Hybrid excitations, called polaritons, emerge in systems with strong light-matter coupling. Usually, they dominate the linear and nonlinear optical properties with applications in quantum optics. Here, we show the crucial role of the electronic component of polaritons in the magnetotransport of a cavity-embedded 2D electron gas in the ultrastrong coupling regime. We show that the linear dc resistivity is significantly modified by the coupling to the cavity even without external irradiation. Our observations confirm recent predictions of vacuum-induced modification of the resistivity. Furthermore, photo-assisted transport in presence of a weak irradiation field at sub-THz frequencies highlights the different roles of localized and delocalized states.The strong light-matter coupling regime [1, 2] is realized when the coupling Ω between photons and a material's excitation of frequency ω exceeds the losses γ tot of both components. An especially interesting situation is attained when quantum fluctuations of the electromagnetic field ground state give rise to the so-called vacuum Rabi splitting of the cavity polaritons. Solid-state systems [3][4][5] have recently proven to be instrumental in achieving the ultimate limit of this kind of coupling. The ultrastrong coupling regime [6-20], realized in the limit of Ω/ω 0.1, exploits the collective nature of the matter excitations [6,21,22] to achieve a peculiar situation where the ground state of the system is constituted by non-trivial quantum vacua [6].The (ultra-)strong coupling regime has so far mostly been investigated by interrogating the photonic component of the polariton quasi-particle weakly probing the coupled system with low photon fluxes [1-5, 7, 9, 10, 13-17, 19, 23-25]. Notable exceptions have been the measurements of the matter part of an exciton polariton condensate with an excitonic 1s-2p transition[26] and a transport experiment in molecules coupled to a plasmonic resonance [18].Recently we pioneered a new experimental platform, the Landau polaritons, to study ultrastrong light matter interactions [15,27] allowing to reach record-high nor-malized light-matter coupling ratios Ω/ω cav > 1 [28]. The inter-Landau level (cyclotron) transition ω c = eB m * (m * : effective electron mass) of a two-dimensional electron gas (2DEG) under strong magnetic field is coupled to a complementary electronic LC resonator [29] at frequencies of 100's of GHz, which plays effectively the role of the optical cavity. This system is especially well suited to study the matter part of ultrastrongly coupled polaritons using low temperature magneto-transport.It was recently proposed theoretically [30] that such transport is actually driven by the bright polariton operator, i.e. the same operator driving the optical response.Here we find experimental evidence consistent with this picture, in which most tellingly the longitudinal resistivity ρ xx bears the signatures of the polariton branches.Further confirmation for polaritonic effects acting on magneto-transport is obtained by observ...

Strong coupling plays a significant role in influencing chemical reactions and tuning material properties by modifying the energy landscapes of the systems. Here we study the effect of vibrational strong coupling (VSC) on supramolecular organization. For this purpose, a rigid-rod conjugated polymer known to form gels was strongly coupled together with its solvent in a microfluidic IR Fabry-Perot cavity. Absorption and fluorescence studies indicate a large modification of the self-assembly under such cooperative VSC. Electron microscopy confirms that in this case, the supramolecular morphology is totally different from that observed in the absence of strong coupling. In addition, the self-assembly kinetics are altered and depend on the solvent vibration under VSC. The results are compared to kinetic isotope effects on the self-assembly to help clarify the role of different parameters under strong coupling. These findings indicate that VSC is a valuable new tool for controlling supramolecular assemblies with broad implications for the molecular and material sciences.

The authors study the interaction of complementary terahertz (THz) split ring resonators with THz surface plasmon polaritons (SPPs) as a function of the meta-atom distance. The THz transmission properties of 15 samples for which the array dimensions are varied keeping the resonator shape constant are investigated. The linewidth of the inductive-capacitive (LC-)resonance is decreasing with increasing meta-atom distance, up to the frequency matching with the first SPP-mode. The SPP-mode couples to the narrow LC-resonance leading to an anti-crossing of the modes. In contrast, the narrow SPP-mode tunes across the broader dipole-like mode in orthogonal polarization. The excitation direction of the SPP-mode is found to lie along the electric field polarization of the THz. www.advopticalmat.de Figure 5. a) Normalized transmission spectra as a function of the lattice constant with a x = a y for the broad dipole-mode excited in x-direction. The calculated SPP-modes that cross the broad dipole-mode are traced in blue solid lines after Equation (2). Black sections denote not sampled lattice constants. The normalized transmission of the dipole-mode as function of the frequency is shown for square lattices in b) for a x = a y = 70 µm and c) for a x = a y = 85 µm. Transmission spectra of rectangular lattices are shown for d) a x = 70 µm and a y = 85 µm and for e) a x = 85 µm and a y = 70 µm.

The prospect of controlling the electronic properties of materials via the vacuum fields of cavity electromagnetic resonators is emerging as one of the frontiers of condensed matter physics. We found that the enhancement of vacuum field fluctuations in subwavelength split-ring resonators strongly affects one of the most paradigmatic quantum protectorates, the quantum Hall electron transport in high-mobility two-dimensional electron gases. The observed breakdown of the topological protection of the integer quantum Hall effect is interpreted in terms of a long-range cavity-mediated electron hopping where the anti-resonant terms of the light-matter coupling Hamiltonian develop into a finite resistivity induced by the vacuum fluctuations. Our experimental platform can be used for any two-dimensional material and provides a route to manipulate electron phases in matter by means of vacuum-field engineering.

The coupling between the optical mode of an LC cavity and a magnetoplasmon is studied by terahertz transmission spectroscopy. The magnetoplasmons are created by etching a high-mobility two-dimensional electron gas into stripes. As a result, we identify three different regimes, depending on the plasmon frequency relative to the cavity frequency. We find a significant coupling to the cyclotron dispersion even in presence of screening of the electric field by the plasmon.