A team of researchers led by the University of Michigan demonstrated an ultrafast all-optical switch in a study published in Nature Communications. They performed this by pulsing circularly polarized light, which twists like a helix, utilizing an optical cavity lined with an ultrathin semiconductor.
Schematic of the optical cavity with a one molecule thick layer of tungsten diselenide (WSe2) at the antinode, the point where the light field intensity is at its maximum. Image Credit: Deng Laboratory, University of Michigan
Modern high-speed internet uses light to transmit large amounts of data quickly and reliably through fiber-optic cables. However, light signals currently encounter a bottleneck when data processing is required. They must convert into electrical signals before being transmitted.
In fiber-optic communication, an apparatus known as an all-optical switch could save time and energy by using light to control other light signals without requiring electrical conversion.
The apparatus may also operate as a standard optical switch, whereby switching a control laser off or on changes the signal beam with the same operation. It could also function as a logic gate known as an Exclusive OR switch, which produces an output signal when one light input turns clockwise while the other is counterclockwise (however, not when the inputs are the same).
Because a switch is the most elementary building block of any information processing unit, an all-optical switch is the first step towards all optical computing or building optical neural networks.
Lingxiao Zhou, Study Lead Author and Doctoral Student, University of Michigan
Optical computing is preferred over electronic computing due to its low loss.
Extremely low power consumption is a key to optical computing's success. The work done by our team addresses just this problem, using unusual two-dimensional materials to switch data at very low energies per bit.
Stephen Forrest, Study Contributing Author and Peter A. Franken Distinguished University Professor, Electrical Engineering, University of Michigan
The researchers used an optical cavity—a collection of mirrors that catch and reflect light repeatedly—to pulse a helical laser at regular intervals, increasing the laser’s power by two orders of magnitude.
The strong, oscillating light causes the electronic bands of the semiconductor’s available electrons to enlarge when a one-molecule-thick layer of tungsten diselenide (WSe2) is embedded within the optical cavity. This nonlinear optical effect is called the optical Stark effect.
This demonstrates that an electron absorbs more energy when it jumps to a higher orbit and emits more energy when it jumps down. This is referred to as blue shifting. As a result, the fluence of the signal light, or the quantity of energy reflected or delivered per unit area, is altered.
The optical Stark effect modulated the signal light and created a pseudo-magnetic field, which affects electronic bands in a manner akin to that of a magnetic field. Its 210 Tesla effective strength was significantly greater than the 100 Tesla strength of the strongest magnet on Earth.
Only electrons with spins aligned with the helicity of the light feel the enormously strong force, which temporarily splits the electronic bands with different spin orientations and directs the electrons in the aligned bands in the same orientation.
By changing the direction in which the light twists, the team rearranged the electronic bands of various spins.
The short-term uniform spin directionality of the electrons in various bands also breaks time-reversal symmetry. Time-reversal symmetry implies energy conservation since the physics underlying a process is the same forward and backward.
A video of electrons spinning would follow the laws of physics whether it was played forward or backward—the electron spinning one way would change into an electron spinning the opposite way with the same energy. This is usually impossible to see in the macroscopic world because energy dissipates through forces like friction.
However, in the pseudo-magnetic field, time reversal symmetry is broken because rewinding causes the electron to spin in the opposite direction and have different energy—and the energy of different spins can be controlled using the laser.
Our results open doors to a lot of new possibilities, both in fundamental science where controlling time reversal symmetry is a requirement for creating exotic states of matter and for technology, where leveraging such a huge magnetic field becomes possible.
Hui Deng, Study Corresponding Author and Professor, Physics and Electrical and Computer Engineering, University of Michigan
Journal Reference:
Zhou, L. et. al. (2024) Cavity Floquet engineering. Nature Communications. doi.org/10.1038/s41467-024-52014-0