What is a Metasurface?

Metasurfaces are nanoscale metamaterials that are ultrathin, planar, and smaller than the wavelength of light. Metasurfaces contain nanostructures and subwavelength features that can change the phase, polarization, and amplitude of an incident lightwave. While there is a major focus on the visible light range, metasurfaces can also be used to manipulate mid- and long-wave infrared wavelengths in aerospace and defense applications.

Metamaterials are synthetic materials composed of nanoscale building blocks known as meta-atoms that are arranged into pillars or cylinders. They possess unique properties that are not found in natural materials. Metamaterials are used to manipulate optical, acoustic, and other electromagnetic waves. Metamaterials have been a rapidly growing field of advanced materials and nanophotonics in recent years.

Metalenses (also known as meta-optics) are a special kind of metamaterial used in different optical components to control and manipulate light. Traditional optics are starting to be replaced by much smaller optical metasurfaces. Metalenses are flat optics, meaning that the lenses and other optical components they are used in have no curvature.

The cylinders or pillars on the surface of metamaterials and metasurfaces can manipulate and control the behavior of different waves. The pillars are arranged into periodic patterns and enable the metasurface to interact in different ways depending on the design.

Metasurfaces come in different shapes and sizes that are all composed of different unit cell building blocks. They can be made of different materials depending on the application and intended optical properties of the metasurface.

Metamaterials gain different abilities depending on their geometry or material makeup. For example, some metamaterials simply adjust the phase of light, whereas others can assist in light propagation.

There are two main types of metasurface: dielectric and plasmonic. All metasurfaces can contain different nanostructures to further customize their topological and optical properties, which enable engineers to create optical devices with multifunctional properties.

Dielectric Metasurfaces

Dielectric metasurfaces are metasurfaces with a high refractive index contrast, in which nanoscale dielectric or semiconductor pillars with square or cylindrical cross sections are surrounded by air. Dielectric metasurfaces tend to have lower absorption losses than plasmonic metasurfaces because they use materials that are transparent at the wavelength of interest, spanning both visible and infrared wavelengths.

Some of the materials used in dielectric metasurface manufacture include:

• Silicon (Si)
• Silicon nitride (Si₃N₄)
• Germanium (Ge)
• Group III-V semiconductors (GaAs, GaP, etc.)
• Lithium niobate (LiNbO₃)
• Titanium dioxide (TiO₂)
• Barium titanate (BaTiO₃)
• 2D material semiconductors (WS₂, GaSe, etc.)

Plasmonic Metasurfaces

Plasmonic metasurfaces are metal-dielectric metasurfaces that contain plasmonic nanoparticles or plasmonic nanostructures (such as antennas) on their surface. These are arranged at distances smaller than the free-space wavelength, or the wavelength of the electromagnetic wave in a vacuum. Plasmonic metasurfaces make use of surface plasmons, which are collective movements of electrons at the boundary between a metal and a dielectric (i.e., an insulating material). Because of their size, surface plasmons can help engineers control and use light at very small scales in applications such as sensing or imaging. Silver and gold are the two most common metals used because their optical properties support surface plasmon behavior.

Plasmonic metasurfaces are similar to photonic crystals, in which the repeating pattern of the metamaterial controls how electromagnetic waves behave. The plasmons on the surface of the metamaterial form when the free electrons in the metal move together in response to light. When light hits the metal, some of its energy is absorbed and makes the electrons oscillate. This resonance behavior couples the electrons to the light waves, allowing the wave to propagate along the metal-dielectric interface in a self-sustaining manner.

Nanostructures in Metasurfaces

Alongside the basic repeating pattern that defines the unit cell of the meta-atoms, metamaterials can contain a range of specially designed nanostructures that help with optimizing their properties. Some examples of this in action include:

• Nanometer antennas on the surface of the metamaterial. These tiny antennas can be either bent or straight, and their shape determines whether it has homogeneous or nonlinear optical properties.
• Small grooves or openings (called nanopatterns and nanoslits) can be made in graphene or metallic films to change the optical field. These features change how light behaves by either confining it or increasing its intensity.
• In some cases, several layers of nanostructured metamaterials can be stacked on top of each other to help reduce reflections and allow light to flow more smoothly through the material. This process is known as impedance matching.

Metasurface geometry with three types of meta-atom (triangle, round, and square pillars)

Metalenses are gaining interest in different applications and industry sectors.

Sensing

Sensing is the biggest and broadest application area of metalenses because their compact size, as well as the multifunctional nature of metamaterial optics, enables them to work well in a variety of applications. 

Engineers working with metalenses can select specific wavelengths and polarizations of light. Using metamaterials, they can integrate ultra-thin sensors into cameras and smartphones, where single photons can be captured to improve picture quality and add multifunctionality to the devices. Additionally, advanced metasurface-based sensors in the defense sector can detect both infrared and visible light, using polarization to filter reflections. The compact size of metalenses is also attractive for medical imaging applications such as endoscopes, in which miniaturized sensors are critical in helping doctors see inside the body.

Automotive

Several applications for metasurfaces are also developing in the automotive industry. One use case that directly crosses over with the sensing field involves the creation of more advanced lidar sensors, which are widely used in advanced driver-assistance systems (ADAS) and autonomous vehicles. The other main application is in very small and flat headlamps to project light more efficiently from the car.

The automotive industry has not yet widely adopted metasurfaces because it is a highly regulated technology area and requires consistent quality among all devices, not including the fabrication challenges of making ultra-small metasurfaces to begin with. However, experts predict it will soon be an area of rapid growth.

