All-optical switching device paves the way for faster fiber optic communications

Modern high-speed internet uses light to quickly and reliably transmit large amounts of data over fiber optic cables, but currently light signals are a bottleneck when data processing is necessary. To do this, they must be converted into electrical signals for processing before being further transmitted.

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

A University of Michigan-led research team demonstrated an ultrafast, all-optical switch by pulsing circularly polarized light, which spins like a helix, through an optical cavity coated with an ultrathin semiconductor. The research was recently published in Nature communication.

The device could function as a standard optical switch, where turning a control laser on or off switches the signal beam with the same polarization, or as a type of logic gate called an Exclusive OR (XOR) switch, which would produce an output signal when one one input is clockwise and the other is counterclockwise, but not when both 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 all optical computing or building optical neural networks,” said Lingxiao Zhou, PhD student in physics at UM and lead author of the study .

The low loss of optical computing makes it more desirable than electronic computing.

“Extremely low power consumption is a key to the success of optical computing. Our team’s work addresses exactly this problem, using unusual two-dimensional materials to switch data at very low energies per bit,” said Stephen Forrest, the Peter A. Franken Distinguished University Professor of Electrical Engineering at UM and contributing author of the study.

To achieve this, the researchers pulsed a spiral laser through an optical cavity (an array of mirrors that capture and reflect light multiple times) at regular intervals, increasing the laser’s power by two orders of magnitude.

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

In addition to modulating the signal light, the optical Stark effect produced a pseudo-magnetic field, which affects electronic bands in the same way as that of a magnetic field. The effective force was 210 Tesla, much stronger 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 the light, temporarily splitting the electronic bands of different spin orientations, causing the electrons in the aligned bands to all be oriented in the same orientation.

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

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

Although we can’t usually observe this in the macroscopic world because of the way energy dissipates due to forces like friction, if you could take a video of electrons spinning, you would obey the laws of physics whether you move them forward or plays backwards: the electron when it spins in one direction turns 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 the electron spinning in the opposite direction has a different energy when rewinding – and the energy of different spins can be controlled via the laser.

“Our results open doors to many new possibilities, both in fundamental science, where controlling time-reversal symmetry is a prerequisite for creating exotic states of matter, and in technology, where harnessing such a huge magnetic field is possible becomes,” said Hui Deng, one of the researchers. professor of physics and electrical and computer engineering at UM and corresponding author of the study.