Observing strongly-coupled Mie polaritons using water droplets

Mie theory, the analytical solution for electromagnetic wave scattering off a spherical particle, provides a powerful approach for understanding scattering spectra in terms of different multipole resonances. While the assumption of spherical symmetry is often merely an approximation, Mie theory can nevertheless give useful insights in more realistic settings such as resonances of cylindrical high refractive index nanopillars.

One setting where spherical scatterers arise quite naturally is in liquids with high surface tension, which promotes the formation of spherical droplets. Remarkably, for the case of water droplets with radii of a few microns, the Mie resonances coincide with the infrared stretching and bending vibrational resonances of the H2O molecule! This leads to strong coupling between electromagnetic and vibrational degrees of freedom leading to the formation of polaritons, as reported in recent work published in Physical Review Letters: Self-Hybridized Vibrational-Mie Polaritons in Water Droplets.

Observing the key signature of strong coupling - Rabi splitting between upper and lower polariton resonances (corresponding to electromagnetic and vibrational oscillations being in or out of phase) - using water droplets is complicated by the non-uniform droplet sizes. Thus, the measured scattering spectrum involved not just a few resonances at specific frequencies, but a distribution of different resonance frequencies dependent on the particles' sizes.

To overcome this, the authors of the study also measured the scattering spectra of droplets of heavy water, where the vibrational modes become red-shifted due to the increased mass of the deuterium atoms. The authors observed that the absorption peaks associated with the strong coupling between vibrational and electromagnetic resonances are also red-shifted.

In addition to applications to the spectra of water droplets in the atmosphere, it will be interesting to explore similar strong coupling phenomena in other high surface tension liquids and applications to polariton chemistry, whereby strong coupling between electromagnetic and molecular degrees of freedom shows promise as a means of controlling rates of chemical reactions.

International Workshop on Polaritons in Emerging Materials

Last week I had the pleasure of attending an IBS PCS International Workshop on Polaritons in Emerging Materials held in Daejeon, Korea. Smaller workshops such a this one (~40 participants) with a more relaxed schedule (40 minutes per speaker and generous coffee/lunch breaks) are great for getting a more in-depth and candid picture of an unfamiliar research field!

One of the hot topics was polaritons in transition metal dichalcogenides - a rapidly-maturing family of two-dimensional graphene-inspired materials. Prof. Myung-Ki Kim from Korea University talked about plasmon resonances in multi-layer TMDs, Prof. Deep Jariwala from the University of Pennsylvania presented experiments with cavity-free polaritonic structures. The high refractive index of 2D materials such as molybdenum disulphide means that they can already exhibit strong light-matter coupling without requiring embedding in a microcavity. Thanks to the different localization of the photonic and electronic degrees of freedom one can form ultra-thin multilayer structures either as thin sheets of the 2D material (with the thickness controlling the electronic band structure), or as lattices formed by multiple non-interacting single sheets. Prof. Su-Hyun Gong (Korea University) presented waveguides based on multilayer tungsten disulphide can achieve tight (nanoscale) light confinement with lower losses compared to conventional plasonic materials such as gold. Expect to see many more works in this area as high-quality and large-area samples of these exotic materials start to become commercially available.

Another active area was optically-driven rotation and localization of exciton-polariton condensates. Dr. Michael Fraser (RIKEN) presented experiments in which a condensate is stirred via incoherent pumping with two slightly-detuned Laguerre-Gaussian layer beams, leading to an asymmetric reservoir density that undergoes a rotation, producing condensates with vortices. Theoretical analyses of vortex generation and turbulence in stirred exciton-polaritons were presented by Dr. Alexey Yulin (ITMO) and Dr. Helgi Sigurðsson (Warsaw), and Dr. Sergei Koniakhin (IBS PCS). Prof. Alberto Amo (Lille) showed that the dynamics of resonantly-driven condensates in lossy lattices can be remarkably counterintuitive - the strongest localization occurs between the pumped sites, not at them!

While not the main theme of the workshop, topological photonics was represented in talks by Profs. Sven Höfling, Sebastian Klembt (both from Würzburg University), Dr. Xingran Xu (NTU), who focused on lasing and non-Hermitian topological phenomena, and Dr. Alexander Cerjan (Sandia National Labs), who showed how real-space topological markers can be used to quantify the robustness of nonlinear topological edge states.

Prof. Fabrice Laussey (Wolverhampton) gave a captivating talk on quantum light and the importance of taking detector bandwidth into account when modelling quantum light sources. Since quantum light is so weak, signals measured using a finite bandwidth filter will inevitably be dominated by the tails of the much stronger pump beam unless homodyne detection is used. Look for quantum correlations in the spectral minima, not the dips! In related talks, Prof. Andrey Moskalenko (KAIST) analyzed entanglement between cavities generated by coherent optical driving, and Prof. Hyang-Tag Lim (KIST) covered experimental generation of multi-mode N00N states.

Most of the talks should become available to watch on the PCS Youtube account at some point.

PhD position openings in topological photonics and turbulence in fluids of light

Two ERC-funded PhD position openings in the group of Alberto Amo in Lille, France on the following subjects:

1. Nonlinear topological phases in lattices of coupled micropillars (experimental)

The goal of this thesis is to study experimentally novel topological phases in photonic lattices. We will employ coupled semiconductor micropillars in which photons can hop from site to site. Taking advantage of the extraordinary photon-photon interactions in this system we aim at observing nonlinear topological effects. This PhD thesis is part of the ERC Consolidator grant EmergenTopo, and will be realized in collaboration with French and international theoreticians.

2. Turbulence in polariton fluids (experimental).

The goal of this thesis is to experimentally investigate the turbulent properties of a polariton fluid when passing and obstacle engineered in the microcavity. In a polariton superfluid, not only friction is strongly reduced, but vortices have very special properties: their circulation is quantised and it can only take integer values. Up to now, the main drawback to study turbulent phenomena has been the need of ultrafast single shot measurements, with time resolution in the picosecond scale. Using cutting-edge time resolved spectroscopic techniques developed in our group, we will study the emergence of vortices and other turbulent phenomena in a polariton fluid. We will investigate turbulence in obstacles of different shapes, including obstacles with the form of a plane wing to answer the question of whether a plane can fly in a superfluid of light.

Application deadline: 10th May 2022
Starting date: 1st October 2022 (negotiable)