Imaging

Like other forms of diffractive optics, light of different wavelengths interacts differently with the metalenses. The different interactions produced are known as chromatic effects. This feature is useful for some imaging applications in which it may be useful to filter out specific colors. On the other hand, strong chromatic aberrations may be undesirable for broadband imaging applications (i.e., operating across a range of wavelengths). Still, developing metalenses with broadband imaging capabilities is an active area of research.

One example of metasurface use in the medical field is in improving the resolution and clarity of images taken with endoscopes. Metasurfaces can cause a phase shift in incoming light to reduce distortions (called monochromatic aberrations) and extend the depth of focus of the endoscope. In conventional camera systems, metasurfaces can combine different polarization measurements inside the camera into a single optical element, which reduces the need for bulky components. Using metasurfaces in cameras has potential in machine vision and remote sensing applications.

AR/VR

Alongside other advanced optical components, metasurfaces improve AR/VR headsets because their thin, light, and flat nature makes them ideal for projecting images inside a headset. The projections use a large-area waveguide, also known as a light guide, to help direct images toward the eye. Light guides are much larger than the small optical waveguides used in broadband communications.

Bulky optical components make up a significant amount of a headset’s weight. To be comfortable to the wearer, AR/VR headset designs need to reduce bulk as much as possible, as added weight induces torque on the wearer’s neck. Metasurfaces could help to bring the weight of these headsets down.

Spectrometry

The strongly chromatic behavior of metalenses, along with their compact size, makes them naturally suitable for spectroscopy applications. Metasurfaces can be used in optical spectroscopy instruments used for characterization and diagnostic applications within the food and beverage industry and medical field.

Engineers designing spectrometers often face a trade-off between resolution and device size, as the focusing elements in spectrometers can introduce optical aberrations. Thin and planar metasurfaces can help create lenses that maintain high resolution over a broad bandwidth while keeping the spectrometer’s size small.

Metasurfaces are designed using the same techniques as conventional semiconductor manufacturing, such as lithography, etching, and bottom-up deposition. Thus, they are very cross-compatible with existing manufacturing techniques used in foundries. However, because of their small size, a very accurate template is required to ensure that every batch of metasurfaces is uniform to ensure high performance.

Creating prototypes for metamaterials is a difficult, expensive, and time-consuming task. Each metamaterial prototype is produced on a small scale, so it’s not always economically advantageous to produce them. There are also big differences between the types of metasurfaces and the electromagnetic waves they are designed to work with. For example, there is interest in metamaterials that can interact with ultraviolet (UV) wavelengths, but they tend to have a lot of optical loss, i.e., the loss of light intensity as it travels through or interacts with a material. Additionally, these types of metamaterials are harder to fabricate than those designed for visible and infrared wavelengths.

Using Simulation in Metasurface Design

One of the more effective ways to design metasurfaces is through simulation instead of multiple prototype iteration cycles. This reduces the number of prototypes needed before manufacturing.

Although metasurfaces are thin, they have a large surface area containing many delicate features at the nanoscale level. This leads to a high computational cost that either requires high-performance computing (HPC) or graphics processing unit (GPU) acceleration to handle the memory demands of the solver algorithms.

Setting up this hardware independently can be an expensive task, which is why partnering with specialist software providers can make the process more affordable and practical. For those who already have the hardware resources, meta-atom libraries are available that provide collections of pillar shapes that can be fabricated, making it easier for engineers to develop more robust designs in-house before manufacturing.

Using Ansys Solutions for Metasurface Design

 The main design challenge of metasurfaces is designing across different size scales (i.e., nanometer-scale unit cells must be arranged into centimeter-scale optics), and each scale requires different simulation techniques.

Another challenge is that metalenses are often subjected to strong chromatic aberrations. This means that metasurfaces tend to work only with their intended wavelength and perform poorly at wavelengths they were not designed for. However, this can be a benefit in some metalens designs, including those used in creating efficient optical filters.

Some designs also enable engineers to create metalenses with weaker, or achromatic, aberrations to bring different wavelengths to the same focal point. In other designs, metasurfaces can facilitate sub-diffraction focusing. There are now tunable metalenses being developed using liquid crystals. So, there are many different design considerations that need to be taken into account.

Ansys, part of Synopsys, offers advanced electromagnetic wave simulation (the Ansys Lumerical platform) and ray tracing software (Ansys Zemax OpticStudio software) that can simulate all the wavelength-dependent effects of a metasurface before a decision is made on the final design. Because of the cross-compatibility of these two tools, data can be imported from the Lumerical platform to OpticStudio software to provide information across all size scales. This way, both simulations can use the same metasurface data to ensure that the results are as robust as possible before the prototype stage begins.

With the high computational cost of simulating metasurfaces, machine learning algorithms are helping make them less computationally intensive. Instead of individually computing each unit cell, a trained metamodel acts as a stand-in for the simulation, reducing computational requirements.

Machine learning is also used in inverse design — a process that works backwards by identifying the material structures with the desired properties first. This is in contrast with traditional design methods, which involve starting with the material first and then identifying its properties. Inverse design with machine learning identifies specific material structures and geometries that have the required properties, making it less expensive to simulate.

Machine learning is also improving metasurface applications. In imaging, it can be used to reconstruct images after they are captured, offering a much higher degree of efficiency and flexibility compared to traditional refractive optics.

To find out more about how simulation can support the design and manufacture of more advanced optical components with metasurfaces, contact our technical team today.

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