Topological photonics - a highway for the development of semiconductor technology

Translated from——spectrum

Lasers, chips and quantum circuits could all benefit from this blurry phenomenon

Topological insulators, a new class of materials that insulate on the inside and conduct electricity on the outside, have sparked interest in their potential for electronics since they were first described in 2007. However, a related but more obscure class of materials, called topological photons, may reach practical applications first.

Topology is the branch of mathematics that studies which aspects of shapes can withstand deformation. For example, an object shaped like a ring can be deformed into the shape of a cup, with the hole in the ring forming the hole in the cup's handle, but cannot be deformed into a shape without a hole.

Using knowledge of topology, researchers have developed topological insulators. Electrons moving along the edge or surface of these materials strongly resist any disturbance that might impede their flow, just as holes in a deformed ring resist any change.

Recently, scientists have designed photonic topological insulators in which light has similar "topological protection." These materials have regular variations in their structure that allow specific wavelengths of light to flow along their surfaces without scattering or loss, even around corners and defects.

Three promising potential uses of topological photonics:

Electron-driven topological lasers operating at terahertz frequencies, shown in this scanning electron microscope image.

The first practical applications of topological lasers in these new materials may be topologically protected lasers. For example, Mercedeh Khajavikhan and her colleagues at the University of Southern California have developed topological lasers that are more efficient than conventional devices and have proven to be more resistant to defects.

The first topological lasers each required an external laser to excite them to work, limiting their practical applications. However, scientists in Singapore and the UK have recently developed a topological laser that can be driven electrically.

The researchers first sandwiched layers of gallium arsenide and aluminum arsenide together to create a wafer. When an electrical current was applied, the wafer glowed brightly.

The scientists drilled a lattice of holes into the wafer. Each hole is like an equilateral triangle with the four corners chopped off. Surrounding the lattice are holes of the same shape, facing in opposite directions.

Topologically protected light on the wafer flows along the interfaces between different groups of holes and emerges from nearby channels as laser beams. The device has proven to be highly resistant to defects, said Qijie Wang, an electrical and optical engineer at Nanyang Technological University in Singapore.

Lasers operate at terahertz frequencies, which are useful for imaging and security screening. Khajavikhan and her colleagues are now developing a lidar that can operate in the near-infrared band, with possible uses in telecommunications, imaging and lidar.

A scanning electron microscope (SEM) image shows a photonic topological insulator developed at the University of Pennsylvania.

By using photons instead of electrons, photonic chips are expected to process data much faster than conventional electronics, which could support high-capacity data routing for 5G and even 6G networks. Photonic topological insulators are of particular interest in photonic chips, where they can guide light around defects.

However, topological protection only works on the exterior of the material, meaning that the interior of a photonic topological insulator is effectively wasted space, greatly limiting how compact such devices can be.

To address this problem, optical engineer Liang Feng of the University of Pennsylvania and his colleagues developed a photonic topological insulator with edges that they could reconfigure so that the entire device could transmit data. They built a 250-micrometer-wide photonic chip and etched elliptical rings on it. By pumping the chip with an external laser, they could change the optical properties of individual rings so that "we can send light anywhere we want in the chip," Feng said -- from any input port to any output port, or even multiple output ports at once.

All in all, the chip carries hundreds of times more ports than today's most advanced photonic routers and switches. The researchers are now developing an integrated way to accomplish this task, rather than requiring the chip to be reconfigured with off-chip lasers.

This artist's rendering shows photons moving through a silicon waveguide protected by topography.

In theory, quantum computers based on qubits are very powerful. But qubits based on superconducting circuits and trapped ions are vulnerable to electromagnetic interference, making it difficult to scale up to useful machines. But qubits based on photons could avoid such problems.

Quantum computers only work if their qubits are "entangled," or linked together to act as one qubit. Entangled states are extremely fragile, and researchers hope that topological protection can protect photonic qubits from scattering and other disturbances that can occur when photons encounter inevitable manufacturing errors.

Photonics scientist Andrea Blanco-Redondo, now head of silicon photonics at Nokia Bell Labs, and her colleagues made lattices of silicon nanowires, each 450 nanometers wide, and arranged them in parallel. The nanowires in the lattice were occasionally separated from the others by two thick slits. This created two different topologies in the lattice, and entangled photons traveling down the boundaries of these topologies were topologically protected, even as the researchers added defects to the lattice. The hope is that this topological protection could help light-based quantum computers solve problems far beyond the capabilities of mainstream computers.

Using topological photonics to create laser beams with unexpectedly good performance 

Fiber laser is the most widely used type of laser. According to forecasts, global fiber laser sales will increase from US$1.59 billion in 2017 to US$2.50 billion in 2020, with a compound annual growth rate of 16.28%. With the rapid development of lasers, countries have never stopped researching laser technology.

In the latest research, Mordechai Segev and his team at the Technion Institute in Haifa, Israel, created a laser beam based on topological photonics in which the light waves are in phase. This means that the energy loss of this technology will be lower, that is, the laser emission efficiency will be higher.

In the experiment, the research team etched a series of circular channels into the surface of a semiconductor material chip and projected infrared light onto the structure from above the chip. These circular channels precisely captured light waves of specific wavelengths and then moved the light waves from one loop to the next to form a photonic system.

However, in photonic systems, the direction of wave propagation is reversible, which can lead to energy loss. Last year, in a study by Boubacar Kanté at the University of California, he used a magnetic field to restrict the propagation of waves to solve this problem; this time, Segev used an asymmetric design of a circular channel, which itself would preferentially screen the propagation of waves in one direction, thus avoiding the problem of energy loss and enhancing or amplifying the circulating light pulses.

There is an essential difference between the two methods. Although Boubacar-Kanté's method forms a laser beam, the use of a magnetic field to confine it more or less weakens the emission energy of the laser beam, while Segev's improvement is much more subtle.

In this regard, Segev said: "This is thanks to topological protection. The system perfectly tells us that imperfections are the most stable."

"Most physicists doubt that topological photonics will be compatible with lasing, so that lasing won't happen, but in fact, these systems are often much easier to work with than the systems we have today."