Trapping Light in Nanoresonators

Schematic illustration of doubling of light frequency using a nanoresonator. (A. Shalaeva / Koshelev et al. Science)

An international team of researchers from ITMO Univer­sity, the Austra­lian National University, and Korea University have experi­mentally trapped an electromagnetic wave in a gallium arsenide nano­resonator a few hundred nanometers in size for a record-breaking time. Earlier attempts to trap light for such a long time have only been successful with much larger resonators. In addition, the researchers have provided experimental proof that this resonator may be used as a basis for an efficient light frequency nano­converter. Scientists have suggested drasti­cally new oppor­tunities for subwave­length optics and nano­photonics – including the develop­ment of compact sensors, night vision devices, and optical data trans­mission tech­nologies.

The problem of mani­pulating the properties of electro­magnetic waves at the nanoscale is of paramount importance in modern physics. Using light, we can transfer data over long distances, record and read-out data, and perform other operations critical to data processing. To do this, light needs to be trapped in a small space and held there for a long period of time, which is something that physicists have only succeeded in doing with objects of a significant size, larger than the wavelength of light. This limits the use of optical signals in optoelectronics. “Usually, to trap light in a certain environ­ment, you’d need structures of a significant size, such as optical fiber cables several kilometers long similar to the ones used today to transfer internet traffic,” explains Kirill Koshelev, a staff member of the Faculty of Physics and Engi­neering. “Another option is to exploit large ordered arrays of nanoparticles operating as a single structure that traps the light.”

Two years ago, an inter­national research team theo­retically predicted a new mechanism that allows scientists to trap light in miniature resonators much smaller than the wavelength of light and measured in hundreds of nanometers. However, until recently, no one had implemented the mechanism in practice. “Experiment is the main criterion in physics. You can develop all kinds of theories, but they’ll stay just theories until confirmed experi­mentally,” says Andrey Bogdanov, a leading research fellow at the Faculty of Physics and Engineering. Then the scientists wanted to prove this hypothesis. First, the concept was developed: gallium arsenide was chosen as the material, being a semi­conductor with a high refractive index and strong nonlinear response in the near-infrared range. Researchers also decided on the most optimal shape for the resonator that would efficiently trap electro­magnetic radiation.

In order to trap light efficiently, the ray must be reflected from the object’s inner boundaries as many times as possible without escaping the resonator. One might assume that the best solution would be to make the object as complex as possible. As a matter of fact, it is just opposite: the more planes a body has, the more likely light is to escape it. The near-ideal shape for this case was a cylinder, which possesses the minimal number of boundaries. One question that remained to be solved was which ratio of diameter to height would be the most effective for trapping of light. After mathe­matical calcu­lations, the hypothesis had to be confirmed experi­mentally.

“Our Korean colleagues made a set of cylindrical resonators out of gallium arsenide, which is one of the most widely used semi­conductor materials in opto­electronics,” says Kirill Koshelev. “The process is as follows: you draw circles of the required diameter on a thin slab, then etch the excess material off and end up with cylinders of the needed size. A set of cylinders with different diameters close to 900 nano­meters were fabricated. For such size they are almost invisible to the naked eye. As our experi­ments have shown, the reference particle had captured light for a time exceeding 200 times the period of one wave oscillation. Usually, for particles of that size the ratio is a five to ten periods of wave oscil­lations. And we obtained 200! That means we have provided experimental proof of a drastically novel physical phenomenon: the trapping of an electro­magnetic wave for a very long time using an isolated nano­particle.”

At the same time, the diameter of the cylinders has a sharp, but not critical effect on how long they trap light. This is important because it will not be possible to achieve a precision of up to one-two nano­meters during mass production of such resonators. There will always be a five-ten nanometers error which, as the experimental results show, is not critical. “We’ve opened a completely new chapter of physics that hadn’t existed before. Our work helps revo­lutionize the entire field of nano-optics and the way it uses meta­materials. Before this, it remained unclear how we could make dielectrics and semiconductors to trap light effi­ciently. Now, we’ve found a way,” explains Yuri Kivshar, research director at the Faculty of Physics and Engineering and head of the Nonlinear Physics Centre at the Australian National Uni­versity.

