All-optical switching device paves the way for faster optical fiber communication

Modern broadband internet uses light to quickly and reliably transmit large amounts of data over fiber optic cables, but currently light signals face a bottleneck when data processing is required. To do this, they must be converted into electrical signals to be processed before further transmission.

A device called an all-optical switch could instead use light to control other light signals without the need for electrical conversion, saving time and energy in fiber optic communication.

A University of Michigan-led research team has demonstrated an ultrafast all-optical switch by pulsing circularly polarized light, which twists like a propeller, through an optical cavity lined with an ultrathin semiconductor. The study was recently published in Natural communications.

The device could function as a standard optical switch, where turning a control laser on or off switches the signal beam of the same polarization, or as a type of logic gate called an exclusive OR (XOR) switch, which would produce an output signal when one light input rotates clockwise and the other counterclockwise, but not when the two inputs are the same.

“Because a switch is the most basic building block of any information processing unit, an all-optical switch is the first step toward any optical computing or building optical neural networks,” Lingxiao said Zhou, a doctoral student in physics at UM and lead author of the study.

The low losses of optical computing make it more desirable than electronic computing.

“Extremely low power consumption is the key to the success of optical computing. The work our team is doing addresses precisely this problem, using unusual two-dimensional materials to switch data at very low energy per bit,” said Stephen Forrest, Peter A. Franken Distinguished Professor. University professor of electrical engineering at UM and contributing author of the study.

To achieve this, the researchers pulsed a helical laser at regular intervals through an optical cavity (a set of mirrors that trap and bounce light multiple times), thereby increasing the laser’s power by two orders of magnitude.

When a one-molecule-thick layer of semiconductor tungsten diselenide (WSe2) is embedded in the optical cavity, the strong oscillating light enlarges the electronic bands of available electrons in the semiconductor – a nonlinear optical effect known as the Stark optical effect. . This means that when an electron jumps to a higher orbital, it absorbs more energy and emits more energy when it jumps down, which is called the blue shift. This in turn changes the fluence of the light signal, the amount of energy delivered or reflected per unit area.

In addition to modulating the light signal, the optical Stark effect produces a pseudo-magnetic field that influences electronic bands in the same way as a magnetic field. Its effective strength was 210 Tesla, far more powerful than Earth’s strongest magnet with a strength of 100 Tesla. The extremely strong force is felt only by electrons whose spins are aligned with the helicity of light, temporarily splitting electronic bands of different spin orientations, directing electrons into the aligned bands all in the same orientation.

The team could change the order of electronic bands of different spins by changing the direction in which light spins.

The brief uniform spin directionality of electrons in different bands also breaks what is called time reversal symmetry. Essentially, time reversal symmetry means that the physics underlying a process is the same forward and backward, implying conservation of energy.

Although we generally cannot observe this in the macroscopic world due to the way energy dissipates through forces such as friction, if you could take a video of spinning electrons it would obey the laws of physics, whether you play it forward or backward: the electron spinning in one direction would transform into an electron spinning in the opposite direction with the same energy. But in the pseudo-magnetic field, the time reversal symmetry is broken because if it is rewound, the electron spinning in the opposite direction has a different energy – and the energy of the different spins can be controlled via the laser.

“Our results open the door to many new possibilities, both in fundamental science, where control of time reversal symmetry is a necessary condition for creating exotic states of matter, and in technology, where it becomes possible to exploit such a huge magnetic field,” said Hui Deng, a research scientist. professor of physics and electrical and computer engineering at UM and corresponding author of the study.