In the future, your glasses may be thinner than paper.

Many people are familiar with lifting the lens. Whether it is the eyes we usually wear, the lenses used for cameraing, or the magnifying glasses used by the elderly, they all belong to the lens. However, these lenses are not concave lenses, but convex lenses. Who has heard of a flat lens?

    Recently, a research team led by a Chinese scholar at the University of Columbia in the United States has developed an ultra-thin flat lens with a thickness of only 1 micron, which is expected to significantly reduce the size and weight of optical devices.

    There are many "superpixels" on the flat lens.

    In daily life, the lenses we use are spherical lenses, which are generally made of optical materials such as optical glass, optical crystals, and optical resins. It uses the principle of optical refraction, using different materials, different spherical surfaces and different spatial positions to achieve light control.

    Light is divided into monochromatic light and complex color light. Simply put, monochromatic light is a kind of color light, and complex color light (such as sunlight) is a mixture of light of various colors.

    Since the light of different colors passes through different media and structures at different speeds, the complex light is split into monochromatic light through the prism, a phenomenon known as the dispersion of light. The rainbow that appears after the rain is a relatively complex dispersion phenomenon caused by refraction and reflection after the sunlight is injected into the air droplets at a certain angle.

    Due to the existence of this phenomenon, ordinary lenses cannot simultaneously focus light of different colors, and chromatic aberration occurs. At present, most imaging systems such as cameras solve the problem of chromatic aberration by stacking multilayer lenses, which results in the current complicated structure and cumbersome camera photographic equipment.

    In solving this problem, the ultra-thin planar lens newly developed by the Southern Group has taken a big step. This flat lens consisting of "superpixels" is thinner than plain paper.

    “The ultra-thin planar lens developed by the team in the south of the country does not use the principle of refraction of traditional light, but uses the phase delay of light in the propagation of the medium to achieve the deflection of light. This is a diffraction principle. The lens is on the plane. There are many 'superpixels' distributed, each 'superpixel' has a size of only about 0.5 microns, is smaller than the wavelength of light, and has a different amount of phase delay. Because the distribution of delay amount is carefully designed, it can be passed to the super The parallel light of the thin lens converges to the same focus, realizing the function of the traditional lens. At present, their team can achieve a very good focusing effect of the near-infrared light of 1.2-1.7 micron wavelength." Hu Yao, an associate professor of Optoelectronics College, Beijing Institute of Technology, told the Science Daily reporter .

    The most important thing is to solve the problem of chromatic aberration

    The ultra-thin planar lens is processed using a semiconductor manufacturing process. For the first time, it focuses on focusing light waves of any color with any polarization state, and its imaging performance can be compared with the first-class complex lens system.

    The processing of traditional lenses requires different processes such as material selection, cutting, rough grinding, fine grinding, polishing, and inspection. This ultra-thin planar lens is processed by photolithography, which is a completely different process flow.

    Hu Yao said that the lithography used in processing ultra-thin planar lenses is currently used for chip processing. This technology can only be photolithographically processed on a flat surface, but because of the relative maturity of lithography, this method can be used for faster conversion applications. In fact, it is not difficult to use a photolithography process to fabricate a device that can realize the basic functions of a lens. The focus of this research is to solve the problem of inconsistent light imaging of different colors, that is, the so-called chromatic aberration problem. This color difference will greatly affect the effect of complex color light imaging, such as the effect of daily sunlight or lighting. If you use a monochromatic light source such as laser, it is not necessary to solve the chromatic aberration problem. The solution of the chromatic aberration problem has made the technology a big step towards practicality.

    At the same time, "The research is done in the infrared band, and there are military projects in the paper project funding list, so it can be guessed that the researchers are considering the military needs for lighter, thinner and more stable applications." Hu shakes.

    In her view, scientists have been working to make lenses thinner and open up multiple research directions on this path. She introduced that Fresnel lenses are a very early method of making lenses thinner and are widely used in everyday life, such as flashlights for mobile phones. The so-called Fresnel lens, also known as a threaded lens, has a smooth surface on one side of the lens and a concentric circle from small to large on the other side. It is made lighter and thinner by mounting several independent sections on a frame. The lens; the aspherical surface and the free-form surface can also reduce the overall optical length of the spherical mirror group, and realize the function of multiple pieces by one piece to improve the image quality while reducing the weight of the optical system; the effect of the lens is realized by the diffraction element, such as the diffraction grating. Holograms and ultra-thin lenses developed by the team; in addition, there are already computational imaging-based, lens-free diffraction imaging techniques used in microscopy and other fields.

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