Polariton Interactions Research Articles

Nonlinear eigenmodes of a non-equilibrium harmonic oscillator

We investigate theoretically the quantum oscillator-like states recently observed experimentally in polariton condensates. We consider a complex Gross–Pitaevskii-type model that includes the effects of self-interactions, and creation and decay of exciton-polaritons. We develop a perturbation theory for approximate solutions to this non-equilibrium condensate model and compare the results with numerically calculated solutions for both repulsive and attractive polariton–polariton interactions. While the nonlinearity has a weak effect on the mode selection, the condensate density profiles are modified at moderate gain strengths. We find the nonlinearity becomes more dominant when a very large gain of polaritons leads to an extended cloud with high condensate densities. Finally, we identify the relation of the observed patterns to the input pump configuration, and suggest this may serve as a generalized NOR gate in the tradition of optical computing.

Phase change dispersion of plasmonic nano-objects.

Phase is an inherent and important feature for coherent processes, which, unfortunately, has not been completely understood for surface plasmon polariton (SPP) and matter interactions. Here we propose a practical approach to extract the phase change dispersion during the interaction between free-space light, SPPs and nanogroove/slit based on far-field information only. Numerical simulation and experimental validation were both presented using nanoslit-groove plasmonic interferometers, agreeing well with theoretical near-field analysis. This approach is generally feasible to extract the intrinsic phase dispersion of other plasmonic nanostructures and can reveal more fundamental features of SPP-matter interactions.

All optical controlled-NOT gate based on an exciton–polariton circuit

We propose an implementation of a CNOT gate for quantum computing based on a patterned microcavity polariton system, which can be manufactured using the modern technological facilities. The qubits are encoded in the spin-coherent polariton states. The structure consists of two wire cavities oriented at 45° with a micropillar between them. The polariton spin rotates due to the Longitudinal–Transverse splitting between polarization eigenstates in the wires. In the pillar, the optically generated circularly polarized polariton macrooccupied state plays the role of the control qubit. Because of the spin-anisotropic polariton interaction, it induces an effective magnetic field along the Z-direction with a sign depending on the qubit value.

Real photons from vacuum fluctuations in optomechanics: The role of polariton interactions

The effect of vacuum fluctuations on the state of a single-cavity optomechanical system, so-called quantum heating, is studied, and several resulting physical phenomena such as polariton thermalization and parametric instabilities are predicted.

Heterodyne spectroscopy of polariton spinor interactions

We report on spinor polariton interactions in GaAs based microcavities. This investigation is carried out by means of heterodyne polarized pump-probe spectroscopy. We show the dependence of the energy renormalization of the lower and upper polariton resonances with cavity detuning for different polariton densities. We use the exciton-photon based Gross-Pitaevskii equation to model the experiment for both lower and upper polariton modes. The theoretical results reproduce qualitatively the experimental observations revealing the magnitude and the sign of the parallel and anti-parallel spin interaction strength. We evidence the strong influence of the biexciton resonance on the anti-parallel spin polariton energy shift and provide the exciton-biexciton coupling constant. We derive our results in the lower polariton basis using Gross-Pitaevskii equation from which, we express analytically the spinor polariton interactions and identify the clear role of the biexciton resonance.

Mid-infrared polaritonic coupling between boron nitride nanotubes and graphene.

Boron nitride (BN) is considered to be a promising substrate for graphene-based devices in part because its large band gap can serve to insulate graphene in layered heterostructures. At mid-infrared frequencies, graphene supports surface plasmon polaritons (SPPs), whereas hexagonal-BN (h-BN) is found to support surface phonon polaritons (SPhPs). We report on the observation of infrared polaritonic coupling between graphene SPPs and boron nitride nanotube (BNNT) SPhPs. Infrared scattering type scanning near-field optical microscopy is used to obtain spatial distribution of the two types of polaritons at the nanoscale. The observation suggests that those polaritons interact at the nanoscale in a one-dimensional/two-dimensional (1D/2D) geometry, exchanging energy in a nonplanar configuration at the nanoscale. Control of the polaritonic interaction is achieved by adjustment of the graphene Fermi level through voltage gating. Our observation suggests that boron nitride nanotubes and graphene can interact at mid-infrared frequencies and coherently exchange their energies at the nanoscale through the overlap of mutual electric near field of surface phonon polaritons and surface plasmon polaritons. Such interaction enables the design of nano-optical devices based on BNNT-graphene polaritonics in the mid-infrared range.

