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In the realm of modern science and engineering, metamaterials and metasurfaces stand as towering pillars of innovation, offering unparalleled control over the behavior of light and other waves. These artificial materials, meticulously crafted at the micro and nano scales, have sparked a revolution across multiple fields, from optics to telecommunications and beyond.
One such innovation that has captured the attention of researchers and engineers alike is optical metasurfaces. These nanostructured materials are paving the way for groundbreaking advancements across various sectors, from communication to data processing and beyond. In this article, we’ll explore how optical metasurfaces are transforming industries through applications such as optical modulators, beam steering LIDAR, radiation control, wireless communication, and optical AI inference chips designed for data center applications.
Metamaterials: Unleashing Exotic Behavior
Metamaterials, crafted through intricate arrangements of artificially structured units, defy the conventions of natural materials. By manipulating the interaction of waves with tailored structures, metamaterials exhibit extraordinary properties not found in nature. For instance, seismic waves can be bent around structures to shield them from earthquakes, while tsunami waves can be diverted away from vulnerable coastal towns. Similarly, sound waves can be meticulously controlled to create soundproof environments, revolutionizing architectural design and acoustic engineering.
The core concept of metamaterials lies in their structural design, where constituent units are meticulously tailored to achieve desired functionalities. These artificial “atoms” and “molecules” can be precisely engineered in terms of shape, size, and composition, enabling fine-tuned control over wave behavior. By arranging these units into specific architectures, metamaterials can exhibit refractive indices ranging from positive to negative, paving the way for unprecedented optical phenomena and wave manipulation.
Challenges and Advances in Metamaterials
While metamaterials offer tantalizing possibilities, their practical implementation faces challenges. Many metamaterial designs require complex fabrication processes and suffer from limited bandwidth or high losses. The quest for a metamaterial capable of operating across the entire visible spectrum remains a significant challenge, necessitating innovations in fabrication techniques and material science.
Moreover, the intricate architectures of metamaterials often lead to extensive losses and fabrication complexities. Traditional designs rely on multiple layers of materials, hindering scalability and integration into practical devices. Despite these challenges, researchers continue to push the boundaries of metamaterial technology, exploring new materials, fabrication methods, and design strategies to unlock their full potential.
Metasurfaces: Two-Dimensional Marvels
In the quest for overcoming the limitations of bulk metamaterials, researchers turned to metasurfaces – two-dimensional counterparts offering compactness, flexibility, and ease of integration. Composed of arrays of subwavelength nanostructures, metasurfaces enable precise control over light waves with unprecedented efficiency. These thin films serve as versatile platforms for encoding complex optical functionalities, including beam steering, focusing, and polarization manipulation.
The beauty of metasurfaces lies in their simplicity and versatility. By tailoring the arrangement of nanoantennas, metasurfaces can modulate the phase and amplitude of light waves with remarkable precision. This capability enables a myriad of applications, ranging from high-resolution imaging to optical communication and sensing.
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.
Applications and Future Prospects
The applications of metamaterials and metasurfaces span a vast array of fields, promising transformative impacts on technology and society. From compact lenses with minimal aberrations to ultra-thin optical components for augmented reality devices, the potential uses are limitless. Metasurfaces, in particular, offer a pathway to lightweight, low-cost optical devices with unparalleled performance and integration capabilities.
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
Optical Modulators: Redefining Data Processing
At the heart of modern data processing lies optical modulators, which manipulate light to encode and transmit information. Optical metasurfaces have revolutionized this field by offering unprecedented control over light waves. These metasurfaces, engineered with subwavelength features, enable precise modulation of light intensity, phase, and polarization. As a result, optical modulators based on metasurfaces exhibit enhanced performance metrics such as speed, efficiency, and compactness, making them ideal for high-speed data transmission and processing in data centers.
1. Dispersionless flat lenses.
Dispersionless flat lenses represent a significant advancement in optical instrumentation, offering the potential to revolutionize traditional lens designs. These lenses address issues such as chromatic aberration and spherical aberration, providing compact and aberration-corrected alternatives that can significantly shrink the complexity and size of optical instruments. By correcting chromatic aberration over a broad wavelength range and mitigating spherical aberration, dispersionless flat lenses pave the way for enhanced optical performance across various applications, promising sharper imaging and improved optical precision.
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 emerge as a solution to the longstanding challenge of developing compact and fast modulators and Spatial Light Modulators (SLMs) operating in the mid-infrared and THz spectral range. These surfaces serve as a versatile platform for creating modulators tailored to these regimes, enabling applications in free-space communications, imaging, LIDAR (Light Detection and Ranging), and homeland security. By facilitating efficient modulation of light in these spectral ranges, metasurfaces offer unprecedented opportunities for enhancing optical communication systems, remote sensing technologies, and security applications.