Besides gallium arsenide, such traps can be made using other dielectrics or semi­conductors, such as, for instance, silicon, which is the most common material in modern micro­electronics. Also, the optimal shape for light trapping, namely the ratio of a cylinder’s diameter to its height, can be scaled up to create larger traps. In order to succeed with their experiment, the researchers had to use a special “doughnut” light beam. “A resonator of a certain shape, such as a cylinder in our case, responds well only to an incident field of a specific shape,” explains Kirill Koshelev. “Our resonators respond to a very specific field confi­guration. Visually, it’s like this: a conven­tional laser beam has the maximum of field intensity at the center of its beam, while ours has the minimum. In cross section it looks as a kind of ring with nearly no electro­magnetic field in the center; however, tight focusing allows the beam to efficiently couple the resonator.”

“Our results include a step-by-step guide to creating such systems and it shows that every step is highly important. To reach maximum effi­ciency, you need to optimize each step: the size of the particle, its shape, the shape of the beam, and so on. All of these steps are sort of multiplied by each other, so if fail in one place, you end up multi­plying everything by zero,” says Andrey Bogdanov. This experiment draws on the ideas presented in the works of John von Neumann and Eugene Wigner, who predicted similar properties of electrons almost a century ago. Even though photons and electrons possess different properties and exhibit different behaviors, the physical laws are universal for both cases.

“We are the first to implement Neumann and Wigner’s physics in optics, or nano-optics to be precise, which is a field that manipulates light using objects of a size comparable to the wavelength of light. The idea is that several electro­magnetic waves with similar oscillation frequencies may exist within the same resonator and interact with each other. If two waves have opposite phases, meaning that the crest of one wave meets the trough of another wave, they’ll cancel each other out and suppress the radiation of light from the object,” says Kirill Koshelev.

The scientists divided their study into two parts: one is an experimental confir­mation of the theory expressed earlier, and the other is an example of how such resonators could be used. For instance, the trap has been utilized for a nano­device capable of changing the frequency, and therefore color, of a light wave. “We used a laser with a certain amount of electromagnetic energy and some of that energy was converted into a different frequency range” says Koshelev. “Simply put, the light was originally infrared and had a wavelength of about 1,500 nanometers; but having interacted with our gallium arsenide particle, it became red and with a wavelength of some 750 nano­meters, meaning it became visible to the human eye. A small particle less than a micro­meter in size allowed us to halve the beam’s wavelength while the efficiency of this conversion was at least 100 times higher than anything previously described. This is a record-breaking conversion effi­ciency at the nanoscale.”

Yet the frequency conversion of electro­magnetic oscil­lations is not the only appli­cation for this technology. It also has potential applications in various sensing devices and even special glass coatings that would make it possible to produce colorful night vision. “If the resonator is able to efficiently trap light, then placing, say, a molecule next to it will increase the efficiency of the molecule’s interaction with light by an order of magnitude, and the presence of even a single molecule can easily be detected experi­mentally. This principle can be used for the develop­ment of highly-sensitive biosensors. Due to the resonators’ ability to modify the wavelength of light, they can be used in night vision devices. After all, even in the darkness, there are electro­magnetic infrared waves that are unseen to the human eye. Trans­forming their wavelength, we could see in the dark. All you’d need to do is to apply these cylinders to glasses or the wind­shield of a car. They’d be invisible to the eye but still allow us to see much better in the dark than we can on our own,” explains Kirill Koshelev. (Source: ITMO)

Reference: K. Koshelev et al.: Subwavelength dielectric resonators for nonlinear nanophotonics, Science 367, 288 (2020); DOI: 10.1126/science.aaz3985

Link: Nonlinear Physics Center, Australian National University, Canberra, Australia • Dept. of Physics and Engineering, ITMO University, St. Petersburg, Russia

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