Oscillatory solitons and time-resolved phase locking of two polariton condensates

When pumped nonresonantly, semiconductor microcavity polaritons form Bose–Einstein condensates that can be manipulated optically. Using tightly-focused excitation spots, radially expanding condensates can be formed in close proximity. Using high time resolution streak camera measurements we study the time dependent properties of these macroscopic coherent states. By coupling this method with interferometry we observe directly the phase locking of two independent condensates in time, showing the effect of polariton–polariton interactions. We also directly observe fast spontaneous soliton-like oscillations of the polariton cloud trapped between the pump spots, which can be either dark or bright solitons. This transition from dark to bright is a consequence of the change of sign of the nonlinearity which we propose is due to the shape of the polariton dispersion leading to either positive or negative polariton effective mass.

Resonant interaction of molecular vibrations and surface plasmon polaritons: The weak coupling regime

Adjusting the free-electron concentration, the surface plasmon frequency of the semiconductor ZnOGa is tuned into resonance with the molecular vibrations of the $n$-alkane tetracontane. Closed molecular films deposited on the semiconductor's surface in the monolayer regime generate distinct signatures in total-attenuated-reflection spectra at the frequencies of the symmetric and asymmetric stretching vibrations of the ${\mathrm{CH}}_{2}$ group. Their line shape undergoes profound changes from absorptive to dispersive and even antiresonance behavior when moving along the surface-plasmon dispersion by the angle of incidence. We demonstrate that this line-shape diversity results from a phase-sensitive perturbation of the surface-plasmon-polariton generation at the molecule-metal interface.

Polaritonic Feshbach resonance

A Feshbach resonance occurs when the energy of two interacting free particles comes to resonance with a molecular bound state. When approaching this resonance, dramatic changes in the interaction strength between the particles occur. Feshbach resonances have been an essential tool to control the atom interactions, which can even be switched from repulsive to attractive [1-4]. Thanks to Feshbach resonances many effects in ultracold atomic gases could be explored [5, 6]. Here we demonstrate a Feshbach resonance based on polariton spinor interactions that characterize the fundamental interaction process in polariton quantum systems. We show the clear enhancement of attractive interactions and the prompt change to repulsive interaction by tuning the energy of two polaritons with anti-parallel spins across the biexciton bound state energy. A mean field two-channel model quantitatively reproduces the experimental results. This observation paves the way for a new tool to tune the polariton interactions and to move forward into quantum correlated polariton physics.

Nonlinear interactions in an organic polariton condensate

Under the right conditions, cavity polaritons form a macroscopic condensate in the ground state. The fascinating nonlinear behaviour of this condensate is largely dictated by the strength of polariton-polariton interactions. In inorganic semiconductors, these result principally from the Coulomb interaction between Wannier-Mott excitons. Such interactions are considerably weaker for the tightly bound Frenkel excitons characteristic of organic semiconductors and were notably absent in the first reported demonstration of organic polariton lasing. In this work, we demonstrate the realization of an organic polariton condensate, at room temperature, in a microcavity containing a thin film of 2,7-bis[9,9-di(4-methylphenyl)-fluoren-2-yl]-9,9-di(4-methylphenyl)fluorene. On reaching threshold, we observe the spontaneous formation of a linearly polarized condensate, which exhibits a superlinear power dependence, long-range order and a power-dependent blueshift: a clear signature of Frenkel polariton interactions.