Beam Steering LIDAR: Enhancing Sensing Capabilities
LIDAR (Light Detection and Ranging) technology plays a pivotal role in applications such as autonomous vehicles, remote sensing, and augmented reality. Optical metasurfaces have unlocked new possibilities in LIDAR systems by enabling dynamic beam steering with unparalleled precision. By manipulating the phase and direction of incident light waves, metasurface-based beam steering devices can rapidly scan the environment, providing real-time 3D imaging and spatial mapping. This advancement holds promise for enhancing the safety and efficiency of various industries reliant on LIDAR technology.
More effective laser control possible via new metasurface system reported in August 2021
In August 2021, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) unveiled a groundbreaking metasurface system capable of effectively controlling various properties of laser light, including wavelength, without the need for additional optical components. Unlike traditional systems relying on individual pillars, this innovative metasurface employs supercells—groups of pillars working together to manipulate different aspects of light. When light from a laser diode interacts with these supercells on the metasurface, it splits into multiple beams, each independently controlled in terms of shape and intensity. The system, comprising a laser diode and a reflective metasurface, creates a laser cavity between them, while simultaneously reflecting part of the light to generate a second beam.
This novel approach not only simplifies optical systems but also enhances their efficiency, offering potential applications ranging from quantum sensing to virtual and augmented reality. By adjusting the position of the metasurface relative to the laser diode, researchers can easily change the emitted wavelength. This precise control over laser light properties is crucial for various technologies, including commercial VR headsets and biomedical imaging. Federico Capasso, the senior author of the paper, highlights the significance of this advancement, emphasizing its potential to revolutionize optical source emission engineering and enable diverse applications, such as scientific instrumentation, augmented reality, and holography.
Radiation Control: Safeguarding Health and Infrastructure
In radiation-sensitive environments such as healthcare facilities and nuclear power plants, controlling the propagation and absorption of electromagnetic radiation is critical. Optical metasurfaces offer a novel solution to this challenge by tailoring the optical properties of materials at the nanoscale. By engineering metasurfaces with specific geometries and compositions, researchers can manipulate the interaction of light with matter, thereby enabling precise control over radiation levels.
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. Radiative cooling metasurfaces present a promising avenue for technological innovation, harnessing thermoregulatory properties inspired by natural mechanisms observed in animals inhabiting extreme environments. These metasurfaces demonstrate the ability to passively cool objects below ambient air temperature by manipulating thermal radiation. By leveraging principles observed in nature, radiative cooling metasurfaces offer potential applications in various fields, including thermal management systems for electronics, passive cooling solutions for buildings, and energy-efficient technologies for harsh environmental conditions. This capability holds immense potential for enhancing radiation shielding, medical imaging, and environmental monitoring systems.
Wireless Communication: Unleashing Bandwidth and Efficiency
The demand for high-speed, reliable wireless communication continues to soar in the era of IoT (Internet of Things) and 5G networks. Optical metasurfaces are poised to revolutionize wireless communication systems by enabling the manipulation of electromagnetic waves across the optical spectrum. Metasurface-based antennas and beamforming devices offer superior performance in terms of bandwidth, efficiency, and beam steering capabilities compared to traditional antenna technologies. As a result, they can support the growing demands of wireless networks while minimizing interference and maximizing data throughput.
Optical AI Inference Chips: Powering Next-Generation Data Centers
In the era of artificial intelligence (AI), the demand for efficient computing hardware capable of handling complex neural network computations is escalating. Optical metasurfaces have emerged as a game-changing technology for AI inference chips designed for data center applications. By leveraging the unique properties of metasurfaces, such as subwavelength light manipulation and high-speed optical processing, these chips can accelerate AI algorithms while minimizing energy consumption and latency. As a result, data centers can achieve unprecedented levels of performance and scalability, paving the way for advancements in AI-driven applications.
Recent advancements in metamaterials and metasurfaces have opened doors to exciting possibilities, from enhanced light-matter interactions to novel photonic devices and beyond. Machine learning techniques, combined with advanced fabrication methods, hold promise for optimizing metasurface designs and accelerating innovation in nanophotonics.
Neurophos, a collaboration between Duke University and Metacept Inc., a metamaterials-focused incubator, recently secured $7.2 million in seed round funding.
The investment aims to propel the commercialization of metamaterial and optical AI inference chips designed for data center applications. Neurophos intends to leverage its high-speed silicon photonics technology to power a metasurface in-memory processor, enabling rapid and efficient AI computations.
