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...
Ultrastrong light-matter coupling allows the exploration of new states of matter through the interaction of strong vacuum fields with huge electronic dipoles. By using hybrid dipole antenna-split ring resonator-based cavities with extremely small effective mode volumes V/λ ≃ 6 × 10 and surfaces S/λ ≃ 3.5 × 10, we probe the ultrastrong light-matter coupling at 300 GHz to less than 100 electrons located in the last occupied Landau level of a high mobility two-dimensional electron gas, measuring a normalized coupling ratio of Ω/ω = 0.36. Effects of the extremely reduced cavity dimensions are observed as the light-matter coupled system is better described by an effective mass heavier than the uncoupled one. These results open the way to ultrastrong coupling at the single-electron level in two-dimensional electron systems.
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.
Spin-split heavy-hole gases in strained germanium quantum wells were characterized by polarisationresolved terahertz time-domain spectroscopy. Effective masses, carrier densities, g-factors, transport lifetimes, mobilities and Rashba spin-splitting energies were evaluated, giving quantitative insights into the influence of strain. The Rashba coefficient was found to lower for samples with higher biaxial compressive strain, while heavy-hole mobilities were enhanced to over1.5 10 6 cm 2 V −1 s −1 at 3 K. This high mobility enabled the observation of the optical quantum Hall effect at terahertz frequencies for spin-split two-dimensional heavy-holes, evidenced as plateaux in the transverse magnetoconductivity at even and odd filling factors.
Extraordinary optical transmission is observed due to the excitation of surface plasmon polaritons in 2-dimensional hexagonal anti-dot patterns of pure Ni thin films, grown on sapphire substrates. A strong enhancement of the polar Kerr rotation is recorded at the surface plasmon related transmission maximum. Angular resolved reflectivity measurements under an applied field reveal an enhancement and a shift of the normalized reflectivity difference upon reversal of the magnetic saturation (transverse magneto-optical Kerr effect-TMOKE). The change of the TMOKE signal clearly shows the magnetic field modulation of the dispersion relation of SPPs launched in a 2D patterned ferromagnetic Ni film. Magneto-plasmonics offer unique possibilities to manipulate light by the use of external magnetic fields. [1][2][3][4] The prevailing choice of materials for fabrication of magnetoplasmonic structures has been combined structures of noble and magnetic metals/dielectrics, such as Au and Co/Iron garnet. 1,5,6 The basic idea behind this choice is the combination of the large plasmon activity of noble metals with the magnetic functionality provided by the additional materials. Another reason for the use of noble metals is the excellent resistance to oxidation, which is required to obtain durable patterned thin films. Ni is an interesting candidate in this context as it forms a thin and self-passivating oxide layer (approximately 1 nm). 7,8 Furthermore, the magneto-optical activity of Ni-based nano-patterns can be enhanced by the presence of surface plasmon polaritons (SPPs). [9][10][11][12][13] The magnetic field can provide the means for control of SPPs, as it has been predicted for noble metals, 14 and explored experimentally in hybrid structures. 2,5,6 Early studies on this effect were targeted towards semiconductor-based SPPs (Ref. 15) but not in metallic systems, where high magnetic fields are required. 16 In pure magnetic materials, the need for high fields is not present as the magneto-optical effects are sufficiently strong.In this Letter, we discuss the influence of an external magnetic field on the SPPs for the case of a pure magnetic metal, such as Ni, patterned in two-dimensions (2D) on a transparent substrate. We examine to what extent the ferromagnetic Ni can be used as a host material for SPPs. We show that the magnetic field induces a modulation of the dispersion of SPPs excitation in Ni.A Ni anti-dot sample was prepared on a double side polished Al 2 O 3 ½11 20 substrate. The patterning was accomplished by the use of self-organization of colloidal polystyrene beads as shadow masks. 9 A 30 nm thick Ni film was deposited on the masked sapphire substrate, using electron-beam evaporation. A snapshot of the procedure is illustrated in Fig. 1, where both the shadow mask and the resulting holes are clearly seen. This process resulted in a well defined Ni layer, decorated by holes of a diameter d ¼ 300 nm, spaced on an hexagonal lattice of periodicity of a ¼ 450 nm. The ratio of the radius to pitch size was determined t...
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