“Light offers a chance to revolutionise electronics. The electrons underlying the basis of this field of science encounter resistance as they move along a metal wire or a semiconductor layer. However, light can propagate through semiconductor crystals practically without losses, so it can be used instead of electrons to streamline logical operations,” says Dr. Jan Suffczyński from the Faculty of Physics, University of Warsaw, co-author of an article published in the prestigious journal Nanophotonics.

The presented results are essential for optical quantum computing and can contribute to creating ultrafast processors used in next-generation computers. Scientists from the Laboratory of Ultrafast Magnetospectroscopy (LUMS) at the Faculty of Physics of the University of Warsaw conducted an experiment in which they used a unique structure, thinner than a human hair, consisting of multi-layer mirrors with crystal layers between them. The research resulted in observing the phenomenon of polariton lasing and parametric scattering of exciton-polaritons in a system of coupled optical microcavities. These seemingly enigmatic issues can be presented in a very illustrative way.

“The operation of mirrors can be explained by the example of a piece of glass, which reflects some of the light but allows the rest to pass through. It is possible to create multiple layers of dozens of such pieces of glass. A small part of the light reflects from each surface between the successive layers, while the rest passes through them. However, if the thickness of the glass is such that the light waves reflected from each successive layer are in phase, these partial reflections will add up so that the entire wave is reflected. Finally, the light will not pass any further, and the multi-layer structure will act as a perfect mirror,” explains Krzysztof Sawicki, the article’s co-author, who will be defending his doctorate at the Faculty of Physics at the University of Warsaw in September. Until recently, this effect was used in dental lamps with a pink-green glow. A mirror mounted in the lamp blocked infrared light and protected the oral cavity from heat – the lamp was only supposed to illuminate it.

Light in microcavities

Scientists  from the Faculty of Physics UW placed a semiconductor crystal layer in the optical microcavity, i.e. in the space between two such mirrors. Then they added one more layer and another mirror, obtaining two coupled microcavities. The entire complex structure was made of cadmium, zinc, and magnesium tellurides in the MBE laboratory, headed by Dr. Wojciech Pacuski. It took over 12 hours to produce the structure, and the thickness of each layer was made with an accuracy of a few nanometres (billionths of a metre). Then, the crystal was illuminated by a laser beam. The crystal absorbed the light and became excited for a short time (some tens of picoseconds, i.e. for several tens of millionths of one-millionth of a second). The semiconductor released this energy by sending a photon that reflected off the mirror and returned to the crystal. The semiconductor captured the light again, releasing energy again.

“When a crystal is constantly exchanging energy a state called a exciton-polariton (or just polariton) emerges. It is a state of a superposition, which is partly the crystal excitation and partly the microcavity excitation,” explains Dr. Jan Suffczyński.

In their research, the scientists, by regulating the power of the laser illuminating the structure, influenced the amount of energy supplied to the microcavity and thus the number of polaritons created. A quantum effect called Bose-Einstein condensation and the associated polariton lasing phenomenon occurred when the polariton density exceeded a specific threshold value.

“This is the first observation of these effects in a system of two coupled optical microcavities. Polariton lasing is an emission with the characteristics of laser emission, but one of its specific properties is that it requires much less excitation power than in the case of a conventional laser,” stresses Krzysztof Sawicki.

Polariton billiards

Dr. Jan Suffczyński says that in the condensate, we can observe a phenomenon that in a simplified manner can be compared to a game of billiards. The condensate is formed by a high density of polaritons that can be imagined as stationary billiards balls. When parametric scattering occurs, two polaritons jump out of the stationary condensate and travel at precisely the same speed, but in opposite directions, i.e. two stationary objects suddenly change into two moving ones with equal and opposite momenta. In the type of scattering described in the article, polaritons do not change their energy, so the principle of conservation of both the energy and momentum are satisfied.

“Parametric scattering was obtained using non-resonant excitation, in which the excitation energy does not match any of the system’s energy levels. Non-resonant stimulation enables spectral separation of the signal from the excitation, which is a promising result from the point of view of practical applications in polaritonic devices, e.g. the fabrication of sources of entangled photons based on polaritons,” adds Krzysztof Sawicki.

It is worth emphasising that the Bose-Einstein condensation of the polaritons and the polariton lasing occur at the two lowest energy levels of a four-level system. This is a surprising result in the context of what has previously been observed in single microcavities, in which condensation took place in the system’s ground state. Emission dynamics measurements have shown that condensates of different energies share the same lasing threshold but do not appear simultaneously, i.e. they form and disappear one by one.

The non-linear effects in coupled optical microcavities described by physicists from the University of Warsaw will be applicable in optical transistors, among others, which will be used to build ultra-fast and highly energy-efficient processors used in new types of computers.