ANN ARBOR, Mich., Oct. 28, 2024 — A University of Michigan-led team demonstrated an ultrafast, all-optical switch by pulsing circularly polarized light through an optical cavity lined with tungsten diselenide (WSe₂), an ultrathin semiconductor.

The device could function as a standard optical switch or could serve as an exclusive OR (XOR) switch, a type of logic gate that produces an output signal when one input signal rotates clockwise and the other input signal rotates counterclockwise.

An all-optical switch would use light to control optical signals without the need for electrical conversion, saving time and energy in fiber optic communications.

“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,” researcher Lingxiao Zhou said.

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A schematic showing 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. Courtesy of Deng Laboratory, Michigan Engineering.

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To develop an optical switch with low loss, the researchers pulsed a helical laser at regular intervals through an optical cavity, increasing the strength of the laser by two orders of magnitude. They embedded a layer of WSe₂ within the optical cavity. The strong light from the laser amplified the electronic bands of the available electrons in the WSe₂ material.

This nonlinear effect is known as the optical Stark effect (OSE) and is a form of Floquet engineering. Although Floquet engineering has shown the potential to manipulate quantum systems coherently, it has been of limited use as a coherent nonlinear optical effect.

OSE causes electrons to absorb more energy when they jump to a higher orbital. When the electrons move to a lower orbital — a phenomenon called blue shifting — they emit more energy. The jumps in electron activity induce changes in the fluence of the signal light.

The researchers engineered the optical cavity to enhance the effective Floquet field by orders of magnitude, enabling Floquet effects at an extremely low fluence of 450 photons per square micron (450 photons/μm²).

At higher fluences, the cavity-enhanced effects of Floquet led to 50 milli-electron volt (50 meV) spin and valley splitting of WSe² excitons corresponding to an enormous time-reversal-breaking, non-Maxwellian magnetic field. The researchers generated a pseudomagnetic field with an effective strength of 210 Tesla (T) — more than twice the strength of Earth’s strongest magnet, which has a strength of 100 T.

The massive force of the pseudomagnetic field was felt only by those electrons whose spins were aligned with the helicity of the laser light. Electronic bands of different spin orientations split temporarily, causing the electrons in the aligned bands to be in the same orientation. The researchers could change the ordering of the electronic bands of different spins by changing the direction the light.

The uniform spin directionality of the electrons in the different bands broke the time reversal symmetry.

Time reversal symmetry is present when a process is the same, and has the same amount of energy, whether it is performed forward or backward. In the pseudomagnetic field, an electron spinning in the opposite direction would have a different energy if it was rewound.

The researchers used the laser to control the energy of different spins.

Using this optically-controlled effective magnetic field, the researchers developed an ultrafast, picojoule, all-optical-chirality XOR gate.

In summary, the researchers demonstrated a nearly two orders of magnitude enhancement of the effective Floquet field intensity in an asymmetric cavity, compared to in free space. This enabled a valley splitting as large as 50 meV that corresponded to time-reversal breaking by a 210 T pseudomagnetic field. It enabled a measurable Floquet effect with a pump fluence as low as 0.12 femtojoules per square micron (0.12 fJ/μm²) and an ultrafast, coherent chirality XOR switch with 15 decibel (dB) on/off switching ratio.

The researchers said that these effects could be further enhanced by reducing the exciton linewidth to the radiative limit and suppressing high-order nonlinear processes.

“Extremely low power consumption is a key to optical computing’s success,” professor Stephen Forrest said. “The work done by our team addresses just this problem, using unusual two-dimensional materials to switch data at very low energies per bit.”

Cavity-enhanced Floquet engineering, or OSE, could enable the creation of steady-state or quasi-equilibrium Floquet bands for a range of materials and applications. It could be broadly applied to optically active materials of different spectral bands to induce pseudomagnetic fields inaccessible by other means, to enable continuous wave Floquet engineering of new quantum phases, and to facilitate ultralow-energy, ultrafast, all-optical switches and sensors.

“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,” professor Hui Deng said.

The research was published in Nature Communications (www.doi.org/10.1038/s41467-024-52014-0).

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