- authors in alphabetical order -

Chiral localisation and artificial magnetic fields in polariton lattices

Alberto Amo
Laboratoire PhLAM – CNRS – Université de Lille, France

Lattices of micropillars are ideal to implement elaborate lattice Hamiltonians for polaritons. Here we will show polariton lattices with exotic Dirac cones and artificial magnetic fields giving rise to photonic Landau levels [1]. The driven-dissipative nature of polaritons allows for the excitation of chiral localised modes whose existence is purely determined by the external driving conditions.

[1] O. Jamadi et al., Direct observation of photonic Landau levels and helical edge states in strained honeycomb lattices, ArXiv:2001.10395 (2020).

Non-equilibrium light-matter condensates

Hui Deng
Randall Lab, University of Michigan

Light-matter interactions are at the heart of quantum electrodynamics. Using the mature, III-Arsenide semiconductor system, we incorporate a designable photonic crystal mirror to control hybrid light-matter coupled modes (polaritons), and use it to study non-equilibrium quantum orders, including a Bardeen-Cooper-Schrieffer like polariton condensate and limit cycles self-oscillations in coupled polariton condensate. Extension to polaritons with unconventional dispersion and in larger lattices may enable the discovery of new many-body quantum states. Using two-dimensional van der Waals crystals with exceptionally strong light-matter interactions, we control the exciton-photon interactions from the incoherent limit to the coherent limit with simple mirrors and laser pulses, showing the promise of the system for novel photonic applications based on coherent light-matter interactions.

Andrew Houck
Princeton University, USA


Polariton-electron interactions in cavity-embedded van der Waals heterostructures

Atac Imamoglu
Institute of Quantum Electronics, ETH Zurich, Switzerland

Two dimensional materials provide new avenues for synthesizing compound quantum systems. Monolayers with vastly different electric, magnetic or optical properties can be combined in van der Waals heterostructures which ensure the emergence of new functionalities; arguably, the most spectacular example to date is the observation of strong correlations and low electron density superconductivity in moire superlattices obtained by stacking two monolayers with a finite twist angle. Optically active monolayers such as molybdenum diselenide provide a different "twist" as they allow for investigation of nonequilibrium dynamics in van der Waals heterostructures by means of femtosecond pump-probe measurements. Moreover, interactions between electrons and the elementary optical excitations such as excitons or polaritons, provide an ideal platform for investigation of quantum impurity physics, with possibilities to probe both Fermi- and Bose-polaron physics as well as mixtures with tunable density of degenerate fermions and bosons.

After introducing the framework we use to describe many-body optical excitations in van der Waals heterostructures, I will describe two recent developments in the field. The first experiment uses pump-probe measurements to demonstrate how exciton-electron interactions lead to strong enhancement of polariton-polariton interactions, as well as to optical gain by stimulated cooling of exciton-polaron-polaritons. The second experiment shows that a tri-layer system, consisting of two semiconducting monolayers separated by an insulating layer, provides an exciting platform for investigating strongly correlated electronic states in moire superlattices using optical spectroscopy.

Molecular strong coupling and novel structures for manipulating light

Stéphane Kéna-Cohen
Department of Engineering Physics, Polytechnique Montreal, Montreal, Canada

In semiconductor geometries that support discrete optical modes, exciton-polaritons form when the light-matter interaction rate exceeds the dissipation rate stemming from both the photon and matter components. In the first part of the talk, we will review earlier work from our group on the condensation of exciton-polaritons in organic microcavities. This phenomenon occurs at high excitation density, when the particle number is such that quantum degeneracy is reached. Molecular polariton condensates have a number of unique features stemming from coupling to molecular vibrations as well as the intrinsic nonlinearities of organic polaritons. We will then discuss the prospects for manipulating the properties of molecules themselves in the strong coupling regime. Here, dark states, which are collective excitations of the molecules that are weakly coupled to light play an essential role. With this in mind, we will review recent experiments on manipulating triplet dynamics as well as organic optoelectronic devices through the formation of polaritons. Finally, we will discuss novel structures designed in our group for manipulating light including dielectric antennas with field enhancements that rival those of their plasmonic counterparts.

