Scientists realize a three-dimensional 'topological' medium for electromagnetic waves

January 9, 2019 by Chong Yidong, School of Physical and Mathematical Sciences, Nanyang Technological University
Credit: CC0 Public Domain

Topological insulators are exotic states of matter that physicists have been intensely studying for the past decade. Their most intriguing feature is that they can be rigorously distinguished from all other materials using a mathematical concept known as "topology." This mathematical property grants topological insulators the ability to transport electric signals without dissipation, via special quantum states called "topological surface states."

However, need not only be realized with electrons. Physicists have also devised insulators, that impart with distinct topological features, allowing light (rather than electric currents) to flow via topological surface states. Unlike electronic topological insulators, photonic topological insulators can operate easily at room temperature, among other advantages. As a result, photonic topological insulators could have applications in future optical devices, such as high-power lasers and optical diodes.

A team of researchers from Nanyang Technological University (NTU), Singapore, and Zhejiang University, China, has announced the development of the world's first three-dimensional (3-D) photonic topological insulator. In a paper to be published in an upcoming issue of Nature, the team reports that a specially-designed 3-D array of resonators can act as a topological insulator for microwaves. They have observed unambiguous evidence for the signature topological surface states, in the form of microwaves that flow effortlessly along 2-D sheets embedded within the 3-D volume of their sample.

"Previous researchers were able to make two-dimensional photonic topological insulators. But in spite of many theoretical proposals over the years, nobody had been able to realize a 3-D version," says Associate Professor Baile Zhang of NTU, who co-supervised the project. He notes that 3-D topological insulators have important capabilities, including the ability to channel topological surface states along all possible spatial directions. In one of their experiments, the researchers showed that microwaves can be guided efficiently along a 2-D surface containing zigzag-like folds.

The team constructed the 3-D photonic topological insulator out of a stack of thin plastic sheets embedded with metal antennas acting as tiny electromagnetic resonators. The key breakthrough was made when they realized how to tailor the resonators to interact with electromagnetic waves in a very specific way, granting the waves the desired topological characteristics.

"Since the sheets are made using well-established technology for printing circuit boards, this design is cheap and simple to implement," explains Professor Hongsheng Chen of Zhejiang University, another co-supervisor of the project. "By comparison, other proposals previously published in the scientific literature involved using non-standard ceramic or magnetic materials, which are very difficult to work with if you want to make a real device."

Dr. Yihao Yang, a postdoctoral researcher at NTU who was the lead author on the paper, said that the team was able to build a compelling scientific case by constructing detailed maps of how electromagnetic waves travel within the photonic topological insulator. "By carefully inserting an electromagnetic field probe into the sample, we measured the field distributions throughout the sample. This allowed us to reconstruct the 'dispersion relations' that serve as the physical signatures of topological insulators," he said.

Associate Professor Yidong Chong, another member of the NTU team, observed that this work is the first realization of a synthetic 3-D topological insulator not based on the flow of electric current. "This is an example of the universality of physics," he said. "A phenomenon arising in one setting, like quantum materials, can be reproduced in another setting, in this case an artificial medium for electromagnetic waves. The key ingredient is that they obey the same equations and theoretical concepts." He suggests that the 3-D photonic topological insulator may provide an interesting setting for studying fundamental physics, as the topological surface states are governed by the same equations as massless 2-D electrons obeying Einstein's theory of relativity.

The current 3-D photonic topological is limited to electromagnetic waves, at relatively low frequencies. "If we can scale it to optical frequencies, that is to say waves of visible light, there could be applications for creating optical computer chips, lasers, and all sorts of interesting optical devices," says NTU's Professor Zhang.

Explore further: A novel topological insulator

Related Stories

A novel topological insulator

October 12, 2018

For the first time, physicists have built a unique topological insulator in which optical and electronic excitations hybridize and flow together. They report their discovery in Nature.

Bismuth shows novel conducting properties

September 12, 2018

A team of international scientists including Maia G. Vergniory, Ikerbasque researcher at DIPC and UPV/EHU associate, has discovered a new class of materials, higher-order topological insulators. Theoretical physicists first ...

Topological photonic crystal made of silicon

January 19, 2017

WPI-MANA researchers derive topological photonic states purely based on silicon, which can lead to the development of new functions and devices through integration with semiconductor electronics

Novel insulators with conducting edges

June 1, 2018

Physicists at the University of Zurich are researching a new class of materials: Higher-order topological insulators. The edges of these crystalline solids conduct electric current without dissipation, while the rest of the ...

Recommended for you

New thermoelectric material delivers record performance

January 17, 2019

Taking advantage of recent advances in using theoretical calculations to predict the properties of new materials, researchers reported Thursday the discovery of a new class of half-Heusler thermoelectric compounds, including ...

Zirconium isotope a master at neutron capture

January 17, 2019

The probability that a nucleus will absorb a neutron is important to many areas of nuclear science, including the production of elements in the cosmos, reactor performance, nuclear medicine and defense applications.

Mechanism helps explain the ear's exquisite sensitivity

January 16, 2019

The human ear, like those of other mammals, is so extraordinarily sensitive that it can detect sound-wave-induced vibrations of the eardrum that move by less than the width of an atom. Now, researchers at MIT have discovered ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.