Scientists from ITMO’s Faculty of Physics and Institute of Photonics in collaboration with researchers from Switzerland and China have created a thin film that can switch light with the speed of up to a billion times a second. The device is essentially an adjustable silicon metasurface that can modulate radiation in the medium infrared range. In the future, it can be used in optical computing, biosensors, and next-get data transfer systems. The results of the study have been published in Nature Communications.

^{Credit: Sonar / photogenica.ru}

Metasurfaces are thin artificial materials used in optics to control light. By accurately calculating the geometry of nanoholes, they can be programmed to perform target functions, such as highly accurate filters for a specific wavelength or superthin lenses for beam focus. Such optics is compact and compatible with standard microchip production processes, which is why it can be integrated into microelectronics. However, usually such structures are developed for a specific task so their properties cannot be altered after production. This makes it impossible to dynamically control light – for instance, to quickly switch the signal on and off in data transfer.

A team of researchers from ITMO’s Faculty of Physics and Institute of Photonics, École Polytechnique Fédérale de Lausanne (EPFL, Switzerland), and Harbin Engineering University (China) has developed the world’s first metasurface that can be modulated by an external signal and whose properties can be controlled in the middle infrared range. As a rule, metasurfaces are made on glass or sapphire substrates, but these materials absorb radiation and weaken the optical signal. In their work, the team suggests a different approach: making a one-micrometer-thin membrane (about 50 times thinner than a human hair) that freely levitates, without laying on the substrate. At such parameters, silicon is transparent and the nanoholes geometry (location, shape, and size) is chosen in a way to allow the entire structure to work as a precise optical resonance filter for radiation of a specific wavelength.

The quality of such a resonator is characterized by the quality factor Q, which indicates how many oscillations the light can complete inside before decaying. Here, the Q factor can reach 3,000. Thanks to this high quality factor and the transparency of the metasurface, it is possible to precisely control an infrared beam: even a weak external perturbation noticeably changes the membrane’s transparency, either attenuating or transmitting the light passing through it. 

“First, the Swiss team of physicists designed the metasurface and conducted calculations of its linear optical properties, for instance, how it will transmit and reflect light depending on the nanohole geometry. Next, we calculated how fast such structures will be able to change their optical properties, for example, switch from transparent to non-transparent state in a fraction of a nanosecond. The structure was fabricated and described in Losanne and the experiments in ultrafast switching were conducted at our joint center with the Harbin Engineering University in Qingdao, China,” shares Mihail Petrov, a senior researcher at ITMO’s Faculty of Physics and head of the research team Quantum Nanophotonics and Optomechanics.

The metasurface can be controlled electrically and optically. In the first case, a voltage of five volts is applied to the membrane, which is a standard level for microchips. The membrane heats up and becomes less transparent to attenuate the beam. In the second method, the membrane is illuminated with a short laser pulse. Free electrons are generated in silicon, reducing its transparency, and the material’s optical properties change almost instantaneously.

“In the electric mode, the speed of switching is limited by the membrane’s heating and cooling time. The optical mode is different: the laser pulse changes silicon properties nearly instantaneously, which is why the switching time is mere nanoseconds and the modulation frequency reaches the order of a gigahertz, that’s a billion switches in a second. This is comparable to the clock speed of contemporary processors, except that in our case signal processing is performed directly by light,” says Sergey Makarov, a chief research associate at ITMO’s Faculty of Physics and the head of the research team specializing in nanophotonics.

Such metasurfaces can become the basis of compact spectrometers that identify the chemical composition of substances, as well as sensitive elements of thermal images and secure optical communication systems where data is encrypted directly in the light beam.

The study was supported by the project Metastructures of Nanophotonics for Optical Computing of the federal program Priority 2030. The work is ongoing: the researchers are studying light polarization control – that is, the direction of its oscillations. In the future, this could multiply the amount of information transmitted through a single optical channel several-fold.

The project was developed by a team from the newly founded ITMO’s Institute of Photonics that is developing infrastructure for artificial intelligence, including optical computing.