Ultrastrong Light-Matter and Matter-Matter Coupling: Dicke Phenomena

Junichiro Kono
Rice University, Houston, Texas, USA

Recent experiments have demonstrated that light and matter can mix together to an extreme degree, and previously uncharted regimes of light-matter interactions are currently being explored in a variety of settings, where new phenomena emerge through the breakdown of the rotating wave approximation [1]. This talk will summarize a series of experiments we have performed in such regimes. We will first describe our observation of ultrastrong light-matter coupling in a two-dimensional electron gas in a high-Q terahertz cavity in a quantizing magnetic field, demonstrating a record-high cooperativity [2]. The electron cyclotron resonance peak exhibited splitting into the lower and upper polariton branches with a magnitude that is proportional to the square-root of the electron density, a hallmark of cooperative vacuum Rabi splitting, known as Dicke cooperativity. Additionally, we have obtained clear and definitive evidence for the vacuum Bloch-Siegert shift [3], a signature of the breakdown of the rotating-wave approximation. The second part of this talk will present microcavity exciton polaritons in a thin film of aligned carbon nanotubes [4] embedded in a Fabry-Pérot cavity. This system exhibited cooperative ultrastrong light-matter coupling with unusual continuous controllability over the coupling strength through polarization rotation [5]. Finally, we have generalized the concept of Dicke cooperativity to demonstrate that it also occurs in a magnetic solid in the form of matter-matter interaction [6]. Specifically, the exchange interaction of N paramagnetic erbium(III) (Er3+) spins with an iron(III) (Fe3+) magnon field in erbium orthoferrite (ErFeO3) exhibited a vacuum Rabi splitting whose magnitude is proportional to N1/2. Our results provide a route for understanding, controlling, and predicting novel phases of condensed matter using concepts and tools available in quantum optics, opening up exciting possibilities to combine the traditional disciplines of many-body condensed matter physics and cavity-based quantum optics.

[1] P. Forn-Díaz, L. Lamata, E. Rico, J. Kono, and E. Solano, Reviews of Modern Physics 91, 025005 (2019).
[2] Q. Zhang et al., Nature Physics 12, 1005 (2016).
[3] X. Li et al., Nature Photonics 12, 324 (2018).
[4] X. He et al., Nature Nanotechnology 11, 633 (2016).
[5] W. Gao et al., Nature Photonics 12, 362 (2018).
[6] X. Li et al., Science 361, 794 (2018).

Many-body cavity QED with multimode resonators

Benjamin Lev
Departments of Physics and Applied Physics, Stanford University, USA

We will present our first experiments involving multimode cavity QED, wherein the cavity-mediated atom-atom interaction is dramatically modified compared to that in a single-mode cavity. We demonstrate the engineering of strong, tunable-range, and sign-changing photon-mediated interactions between atoms and between atomic spins. Other results include the observation of a 'spinor density-wave polariton' condensate and cavity-assisted dynamical spin-orbit coupling. Together, these results pave the way for future experimental access to nontrivial quantum many-body phases and transitions in driven-dissipative quantum systems, synthetic dynamical gauge fields, and the ability to study nonequilibrium spin glasses and implement quantum neural networks such as associative memories.

Non-equilibrium light-matter condensates

Peter B. Littlewood
James Franck Institute, University of Chicago, USA

Engineering of optical microcavities makes use of the mixing of electronic excitations with photons to create a composite boson called a polariton that has a very light mass, and experiments provide good evidence for a high-temperature near-equilibrium Bose condensate. Polariton systems also offer an opportunity to use optical pumping to study quantum dynamics of a many body system outside equilibrium, in a new kind of cold atom laboratory. We suggest that a new kind of dynamical phase transition is available in these two-component condensates, associated with the presence of a many-body exceptional point that has degenerate real eigenvalues corresponding to coalescing solutions – a dynamical-systems equivalent to a critical point of a conventional phase transition. We think that this model is generalizable to non-reciprocal models of classical dynamics, being a new source of complexity in active matter systems.

Manipulating molecules with strong light-matter coupling

Andrew J. Musser
Department of Chemistry & Chemical Biology, Cornell University, USA

The interaction of organic semiconductors with confined light fields offers one of the easiest means to tune their material properties. In the regime of strong light-matter coupling, semiconductor exciton and cavity photon states hybridize to form exciton-polaritons. In organic systems these light-matter hybrids are tuneably separated by as much as 100’s of meV from the parent exciton, enabling radical alteration of the energetic landscape. The effects of strong coupling can be profound, with reports of long-range energy transfer, enhanced carrier mobility and altered chemical reactivity. Theoretical work is increasingly focused on the potential of polaritons to manipulate electronic dynamics in the excited state, but experimental realisation has proved challenging. We have recently demonstrated the ability to manipulate triplet photophysics in singlet exciton fission materials in the strong coupling regime. Within microcavities, we dramatically enhance the emission lifetime and increase delayed fluorescence by >100%, which we attribute of direct transfer from a reservoir of fully ‘dark’ triplet pairs to the bright polaritons – providing new insights into the interactions between polaritons and dark electronic states. Indeed, with this approach we can create entirely new radiative pathways, turning completely dark states bright and opening new scope for microcavity-controlled materials.

