A nanostructure made of silver and an ultrathin semiconductor layer can be transformed into a fast-switching mirror—in principle, an optical transistor that switches about 10,000 times faster than its electronic equivalent. This effect is described by an international team led by physicist Prof. Dr. Christoph Lienau of the University of Oldenburg in the current issue of the journal Nature Nanotechnology. According to the researchers, these ultrafast light switches are of particular interest for optical data processing.
The team's goal was to find a material whose reflective properties could be specifically modified—or "switched"—by a laser in a period of just a few femtoseconds. A femtosecond is one millionth of a billionth of a second. For the study, the researchers used a thin silver plate, on whose surface they milled a grid of parallel grooves about 45 nanometers (billionths of a meter) wide and deep. Researchers at the University of Cambridge (UK) applied an extremely thin semiconductor layer to it. The film of semiconductor material, tungsten disulfide, consisted of a single monolayer of the crystal, meaning it was only three atomic diameters thick.
Nanostructure with an unusual reaction to light.
Thanks to this combination, the nanostructure exhibited an unusual reaction to light. “Neither material on its own exhibits a switching effect,” Lienau emphasizes. However, when combined in a nanostructure, both materials react in a completely new way, which is why the researchers call it an active metamaterial. Incident light can be stored on the surface of the nanostructure for about 70 femtoseconds in the form of a special quantum state, called a polariton exciton-plasmon, before being reflected.
In this state, which possesses properties of both light and matter, the light propagates along the surface of the semiconductor layer in the form of so-called plasmonic waves. During this process, it interacts intensely with the electron-hole pairs in the semiconductor layer, known as excitons.
“During this storage time, we were able to specifically control the reflectivity of the layer,” explains Dr. Daniel Timmer of the Institute of Physics in Oldenburg, who was the first author of the study along with Dr. Moritz Gittinger. The researchers used an external laser pulse to modify the intensity of the interaction between the excitons and the plasmonic wave. In the first experiments, the team managed to modify the brightness of the reflected light by up to 10%, a surprisingly high value that can likely be increased by optimizing the material.
Timmer and Gittinger investigated the effect using two-dimensional electron spectroscopy (2DES). This experimentally demanding method allows observation of quantum interaction processes with a temporal resolution of just a few femtoseconds, as if viewing a film. Recently, a team led by Lienau succeeded in significantly simplifying the application of 2DES, making it usable for further studies. “In this work, we were able, for the first time, to examine a metamaterial of this type using light pulses shorter than the observed switching process itself,” Lienau points out. This allowed us to record the different stages of the phenomenon in intervals of just a few femtoseconds.
Potential applications: chip manufacturing, sensors, and quantum computers.
“Our results are of great interest for the development of ultrafast light switches at the nanoscale,” emphasizes Lienau. One potential application is optical data processing. “The amount of information that can be transmitted per unit of time would increase dramatically with this type of switch ,” he explains. By comparison, the switching time of electronic transistors, which are used millions of times in computers or LED televisions, is about a thousand times greater.
From a physical standpoint, optical technologies are therefore the only way to further increase the clock speed of conventional computers. Optical nanoswitches could also offer exciting possibilities in chip manufacturing, optical sensors, or quantum computers. Lienau concludes: “The most important task will be to design, fine-tune, and optimize active metamaterials so that these applications can become a reality.”
In addition to the Oldenburg team, researchers from the University of Cambridge (UK), the Polytechnic University of Milan (Italy), and the Technical University of Berlin participated in the study.
