A laser pulse is much like a coloured drop of light which travels undisturbed in the vacuum. Photonic microcavities can be designed to trap such a drop of photons and make them to resonate with the electronic oscillations of a very thin semiconductor, in order to instantaneously create upon the drop entrance a mixed fluid of light and matter, bearing the speed of the photons and the interactions typical of electrons. Such a hybrid fluid has both the features of a classical liquid, as the ones we are used to, and at the same time follows the laws of the quantum world.

Such experiments have been published in the Nature Communications, after their implementation in the laboratories at the Institute for Nanotechnology of the Italian National Council of Researches (CNR Nanotec) based in the southern Italy (city Lecce).

“Among the fascinating properties of the interacting fluids of light and matter, composed of particles known as polaritons, there are the high coherence, the ultrafast response times and the strong non-linear interactions. Such interactions make possible for example the transformation of the fluid into a Bose-Einstein condensate, known also as the ‘fifth state of matter’, a single collective wave state where all the particles oscillate synchronously”,

explains Lorenzo Dominici researcher at CNR Nanotec, to the Science News Journal, who lead the experiment adding:

“We have observed a very intriguing and unexpected effect: the implosion of the two-dimensional fluid in a central point, many times denser and thinner than the drop initially created by the laser pulse, together with the external irradiation of ring-waves. It resembles a collapsing singularity or a condensation in real-space. We like to refer to this new phenomenology as the ‘polariton backjet’ “.

Daniele Sanvitto, coordinator of the experimental team at CNR Nanotec Lecce, points out that:

“What we have observed is surprisingly similar to the liquid backjet which is created upon throwing a drop or a small rock onto a water surface, but different since it is happening on the scale of thousandths of millimeters and in the time of billionths of a billionth of a second, involving an hybrid quantum fluid of light and electrons.”

Fabrice Laussy, Ramón Y Cayal researcher leading the group of Quantum Polaritonics at the Universidad Autónoma de Madrid, theoretically investigated the ‘polariton backjet’ effect, confirming that:

“We are looking at a spectacular phenomenology observed within a polariton bosonic condensate. Even though these quasi-particles have repulsive interactions and it would be expected for the inital drop to expand all around, they draw closer together under the sudden quench that follows a femtosecond pulse”.

Prof. Alexey Kavokin, the renamed scientist who theoretically explained the effect, who is the co-founder and Scientific Director of the Mediterranean Institute of Fundamental Physics, says that:

“The self-trapping of a polariton condensate is a smoking gun for formation of collective bosonic polaron state, that may be used in future quantum simulators based on lattices of bosonic condensates of exciton-polaritons”.

The extreme localization and enhancement of the polaritonic field, explain the researchers in Lecce, is very interesting in view of possible applications. Daniele Sanvitto adds:

“Some examples are represented by the new kind of so-called polariton lasers, or the use as sub-micrometric light emitting pixel for high-resolution displays. The ultrafast response time makes the polariton fluids of light and matter very appealing also in the field of optical computation and memories”.

The work involved the international collaborations with theoretical groups of different countries such as the Universidad Autonoma de Madrid in Spain, Saint Petersburg State University in Russia, University of Southampton in the UK, the Institute of Physics of the Polish Academy of Sciences and the SPIN-CNR institute in Roma.

Object: Dynamical study of a fluid of light and matter suddenly created in a regime of high density.

‘Real-space collapse of a polariton condensate‘, L. Dominici, M. Petrov, M. Matuszewski, D. Ballarini, M. De Giorgi, D. Colas, E. Cancellieri, B. Silva Fernandez, A. Bramati, G. Gigli, A. Kavokin, F. Laussy, D. Sanvitto. Nature Communications 6, 8993 (2015)