High-angle optically accessible Brewster cavity exciton-polaritons

We report on the observation of the strong-coupling regime between quantum well excitons and a high incidence “Brewster cavity mode” previously identified as the generalized Brewster angle condition in multilayer structures [H. F. Mahlein, J. Opt. Soc. Am. 64, 647 (1974)]. This propagating mode is inside the light cone and therefore can be accessed from the top side of the sample without the need for prism or grating coupling methods. The observed anticrossing is clear evidence of the strong light-matter coupling regime. All the results are accurately reproduced by transfer matrix simulations. These results demonstrate the high potential of such structures for the study of propagating polaritons at high k, which could be harnessed for the realization of polaritonic circuit devices.Financial support from the bilateral Greece-Russia Polisimulator project cofinanced by Greece and the EU Regional Development Fund, Russian Science Foundation Grants No. 19-72-20120, UK EPSRC EP/L027151/1, EP/N016920/1 grants are acknowledged. The research was cofinanced by Greece and EU-ESF Fund through HRDELL 2014-2020 program (project MIS 5004464)

Measurement of local optomechanical properties of a direct bandgap 2D semiconductor

Strain engineering is a powerful tool for tuning physical properties of 2D materials, including monolayer transition metal dichalcogenides (TMDs)—direct bandgap semiconductors with strong excitonic response. Deformation of TMD monolayers allows inducing modulation of exciton potential and, ultimately, creating single-photon emitters at desired positions. The performance of such systems is critically dependent on the exciton energy profile and maximum possible exciton energy shift that can be achieved under local impact until the monolayer rupture. Here, we study the evolution of two-dimensional exciton energy profile induced in a MoSe2 monolayer under incremental local indentation until the rupture. We controllably stress the flake with an atomic force microscope tip and perform in situ spatiospectral mapping of the excitonic photoluminescence in the vicinity of the indentation point. In order to accurately fit the experimental data, we combine numerical simulations with a simple model of strain-induced modification of the local excitonic response and carefully account for the optical resolution of the setup. This allows us to extract deformation, strain, and exciton energy profiles obtained at each indentation depth. The maximum exciton energy shift induced by local deformation achieved at 300 nm indentation reaches the value of 36.5 meV and corresponds to 1.15% strain of the monolayer. Our approach is a powerful tool for in situ characterization of local optomechanical properties of 2D direct bandgap semiconductors with strong excitonic response

Graphene on hexagonal boron nitride as a tunable hyperbolic metamaterial

Hexagonal boron nitride (h-BN) is a natural hyperbolic material1, in which the dielectric constants are the same in the basal plane (ε[superscript t] ≡ ε[superscript x] = ε[superscript y]) but have opposite signs (ε[superscript t] ε[superscript z ]< 0) in the normal plane (ε[superscript z]). Owing to this property, finite-thickness slabs of h-BN act as multimode waveguides for the propagation of hyperbolic phonon polaritons—collective modes that originate from the coupling between photons and electric dipoles in phonons. However, control of these hyperbolic phonon polaritons modes has remained challenging, mostly because their electrodynamic properties are dictated by the crystal lattice of h-BN. Here we show, by direct nano-infrared imaging, that these hyperbolic polaritons can be effectively modulated in a van der Waals heterostructure composed of monolayer graphene on h-BN. Tunability originates from the hybridization of surface plasmon polaritons in graphene with hyperbolic phonon polaritons in h-BN so that the eigenmodes of the graphene/h-BN heterostructure are hyperbolic plasmon–phonon polaritons. The hyperbolic plasmon–phonon polaritons in graphene/h-BN suffer little from ohmic losses, making their propagation length 1.5–2.0 times greater than that of hyperbolic phonon polaritons in h-BN. The hyperbolic plasmon–phonon polaritons possess the combined virtues of surface plasmon polaritons in graphene and hyperbolic phonon polaritons in h-BN. Therefore, graphene/h-BN can be classified as an electromagnetic metamaterial as the resulting properties of these devices are not present in its constituent elements alone

