Novel quasi-particles, so-called polaritons, can be formed in an optically active semiconductor material due to mixing of photons and excitons (an exciton is an analogue of hydrogen in condensed matter). While two photons colliding in free space do not interact, polaritons strongly repel due to the exciton component in their wavefunctions. The Sheffield group has demonstrated that polariton-polariton interactions are several orders of magnitude stronger than effective photon-photon interactions in any other ultrafast photonic materials, where light is weakly coupled to matter. The efficient polariton-polariton scattering can be utilised for development of novel ultrafast light sources, frequency mixers and converters as well as scalable and compact devices performing control of light by light on a very fast timescale and at very low signal intensities (potentially at a single photon level). Potentially, this may have a strong impact on development of novel photonic signal processing and quantum technology hardware. The polariton platform is also well suited to the study of fundamental many-body phenomena ranging from Bose-Einstein condensation, superfluidity and solitons to quantum correlated phases in important physical systems, such as photonic analogues of topological insulators or quantum Hall systems.
So-far phenomena due to polariton interactions have been explored in GaAs microresonators only at 4-50 K. Our proposal capitalises on the recent demonstration of ultraviolet polaritons in waveguides based on AlGaN/GaN material, where polaritons are robust at 300 K given the large exciton binding energy and highly efficient exciton-photon coupling; this provides an opportunity to explore the novel room temperature physics of interacting polaritons and to bring polariton applications to reality.
In order to explore the fundamental physics of interacting GaN polaritons we will address polariton solitons, i.e non-spreading wavepackets stabilised by the nonlinearity. These interactions depend on many factors, such as the exciton Bohr radius, binding energy and the exciton fraction in the polariton wavefunction. A weak light pulse propagating in a polariton waveguide broadens with time, as different frequency components propagate with different velocities. By contrast, by increasing the pulse intensity polariton interactions are expected to cancel the spreading leading to formation of a soliton. Given the giant polariton nonlinearities solitons are expected to form at ultra-low thresholds and on a very short length-scale of ~10 micrometers.
Another advantage of GaN-based waveguides over GaAs counterparts is that exciton-photon hybridisation occurs over a broad range of frequencies enabling polariton-polariton scattering from a spectrally narrow pulse to a broad quasi-continuum of states. As a result very short UV polariton soliton pulses with a duration down to 10's femtoseconds are anticipated. The outcome of this research would also enable realisation of novel ultraviolet broadband pulsed sources and frequency converters operating at very low thresholds, which are important for many spectroscopy applications in biophotonics and molecular photochemistry.
Finally, we will demonstrate a prototype of a compact ultrafast all-optical polariton switch by exploiting nonlinear interactions between the polariton pulses with different central frequencies propagating in a photonic circuit based on coupled waveguides. These interactions induce nonlinear phase shifts in the optical signals enabling routing and switching of the ultrafast optical pulses. Given the very fast response of polariton system such a switch is expected to operate at THz rates. We note that exciton-polaritons in GaN-based nanostructures are also very robust against screening by hot electron-hole carriers, enabling study of the amplification of propagating polaritons in the presence of optical gain. This is essential for scalability of polariton devices.