (PVA) polymer. The dispersion and size distribution of N@C-dots embedded within PVA have been investigated using electron microscopy. The RI of PVA-N@C-dots can be enhanced by increasing the doping concentration of highly fluorescent C-dots (quantum yield 44%). This is demonstrated using ultraviolet–visible (UV–visible), photoluminscence, Raman, and Fourier transform infrared (FTIR) spectroscopy measurements. The Mie scattering of light on N@C-dots was applied for developing the relationship between RI tuning and absorption cross section of N@C-dots. The extinction cross section of N@C-dot thin films can be rapidly enhanced by either tuning the RI or increasing the concentration of N@C-dots. The developed method can be used as effective RI contrast for various applications such as holography creation and bioimaging.

Refractive-Index Tuning of Highly Fluorescent Carbon Dots

We report on an 800 nm femtosecond laser beam induced giant refractive index modulation and enhancement of near-infrared transparency in topological insulator material Bi2Te3 thin films. An ultrahigh refractive index of up to 5.9 was observed in the Bi2Te3 thin film in near-infrared frequency. The refractive index dramatically decreases by a factor of similar to 3 by an exposure to the 800 nm femtosecond laser beam. Simultaneously, the transmittance of the Bi2Te3 thin films markedly increases to similar to 96% in the near-infrared frequency. The Raman spectra provides strong evidences that the observed both refractive index modulation and transparency enhancement result from laser beam induced photooxidation effects in the Bi2Te3 thin films. The Bi2Te3 compound transfers into Bi2O3 and TeO2 under the laser beam illumination. These experimental results pave the way towards the design of various optical devices, such as near-infrared flat lenses, waveguide and holograms, based on topological insulator materials.

https://iopscience.iop.org/article/10.1088/2053-1591/aa9c94/pdf

Photo-oxidation-modulated refractive index in Bi2Te3 thin films

We report on an 800 nm femtosecond laser beam induced giant refractive index modulation and enhancement of near-infrared transparency in topological insulator material Bi2Te3 thin films. An ultrahigh refractive index of up to 5.9 was observed in the Bi2Te3 thin film in near-infrared frequency. The refractive index dramatically decreases by a factor of ~ 3 by an exposure to the 800 nm femtosecond laser beam. Simultaneously, the transmittance of the Bi2Te3 thin films markedly increases to ~ 96% in the near-infrared frequency. The Raman spectra provides strong evidences that the observed both refractive index modulation and transparency enhancement result from laser beam induced photooxidation effects in the Bi2Te3 thin films. The Bi2Te3 compound transfers into Bi2O3 and TeO2 under the laser beam illumination. These experimental results pave the way towards the design of various optical devices, such as near-infrared flat lenses, waveguide and holograms, based on topological insulator materials.

Photo-induced change of refractive index and transparency in Bi2Te3 films

A profound problem in existing photorefractive polymers is discovering the efficient sensitizer in order to achieve a large refractive index modulation to realize a glass free holographic three-dimensional display. We experimentally demonstrate the graphene-oxides can enhance the photorefractivity of photorefractive polymers. The enhanced photorefractivity paves the way for a variety of three-dimensional holographic display on the reversible refractive-index modulations.

Amit Sahu

Papers

Athermally photoreduced graphene oxides for three-dimensional holographic images

Refractive-Index Tuning of Highly Fluorescent Carbon Dots

Photo-oxidation-modulated refractive index in Bi2Te3 thin films

Photo-induced change of refractive index and transparency in Bi2Te3 films

Graphene-oxides Photorefractive Polymers