Large Rabi Splitting Research Articles

Graphene Plasmonics: A Platform for Strong Light–Matter Interactions

Graphene plasmons provide a suitable alternative to noble-metal plasmons because they exhibit much tighter confinement and relatively long propagation distances, with the advantage of being highly tunable via electrostatic gating. Here, we propose to use graphene plasmons as a platform for strongly enhanced light-matter interactions. Specifically, we predict unprecedented high decay rates of quantum emitters in the proximity of a carbon sheet, observable vacuum Rabi splittings, and extinction cross sections exceeding the geometrical area in graphene nanoribbons and nanodisks. Our theoretical results provide the basis for the emerging and potentially far-reaching field of graphene plasmonics, offering an ideal platform for cavity quantum electrodynamics, and supporting the possibility of single-molecule, single-plasmon devices.

Dephasing of Triplet-Sideband Optical Emission of a Resonantly DrivenInAs/GaAsQuantum Dot inside a Microcavity

Detailed properties of resonance fluorescence from a single quantum dot in a micropillar cavity are investigated, with particular focus on emission coherence in the dependence on optical driving field power and detuning. A power-dependent series over a wide range reveals characteristic Mollow triplet spectra with large Rabi splittings of |Ω|≤15 GHz. In particular, the effect of dephasing in terms of systematic spectral broadening ∝Ω(2) of the Mollow sidebands is observed as a strong fingerprint of excitation-induced dephasing. Our results are in excellent agreement with predictions of a recently presented model on phonon-dressed quantum dot Mollow triplet emission in the cavity-QED regime.

Exciton-polariton microphotoluminescence and lasing from ZnO whispering-gallery mode microcavities

Hexagonal ZnO microrods were employed as whispering-gallery mode (WGM) optical microcavities to investigate exciton-polariton microphotoluminescence and lasing emission. Using a confocal microphotoluminescence system, the exciton-polariton emission with a large Rabi splitting of about 90 meV was observed from a ZnO microrod with the diameter of 9.38 μm. The spatial-resolved spectra demonstrated a collective nonlinear blueshift in the WGM resonance peaks along a tapered microrod and proved the anticross dispersion property of the exciton-polariton. Furthermore, the exciton-polariton WGM lasing was stimulated and blueshifted in the strong coupling region under the excitation of a 355 nm nanosecond pulsed laser.

Ultrafast manipulation of the Rabi splitting in metal‐molecular aggregate hybrid nanostructures

AbstractWe report on an experimental study of the optical properties of a metal‐molecular aggregate hybrid nanostructure consisting of a gold nanoslit array coated with a thin J‐aggregated dye layer. Our experiments reveal a strong coupling between the surface plasmon polaritons (SPPs) excited on a nanoslit array and the aggregate excitons with a large Rabi splitting energy (ΩR). Ultrafast excitation drastically alters the optical response of the hybrid nanostructure inducing transient switching from the strong to weak the coupling regime due to the bleaching of the exciton resonance. The externally controlled switching is fully reversible and occurs on sub‐ps timescale. The results are explained within a phenomenological two coupled oscillator model. Such a strong interaction can, e.g., be used to fabricate ultrafast all‐optical plasmonic switches or tunable mirrors. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characteristics of exciton-polaritons in ZnO- based hybrid microcavities

Wide bandgap semiconductors are promising materials for the development of polariton-based optoelectronic devices operating at room temperature (RT). We report the characteristics of ZnO-based microcavities (MCs) in the strong coupling regime at RT with a vacuum Rabi splitting of 72 meV. The impact of scattering states of excitons on polariton dispersion is investigated. Only the lower polariton branches (LPBs) can be clearly observed in ZnO MCs since the large vacuum Rabi splitting pushes the upper polariton branches (UPBs) into the scattering absorption states in the ZnO bulk active region. In addition, we systematically investigate the polariton relaxation bottleneck in bulk ZnO-based MCs. Angle-resolved photoluminescence measurements are performed from 100 to 300 K for different cavity-exciton detunings. A clear polariton relaxation bottleneck is observed at low temperature and large negative cavity detuning conditions. The bottleneck is suppressed with increasing temperature and decreasing detuning, due to more efficient phonon-assisted relaxation and a longer radiative lifetime of the polaritons.

