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ITMO develops 'chemically-synthesized' optical switch

20 May 2020

Scientists at ITMO, St Petersburg, say the switch, based on a metal-organic framework, could support ultrafast memories.

Scientists at ITMO University, St Petersburg, Russia, say they have demonstrated how to create, using a femtosecond laser, an all-optical switch based on a metal-organic framework that can be synthesized in vitro and is usually used in chemistry for gas absorption. The research has been published in Angewandte Chemie (the international edition of Applied Chemistry).

The team of Valentin Milichko and Nikita Kulachenkov say that the scientific community has been researching metal-organic frameworks (MOFs) for many years. “This is a class of functional materials which combine the properties of crystal lattice substances and organic compounds,” they state in the ITMO announcement. “From the former, they inherited a defined structure, from the latter – a large number of chemical bonds that enable them to enter into different chemical reactions, serve as catalysts or absorb gases.”

At some point, scientists expanded their focus to not only chemical but also optical properties of such complex materials, continue the ITM scientists. “If you add an optically-active organic molecule to their structure, they will start interacting with light differently. If illuminated by a laser, the molecules will rearrange, and they will become opaque; isomerization.” But this process takes place over a relatively long period of time, from several minutes to several days, which, it would seem, prevent practical application of such MOF crystals.

The idea underlying the Angewandte Chemie paper arose at ITMO, in the Faculty of Physics and Engineering’s Russian-French laboratory headed by Valentin Milichko. An international group of scientists from Russia, France, and the Netherlands, led by ITMO, decided to test whether it could be possible to manage the optical properties of not special MOFs but the very common metal-organic frameworks that have long been used in the chemical industry.

“We decided, why not use a group of MOFs that demonstrate the property of changing their structure under external stimuli such as pressure, temperature or others,” says Nikita Kulachenkov, a junior research associate at ITMO University and one of the paper’s authors. “Among these metal-organic frameworks was HKUST-1. It was very well-researched in the field of gas absorption, but no one could ever have thought that its properties, and consequently its structure, could undergo significant changes when exposed to light.”

Hundredfold reduction

“The number of photons passing through the MOF decreased by about a hundred times,” explained Nikita Kulachenkov. “The switch-over period amounted to several dozen milliseconds. This is two to three orders better than offered by existing MOF-based organic systems.”

This change has the following explanation: the femtosecond impact generated by the infrared laser is enough to evaporate the water from the metal-organic framework. This leads to the MOF becoming less transparent to thelight.

But once the light is switched off, the framework reabsorbs water molecules from the air and returns to its initial state. “In our paper, we demonstrate that with the increase of humidity, for example, when the crystal is placed in water, the switching effect occurs much faster,” said Kulachenkov.

He added, "Physicists, engineers and programmers worldwide are increasingly talking about the prospects of optical memory elements. It is expected that computing elements operating on photons would work faster, more efficiently, and most importantly, consume less energy. But in order to get closer to achieving this, we need to solve a number of theoretical and engineering problems. One of them is achieving reliable, energy-efficient, and low-cost light control.

“All of today’s digital electronics are built on triggers. These are devices for switching between the two binary. For optical devices, which might in the future take the place of our electronic devices, we also need this special switch.”

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