Light switches made of ultra-thin semiconductor layers

A nanostructure made of silver and an atomically thin semiconductor layer can be turned into an ultrafast switching mirror device that may function as an optical transistor – with a switching speed around 10,000 times faster than an electronic transistor. An international team of researchers led by University of Oldenburg physicist Professor Dr. Christoph Lienau describes this effect in a paper published in the current issue of Nature Nanotechnology. Ultrafast light switches offer interesting prospects for optical data processing, the researchers explain.

The team’s goal was to find a material whose reflective properties could be manipulated, or “switched”, within a few femtoseconds using a highly focused laser beam. One femtosecond is equal to one millionth of a billionth of a second. For their experiments the researchers used an ultra-thin silver “nano-slit array”, into the surface of which they milled a grid of parallel grooves of approximately 45 nanometres (or billionths of a metre) in width and depth. Members of the research team from the University of Cambridge (UK) then applied a monolayer of the semiconductor crystal tungsten disulphide just three atoms thick to the surface of this structure.

Plasmon-polaritons exhibit properties of both light and matter

With this combination, the nanostructure displayed an unusual reaction to light: “Taken separately, neither of the two materials exhibits a switching effect,” Lienau explains. However, when combined in a hybrid nanostructure they react very differently to light, turning into what is known as an “active metamaterial”. Light that hits the nanostructure’s surface can then be stored in a hybrid quantum state known as an exciton-plasmon polariton for around 70 femtoseconds before it is reflected. In this state, which exhibits properties of both light and matter, the light propagates across the surface of the semiconductor layer in the form of plasmon waves, producing a strong interaction with the bound electron-hole pairs of the semiconductor layer, the excitons.

“During this storage phase we were able to control the reflectivity of this layer,” says Dr Daniel Timmer from the University of Oldenburg’s Institute of Physics, who was lead author of the study together with Dr Moritz Gittinger. The researchers used an external laser pulse to modify the strength of the interaction between the excitons and plasmon waves. They were already able to change the brightness of the reflected light by up to 10 percent in their first experiments – a surprisingly high value that could potentially be further boosted with optimised materials.

Timmer and Gittinger investigated the effect using two-dimensional electronic spectroscopy (2DES). This complex technique makes it possible for scientists to observe quantum physical interactions with a time resolution of just a few femtoseconds, as if they were watching a film. A team led by Lienau recently came up with a trick to simplify the 2DES procedure and thus extend its use to other studies. “In the current study, we were able to investigate a metamaterial for the first time by using light pulses that were shorter than the observed switching process,” Lienau explains. The scientists were thus able to record the different stages of the phenomenon at intervals of just a few femtoseconds.  

These switches could dramatically increase the amount of information that can be transmitted per unit of time

“Our findings are of particular interest if we want to make ultrafast light switches on the nanoscale,” says Lienau, citing the field of optical data processing as a potential area of application. “With these switches, the amount of information that can be transmitted per unit of time would increase dramatically.” For comparison, the switching time of the electronic transistors currently used in millions of computers and LED televisions is about a thousand times longer. From a physics perspective, optical technologies are therefore the only way to further increase the clock speed of conventional computers, Lienau explains, adding that nanoscale ultrafast light switches could also open up interesting possibilities for chip manufacturing, optical sensors or quantum computers. “The main task will be to design, tailor and optimise active metamaterials in such a way as to make these applications possible,” he emphasises.  

In addition to the Oldenburg team, researchers from the University of Cambridge (UK), the Politecnico di Milano (Italy) and the Technische Universität Berlin participated in the study.

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