Toward polariton lasing in a zinc oxide microcavity: Design and preliminary results

We report the observation of the strong coupling regime in a half ZnO cavity. A large Rabi splitting of 130 meV is seen as well as a bottleneck in the exciton-polariton relaxation due to a shorter cavity-photon lifetime. The deposition of the top mirror is expected to yield a Q factor of 620 and the structure would then fulfill the main criteria required for room temperature polariton lasing to occur. This conclusion is supported by simulations based on the solution of Boltzmann equations leading to a polariton density threshold of 1016 cm−3.

Fabrication, characterization and its local reflection properties of a metal-mirror microcavity with high concentrated PIC J-aggregates

We have investigated reflection properties of light-matter strong coupling in a planar metal-mirror microcavity with highly concentrated PIC J-aggregates. Large vacuum Rabi-splitting ranging from 100 to 250 meV is obtained depending on the concentration of the J-aggregates. The factors for providing these large Rabi-splitting will be discussed based on its concentration dependence. We also present our recent study focused on microscopic reflection properties of the microcavity. We have improved microscope optics which enables us to measure local reflection spectra within 0.3-μm-diameter area. Observation of incident-light-angle dependence becomes possible. We found existence of micrometer-scale inhomogeneity in the Rabi-splitting (e.g. ±10% in a 17μm×17μm region), which will be interpreted mainly by the spatial distribution of J’s in the active layer.

Large vacuum Rabi splitting in ZnO-based hybrid microcavities observed at room temperature

Wide-band gap ZnO semiconductors are attractive materials for the investigation of microcavity exciton polaritons due to the large exciton binding energy and oscillator strength. We report the growth and characterization of bulk ZnO-based hybrid microcavity. The phenomenon of strong exciton-photon coupling at room temperature has been observed in the ZnO-based hybrid microcavity structure, which consists of 30 pair epitaxially grown AlN/AlGaN distributed Bragg reflector (DBR) on the bottom side of the 3/2λ thick ZnO cavity and 9 pair SiO2/HfO2 DBR as the top mirror. The cavity quality factor is about 221. The experimental results show good agreement with theoretically calculated exciton-polariton dispersion curves based on transfer matrix method. From the theoretical and experimental exciton-polariton dispersion curves with two different cavity-exciton detuning values, the large vacuum Rabi splitting is estimated to be about 58 meV in the ZnO-based hybrid microcavity.

Theory of exciton-polariton lasing at room temperature in ZnO microcavities

We use the semiclassical Boltzmann equations to describe the exciton-polariton relaxation in a bulk ZnO microcavity at room temperature. We present kinetic phase diagrams which report the lasing threshold versus the main sample parameters, such as the polariton lifetime or the value of the Rabi splitting. We find that the polariton laser is operating close to the thermodynamic equilibrium in most cases which makes advantageous the use of microcavities showing very large Rabi splittings as it is the case in ZnO structures. We find that room temperature polariton lasing could be observed in the samples presently available.

Particularities of surface plasmon–exciton strong coupling with large Rabi splitting

This paper presents some of the particularities of the strong coupling regime occurring between surface plasmon (SP) modes and excitons. Two different active materials were deposited on a silver film: a cyanine dye J-aggregate, and a two-dimensional layered perovskite-type semiconductor. The dispersion relations, which are deduced from angular resolved reflectometry spectra, present an anticrossing characteristic of the strong coupling regime. The wavevector is a good parameter to determine the Rabi splitting. Due to the large interaction energies (several hundreds of milli-electron-volts), the calculations at constant angle can induce an overestimation of the Rabi splitting of more than a factor of two. Another property of polaritons based on SP is their nonradiative character. In order to observe the polaritonic emission, it is thus necessary to use particular extraction setups, such as gratings or prisms. Otherwise only the incoherent emission can be detected, very similar to the bare exciton emission.

Large vacuum Rabi splitting in a multiple quantum well GaN-based microcavity in the strong-coupling regime

