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From digital to optical

May 18, 2020

Today, scientists from all over the world face the task of creating optical switches. These devices will allow for transmitting information in binary code with the help of light, which in the future will be useful for the development of ultrafast optical memory cells. ITMO University scientists have demonstrated how to create, using a femtosecond laser, an all-optical switch based on a metal-organic framework which can be synthesized in vitro and is usually used in chemistry for gas absorption. The research has been published in the journal Angewandte Chemie.

Physicists, engineers and programmers from all over the world are increasingly talking about the prospects of so-called optical memory elements. These might replace modern devices, in which information is processed based on the movement of electrons. It is expected that computing elements operating on photons will work faster, more efficiently, and most importantly, consume less energy. But in order to get closer to bringing these bold plans to life, it is necessary 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 so-called triggers," explains Nikita Kulachenkov, a junior research associate at ITMO University and one of the paper's authors. "These are devices for switching between two states, 0 and 1. For optical devices which might in the future take the place of our electronic devices, we also need a special switch."

One of the options for such a switch comes in the form of metal-organic frameworks (MOFs). This is a class of functional materials which combine the properties of crystal lattice substances and organic compounds. But for the purposes of developing optical computing devices, the most important aspect is that some MOFs contain special photochromic compounds capable of changing their optical properties when exposed to light. This process, however, usually takes place over a relatively long period of time, from several minutes to several days, which puts significant limitations on the practical application of such structures as switchers.

A group of scientists from ITMO University's Russian-French laboratory, headed by Valentin Milichko, decided to take a different path - the researchers used standard metal-organic frameworks that don't contain any photochromic compounds and have for a long time 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. "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."

Experiments with HKUST-1 metal-organic frameworks have shown that when subjected to an ultra-short pulse of an infrared laser, this MOF suddenly starts to transmit less light. "The number of photons passing through the MOF decreased by about a hundred times," explains 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."

Physics-wise, this change has the following explanation - the femtosecond impact generated by the infrared laser is enough to, in effect, evaporate the water from the metal-organic framework. This leads to the MOF becoming less transparent for the laser-emitted light. But once you turn off the light, the framework reabsorbs water molecules from the air and returns to its initial state.

ITMO University

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