Lens is a basic optical element that is widely used in daily life, such as cameras, glasses, microscopes, etc. Conventional lenses have different refractive indices for light of different wavelengths, and therefore cannot focus light of various colors to the same point, resulting in chromatic aberration and image distortion.
Focusing the entire visible spectrum and white light is very challenging, because light of different wavelengths move at different speeds in the material. For example, red light will pass through glass faster than blue light. When two kinds of light reach the same position at different times, two different focal points are produced. We call this phenomenon chromatic aberration.
In order to solve the problem of chromatic aberration, traditional imaging systems stack multiple curved lenses of different thicknesses and materials. No matter how thin and compact it is, the image will be distorted and unclear. This is why high-power microscopes and telephoto lenses have made the lenses so large due to the physical rules of the lens that cannot be broken. However, this solution comes at the cost of increasing system complexity and weight.
However, a new type of lens can solve the above problems, which is the meta-lens metalens. The meta-lens has a flat surface with nano-structures, that is, the metasurface, which can use nano-structures to condense light to project incident light to a desired place.
The superstructure lens is light, thin and compact, and its function greatly exceeds the traditional lens. It is a plane that uses nanostructures to condense light to avoid chromatic aberration, that is, the superstructure surface.
Compared with traditional lenses, the weight of optical nano-material flat lenses called metasurfaces is greatly reduced. When the sub-wavelength nanostructure of the metasurface forms a specific repetitive pattern, it can simulate the complex curvature of the refracted light, it is not bulky and the traditional lens is not bulky, and the ability to focus light is improved while reducing distortion.
As a revolutionary technology in the optical field, it is expected to completely subvert the cumbersome lens set in the traditional optical system, making mobile phones, cameras, surveillance cameras and other products smaller, thinner and lighter.
As a unique photonic technology, metasurfaces can precisely manipulate the wavefront of light on an unprecedented scale to produce many interesting and peculiar optical phenomena, which has stimulated researchers' extensive research interest in the field of planar optics.
The local interaction of light with the polarized atoms arranged on the nanoscale provides control of the phase, amplitude and polarization state of the wavefront passing through the structured plane. With the current manufacturing technology, the phase, amplitude, and polarization state can be engineered, so that the scattered field can be locally controlled, and the optical flow can be molded to create an optical effect that natural materials can't match. Compared with traditional refractive optical elements, this technology has shown promise to achieve a fundamental change in the form-function relationship.
For more than 500 years, humans have mastered the optical imaging technology of making glass into lenses to refract light, and then bend or combine these lenses to enlarge and clear images at close and long distances. However, in the past ten years, Federico Capasso, a scientist at Harvard University, pioneered the field of planar optics research, and first published research on meta-lens in 2014.
Immediately afterwards, Capasso led the research team in 2016 to propose a meta-lens technology. This new type of lens covered with nano-pillars can be 100,000 times thinner than ordinary lenses, and has the advantages of easy production and low cost.
The research team replaced traditional lenses by designing planar optical metasurfaces, and used millions of tiny, thin and transparent quartz column arrays to diffract and shape the flow of light. This is roughly the same way as a glass lens, but it is not inherently restricted by "aberration" like glass.
The meta-lens uses a flat surface with nanostructures to focus light, and realizes an optical revolution by replacing the bulky curved lens currently widely used in optical devices with a simple, flat surface. This technology was rated as one of the best discoveries of Science in 2016.
This technology has been rated as one of the top ten emerging technologies by the World Economic Forum (WEF) and Scientific American (SciAm) in 2019.
It shows that these smaller and sharper lenses will soon appear in camera phones, sensors, fiber optic lines, and medical imaging equipment such as endoscopes. The World Economic Forum commented: "The ability to make the lenses used in mobile phones, computers and other electronic devices smaller is beyond the capabilities of traditional glass cutting and glass bending techniques… These tiny, thin and flat lenses can replace existing bulky lenses. The glass lens can further miniaturize sensors and medical imaging equipment."