Chinese scientists achieve three-state light in hair-thin silicon device

Microcavity tightly traps light while efficiently exchanging energy, enabling stable multi-state optical operation

A group of researchers has successfully induced a beam of light to exhibit three stable states within a silicon photonic crystal microcavity measuring merely 20 micrometers in diameter, thinner than a human hair, according to a recent research article published in the journal Nature Nanotechnology.

Enabling a beam of light to hold an additional state beyond the binary "on" and "off" is a key milestone on the path to making photonic computing a practical reality.

The researchers from Peking University in Beijing and Harbin Engineering University in northeast China achieved multistability requiring only 240 microwatts for the switching power, less than the power consumption of a typical laser pointer. They have, notably, built a prototype multi-valued optical memory device based on this mechanism.

Tristability is a specific form of multistability. The latter refers to the ability of a system to maintain multiple stable states under the same system parameters and external conditions, and to switch flexibly among these states via external stimuli or perturbations.

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In the context of light, multistability implies that a single storage unit can hold more information. This holds substantial importance for both photonic computing and optical data storage.

However, on micro- and nano-scale chips, the non-linear effects of light are extremely weak, making it exceedingly challenging to design devices that can stably hold light in multiple states.

To address this challenge, the researchers leveraged a physical mechanism known as "near-exceptional-point coupling."

They designed two resonant modes in a photonic crystal microcavity. When the system is driven toward a special state called an "exceptional point," the two modes become highly coupled, with their wavelengths drawing close and their linewidths converging.

This state allows the microcavity to confine light tightly while efficiently exchanging power with the outside environment, paving the way for optical multistability.

In the experiment, the microcavity fabricated by the researchers achieved a quality factor as high as one million, allowing light to oscillate repeatedly inside it before decaying slowly. At an extremely low input power of 240 microwatts, the system exhibited clear tristability.

By finely tuning the input optical power or wavelength, the system can switch quickly and reliably among the three states.

This research demonstrates that through ingenious physical design, even on an extremely small silicon chip, light's nonlinear effects can be regulated and induced to display a rich variety of multistability.

This provides a new fundamental component for the future development of optical neural networks and neuromorphic computing processors capable of handling more complex information.