Optical Metasurfaces: Applications in Imaging and Sensing

By Owais AliReviewed by Lexie CornerSep 12 2024

Optical metasurfaces have emerged as promising solutions to the limitations posed by bulky optical elements. They provide a compact, efficient method for light manipulation, offering advanced control over phase, polarization, and emission compared to conventional refraction and propagation techniques. This article overviews optical metasurfaces, their diverse applications in imaging and sensing technologies, and recent advancements in these fields.

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Optical Metasurfaces: Background

Optical metamaterials have fascinated physicists and engineers for decades. While theoretical discussions began in the 1940s, significant experimental progress only occurred in the last 20-30 years, driven by advances in nanotechnology and semiconductor manufacturing.

Early theories focused on 3D bulk nanostructured materials, but practical efforts quickly shifted to 2D nanostructured optical elements, known as optical metasurfaces. These 2D metasurfaces have revolutionized light manipulation by enabling rapid nanoscale changes in phase and amplitude, offering unprecedented control over light.¹

How Do Optical Metasurfaces Work?

Optical metasurfaces are artificially engineered materials composed of light-scattering dielectric or metallic nanostructures arranged in two-dimensional or quasi-two-dimensional patterns. These surfaces offer a compact and sophisticated platform for controlling light's polarization, phase, and amplitude with high precision.

Unlike traditional optics that rely on light refraction and propagation, optical metasurfaces manipulate light through the scattering of nanostructures. These nanostructures resonate with incoming light and re-emit it with controlled phase, polarization, and spectral properties, enabling precise shaping of light waves.

This level of control allows for enhanced spectral selectivity, wavefront and polarization control, and improved light radiation and detection.²

Advantages Over Traditional Optical Components

The key advantage of optical metasurfaces is their ability to achieve complex optical functions within a thin, planar geometry. This offers exceptional flexibility in customizing optical responses, allowing for functionalities that are challenging or impossible with conventional optics.

Optical metasurfaces can perform multiple optical functions simultaneously within a single layer, leading to more efficient and compact systems by reducing the need for numerous discrete components. They can be designed to operate across a wide range of wavelengths, from visible light to infrared, enhancing their versatility for various applications.³

Applications in Imaging

Super-Resolution in Advanced Imaging Techniques

Optical metasurfaces are transforming imaging technologies by providing enhanced resolution and miniaturization. They can overcome the diffraction limit, enabling super-resolution imaging in systems like optical coherence tomography (OCT) and two-photon microscopy.

In plasmonic structured illumination microscopy (PSIM), optical metasurfaces with periodic slit arrangements enhance resolution by a factor of 2.6 compared to conventional methods.

Two-photon microscopy also benefits from metasurfaces, with a thin double-wavelength metasurface objective lens achieving a high numerical aperture (NA) of 0.5 for compact and high-resolution imaging.

Similarly, in OCT, optical metasurfaces enhance imaging resolution and extend the depth of focus (DOF) to 211 μm and 315 μm, outperforming traditional lenses while reducing component size.⁴

Multi-Color Holograms

Optical metasurfaces also enhance computational imaging by facilitating advanced applications like ghost and complex optical field imaging. They simplify setups and improve performance in systems that extract hidden information from scattered light or complex speckle patterns.

Additionally, researchers have developed high-efficiency, multi-color holograms by encoding complex phase distributions onto a single optical metasurface layer, achieving over 90 % resolution in color imaging with chromatic aberration correction.^2,4

High-Resolution Polarization and Hyperspectral Imaging

In functional imaging, optical metasurfaces enable advanced polarization and hyperspectral imaging. They provide efficient polarization analysis without traditional filters, enhancing the capability to measure and analyze polarization states.

Hyperspectral imaging also benefits from optical metasurfaces that support high-quality factor resonances for ultrasensitive detection of molecular changes, allowing for detailed spectral analysis and enhanced biosensing capabilities.⁴

Applications in Sensing

The unique properties of optical metasurfaces have also led to their widespread adoption in biosensing, gas sensing, and chemical sensing. These metasurface-based sensors offer higher sensitivity, selectivity, and detection limits than conventional plasmonic sensors and also facilitate miniaturization and integration into portable devices.

SPR Sensors for Viral Diagnostics

A key application of optical metasurfaces is their use in enhancing traditional plasmonic sensors for biomarker detection, significantly improving the accuracy of disease diagnosis.

Recent studies have shown that integrating metasurfaces into surface plasmon resonance (SPR) sensors greatly enhances sensitivity and detection limits. These advanced SPR sensors have successfully detected viruses like hepatitis B, Zika, and SARS-CoV-2, demonstrating the potential of metasurface-based biosensors for rapid and precise disease diagnosis.

