Capturing Light in a Quasicrystal

Crystals are solids with a periodic structure – when atoms are displaced, they take the exact places of other atoms, in which the latter were before the shift. This fact was scienti­fically proved at the beginning of the 20th century. It gave rise to modern solid-state physics and also laid the foundation for the development of semi­conductor technologies. “Computers, smartphones, LED bulbs, lasers – everything we can’t imagine our day-to-day lives without,” says Mikhail Rybin, Associate Professor at ITMO’s Faculty of Physics and Engineering, “was designed thanks to the fact that we understand the nature of the crystalline structure of semi­conductor materials. The theory of periodic structures allows us to conclude that waves – be it light, electrons, or sound – can only move in two ways. Either the wave propagates forward in the crystal, or it rapidly fades at the frequencies of the so-called band gap. There are no other options and it greatly simplifies the laws of particle propa­gation while faci­litating engi­neering tasks”.

First direct experimental observation of intrinsic light localization in defect‐free quasicrystals. (Source: ITMO / AOM, Wiley)

However, some devices require the crystal to not transmit the wave and not extinguish it either, but to retain it in itself for some time – something like a light trap is needed. “For example, for laser or sensors operation, the wave must pass through the working area of the device several times to enhance the efficiency of its inter­action with an active element,” explains Rybin. “It’s especially crucial to create such a “trap” for light because it is very difficult to keep it in a small area. It is an important techno­logical challenge for modern physics”.

Ideally, the entire material should take on the role of a trap, because the more light is captured, the more efficient the interaction of the wave with the active substance will be. However, in the case of a crystal, it is not possible. As stated earlier, it can only extinguish the wave or let it pass through. “Alternatively, there is a possi­bility of localizing light in disordered structures, for example, in powders,” notes Rybin. “However, we cannot achieve repro­ducibility in such systems. In one sample, the particles were arranged in one way, and in another – completely differently. For applied tasks, you need something suitable for mass production of the same devices”.

There is also a third way. We can use an intermediate type of materials in which the particles do not form periodic lattices, as it happens in crystals, but at the same time have a mathe­matically strict ordering. These structures are called quasi­crystals, they were discovered in the 1980s and have been studied by physicists ever since. “Since there is no periodicity in quasi­crystals,” says Rybin, “there is also no restriction that the wave can either pass directly without loss or disappear quickly. A paper published in 2017 predicted the pheno­menon of light locali­zation in quasi­crystalline structures, and we had to confirm it experi­mentally.”

During almost 40 years of studying quasi­crystals, physicists have understood their structure and learned to model it on a computer. The problem is that such quasi­crystals are not so easy to synthesize on the microlevel. “That’s when the development of technology comes to our rescue,” says Artem Sinelnik, a PhD student at the Department of Physics and Engineering. “At our faculty, there is a setup for three-dimen­sional nano-printing, where a voxel is about half a micron, which is a hundred times smaller than a human hair. With its help, we created the structure of a quasicrystal with a complex structured distribution of the material in three-dimen­sional space.”

After creating the samples, the scientists started their preliminary study. They analyzed the surface quality with an electron micro­scope. Then, they proceeded to optical measurements to confirm that the internal capacity of the sample really does have a quasi­crystalline structure. “After that, we did an experiment,” explains Sinelnik, “a short light pulse was sent to the quasi­crystal and the so-called afterglow was measured. As it turned out, light exits our samples with a delay, that is, the wave is held inside for quite a long time. Thus, we have confirmed the ability to capture light in a three-dimen­sional polymer quasicrystal.”

For now, the work is solely fundamental. It demons­trates the main optical properties of polymer quasi­crystals, created by using three-dimensional nano-printing, and their ability to localize light. However, as the authors note, the study may be applied in the future. “For example, usually a laser is designed based on the fact that we have an active medium in which light gets localized via a suffi­ciently large external resonator,” explains Rybin. “In this work, we have shown that a quasicrystal can combine the functions of an active medium and a resonator in a single structure.” (Source: ITMO)

Reference: A. D. Sinelnik et al.: Experimental Observation of Intrinsic Light Localization in Photonic Icosahedral Quasicrystals, Adv. Opt. Mat., online 22 September 2020; DOI: 10.1002/adom.202001170

Link: Dept. of Physics, ITMO University, St. Petersburg, Russia

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