Why can't the camera of the mobile phone press in when it always protrudes?

    I don't know since when the lens module protruding from the back of people's mobile phones has become commonplace. No one roast about why the lens protrudes so much from the body. Without a mobile phone case, some mobile phones can even slide off the table due to the vibration of the message reminder because the lens module protrudes too much. From cell phones to today's smart phones, mobile phones have become so thin over the years, can't we make the lens a little thinner?

Not really. The miniaturization of electronic components has gradually brought us into the information age. We can build a complex structure comparable to the whole city on a chip smaller than a fingernail, so that countless currents can move in nanosecond time scales in full accordance with the rules set by engineers. However, the lens we use today, in addition to the improvement in imaging quality such as light input, phase difference and color difference, is not fundamentally different in principle from the lens Louis Daguerre used when he first photographed people in 1839. For the imaging optical system, there has been no essential progress in technology, as if the basic technology was locked by "Zhizi".




When imaging with a traditional lens, the whole lens can be equivalent to a convex lens. But the lens cannot be composed of only one convex lens, because it has aberration and chromatic aberration. Therefore, the lens needs to be composed of multiple lenses, each of which has its own responsibilities, some responsible for deflecting light, some responsible for eliminating color difference, and some responsible for eliminating distortion. Each lens needs to go through a complex grinding process, and the assembly also needs high precision. After all, optics is the most precise subject that human beings have mastered. The lithography machine that makes chips and the laser interferometer that detects gravitational waves are all optical instruments - behind precision, there is a high cost.

The application of cameras is more and more extensive, and our demand for high-quality images is increasing day by day. Both autonomous driving and obstacle avoidance of unmanned aerial vehicles require a large amount of imaging data. Even if the lens size of mobile phones is small now, it can also rely on mass production of assembly lines to reduce costs. However, limited by the principle of traditional optical lens, it must be realized by multiple lenses, and the thickness and cost can never be reduced to a satisfactory level.

Hyperlens

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

Seeing "meta", people think of the metauniverse at most. But in fact, this term has been used in the field of materials science for a long time. The "metalens" of the super lens are also derived from the concepts of metamaterial and metasurface. The word "metamaterial" comes from the Greek "meta", which means "beyond". Metamaterials go beyond the scope of ordinary materials and have properties that ordinary substances do not have. Metamaterial is not so much a substance as a special artificial structure composed of conventional substances such as metal, silicon and plastic. If this structure is regarded as a substance as a whole, it may have special properties, such as negative refractive index.

The size of the microstructure of metamaterials determines what wavelength of light it can interact with. If the microstructure is tens of hundreds of nanometers, it is a visible light metamaterial. At the same time, in order to improve the transmittance of light, all microstructures can be made on a two-dimensional surface, and the metamaterial becomes a super surface. Each microstructure looks like a tiny pillar, which acts as a waveguide. The super surface can change the direction of light propagation, and it is used as a lens, which is called a super lens.




An example of a possible super lens mode collected by an electron microscope. (image source: Science · 18 Jul 2014 · Vol 345, issue 6194 · PP. 298-302)

Generally speaking, in order to image, the optical system needs the ability of focusing. Light is an electromagnetic wave, which has phase property. The plane composed of electromagnetic waves with 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 super lens adjusts the shape of the wavefront to the convergent shape, its effect is equivalent to a convex lens, which can be imaged.

Traditional lens is a lens that needs fine grinding, while super lens is a kind of ultra-thin planar structure. The lens with thickness will produce chromatic aberration due to the different refractive index of different color light of the material, while the super surface is ultra-thin because of itself. Light of all wavelengths passes through the lens almost at the same time, and will not produce chromatic aberration. It is actually an achromatic lens. What's more, it's not very difficult to produce super surface. The improvement of the manufacturing capacity of microstructure is the main driving force for the progress of electronic technology in the past few decades. In fact, the super surface can be produced in large scale by the existing semiconductor foundry.


Therefore, if the technology of super lens is mature, we only need to stack the sensor that senses light, the glass that provides thickness and the super lens that bends light together, and we can get a nearly perfect lens. It can image without chromatic aberration, without complex lens group structure, and its thickness is much thinner - and the cost is lower.
Curse of meta


owever, the word meta seems to have a magic spell. Everything associated with it looks so promising, but it seems that it is not close to landing in real life. Super lens technology has always given people a sense that gimmicks are greater than reality, and few people can really give the actual commercial time of super lens technology. However, this phenomenon is changing rapidly.



Last week, Li Tao's team at Nanjing University used the super lens technology to produce an ultra-thin, high-quality integrated single-layer lenses array wide-angle camera (miwc). Relevant results were published in optica magazine. Miwc camera size is 1 × one × 0.3 cm, angle of view is 120 °. Compared with the previous single super lens camera, the super lens array in miwc camera can make up for the degradation of image quality at the edge of different super lenses, and achieve higher imaging quality. At the same time, because the camera is only composed of CMOS optical sensor and super lens array, it is expected to reduce costs in mass production. In the future, the research team plans to increase the diameter of a single super lens in the array from 0.3 mm to 1 to 5 mm, so as to further improve the imaging quality.

When taking portrait photos, people often need the shallow depth of field brought by the "big aperture" to blur the photo background and highlight the main body. However, for data acquisition, the greater the depth of field of the camera, the better. It is the most ideal state to be able to see distant and near objects at the same time. Yesterday, the Xu Ting team of Nanjing University published a document on nature · communication. Inspired by the compound eye of trilobites, they developed a super deep field micro camera with a bifocal lens using super lens technology, which can clearly image objects within 3 cm and 1.7 km away from a single photo at the same time. Relevant research was published in nature communication.


Rome was not built in a day, and the development of super lens also takes time. Now, the development of hypersurface is faster and faster. Maybe we can expect that with the help of hypersurface technology, we can return to the era when the mobile phone lens is not prominent.