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27
2019
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02
3D optical topological insulator: let a beam of light run out of a "Z" bend
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Light travels along a straight line. This is a sentence written in our common sense. Scientists have a way to make light turn. Zhejiang University and Singapore Nanyang Technological University collaborated to build the world's first three-dimensional optical topological insulator. On the "highway" of three-dimensional materials, a beam of light ran out of the "Z" shape perfectly. Related papers were recently published in the journal Nature.
If the light is like water, there will be a hidden miracle.
There are many interesting phenomena in the light bending, and invisibility cloak is one of them. In 2003, Chen Hongsheng, a co-author of the paper, produced a stealth device in the visible light band, which allowed goldfish, cats and other animals to squat in front of the eyes.
"We can make the light flow like water without scattering on the surface of the object, and like a stream flowing through the stone, going around the shape of the stone, and continuing to follow the original direction of propagation." Chen Hongsheng said that without scattered light, The human eye can't recognize the object.
In the invisible list of scientists, the impurities and defects of materials occupy an important position. When electromagnetic waves propagate through the optical waveguide or at the interface of the medium, impurities and defects encountered on the way can cause electromagnetic waves to scatter, resulting in a decrease in transmission efficiency. "If you can design a new type of waveguide, let these scattering factors be 'invisible', it will greatly improve the transmission efficiency, and there will be significant application prospects in the future." Chen Hongsheng said.
In many devices, electromagnetic waves must be bent around. "In the current technical system, once the turning range is large, electromagnetic waves will scatter, affecting transmission efficiency. The small turning range is not conducive to saving space." A scientist engaged in electromagnetic wave research believes that this is the realization of future photonic chips. A huge challenge. "We don't want to scatter when we want to make a sharp turn."
From electronics to photons, research has been improving
Dr. Yang Yihao, the first author of the paper and the School of Information and Electronic Engineering of Zhejiang University, said that topological insulators, the hottest material of condensed matter physics, are the inspiration for this research. A topological insulator is a surface-conducting, internally-insulating material that allows electrons to travel around the surface of the material without being "passed" inside the material.
When the famous scientist Zhang Shouyi introduced the topological insulator to the public, he used the "highway" as a metaphor: the movement of electrons in the chip is like a sports car driving in the market, constantly colliding and generating heat. Topological insulators seem to set up highways for electronics, allowing electronics to run on a "one-way lane."
Electronic "highway", can photons run? In 2005, Duncan Haldane (2016 Nobel Laureate in Physics) at Princeton University conducted a theoretical experiment to extend the theory of topological insulators to optical systems until the theory of optical topological insulators was officially released in 2008.
In 2009, MIT scientists first realized two-dimensional optical topological insulators through experiments, and opened up experimental research on optical topological insulators.
Currently, experimental studies on optical topological insulators are still limited to two-dimensional materials. In 2017, the team of Professor Alexander of the City University of New York presented the design theory of three-dimensional optical topological insulators without magnetic materials. "We are concerned about this work, but its parameters are very demanding." Yang Yihao said.
The joint research group of Zhejiang University and Nanyang Technological University of Singapore began to try to build a new experimental system. This is the first time in the scientific community to experiment with optical three-dimensional topological insulators.
"The problem of heat generation in electronic chips, topological insulators give a good solution; the information dissipation problem of photonic chips, scientists hope to give solutions through optical topological insulators." Yang Yihao said.
Build a "Z" shaped highway to let the photon turn "run"
From electronic systems to photonic systems, from two-dimensional to three-dimensional, there are many essential differences in research objects, and the experiment has encountered unprecedented difficulties. In the beginning, they didn't even have ready-made experimental equipment to measure.
Yang Yihao skillfully designed a unit structure composed of a plurality of open resonators. "This is the roadbed of the highway, and it is also the key to the success of the experiment." Chen Hongsheng said. Finally, the joint research group realized for the first time a three-dimensional optical topology insulator with a wide-band topology energy gap.
The "highway" of the three-dimensional world photon is "Z" shaped. When the surface wave propagates through the interface, it can bypass the Z-shaped corner without any trouble. "By imaging the internal and surface electromagnetic field distribution of the material, we observed the three-dimensional energy gap of the material and the surface state of the two-dimensional Dirac cone - these are the key features of the three-dimensional optical topological insulator." Yang Yihao said.
"For surface waves, these corners are like invisible, and they can bypass the corners for efficient propagation, which is the topological protection feature that benefits from 3D optical topological insulators," said Chen Hongsheng. This is the magic of the "Photon Highway". "On this highway, no matter how tortuous the road, the photon can go forward." Yang Yihao said that this can avoid the problem of light scattering caused by scattering of information.
"Our work for the first time gives the three-dimensional photonic bandgap a topological nature, that is, in the future, a three-dimensional topological photonic crystal can be used to control photons like a three-dimensional topological insulator." Co-investigator, Professor Zhang Baile of Nanyang Technological University, Singapore said .
Chen Hongsheng believes that this study extends the three-dimensional topological insulator from the Fermion system to the boson system for the first time, and may be applied to three-dimensional topological optical integrated circuits, topological waveguides, optical delay lines, topological lasers, and other surface electromagnetic wave control devices.
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