Experimental demonstration of a transparent graphene millimetre wave absorber with 28% fractional bandwidth at 140 GHz

The development of transparent radio-frequency electronics has been limited, until recently, by the lack of suitable materials. Naturally thin and transparent graphene may lead to disruptive innovations in such applications. Here, we realize optically transparent broadband absorbers operating in the millimetre wave regime achieved by stacking graphene bearing quartz substrates on a ground plate. Broadband absorption is a result of mutually coupled Fabry-Perot resonators represented by each graphene-quartz substrate. An analytical model has been developed to predict the absorption performance and the angular dependence of the absorber. Using a repeated transfer-and-etch process, multilayer graphene was processed to control its surface resistivity. Millimetre wave reflectometer measurements of the stacked graphene-quartz absorbers demonstrated excellent broadband absorption of 90% with a 28% fractional bandwidth from 125-165 GHz. Our data suggests that the absorbers’ operation can also be extended to microwave and low-terahertz bands with negligible loss in performance

Tunable optical nonlinearity for transition metal dichalcogenide polaritons dressed by a Fermi sea

This is the final version. Available from the American Physical Society via the DOI in this record. We study the system of a transition metal dichalcogenide (TMD) monolayer placed in an optical resonator, where the strong light-matter coupling between excitons and photons is achieved. We present a quantitative theory of the nonlinear optical response for exciton-polaritons for the case of a doped TMD monolayer, and analyze in detail two sources of nonlinearity. The first nonlinear response contribution stems from the Coulomb exchange interaction between excitons. The second contribution comes from the reduction of Rabi splitting that originates from phase space filling at increased exciton concentration and the composite nature of excitons. We demonstrate that both nonlinear contributions are enhanced in the presence of free electrons. As free electron concentration can be routinely controlled by an externally applied gate voltage, this opens a way of electrical tuning of the nonlinear optical response.ESPR

Whispering-gallery exciton polaritons in submicron spheres

We show that a semiconductor sphere with a radius comparable with the wavelength of light at the exciton resonance frequency can behave as a high-quality three-dimensional microcavity or "polariton dot." We obtain numerical results for gallium arsenide submicron spheres and demonstrate that, in contrast to a larger sphere or planar microcavity, they can simultaneously possess both large quality factor and high finesse. In the strong-coupling regime the Rabi splitting approaches the bulk polariton splitting.

Polarization beats in a pillar microcavity

The beats of the Stokes luminescence parameters in pillar semiconductor microcavities are theoretically analysed. The beats are originated by a slight in-plane anisotropy of the pillar. The influence of the coherence time of exciton polaritons on the decay rate of polarization oscillations of the emission of light by the cavity is revealed. This link is essential for studies of the dynamic properties of polariton condensates in pillar microcavities

Spin-valley dynamics in alloy-based transition metal dichalcogenide heterobilayers

Van der Waals heterobilayers based on 2D transition metal dichalcogenides have been recently shown to support robust and long-lived valley polarization for potential valleytronic applications. However, the roles of the chemical composition and geometric alignment of the constituent layers in the underlying dynamics remain largely unexplored. Here we study spin-valley relaxation dynamics in heterobilayers with different structures and optical properties engineered via the use of alloyed monolayer semiconductors. Through a combination of time-resolved Kerr rotation spectroscopic measurements and theoretical modeling for Mo1 − xWxSe2/WSe2 samples with different chemical compositions and stacking angles, we uncover the contributions of the interlayer exciton recombination and charge carrier spin depolarization to the overall valley dynamics. We show that the corresponding decay rates can be tuned in a wide range in transitions from a misaligned to an aligned structure, and from a hetero- to a homo-bilayer. Our results provide insights into the microscopic spin-valley polarization mechanisms in van der Waals heterostructures for the development of future 2D valleytronic devices