With the newly acquired funds, Neurophos plans to advance the production of its proprietary metasurface, which functions as a tensor core processor. Additionally, the company intends to expand its team of engineers based in Austin, Texas. Notably, Neurophos’ metamaterial-based optical modulators boast a size reduction of over 1000 times compared to conventional foundry process design kits, facilitating more than 1 million trillion operations per second. The technology merges an optical metasurface with silicon photonic computing, employing a compute-in-memory (CIM) processor architecture powered by high-speed silicon photonics to accelerate matrix-matrix multiplications, a fundamental operation in neural network computations.
The metasurface-enabled optical CIM elements offer a remarkable reduction in size compared to traditional silicon photonics modulators, enabling on-chip processing of significantly larger matrices. This breakthrough leads to an unparalleled increase in computational density, presenting promising prospects for high-performance computing applications. Patrick Bowen, CEO of Neurophos, highlighted the revolutionary impact of leveraging metamaterials within a standard CMOS process, resulting in an 8000-fold reduction in optical processor size and promising substantial improvements over current GPU capabilities.
Neurophos emerged from Metacept, an incubator spearheaded by David Smith, the James B. Duke Professor of Electrical and Computer Engineering at Duke University. Metacept focuses on fostering metamaterials-based companies and collaborates closely with Smith’s research group at Duke. The seed funding round was led by Gates Frontier, with support from partners such as MetaVC Partners, which initially funded Neurophos and granted an exclusive license to the fund’s metamaterials IP portfolio for optical computing. Furthermore, Neurophos has joined Silicon Catalyst, an incubator/accelerator specializing in semiconductor technology, to further advance its innovative solutions.
New material platforms for metasurfaces.
Recent research has been actively exploring new material platforms tailored for metamaterials and metasurfaces, focusing on properties like low losses, tunability, high melting points, and compatibility with CMOS technology. Transition-metal nitrides, such as TiN, offer optical properties comparable to gold but with significantly higher melting points, making them suitable for metasurface applications requiring high optical intensity. Transparent conducting oxides (TCOs), like indium-tin-oxide, provide control over the spectral location of the epsilon-near-zero point, leading to enhanced optical near-fields that can be harnessed for optical modulation and nonlinear optics.
Furthermore, phase-change materials, such as chalcogenide alloys, known for their use in rewritable optical media, offer intriguing opportunities for metasurface applications. These materials can switch between amorphous and crystalline states using laser or electrical current pulses, enabling functionalities like all-optical, non-volatile metasurface switches and high-resolution solid-state displays. Additionally, materials like SmNiO3, a phase-change perovskite nickelate, exhibit reversible refractive index changes over an ultra-broad spectral range, from visible to long-wavelength mid-infrared. This unique property, driven by strong electron correlation effects, opens avenues for creating diverse active photonic devices leveraging this new mechanism.
Metasurfaces Enable Low-Loss Integrated Photonics Platform
In September 2019, a research team from the University of Delaware introduced an innovative integrated photonics platform utilizing metasurfaces to enable low-loss on-chip optical signal processing. Their platform, incorporating a high-contrast transmitarray (HCTA)-based metalens embedded with hundreds of air slots on a silicon-based chip, achieved remarkable performance metrics. The 1D metalens demonstrated a high numerical aperture of up to 2.14, capable of focusing light within 10 µm with less than 1 dB loss, and enabled computational tasks such as Fourier transformation and spatial differentiation by stacking multiple layers of the HCTA-based metalenses. The platform, compatible with foundry fabrication, offered ultracompact dimensions and broadband operation, addressing key requirements for energy-efficient optical communications.
This pioneering work represents the first application of low-loss metasurfaces on an integrated photonics platform, heralding a significant advancement in on-chip optical processing capabilities. Notably, the compact and alignment-free design of the device, coupled with its scalability and compatibility with foundry fabrication processes, distinguishes it from conventional free-space optics platforms. The researchers anticipate broader applications, envisioning its integration with multimode waveguides for mode transformation in mode division multiplexing systems, and foresee its potential use in on-chip spectrometers, light detection and ranging devices, and diffractive optical computational circuits. Despite existing technical challenges, the platform promises faster operation and serves as a promising starting point for further advancements in on-chip optical systems.
