16 August 2017

​The recent review by members of ITMO’s International Research Center for Nanophotonics and Metamaterials and their foreign colleagues is a detailed classification of the latest results in the field of creating light-adjusted nanostructures from different materials. Such nanostructures and the respective methods for controlling light can have a wide application in recording optical information and ultrafast processing of signals. The report will be published in the annual release of the Laser&Photonics Reviews magazine

The prestigious magazine by the Wiley German publisher has been around for ten years already, and is deemed one of the leading science magazines in the field of photonics and laser technology. For Anastasia Zalogina, PhD student at the International Research Center for Nanophotonics and Metamaterials, this publication will be her debut on the international scientific arena, and for her colleague, Sergei Makarov, it will be his third publication in the renowned magazine. What's more, the publication was prepared on a special invitation for the annual release of the Laser&Photonics Reviews magazine commemorating its 10th anniversary.

"In 2017, Laser&Photonics Reviews, which is one of the best science magazines in the field of photonics, celebrates its 10th anniversary. For the anniversary issue, the magazine invited the best authors who've greatly contributed to the field of fundamental and applied photonics. The goal of the issue is to publish a series of high-quality articles that will provide a review of the current progress of modern photonics, as well as discuss the field’s new trends. One will have the opportunity to see the online version of the issue right after the actual celebration of the magazine's anniversary in 2017," notes Yuri Kivshar, the issue's curator and professor emeritus of ITMO University.

All of the authors for the anniversary issue believe that this review will have an impact on modern photonics, as it will be a unique generalization of modern knowledge in the field of nanostructures, new optical materials, nanoscale lithography and the related fields. The authors hope that it will help researchers from all over the world be more conversant in the multitude of different approaches that help control light on a nano-scale.

A surface colored by irreversible adjustment of single nanoparticles' optical properties

"What will happen to a material if we light it with very weak light on it, like when sun shines on glass? In our everyday lives, we see that light has almost no effect on the objects surrounding us. The light passes through a material, scatters or reflects. On the other hand, everyone knows that if we apply really intense light to a material, it can melt or even vaporize. Yet, what about between these two extremes? It turned out that this range has a great potential in terms of how it can be applied," shares Sergei Makarov.

If one applies an ultrashort laser impulse of a particular intensity to a material, it is possible to quickly change the material's properties, which can then return to their initial state in a very short amount of time. This change greatly depends on the material's initial properties, which has become a popular research field since the invention of lasers that can generate ultrashort impulses.

Researchers hope to apply this effect in developing ultrafast modulators in optical computers that will replace their electronic counterparts. As of now, many major companies (IBM, for instance) are working on projects that have to do with creating optical transistors that can greatly increase processing speed. Every now and then, there's news of the invention of fully optical modulators, yet the size of them are still significantly larger than the sizes of electronic devices. The problem lies in the photon's size - it is much bigger than the size of an electron. Thus, to develop compact-size optical chips, nanoscale optical modulators first need to be created, and this is one of the topics of the new review that will be published in Laser&Photonics Reviews.

Apart from summarizing the literature on this subject, the review's authors also mentioned their own contribution to this field. For instance, in 2015 ITMO University scientists showed that it is possible to create an optical modulator on a single silicon nanoparticle, with a  size of  less than 200 nm. Last year, they proposed an ultrafast compact router based on two silicon nanoparticles. If such nanoparticles are to be organized in a periodic manner, they become a metasurface that can provide for more effective modulation or routing of the optical signal.

Yuri Kivshar is one of the pioneers of the field of non-linear metasurfaces and metamaterials; in his laboratory at the Australian National University, they actively research the application of metasurfaces in holography and modulation of optical signals. According to the researchers' forecasts, in future it will be possible to create color holograms that will be able to change dynamically, which will become the groundwork for creating 3D displays.

Using nanostructures in ultrafast information processing was not the only field summarized in the review. Surely, by increasing the light's intensity one can irreversibly change a material's properties in a particular area, which is widely used in information recording, for instance. Yet, the diffraction limit narrows down the efficiency of recording information with a focused laser beam, and the way to further increase the recording density is to apply the beam to surfaces with nanostructures, not flat surfaces used in CD and DVD disks. For instance, in one of the recent articles by the review's authors they showed that ultrashort laser impulses can be used to gradually change the color of metal-dielectric particles. The color changed due to a local melting of the metal part of the nanoparticle and change in its form. And in the current review, the authors systemized the results of similar research works that used different materials and nanostructure designs.

"Apart from choosing the intensity of laser radiation, we can also make use of a variety of materials, for example semiconductors (silicon, gallium arsenide, etc.) and metals (gold, silver and others), as well as new materials - organic-inorganic ones or even two-dimensional ones like graphene. Each material has a different response to light, hence, some are better for ultrafast modulation of signals, others - for information recording. In our review, we explain which materials are best for particular applications, and that will be useful for researchers who don't have much experience in materials science," explains Sergei Makarov.

In their review, the scientists also described different nanostructure architectures that have radically different optical properties.

"The third aspect is the choice of the nanostructure type. Adding a particle to another (creating a dimer) leads to the amplification of localization of incident light and system's sensitivity to changes in the material's optical properties, as well as changes the speed of modulation. Thus, the three properties altogether (laser radiation intensity, the type of material and the type of nanostructure) provide for a wide range of opportunities. Our goal was to not just list them, but lay some groundwork for researchers who now have the opportunity to easily check which types of nanostructures are best used with certain materials and at different light intensity. For instance, writing this review was most useful for me, as my main research field as a PhD student is the use of special dielectric nanostructures for effective control of light radiation from single photons," shares Anastasia Zalogina.