The direction that light is scattered by a single silicon nanoparticle can be controlled by the intensity of an incoming laser pulse, allowing the nanoparticle to act like an optical transistor. Illustration: Nano Letters.
The direction that light is scattered by a single silicon nanoparticle can be controlled by the intensity of an incoming laser pulse, allowing the nanoparticle to act like an optical transistor. Illustration: Nano Letters.

Physicists from the Department of Nanophotonics and Metamaterials at ITMO University have experimentally demonstrated the feasibility of creating an optical version of a transistor from a single silicon nanoparticle. Because transistors are some of the most fundamental components of computing circuits, the results of this study could prove crucial for the development of optical computers, where transistors must be very small and ultrafast at the same time. The study is published in Nano Letters.

The performance of modern computers, which use electrons as signal carriers, is largely limited by the time needed to trigger the transistor – usually around 0.1–1 nanoseconds. Next-generation optical computers, however, will rely on photons of light to carry the signal, greatly increasing the amount of information that can pass through a transistor per second. For this reason, the creation of an ultrafast and compact all-optical transistor is considered to be essential for the development of optical computing. Such an all-optical transistor would need to control the propagation of an optical signal beam by means of an external control beam within several picoseconds, making it a thousand times faster than current transistors.

In the study, a group of Russian scientists from ITMO University and St. Petersburg Academic University proposed a completely new approach for designing such optical transistors, having made a prototype using just a single silicon nanoparticle.

The scientists found they could dramatically alter the properties of the silicon nanoparticle by irradiating it with an intense and ultrashort laser pulse. By providing ultrafast photoexcitation of electron-hole pairs on the silicon nanoparticle, the laser changes the nanoparticle’s dielectric permittivity for a few picoseconds, acting just like a control beam.

This abrupt change in the optical properties of the nanoparticle offers the possibility of controlling the direction in which incident light is scattered. For instance, the direction of nanoparticle scattering can be changed from backward to forward on picosecond timescales, depending on the intensity of the incident laser pulse. This turns the nanoparticle into an ultrafast switch that could form the basis for an all-optical transistor.

"Generally, researchers in this field are focused on designing nanoscale all-optical transistors by means of controlling the absorption of nanoparticles, which, in essence, is entirely logical,” explains Sergey Makarov, lead author of the study and senior researcher at the Department of Nanophotonics and Metamaterials. “In high absorption mode, the light signal is absorbed by the nanoparticle and cannot pass through, while out of this mode the light is allowed to propagate past the nanoparticle. However, this method did not yield any decisive results. Our idea is different in the sense that we control not the absorption properties of the nanoparticle, but rather its scattering diagram. Let's say, the nanoparticle normally scatters almost all incident light in the backward direction, but once we irradiate it by a control pulse, it becomes reconfigured and starts scattering light forward."

The choice of silicon as a material for the optical transistor was not accidental. Optical transistors ideally need to be made of inexpensive materials appropriate for mass production and capable of changing their optical properties in several picoseconds without overheating.

"The time it takes us to deactivate our nanoparticle amounts to just several picoseconds, while to activate it we need no more than tens of femtoseconds,” says Pavel Belov, co-author of the paper and head of the Department of Nanophotonics and Metamaterials. “Now we already have experimental data that clearly indicates that a single silicon nanoparticle can indeed play the role of an all-optical transistor. Currently we are planning to conduct new experiments, where, along with a laser control beam, we will introduce a useful signal beam.”

This story is adapted from material from ITMO University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.