An $\mathrm{Al}\mathrm{In}\mathrm{N}∕\mathrm{Al}\mathrm{Ga}\mathrm{N}$ microcavity (MC) containing a $\mathrm{Ga}\mathrm{N}∕{\mathrm{Al}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{N}$ multiple quantum well (MQW) structure is investigated through room temperature (RT) photoluminescence and reflectivity experiments. A vacuum Rabi splitting as large as $50\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ at RT is reported for this MC structure, the highest value reported so far for a semiconductor MC containing QWs. This is shown to result from a geometry combining a state of the art nitride-based MC with a MQW system where the built-in electric field has a reduced impact on the oscillator strength of optical transitions. The contribution of Bragg modes seen in optical spectra is well accounted for by transfer matrix simulations. In addition, the sensitivity of the present system to the tuning between the various optical components of the microcavity (bottom and top Bragg reflectors and active cavity region) to maximize the strong-coupling regime is also shown through simulations. Prospects regarding the nonlinear polariton emission from such a structure indicate that this type of MCs could potentially sustain ultrafast polariton parametric amplification up to $440\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, thanks to an increased exciton binding energy. More generally, it is predicted that, owing to a large exciton saturation density in excess of $1\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ per QW, such a MC structure would be suitable for the observation of nonlinear effects associated with cavity polaritons (polariton lasing and polariton Bose-Einstein condensates) at RT and above.

Optical and excitonic properties of ZnO films

Optical and excitonic properties of ZnO heterostructures are studied in order to observe the strong light-matter coupling in this material as it has been recently demonstrated in GaN. The optical index of ZnO is first studied as a function of wavelength through spectroscopic ellipsometric and reflectivity experiments on ZnO layers grown by molecular beam epitaxy and deposited on two different substrates: sapphire and silicon with an AlN buffer layer. The main features of the excitons: energy, oscillator strength and broadening are deduced. From the knowledge of these properties, a microcavity is then designed. The ZnO active layer is embedded between AlGaN/AlN and dielectric Bragg mirrors. The calculation of the reflectivity spectra as a function of the incident angle attests the strong coupling and a large Rabi splitting of 110 meV is expected in such a cavity.

Polariton kinetics for bosonic condensation in CdTe-microcavities for various Rabi splittings and detunings

The applied Boltzmann relaxation kinetics for stationary and ps-pulsed nonresonant pumping takes the polariton–acoustic phonon and the polariton–polariton scattering into account. For cavities with a vacuum Rabi splitting around Ω = 8 meV , we find for ps-pulse excitation a transient bosonic condensation in the ground state. For microcavities with a large vacuum Rabi splitting of Ω = 26 meV we find for zero detuning, due to the steep increase of radiative losses, a condensation with a macroscopically occupied state in the wavenumber region at which the Bragg mirrors become transparent. With positive detunings which increase the scattering rates, the finite-size Bose–Einstein condensation in the ground state is restored.

Microcavity polariton kinetics for bosonic condensation and lasing in II-VI compound materials

We investigate with a Boltzmann approach the relaxation kinetics of II-VI compound microcavity polaritons after a nonresonant pump pulse for the example of CdTe. In these polar materials large vacuum Rabi microcavity splittings guarantee robust polariton modes in the strong-coupling regime with an extremely small effective mass at the bottom of the lower branch, but with very short cavity lifetimes. Our kinetic treatment for stationary and picosecond-pulsed pumping takes the polariton--acoustic-phonon as well as the polariton-polariton scattering into account. For cavities with a vacuum Rabi splitting around $\ensuremath{\Omega}=10\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, we find for nonresonant picosecond-pulse excitation a relaxation scenario which leads to a transient bosonic condensation in the lowest polariton state and coherent laser action at densities smaller than the exciton saturation density. For microcavities with a large vacuum Rabi splitting of $\ensuremath{\Omega}=26\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ we find due to the steep increase of radiative losses and the simultaneous steep decrease of the scattering rates with decreasing energy a nonequilibrium phase transition with a macroscopically occupied state in the bottleneck region at a finite transverse momentum and a connected coherent laser emission under an angle of 17\ifmmode^\circ\else\textdegree\fi{}. All of our results for medium and large Rabi vacuum splitting are in good qualitative agreement with corresponding observations of Dang and co-workers

Properties of surface plasmons strongly coupled to excitons in an organic semiconductor near a metallic surface

The properties of organic semiconductor excitons in strong interaction with surface plasmons are described. For this purpose, a $J$-aggregated cyanine dye layer is deposited on a silver film and studied with reflectometry experiments. The dependence of the Rabi splitting with the square root of the dye layer absorption and the width inversion of the reflectometry lines at the resonance are observed. The experimental spectra are well fitted using a transfer matrix method with a complex index. The comparison between experiments and simulations for layers in different configurations is qualitatively related to the role of the plasmon polarization in the interaction with the aggregated dye chains. Finally, large Rabi splitting of $300\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ are observed for pure dye layers. Characteristic behavior of the large coupling and the effect of asymmetrical absorption line shape are analyzed.

