A review of metasurfaces: physics and applications (1605.07672v1)

Abstract: Metamaterials are composed of periodic subwavelength metal/dielectric structures that resonantly couple to the electric and/or magnetic components of the incident electromagnetic fields, exhibiting properties that are not found in nature. 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. Metasurfaces enable a spatially varying optical response, mold optical wavefronts into shapes that can be designed at will, and facilitate the integration of functional materials to accomplish active control and greatly enhanced nonlinear response. This paper reviews recent progress in the physics of metasurfaces operating at wavelengths ranging from microwave to visible. We provide an overview of key metasurface concepts such as anomalous reflection and refraction, and introduce metasurfaces based on the Pancharatnam-Berry phase and Huygens' metasurfaces, as well as their use in wavefront shaping and beam forming applications, followed by a discussion of polarization conversion in few-layer metasurfaces and their related properties. An overview of dielectric metasurfaces reveals their ability to realize unique functionalities coupled with Mie resonances and their low ohmic losses. We also describe metasurfaces for wave guidance and radiation control, as well as active and nonlinear metasurfaces. Finally, we conclude by providing our opinions of opportunities and challenges in this rapidly developing research field.

• The paper demonstrates metasurfaces' ability to control electromagnetic waves via subwavelength structures and tailored phase discontinuities.
• It details advanced fabrication methods and designs, including Pancharatnam-Berry phase and Huygens’ metasurfaces, to achieve broadband performance.
• Practical applications such as flat lenses, holograms, and beam steering devices highlight the potential of metasurfaces in modern optical systems.

An Overview of Metasurfaces: Physics and Applications

Metasurfaces, the two-dimensional analogs of metamaterials, have emerged as a focal point in the scientific community for their ability to manipulate electromagnetic waves with subwavelength-thick structures. This review comprehensively outlines the various aspects of metasurfaces, covering theoretical underpinnings, fabrication techniques, and practical applications ranging from the microwave to the visible spectrum.

The paper begins by addressing the limitations of traditional three-dimensional metamaterials, which suffer from high losses, strong dispersion, and complex fabrication processes. Metasurfaces offer a compelling solution by simplifying these challenges into a two-dimensional form that allows for greater ease in fabrication and operational efficiency, thereby reducing ohmic losses and enabling anomalous reflection and refraction of light.

Key Concepts and Progress in Metasurface Physics

The fundamental principle behind metasurfaces is their ability to impart spatially and spectrally varying optical responses—such as phase, amplitude, and polarization—on an incident wavefront. The paper elucidates critical concepts such as the generalized laws of reflection and refraction, which extend classical Snell's laws to account for designed phase discontinuities across metasurfaces.

An important aspect covered is the Pancharatnam-Berry phase, which enables wide-phase control through geometric phase manipulation, inherently providing broadband operational capabilities. The Huygens' metasurfaces are presented as a means for achieving near-unity transmission efficiencies by matching surface impedance with free space, facilitating applications that range from beam steering to focusing.

Metasurface Applications and Numerical Findings

The authors detail various applications of metasurfaces in optical devices, showcasing flat lenses, holograms, and beam forming implementations. These devices utilize phase-gradient metasurfaces to achieve functions such as focusing, imaging, and holographic display with minimal spatial footprint and enhanced integration capability in optical systems.

Noteworthy numerical achievements include metasurfaces demonstrating ultra-high efficiency, with some reflect-array designs achieving 80% efficiency in specific anomalous reflection applications. Furthermore, the introduction of dielectric metasurfaces promises reduced losses due to their low ohmic nature and the potential for unique functionalities through Mie resonances.

Active and Nonlinear Metasurfaces

Active metasurfaces, which integrate with materials like graphene and phase-change substances, open new avenues for dynamic modulation and tunability across various spectral ranges. The paper discusses electrically tunable metasurfaces, which provide rapid, reversible changes to optical responses, thereby facilitating advancements in communication technologies and imaging systems.

Nonlinear metasurfaces further enhance interaction with light by leveraging high field enhancements within subwavelength elements, paving the way for strong nonlinear phenomena and efficient second-harmonic generation.

Challenges and Future Directions

Despite the astonishing progress, the field continues to face challenges, particularly in scaling efficiency to commercial applications or integrating metasurfaces with existing technologies. Future developments may focus on overcoming intrinsic material losses, improving fabrication techniques to achieve higher spatial resolutions, and exploring new materials like transition-metal nitrides or phase-change substances for broader tunability and spectral range.

In conclusion, metasurfaces represent a significant advancement in the manipulation of electromagnetic waves, with theoretical and practical implications across optics and photonics. Their ability to offer custom-tailored properties promises transformative applications in imaging, communication, and beyond, driving continued research and development in this vibrant field.