Modern optical devices need to constantly change their characteristics of interaction with light. To do this, various mechanical devices that move the lenses, rotate reflective surfaces, and move the laser emitters are used. An international group of scientists, which included employees of ITMO University and Exeter University, has proposed a new metamaterial that can change its optical characteristics without any mechanical influences. This can significantly increase reliability and reduce the cost of manufacturing complex optical devices.
The results of the research fell on the cover of Optica magazine. The rapid development of science in recent decades has given mankind a very wide selection of new materials. Now the creators of complex mechanisms need to adapt less and less to the limitations that traditional materials impose on their imagination.
The so-called metamaterials open up enormous prospects in this sense, the creation of which is being worked on in particular at ITMO University. Due to the complex structure of the constituent elements, the functionality of such structures is less limited by the properties of the materials from which they are made. Metamaterials can be voluminous, or can be flat – in this case they are called metasurfaces.
“Metasurfaces allow you to achieve so many interesting effects in controlling light”, says Ivan Sinev, Senior Researcher at the New Physical Engineering University at ITMO University. – However, they have a problem – all their properties are laid at the time of production and remain unchanged. For practical applications, I would like to manage these properties not only at the time of creation but also as they are used”.
In search of material for such adaptive optics, researchers from ITMO University, who have extensive experience working with silicon metasurfaces, teamed up with colleagues from the British University of Exeter, who have long been researching materials with phase memory. Such substances include, for example, compounds of germanium, antimony, and tellurium (GeSbTe), which are often used in DVDs.
“We made calculations of what a new silicon-based composite material should look like,” says Pavel Trofimov, an engineer at the New Fiztekh, “Our GeSbTe insert is presented as a thin layer between two silicon layers. It turns out such a sandwich – first silicon is sprayed onto the initial substrate, then a layer of material with phase memory, then silicon again. ”
Then, using electronic lithography, scientists obtained arrays of microscopic hybrid discs – a metasurface with which they had already worked in the laboratory, checking its properties for controlling light. As expected, the combination of the two materials gave a very important effect – the transparency level of the resulting surface could be changed during the experiment.
The fact is that the silicon disk has two optical resonances in the near-infrared zone, which allow it to reflect the infrared beam directed to the surface especially strongly. The GeSbTe layer allowed under certain conditions to “turn off” one of these resonances, making the disk almost completely transparent to light in the near-infrared spectrum.
Materials with functional memory have two states – crystalline, with a rigidly ordered structure of atoms, and amorphous. If the GeSbTe layer located in the center of the metamaterial is in a crystalline state, then the second resonance will disappear, but if it is in an amorphous state, the disk will still reflect infrared rays.
“To switch the metasurface between the two states, we used a pulsed laser with a sufficiently high energy,” says Pavel Trofimov, “a short laser pulse heats the GeSbTe layer to its melting point, after which it quickly cools and amorphized. If the disk is affected by a series of short pulses, then it cools more slowly, freezing in the crystalline structure”.
The properties of the new metasurface can come in handy for a wide variety of applications. First of all, this is the creation of lidars, devices that scan space using radiation, and receiving infrared pulses reflected by objects. Also, the potential principle of their creation can be taken as a basis in the production of special ultra-thin lenses for photo lenses, for example, installed in mobile phones.