Vacuum-field Rabi splitting in semiconducting core-shell microsphere

The optical properties of a semiconducting core-shell microsphere are studied theoretically. Exciton states confined in the shell region strongly and selectively couple with the whispering gallery modes (WGMs), resulting in a large well-defined vacuum-field Rabi splitting in the optical spectra. The spectral shape is discussed in detail by decomposing the scattering cross section into exciton, background, and their interference contributions. If the thickness of the shell is sufficiently small, the Rabi splitting is proportional to the square root of the shell volume and the normalized intensity of the WGM at the shell region. Further, it is found that the WGM with smaller angular momentum results in a larger Rabi splitting.

Strong exciton-photon coupling in a length tunable optical microcavity with J-aggregate dye heterostructures

We report the incorporation of thin films of a cyanine dye J aggregate into a versatile, length tunable, optical microcavity. The dense J-aggregate layers give an optical response that can be modified by embedding them at specific positions within heterostructures of dielectric and metal layers. The microcavities are composed of separate gold mirrors, which can be individually nanopositioned, and give sharp resonant modes in the red/near-infrared region of the spectrum. With the dye layer favorably placed, anticrossing behavior is observed as the cavity modes are successively swept through the absorption resonance. Large Rabi splittings of up to 170meV are achieved at room temperature, agreeing well with predictions from a transfer-matrix model. These strongly coupled microcavities pave the way for microelectromechanical systems-integrated microdevices with tailored nonlinear optical properties.

Fast polariton relaxation in strongly coupled organic microcavities

We consider the regime of strong light-matter coupling in an organic microcavity, where large Rabi splitting can be achieved. As has been shown, the excitation spectrum of such a structure, besides coherent polaritonic states, contains a number of strongly spatially localized incoherent excited states. These states form the majority of the excited states of the microcavity and are supposed to play the decisive role in the relaxation dynamics of the excitations in the microcavity. We consider the non-radiative transition from an incoherent excited state into one of the coherent states of the lower polaritonic branch accompanied by emission of a high-energy intramolecular phonon. It is shown that this process may determine the lifetime of incoherent excited states in the microcavity. This observation may be important in the discussion of pump–probe experiments with short pulses. This process may also play an important role for the population of the lowest energy states in organic microcavities, and hence in the problem of condensation of cavity polaritons.

Magnetic-field effects on exciton–polaron in a CdSe quantum disk

We study theoretically the interaction between excitons and longitudinal optical (LO) phonons in a cylindrical disk-like semiconductor quantum dot under an applied magnetic field. Due to the intensity of the interaction in the strong coupling regime, a composite quasi-particle called exciton–polaron is formed. We focus on the effect of the disk size and an external magnetic field on the exciton–phonon interaction energy and the exciton–polaron modes. The numerical computation for a CdSe quantum disk have shown that the exciton–phonon interaction energy is very significant and is even dominant when the disk height is small, which leads to a large Rabi splitting between the exciton–polaron modes. We investigate also the effect of the temperature on the integrated photoluminescence (PL) intensity, and show that at relatively high temperature the LO phonons have a noticeable effect on it. This physical parameter also shows a great dependence on quantum disk size and on magnetic field.

Photonic devices based on wide gap semiconductors for room temperature polariton emission

Planar semiconductor microcavities have found increasing attention since they allow to enhance and control the interaction between light and excitons. When the coupling between photon and exciton is strong enough, polaritons are formed which are observed in a pronounced Rabi-splitting in the cavity spectra. The strong exciton-light coupling regime, necessesary for polariton-based applications depends on the oscillator strength and the exciton binding energy. The wide-gap II–VI (CdTe,ZnSe and CdSe) as well as the group III-nitrides (GaN,InN and AlN) semiconductors excellently fulfil these conditions. We observed a large Rabi-splitting (about 44 meV) with ZnSe-based semiconductor microcavities containing four strained (Zn,Cd)Se quantum wells and ZnS/YF3 distributed Bragg-reflectors. Measurements of the reflectivity and of the photoluminescence revealed clear evidence of the strong coupling regime. We also report on first experiments to optimise the structural and optical properties of (Al,Ga)N/GaN quantum wells. The results obtained so far show that these structures may be used for the investigation of polariton Rabi-splitting at room temperature. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)