Air Quality Monitoring

Unlike traditional gas sensors, which struggle with slow response times and sensitivity to environmental changes, optical metasurface-based sensors offer rapid and highly sensitive detection by leveraging plasmonic effects in metal-insulator-metal (MIM) structures.

For instance, metamaterial perfect absorber (MPA) sensors exhibit a sensitivity of 22.4 ± 0.5 ppm·Hz^−0.5 for gases like carbon dioxide and butane due to enhanced analyte interaction.

This exceptional sensitivity, combined with their compact size and quick response times, makes these sensors ideal for extensive sensor networks. They provide real-time, high-resolution data on air quality and greenhouse gases in urban, industrial, and natural settings.

Non-Destructive Real-time Chemical Monitoring

Optical metamaterial-based sensors, integrated with microfluidic channels and double split-ring resonators, enable real-time, label-free monitoring of multiple chemical concentrations in liquid or gaseous environments. Their ability to tailor optical responses to specific chemical signatures makes them highly valuable for environmental monitoring, food safety testing, and industrial process control.⁵

Recent Advancements and Emerging Trends

The field of optical metasurfaces is rapidly advancing, with recent breakthroughs significantly enhancing imaging and sensing applications.

Single-Shot Polarization Imaging

Harvard researchers recently developed a compact, single-shot polarization imaging system using Mueller matrix imaging with nano-engineered optical metasurfaces. The results are published in Nature Photonics.

This innovative system replaces traditional complex setups that rely on multiple rotating plates and polarizers with a simpler design that captures and analyzes polarized light in a single step.

The proposed approach simplifies the design by eliminating moving parts. It has the potential for real-time applications such as live tissue microscopy, polarimetric endoscopy, retinal scanning, and non-invasive cancer imaging.⁶

Reconfigurable Metasurfaces for Adaptive Sensing Systems

There is a growing trend in developing reconfigurable optical metasurfaces that dynamically switch between optical functions.

In a recent study published in Nature Communications, researchers designed a flexible sensor that can rapidly switch between high-contrast edge detection and detailed infrared imaging.

This was achieved by utilizing the phase transition properties of vanadium dioxide, which alters the metasurface's optical characteristics in response to temperature changes.

Such reconfigurability opens new possibilities for adaptive imaging and sensing systems, including remote crop monitoring and quantitative phase microscopy, allowing them to adjust their functionality based on environmental conditions or user needs.⁷

Future Outlooks

Optical metasurfaces have significantly advanced imaging and sensing technologies, providing unprecedented control over light manipulation within ultra-thin, compact structures. This has led to new capabilities and enhancements in applications ranging from super-resolution microscopy to highly sensitive chemical detection.

As fabrication techniques continue to advance, metasurface-based devices are set to transform industries, including consumer electronics, medical devices, and scientific research.

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References and Future Outlooks

1. Kuznetsov, AI., et al. (2024). Roadmap for optical metasurfaces. ACS Photonics. https://doi.org/10.1021/acsphotonics.3c00457
2. Neshev, D., Aharonovich, I. (2018). Optical metasurfaces: new generation building blocks for multi-functional optics. Light: Science & Applications. https://doi.org/10.1038/s41377-018-0058-1
3. Pertsch, T., Xiao, S., Majumdar, A., Li, G. (2023). Optical metasurfaces: fundamentals and applications. Photonics Research. https://doi.org/10.1364/PRJ.487440
4. Lee, D., Gwak, J., Badloe, T., Palomba, S., Rho, J. (2020). Metasurfaces-based imaging and applications: from miniaturized optical components to functional imaging platforms. Nanoscale Advances. https://doi.org/10.1039/C9NA00751B
5. Tabassum, S., Nayemuzzaman, SK., Kala, M., Kumar Mishra, A., Mishra, SK. (2022). Metasurfaces for sensing applications: Gas, bio and chemical. Sensors. https://doi.org/10.3390/s22186896
6. Zaidi, A., Rubin, NA., Meretska, ML., Li, LW., Dorrah, AH., Park, JS., Capasso, F. (2024). Metasurface-enabled single-shot and complete Mueller matrix imaging. Nature Photonics. https://doi.org/10.1038/s41566-024-01426-x
7. Cotrufo, M., Sulejman, SB., Wesemann, L., Rahman, MA., Bhaskaran, M., Roberts, A., Alù, A. (2024). Reconfigurable image processing metasurfaces with phase-change materials. Nature Communications. https://doi.org/10.1038/s41467-024-48783-3

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