What is a metalens and what’s it good for?
A metalens is a cutting-edge optical lens that’s flat and lightweight, replacing traditional bulky curved lenses with a specially designed surface featuring nanoscale structure. Metalens can be used to improve optical system performance and dramatically reduce system size and weight.
A metalens is a single thin, flat structure. In many instances, a single metalens can deliver the same performance as multiple conventional optical components.
It includes multiple waveguides in the shape of pillars, approximately 600 nm long, made from various dielectric materials and arranged side by side (Figure 1). The flat planar structure means that all wavelengths of light pass through a metalens simultaneously, resulting in no chromatic aberrations and making them achromatic.
Figure 1. A metalens is a planar array of pillar-shaped waveguides. (Image: Ansys)
While the basic design is achromatic, metalenses can be produced with tunable dispersions and can manipulate how specific various wavelengths (colors) of light are dispersed. Traditional lens designs have fixed dispersions.
Fabricating metalenses
For near-infrared applications, metalenses are often fabricated using a silicon-on-glass substrate. Metalenses for visible wavelengths are primarily fabricated using dielectric materials, such as titanium dioxide (TiO2), gallium nitride (GaN), and silicon nitride (SiN), which have high refractive indices in the visible spectrum.
Metasurfaces can be mass-produced using mature CMOS fabrication nodes readily available in foundries, enabling the wafer-scale integration of metalens-based optical systems. A typical metalens consists of millions of subwavelength unit-cells, also called meta-atoms.
Those meta-atoms can delay the phase of light in a highly controlled and predictable manner, creating the same phase profile as a classic curved lens structure (Figure 2). Meta-atoms typically measure tens of nanometers. For example, metalens designs with SiN nano-posts on a fused silica substrate, and feature widths between 100 nm and 310 nm, can be used for applications in the visible to near-infrared (NIR) wavelength range.
Figure 2. Metalenses (right) can focus light just like a conventional optical element (left). (Image: Synopsys)
Numerical apertures are important
Metalenses have superior numerical apertures (NAs) compared with traditional lens designs. The NA of a lens describes its ability to collect light and resolve small details. It’s a dimensionless quantity and is critical in determining the performance of lenses, especially in microscopy.
A higher NA indicates better performance. A high NA lens can gather more light and produce a brighter image. A high NA also indicates the ability to distinguish between closely spaced objects and deliver highly detailed images.
To illustrate the importance of high NA, a study was done based on fluorescence correlation spectroscopy (FCS) of Alexa Fluor 647, a bright, far-red fluorescent dye commonly used in biological and chemical research for labeling molecules.
The performance comparison was done using a metalens with an NA of 0.6, two single-element aspheric lenses with NAs of 0.18 and 0.54, and a series of achromatic lenses with NAs from 0.15 to 1.2 (Figure 3). The aspheric lenses could not detect the dye samples, while the performance of the metalens was comparable to that of the much larger achromatic objective lenses.
Figure 3. A metalens with an NA of 0.6 is much smaller than aspheric or achromatic lenses (a), and the relatively small aspheric lenses had a brightness of zero and were not able to detect the dye molecules (b). (Image: nature communications)
Metalenses and VCSELs
The compact sizes and high performance of metalenses are being combined with vertical cavity surface-emitting lasers (VCSELs) to create more compact and versatile light sources. VCSELs are semiconductor lasers that emit light vertically from the surface of a wafer.
In VCSEL-illuminated applications, a single metalens can perform functions that would otherwise require several refractive surfaces, leading to ultra-compact and simpler-to-assemble modules. When combined with VCSELs, metalenses can be fabricated using a gallium arsenide (GaAs) film.
A few of the things a metalens and VCSCL system can do include:
- Metalenses can shape VCSEL beams to create desired illumination patterns, such as dot arrays for facial recognition and simultaneous localization and mapping (SLAM), and line patterns for autonomous mobile robot (AMR) obstacle avoidance.
- Metalenses can reduce the divergence angle of the light emitted by a VCSEL, thereby collimating it and enhancing its effectiveness in applications such as LIDAR and endoscopy.
- Metalenses can improve laser beam uniformity and contrast compared with conventional techniques for beam shaping.
Summary
A metalens is a planar structure of thousands, or millions, of pillar-shaped optical waveguides. They can perform the same functions as conventional optical elements and support increased flexibility. Even though they are tiny, they have high NAs needed for bright and detailed images. Since they are fabricated using conventional CMOS processes, metalenses can be easily integrated with VCSELs, combining the strengths of both technologies to provide unique optical system capabilities.
References
Concept of a Convex on-Chip Metalens as a Miniature Sensor of Fluorescence of Single Molecules, Methods of Physical Research
Dual-wavelength metalens enables Epi-fluorescence detection from single molecules, nature communications
Flat Lenses Made of Nanostructures Transform Tiny Cameras and Projectors, IEEE Spectrum
How metalenses are reshaping the future of optics, Radiant Vision Systems
Meta Optical Elements (MOE), NIL Technology
What are Meta-optics, Metalenz
What Is a Metalens?, Synopsys
What is a Metalens and How Do They Work?, Ansys
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