——A gentleman's nature is not different, and he is good at faking things

Ever since the dawn of humans, they have been using tools as an extension of their own bodies.

The lens as an extension of the eye system can be traced back to the Nimrud lens milled from quartz in the ancient Assyrian Empire 2700 years ago, and its magnification is about three times.

Nimrud lens [Image source network]

In 1608, Dutch spectacle craftsmen discovered that a convex lens + a concave lens combination could produce the effect of magnifying images, and used this as a principle to manufacture the first recorded optical telescope. The following year, Galileo improved the original design and used it for astronomical purposes for the first time.

Galileo Telescope [Image source network]

With the continuous development of design and processing technology, in modern life, lenses are widely used in our production and life, so wide that everyone's impression of it is solidified, and it has become a necessary tool for everyone but not valued. Such as our cameras, glasses, more sophisticated instruments such as optical microscopes, lithography machines, etc. are inseparable from the control of light by lenses.

Lithography machine

Mobile phone lens group

Light microscope

Camera lens group

However, the lenses we use today, except for improvements in imaging quality such as light intake, phase difference, chromatic aberration, etc., are not fundamentally different in principle from the lenses used by Louis Daguerre when he first photographed people in 1839. For imaging optical systems, technology has not progressed intrinsically, as if the underlying technology has been locked by "Chiko".

The first photograph of a person was taken in 1839. Photo by Louis Daguerre/public domai

They all inevitably face the same problem chromatic aberration; When the polychromatic light enters the prism, because it has different refractive indices for various frequencies of light, the propagation direction of various colors has different degrees of deflection, so it is dispersed when leaving the prism, forming which will lead to image distortion and low resolution during imaging.

Color difference diagram [Photo source Baidu Encyclopedia]

Color difference causes image distortion and because the basic principle of lenses is to rely on different thicknesses to control light, complex imaging systems require multiple curved lenses of different thicknesses and materials to be superimposed on top of each other. This inevitably leads to a large and complex system. In addition, in the imaging of traditional lenses, the whole lens can be equivalent to a convex lens. But a lens cannot consist of just one convex lens, because it has aberrations and chromatic aberrations. So the lens needs to be composed of multiple lenses, each one has its own responsibility, some responsible for deflecting light, some responsible for eliminating color aberration, and some responsible for eliminating distortion. Each piece of lens needs to go through a complex grinding process, and the assembly needs to be extremely precise. After all, optics is the most sophisticated subject that human beings master, the lithography machine that makes chips, the laser interferometer that detects gravitational waves, are all optical instruments - behind the precision, is the high cost.

As cameras become more and more widely used, our need for high-quality images is increasing day by day. Whether it is autonomous driving, or drone obstacle avoidance, it needs a lot of imaging data. Even if the size of the mobile phone lens is smaller now, it can rely on mass production on the assembly line to reduce costs. However, due to the limitation of the traditional optical lens principle, it must be realized by multiple lenses, and the thickness and cost can not be reduced to a satisfactory degree.

Complex imaging System [Image source Network]

In recent years, a new type of lens has been able to solve this problem - the metasensor proposed by Capasso at Harvard University, which breaks the limitation of traditional lenses using only material thickness to control light. Once proposed, it quickly became a research hotspot, and in 2016, it was also honored as the cover article of the internationally renowned journal Science, and in 2019, it became one of the top ten scientific advances in world economic theory.

What we need is not the lens itself, but the image that the lens ultimately presents on the sensor. If there is any thin and simple structure can replace the traditional lens, it is naturally best. And metalens is such an optical instrument.

When you see "meta", the most people think of is the meta-universe. But in fact, the field of materials science has been using the term for a long time. The "metalens" of metamaterial is also derived from the concepts of metamaterial and metasurface. The word "Metamaterial" comes from the Greek word "meta", meaning "beyond". Metamaterials go beyond the scope of ordinary materials and have properties that ordinary matter does not have. Metamaterial is not so much a matter as a special man-made structure made of conventional materials such as metals, silicon and plastics. If this structure is considered as a matter as a whole, it may have special properties, such as possibly having a negative refractive index.

An example of a possible hyperlens mode captured by an electron microscope. (photo credit: Science)

The scale of a metamaterial's microstructure determines what wavelengths of light it can interact with. If the microstructure is tens of hundreds of nanometers in scale, it is a metamaterial of visible light. At the same time, in order to increase the transmittance of light, all the microstructure can be placed on a two-dimensional surface, and the metamaterial becomes a metasurface, where each microstructure looks like a tiny pillar, acting like a waveguide. The metasurface can change the direction of light propagation, and use it as a lens, that is, a super lens.

An example of a possible hyperlens mode captured by an electron microscope. (Photo credit: SCIENCE · 18 Jul 2014 · Vol 345, Issue 6194 · pp. 298-302)

In general, optical systems need the ability to focus light in order to image. Light is a kind of electromagnetic wave, wave has phase property, and the plane composed of electromagnetic waves in the same phase is called wave front. The microstructure on the super lens can adjust the phase of the incident electromagnetic wave according to the shape and arrangement, so as to control the shape of the wave front. As long as the microstructure of the superlens adjusts the shape of the wave front to the shape of convergence, the effect is equivalent to a convex lens, and you can image.

While a traditional lens is a lens that requires fine grinding, a super lens is an ultra-thin flat structure. A lens with thickness will produce chromatic aberration because of the different refractive index of the material to different colors of light, and the metasurface because of its ultra-thin, all wavelengths of light almost simultaneously through the lens, will not produce chromatic aberration, it is actually an achromatic lens. And, on the plus side, metasurfacements aren't really that hard to produce. The improvement of the manufacturing capacity of microscopic repetitive structures has been the main driving force for the advancement of electronic technology in the past few decades. In fact, metasurfaces can be mass-produced by existing semiconductor foundries. So, if the technology for a super-lens matures, we just need to stack a light-sensing sensor, a glass that provides thickness, and a super-lens that bends the light together to get a near-perfect lens. It produces images without chromatic aberration, has no complex mirror structure, is much thinner - and costs less.