New photonic integrated chip could enable developments for many optical technologies
Penn State researchers, led by Assistant Professor Xingjie Ni, have developed a novel photonic integrated chip that combines the advantages of photonic integrated circuits (PICs) and metasurfaces. This hybrid architecture enables high controllability of light while maintaining integration on a small chip, addressing the limitations of previous options for light control. By incorporating metasurfaces onto a PIC chip, the team can drive the metasurfaces using guided waves inside the PICs, facilitating routing of light among different metasurfaces and performing multiple complex functions on a single chip. This breakthrough technology has broad applications across optical communications, LiDAR, free-space optical interconnects, and virtual and augmented reality displays.
The hybrid photonic architecture developed by the researchers offers a flexible and modular design, allowing for the establishment of a library of building blocks for consistent functional components across various devices or systems. The successful combination of integrated photonics and metasurfaces leverages the complementary capabilities of both technologies, paving the way for multifunctional PIC devices with flexible access to free space and fully integrated wave-driven metasurfaces. Funding for this research was provided by various organizations, including the Gordon and Betty Moore Foundation, NASA, 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 significantly enhance the conversion of laser pulses into high-harmonic generation, paving the way for advanced scientific tools in high-resolution imaging and the study of attosecond-scale physical processes. Unlike traditional gas sources, the engineered ultrathin resonant gallium phosphide metasurface offers a more efficient solution for generating extreme ultraviolet (EUV) light and x-rays. This metasurface overcomes common challenges associated with high harmonic generation in gases and solids by allowing harmonics of all orders without re-absorption and interacting effectively with the laser pulse’s full spectrum.
Through meticulous engineering and custom nanofabrication, the researchers achieved nanostructures capable of generating both even and odd harmonics across a wide range of photon energies. This breakthrough enables scientists to observe molecular and electronic dynamics within materials with a single laser shot, preserving samples that might otherwise degrade under multiple high-powered shots. The study, which marks the first observation of high-harmonic generated radiation from a single laser pulse, showcases the metasurface’s ability to withstand high powers and opens new avenues for studying matter at ultrahigh fields. Moving forward, the team aims to enhance high-harmonic devices and facilities by stacking nanostructures together to replace solid-state sources like crystals.
Machine learning enhances light-matter interactions in dielectric nanostructures
The recent publication in Advanced Photonics titled “Enhanced light-matter interactions in dielectric nanostructures via machine-learning approach” heralds a significant breakthrough in photonics research. By harnessing machine-learning techniques, researchers have unlocked the potential to optimize metasurfaces for nonlinear optics and optomechanics, offering a pathway to revolutionize a wide array of photonic devices and applications. These advancements hold promise for enhancing optical sensing capabilities, facilitating optoacoustic vibrations, and refining narrowband filtering techniques.
Metasurfaces, which serve as versatile platforms for manipulating light at the nanoscale, have garnered increasing attention in recent years due to their adaptability to diverse applications. From enabling superlenses to creating tunable images and holograms, metasurfaces offer unprecedented control over the scattering, color, phase, and intensity of light. Anatoly Zayats, Co-Editor-in-Chief of Advanced Photonics and a leading figure in the field of nanophotonics, underscores the significance of this research advancement. By leveraging machine learning, researchers can efficiently navigate the complex parameter space inherent in optimizing metasurfaces and metamaterials, ultimately achieving superior performance beyond the constraints of traditional optimization approaches. This novel methodology promises to accelerate advancements in photonics, paving the way for transformative innovations in various fields reliant on precise light-matter interactions.
Conclusion
In conclusion, optical metasurfaces represent a paradigm shift in the field of photonics, unlocking a myriad of possibilities across diverse industries. From enhancing data processing and sensing capabilities to safeguarding health and infrastructure, and powering next-generation communication and computing systems, the potential applications of optical metasurfaces are boundless.
As researchers continue to explore the frontiers of metamaterial science, we can anticipate a future where light and waves are harnessed with unprecedented precision and efficiency. Whether revolutionizing telecommunications, enabling next-generation sensors, or transforming healthcare, the journey of metamaterials and metasurfaces is one of endless discovery and innovation. Through collaborative efforts and interdisciplinary research, we pave the way towards a brighter, more interconnected future.
References and Resources also include:
https://www.photonics.com/Articles/Metasurface_for_Nonlinear_Manipulation_Could/p4/a64253
http://iopscience.iop.org/article/10.1088/0034-4885/79/7/076401/ampdf
https://www.photonics.com/Article.aspx?AID=65278
https://eurekalert.org/pub_releases/2020-04/ssfo-mle043020.php
https://news.engr.psu.edu/2020/ni-xingjie-photonic-integrated-chip.aspx
https://www.photonics.com/Articles/Nanostructures_Allow_High_Harmonic_Generation/a67196
