First Ever Subwavelength Dielectric Resonators for Light Trapping at Nanoscale

Scientists have designed the very first subwavelength dielectric resonators for light trapping at nanoscale that seems to be a simple silicon cylinder which is a hundred times thinner than a strand of human hair.

Credit: ITMO University

Such a structure has the potential to trap light ten times longer than any standard resonator. This new resonator, along with its small size and simple shape, is considered to be a promising basis for the design of powerful biosensors, nanolasers, and different light transmitting devices. The results have been featured in Physical Review Letters.

One of the most vital tasks of modern optics refers to localizing light within a photonic system. For this task, resonators generally based on dielectrics and metals are used. A significant feature of these devices is how long a light wave spends within the system prior to being emitted. Scientifically, this time can be referred to as lifetime of optical mode. Metallic resonators contain strong losses which restrict applications of photonic devices since metals have free electrons.

A new way to enhance the effectiveness of optical resonators has been discovered by researchers from ITMO University, the Ioffe Institute and the Nonlinear Physics Center at the Australian National University. This can be attained by developing a structure in which incoming light creates two waves of the same frequency yet varied phases. In such a situation, destructive interference takes place by suppressing outgoing light waves, thus avoiding the escape of light from the system.

In conventional resonators, trapped light tends to damp gradually due to radiation or material absorption. However, once interference occurs, the radiation escape becomes impossible, so that we can trap light for a long time. We call such states bound states in the continuum.

Dr. Mikhail Rybin, Senior Fellow at the Department of Dielectric and Semiconductor Photonics at ITMO University.

On a theoretical basis, the bound states in the continuum can be present only in extremely long resonators developed from non-absorbing materials. Despite the fact that presently there is no way of developing an infinite structure, scientists succeeded to get closer to this concept. Researchers were able to identify the resonator parameters which allow the light lifetime to be similar to one projected for an infinite model. This continues until the limitations linked to the absorption of the material and final structure come into force.

Our resonator does not allow to capture the light permanently as it does not provide an ideal interference. However, we can significantly suppress the energy leakage and therefore keep the light ten times more efficiently than conventional resonators of comparable sizes.

Dr. Mikhail Rybin, Senior Fellow at the Department of Dielectric and Semiconductor Photonics at ITMO University.

It could be possible to observe such an effect in a simple silicon cylinder with a specific ratio of the height to radius. The cylinder size will be able to reach 400 nm which is considered to be less than visible spectrum wavelengths. The authors highlight that the small size will be favorable for the development of miniature lasers on optical sensors and microchips with high precision.

For its functioning, a laser needs the light to be repeatedly passed through the same atom. The better the resonator is, the less light emitting atoms we need. The smaller it is, the more light sources we can place on an optical chip. In this way, a laser becomes more powerful, while technology of its construction becomes easier. The same applies to various antennas and sensors, among other applications, we consider frequency conversion processes and even night vision. We can cover glasses with a layer of such resonators making the world visible in the dark.

Yuri Kivshar, Research Director of the International Center for Nanophotonics and Metamaterials of ITMO University and Distinguished Professor of the Australian National University.

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