Russian and Danish researchers have made a first-ever experimental observation of a plasmon nanojet. This physical phenomenon enables nanoscale focusing of light and, theoretically, allows engineers to bypass one of the fundamental limitations of the ordinary converging lens. Tight compression of light waves is necessary to use them as signal carriers in compact devices that would work much faster than today’s electronics. The study is published in the journal Optics Letters.
Before laser pointers became available, the heroes of romance novels had to make do with small rocks they would throw into a beloved’s window to indicate their presence. Among the numerous drawbacks of rocks as signal carriers is their mass, which means sending a message requires an effort and time. While the electron does not weigh as much as a rock, it still cannot be put in motion instantaneously. If we could replace the electrons in microcircuits with photons — the massless particles of light — the resulting devices would operate much faster.
What prevents engineers from abandoning electronic chips in favor of their photonic analogues is the need for miniaturization. With today’s technology, such optical devices would have an enormous size. To make them smaller, engineers require a way to control photons on such a small scale that the light wave itself has to be localized, squeezed into a minimum space. Ideally, the light needs to be focused into a spot smaller than 50% of the original wavelength. While this feat is impossible in the classical optics due to what’s known as the diffraction limit, modern research has already found several ways around it. And the newly observed plasmon nanojet is likely to become one of them.
A team of Russian and Danish physicists has created a focusing component, or nanolens, capable of converting light into electromagnetic waves of a special kind, compressing it to 60% of the initial radiation wavelength. This new contraption is made up of a square piece of dielectric material 5 by 5 micrometers in size and 0.25 micrometers thick.
Illuminating the grating in the gold film with a laser generates excitations known as surface plasmon polaritons(SPP), which travel along the metal’s surface. These SPPs are essentially two kinds of waves coupled to each other and propagating together. First, there’s the collective oscillation of electrons in gold — the plasmon part — and then there’s also a surface light wave called a polariton. The point of converting light to SPPs is that there are ways to focus them to a greater extent than the initial laser pulse.
The study has demonstrated a new and efficient mechanism for strongly localizing radiation and manipulating it on the nanoscale, which is a prerequisite for densely packing optical components in photonic and plasmonic devices that would operate much faster than conventional electronics.
News Source: MIPT