Dynamical Casimir Emission from Polariton Condensates

We study theoretically the dynamical Casimir effect in an exciton-polariton condensate that is suddenly created by an ultrashort laser pulse at normal incidence. As a consequence of the abrupt change of the quantum vacuum, Bogoliubov excitations are generated. The subsequent evolution, governed by polariton interactions and losses, is studied within a linearized truncated Wigner approximation. We focus in particular on the momentum distribution and spatial coherence. The limiting behavior at large and small momenta is determined analytically. A simple scaling relation for the final condensate depletion as a function of the system parameters is found and the correlation length is shown to depend linearly on the condensate depletion.

Wannier-Stark effect and electron-phonon interaction in macroporous silicon structures with SiO2 nanocoatings

AbstractWe investigated the contribution of electron-phonon interaction to the broadening parameter Γ of the Wannier-Stark ladder levels in oxidized macroporous silicon structures with different concentration of Si-O-Si states (TO and LO phonons). The obtained value of the Wannier-Stark ladder parameter Γ is much less than the djacent level energy evaluated from giant oscillations of resonance electron scattering on the surface states. We determined the influence of broadening on the oscillation amplitude in IR absorption spectra as interaction of the surface multi-phonon polaritons with scattered electrons. This interaction transforms the resonance electron scattering in samples with low concentration of Si-O-Si states into ordinary scattering on ionized impurities for samples with high concentration of Si-O-Si states. The transformation takes place at the scattering lifetime coinciding with the period of electron oscillations in the surface electric field.

Toward stimulated interaction of surface phonon polaritons

Thermal emission spectra mediated by surface phonon polariton are examined by using a theoretical model that accounts for generation processes. Specifically, the acoustic phonon fusion mechanism is introduced to remedy theoretical deficiencies of the near thermal equilibrium treatments. The model clarifies the thermal excitation mechanism of surface phonon polaritons and the energy transfer path under non-zero energy flow. When applied to GaAs and SiC semi-infinite surfaces, the nonequilibrium model predicts that the temperature dependence of the quasi-monochromatic peak can exhibit distinctly different characteristics of either sharp increase or slow saturation depending on the materials, which is in direct contrast with the estimate made by the near-equilibrium model. The proposed theoretical tool can accurately analyze the nonequilibrium steady states, potentially paving a pathway to demonstrate stimulated interaction/emission of thermally excited surface phonon polaritons.

Ultrathin nanostructured metals for highly transmissive plasmonic subtractive color filters.

Plasmonic color filters employing a single optically-thick nanostructured metal layer have recently generated considerable interest as an alternative to colorant-based color filtering technologies, due to their reliability, ease of fabrication, and high color tunability. However, their relatively low transmission efficiency (~30%) needs to be significantly improved for practical applications. The present work reports, for the first time, a novel plasmonic subtractive color filtering scheme that exploits the counter-intuitive phenomenon of extraordinary low transmission (ELT) through an ultrathin nanostructured metal film. This approach relies on a fundamentally different color filtering mechanism than that of existing plasmonic additive color filters, and achieves unusually high transmission efficiencies of 60 ~ 70% for simple architectures. Furthermore, owing to short-range interactions of surface plasmon polaritons at ELT resonances, our design offers high spatial resolution color filtering with compact pixel size close to the optical diffraction limit (~λ/2), creating solid applications ranging from imaging sensors to color displays.

Polariton condensation in an optically induced two-dimensional potential

We demonstrate experimentally the condensation of exciton polaritons through optical trapping. The nonresonant pump profile is shaped into a ring and projected to a high quality factor microcavity where it forms a two-dimensional repulsive optical potential originating from the interactions of polaritons with the excitonic reservoir. Increasing the population of particles in the trap eventually leads to the emergence of a confined polariton condensate that is spatially decoupled from the decoherence inducing reservoir, before any buildup of coherence on the excitation region. In a reference experiment, where the trapping mechanism is switched off by changing the excitation intensity profile, polariton condensation takes place for excitation densities more than two times higher and the resulting condensate is subject to much stronger dephasing and depletion processes.