Polaritons for neuromorphic computing

Daniele Sanvitto
CNR – NANOTEC, Institute of Nanotechnology, Lecce, Italy

Exciton-polaritons, mixed states of photons and excitons, have attracted a great deal of interest both from a fundamental point of view, with the observation of quantum macroscopic phenomena, and, given the possibilities they can offer, for the realisation of all-optical devices with limitless advantages in terms of energy consumption, dissipation-less operation and high clock frequencies [1].

After showing some of the most intriguing characteristics of polaritons in semiconductor microcavities, we will focus on the use of polariton systems as semiconductor-based platforms for the realisation of an image recognition system based on a reservoir computing array of polariton nodes [2].

We have studied several schemes to best exploiting the strong polariton nonlinearities in a network of almost degenerate polariton states. We used the MNIST database of handwritten numbers as a benchmark to test the efficiency of the network against the number of training dataset as well as the network dimension.

Using quasi-resonant excitation schemes, we obtained extremely unexpected and startling results. Compared to previous works on hardware implementation of neuronal network schemes we could show a higher success rate in a system that offers the fastest computational speeds. Moreover, despite a smaller set of training data, such an exciton-polariton-based platform demonstrated to outperform even linear classification algorithms working with the full MNIST database.

[1]. D. Sanvitto, & S. Kena-Cohen, “The road towards polaritonic devices”. Nat. Mater. 15, 1061–1073 (2016).
[2]. D. Ballarini, A. Gianfrate, R. Panico, A. Opala, S. Ghosh, L. Dominici, V. Ardizzone, M. De Giorgi, G. Lerario, G. Gigli, T.C.H. Liew, M. Matuszewski, D. Sanvitto “Polaritonic neuromorphic computing outperforms linear classifiers”. arXiv:1911.02923 (2019).

Bose-Einstein condensation and stimulated thermalization of polaritons in plasmonic lattices

Päivi Törmä
Department of Applied Physics, Aalto University, Finland

Bose-Einstein condensation has been realized for various particles or quasi-particles, such as atoms, molecules, photons, magnons and semiconductor exciton polaritons. We have experimentally realized a new type of condensate: a BEC of hybrids of surface plasmons and light in a nanoparticle array [1]. The condensate forms at room temperature and shows ultrafast dynamics. We utilized a special measurement technique, based on formation of the condensate under propagation of the plasmonic excitations, to monitor the sub-picosecond thermalization dynamics of the system. Recently, we have achieved such Bose-Einstein condensation also at the strong coupling regime, and shown by varying the lattice size that the thermalization in these systems is a simulated process that occurs in 100 femtosecond scale [2]. This new platform is ideal for studies of differences and connections between BEC and lasing [3,4,5]. While usually lasing in nanoparticle arrays occurs at the centre of the Brillouin zone, we have now demonstrated lasing also at the K-point [6]. The lasing mode can be identified with the help of group theory. Clear lasing is observed despite a narrow band gap at the K-point, which is promising considering future studies of topological photonics.

[1] T.K. Hakala, A.J. Moilanen, A.I. Väkeväinen, R. Guo, J.-P. Martikainen, K.S. Daskalakis, H.T. Rekola, A. Julku, P. Törmä, Nature Physics, 2018, 14, 739.
[2] A.I. Väkeväinen, A.J. Moilanen, M. Necada, T.K. Hakala, K.S. Daskalakis, P. Törmä, arxiv:1905.07609, 2019.
[3] T.K. Hakala, H.T. Rekola, A.I. Väkeväinen, J.-P. Martikainen, M. Necada, A.J. Moilanen, P. Törmä, Nature Communications, 2017, 8, 13687.
[4] H.T. Rekola, T.K. Hakala, and P. Törmä, ACS Photonics 2018, 5, 1822.
[5] K.S. Daskalakis, A.I. Väkeväinen, J.-P. Martikainen, T.K. Hakala, P. Törmä, Nano Letters, 2018, 18, 2658.
[6] R. Guo, M. Necada, T.K. Hakala, A.I. Väkeväinen, P. Törmä, Physical Review Letters, 2019, 122, 013901.