The problem being investigated---Intense light beams propagating in nonlinear defocusing media behave identically to fluids. The governing evolution equations for small oscillations on top of the light beam are indeed the same as those for waves propagating on the surface of a fluid. This raises the fascinating perspective of studying fluid dynamics using light. These fluids made of light are referred to as `photon fluids' and have been shown to exhibit remarkable properties such as superfluidity and condensation (similar to Bose-Einstein condensation (BEC)). In this project we used photon fluids and gasses for a completely new and exciting application, namely for the creation of artificial flowing spacetime geometries otherwise thought to be the object of more complex and less (experimentally) accessible theories such as event horizons and expanding spacetimes.
The advantage of photon fluids over real fluids in this context is related to the great precision with which the fluid flow is determined by controlling the spatial phase profile of the laser beam. This allows us for example to also include angular momentum in our black holes, something that has never been done before. In essence, we study photon fluids with vorticity and look for novel effects including so-called superradiant amplification at the expense of the energy of the black hole. This effect has been predicted as a truly efficient mechanism by which energy may be extracted from a black hole yet has only very recently attracted experimental attentions.
This Project has developed these ideas and accounted for the full experimental complexity, thus paving the way for experiments that will also be carried out at the host institution.
These studies have also bridged the gap between BEC physics and photon fluid physics and in particular has focused on the presence of superfluidity within the photon fluid and novel methods for detecting related effects such as frictionless flow around obstacles.
Finally, we developed a quantised model of optical/ matter perturbations including dispersion and absorption applicable for a wide range of systems including for photon fluids, paving the way for the study of true quantum effects in these artificial fluids.
These studies have on the one the hand pushed the boundaries of general relativity applied to condensed matter systems and on the other paved the way for room-temperature superfluid physics.
The major scientific objectives were as follows:
-To understand if and under which conditions the photon fluid configurations which possess the necessary features for superradiant scattering (the analogue of an `ergo region' from gravitational terminology) are possible.
-To instigate a study of the elementary excitations on top of such a background flow, their dispersive and non-local dynamics and the possibility that `photonic sound waves' might be capable of being superradiantly scattered.
-Attain a detailed and working understanding/knowledge of the current experimental situation as pertains to the production and control of optical configurations necessary for the simulation of superradiance.
-Appropriately modify the current theoretical models for superradiance in order to describe experimentally feasible scenarios and conditions.
-Describe the optimal feasible experimental conditions required to sustain and observe other new physics predicted by the new analytic models.
-Formulate a workable quantum theory of elementary optical-matter excitations in the presence of dispersion and absorption.
All these objectives were achieved during the Project.
Importance of this work---The group of Prof. Faccio at HWU are in the process of building a world-wide centre of excellence for QFT analogues in Europe. The main challenge in this field is precisely the difficulty in bringing together state of the art QFT expertise with state of the art experiments and, most importantly, building a common knowledge base and language. This project therefore represented a unique opportunity to create a research environment that is missing in other groups worldwide and will be unique to this European group. As part of the scheduled research activities associated with this Project new technologies were required to manipulate and model optical systems in new ways and under new conditions that have not been achieved before due to their `extra-optical' underlying motivations. There is a possibility that such activities will result in patentable technologies in the future with the potential for wider implications in European society.