Stress-modulated exciton polariton Josephson effect in a semiconductor microcavity

We demonstrate that the exciton polariton Josephson effect can be effectively modulated by a uniaxial stress in a semiconductor microcavity. By increasing the stress, we can show there are two critical transition stresses, which correspond to the 0-π and dc-ac transition, respectively. We find that the ac Josephson frequency increases with the stress and the macroscopic quantum self-trapping can appear by regulating the stress. We also find that the Shapiro steps emerge under appropriate conditions, but some steps are suppressed. In contrast to a conventional superconducting tunnel junction, it is exposed that all the rich phenomena come from the competition between the interaction of exciton polaritons and the energy difference induced by the stress. It is feasible that these results can be experimentally verified.

Polariton–polariton collinear interaction in semiconductor microcavities

It is shown that the existence of a small polariton mass near k=0 and a large exciton mass leads to a special type of polariton–polariton interaction: a collinear scattering, which has no analogs in bulk polariton scattering. This scattering process governed with energy and momentum conservation laws is very sensitive to the population of the k=0 state, which plays a main role in nonlinear relaxation of polaritons.

Optical parametric oscillation in one-dimensional microcavities

We present a comprehensive investigation of optical parametric oscillation in resonantly excited one-dimensional semiconductor microcavities with embedded quantum wells. Such solid-state structures feature a fine control over light-matter coupling and produce a photonic/polaritonic mode fan that is exploited for the efficient emission of parametric beams. We implement an energy-degenerate optical parametric oscillator with balanced signal and idler intensities via a polarization-inverting mechanism. In this paper, we (i) precisely review the multimode photonic/polaritonic structure of individual emitters, (ii) provide a thorough comparison between experiment and theory, focusing on the power and the threshold dependence on the exciton-photon detuning, (iii) discuss the influence of inhomogeneous broadening of the excitonic transition and finite size, and (iv) find that a large exciton-photon detuning is a key parameter to reach a high output power and a high conversion efficiency. Our study highlights the predictive character of the polariton interaction theory and the flexibility of one-dimensional semiconductor microcavities as a platform to study parametric phenomena.

Interaction of light and surface plasmon polaritons in Ag Islands studied by nonlinear photoemission microscopy

Two photon photoemission microscopy was used to study the interaction of femtosecond laser pulses with Ag islands prepared using different strategies on Si(111) and SiO₂. The femtosecond laser pulses initiate surface plasmon polariton (SPP) waves at the edges of the island. The superposition of the electrical fields of the femtosecond laser pulses with the electrical fields of the SPP results in a moiré pattern that is comparable despite the rather different methods of preparation and that gives access to the wavelength and direction of the SPP waves. If the SPPs reach edges of the Ag islands, they can be converted back into light waves. The incident and refracted light waves result in an interference pattern that can again be described with a moiré pattern, demonstrating that Ag islands can be used as plasmonic beam deflectors for light.

Double-boson stimulated terahertz emission in a polariton cascade laser

A system of equidistant polariton states interacting with a terahertz electromagnetic field are considered that are localized in a cavity for terahertz radiation. Accumulation of terahertz photons in the cavity, together with boson stimulation of the transitions between polariton levels, leads to intense radiative transitions between terahertz levels. A concept of double-boson stimulated transitions between polariton levels in the boson cascade laser is proposed. We study a possibility of using the boson cascade laser in generation of terahertz radiation. The system exhibits threshold dependence of the intensity of terahertz radiation on pumping. The quantum effectiveness of the boson cascade laser can exceed unity when pumping is performed above the threshold. The interaction of polaritons with the cavity leads to an increase in the threshold of pumping and a decrease in quantum effectiveness.