A few days ago, a research team from the Massachusetts Institute of Technology (MIT) said in a paper published in the journal Nature that for the first time, some "singular points" were used in Dirac cones to make "singular rings." "Singular points" can produce some important anti-intuition phenomena, can be applied to the preparation of powerful laser devices and other fields. What is a singular ring and singularity, why it can bring strange phenomena, how to put it into practical use? The first author, Massachusetts Institute of Technology Department of Physics postdoctoral interview with Science and Technology Daily reporter eleven answered these doubts, let us have a glimpse of the mystery of this special physical state.
Dirac cone generated within the singular ring
"Singularities are a special phenomenon that occurs in non-Hermitian systems," said Zhen. Usually we study the wave system are Hermitian, such as quantum physics, electromagnetics and other learning in university physics. In contrast, the academic community knows little about non-Hermitian systems. The conservation of energy and the completeness of eigenstates are two of the most important properties of the Hermitian system, and these properties may not be correct for non-Hermitian systems. For example, when a physical system is at a singular point, its multiple eigenstates collapse into one, that is, the eigenstate is no longer complete. At this time there will be a lot of novel physical phenomena.
"Everyone wants to study singularities, and the first question is how to systematically generate such singularities." We discovered a novel way: to use Dirac cones. "In 1928, the British physicist Paul Dirac proposed If the Dirac equation of the relativistic particle state is drawn in the three-dimensional coordinate system, the energetic-momentum function of the massless particle fitting the Dirac equation is conical, called Dirac cones. Zhen Bo explained: "The Miraculous Dirac cone system, the two vertebral head pairs at one point; and non-Hermitian, two pointed head was crushed: inside a circle , The heads of both cones become flat. "In this circle, the Zhenbo team names it singular and each point on it is a singular point.
Singular point ring to bring strange phenomenon
When the system is near singularities, there are many seemingly counterintuitive physical phenomena. Zhen Bo cited such an example: constantly add to a piece of transparent glass absorbing material, while measuring how much light through the glass. The usual intuition is that the more light absorber material is added, the less light is transmitted through the glass. But the reality is that when a certain amount of light-absorbing material is added, the singularity appears, and if more light-absorbing material is added, a strange phenomenon occurs: on the contrary, more light can pass through the glass.
The reason why such counter-intuitive phenomena have arisen is due to the localization of the vibrational modes of light. When the light absorbing material is small, the vibration mode of light is dispersed, and the light may be present in the absorbing material as well as in the transparent material. When there are many light-absorbing materials, the vibrational modes of light are localized. Some patterns are only fixed in the absorption part, the other part of the pattern is only fixed in the transparent part. At this time, the more light absorbing materials are added, the more localized the localized patterns are and the more transparent the pattern is, the more transparent the glass is. The turning point of light vibration mode from non-localization to localization is a manifestation of singular points and also a very important experimental application of singular points.
Let singular point ring play a big role
How to apply these interesting singularities? We know that singularities can only occur in non-Hermitian systems and the simplest way to have non-Hermitian systems is to introduce loss. Zhen Bo pointed out that the loss can be broadly divided into two types: absorption loss and radiation loss. The loss mechanism used by professional absorbing materials is absorption loss; the acoustic waves are transmitted to the distant and gradually weakened, and the loss mechanism is the radiation loss. Both types of losses exist in a wide range of fluctuating systems.
"Most of the loss mechanisms used in previous studies are absorption losses, which often have a detrimental effect on the equipment used, and our study uses radiation loss." Jenow said the most common application of radiation loss to light waves is the laser pointer. There is a laser in the laser pen, which is the cavity that resonates. Cavity with radiation loss, making the light ran outside the resonant cavity to form a spot. "The laser pointer spot only exists under the condition of radiation loss, so we can see that radiation loss is very useful, and in some cases it is even necessary.We use the principle of radiation loss to generate and study singularities. "
As an application example of singular point ring, Zhenbo's research presents a new design scheme of photonic crystal lasers. Photonic crystals are nanomaterials that can penetrate some wavelengths of light by introducing some periodic structure, while other wavelengths can not penetrate the light. Zhen Bo said: "In recent years, the rapid development of new scientific computing simulation system and sample preparation capabilities, making photonic systems, especially one-dimensional and two-dimensional periodic photonic crystals in the experiment has made great achievements, one of which is the photon Development of crystal lasers. "Photonic crystal lasers refer to electronically controlled lasers that utilize photonic crystals as resonators, and those of the third and fifth family semiconductor quantum wells as amplifiers. At present, the maximum output power of this laser can already reach 1.5 watts. And one of the important reasons to limit the output power to further increase is that there are many unwanted resonance modes that cause the performance of the laser to be degraded. Zhen Bo said: "We introduced a circle of singularities in the laser, or singular ring, so that those redundant resonant mode is difficult to compete with the resonant mode we want. The results show that using our Singular ring design, the existing photonic crystal laser output power can continue to increase about 10 times.
Zhen Bo told reporters: "In recent years, researchers have made many important breakthroughs in the study of non-Hermione wave system and the related symmetry inversion of parity time, especially in the study of non-Hermitian photonics." He The study is an example. Dilemma, looking forward to more future research for us to open the door to a deeper understanding of non-Hermit world.
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