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Researchers from ITMO University and their international colleagues have developed the first three-dimensional dynamic model of an interaction between light and silicon nanoparticles. They used a supercomputer with graphic accelerators for the calculations. Results showed that when exposed to short, intense laser pulses, silicon particles temporarily lose their symmetry and their optical properties become strongly heterogeneous. Such a change in properties depends on particle size; therefore it can be used to control light at nanoscale and in ultrafast information processing devices. The study was published in Advanced Optical Materials.
Improvement of computing devices today focuses on increasing information processing speeds. Nanophotonics is one of the sciences that can solve this problem by means of optical devices. Although optical signals can be transmitted and processed much faster than electronic ones, first, it is necessary to learn how to quickly control light on a small scale. For this purpose, one could use metal particles. They are efficient at localizing light, but weaken the signal, causing significant losses. However, dielectric and semiconducting materials, such as silicon, can be used instead of metal.
Silicon nanoparticles are now actively studied by researchers all around the world, including those at ITMO University. The long-term goal of such studies is to create ultrafast, compact optical signal modulators. They can serve as a basis for computers of the future. However, this technology will become feasible only once we understand how nanoparticles interact with light.
"When a laser pulse hits the particle, a lot of free electrons are formed inside,” explains Sergey Makarov, head of ITMO’s Laboratory of Hybrid Nanophotonics and Optoelectronics. “A region saturated with oppositely charged particles is created. It is usually called electron-hole plasma. Plasma changes optical properties of particles and, up until today, it was believed that it spreads over the whole particle simultaneously, so that the particle’s symmetry is preserved. We demonstrated that this is not entirely true and an even distribution of plasma inside particles is not the only possible scenario.”
Scientists found that the electromagnetic field caused by an interaction between light and particles has a more complex structure. This leads to a light distortion which varies with time. Therefore, the symmetry of particles is disturbed and optical properties become different throughout one particle.
"Using analytical and numerical methods, we were the first to look inside the particle and we proved that the processes taking place there are far more complicated than we thought,” says Konstantin Ladutenko, staff member of ITMO’s International Research Center of Nanophotonics and Metamaterials. “Moreover, we found that by changing the particle size, we can affect its interaction with the light signal. This means we might be able to predict the signal path in an entire system of nanoparticles.”
In order to create a tool to study processes inside nanoparticles, scientists from ITMO University joined forces with colleagues from Jean Monnet University in France.
“We developed analytical methods to determine the size range of the particles and their refractive index which would make a change in optical properties likely. Afterwards, we used powerful computational methods to monitor processes inside particles. Our colleagues performed calculations on a computer with graphics accelerators. Such computers are often used for cryptocurrency mining. However, we decided to enrich humanity with new knowledge, rather than enrich ourselves. Besides, bitcoin rate had just started to go down then,” adds Konstantin.
Devices based on these nanoparticles may become basic elements of optical computers, just as transistors are basic elements of electronics today. They will make it possible to distribute and redirect or branch the signal.
"Such asymmetric structures have a variety of applications, but we are focusing on ultra-fast signal processing,” continues Sergey. “We now have a powerful theoretical tool which will help us develop light management systems that will operate on a small scale – in terms of both time and space".