Propagation Of Exciton-polaritons Research Articles

Exciton mediated self-organization in glass driven by ultrashort light pulses

We propose an exciton-polariton-mediated self-organization effect in transparent SiO2 glass under intense femtosecond light irradiation. Interference and dipole-dipole interaction of polaritons causes formation of gratings of dielectric polarization. Due to an ultrafast exciton self-localization into a quasicrystal structure, the polariton gratings remain frozen in glass and a permanent three-dimensional image of exciton-polariton gas is created. We show that coherent effects in propagation of exciton-polaritons can serve as a tool for nanostructuring and fabrication of 5-dimensional optical memories in glass, opening new horizons for polaritronics.

Ultra-Compact Photonic Circuit Components based on Propagation of Exciton Polaritons in Organic Dye Nanofibers

not Available.

Motion of Spin Polariton Bullets in Semiconductor Microcavities

The dynamics of optical switching in semiconductor microcavities in the strong coupling regime is studied by using time- and spatially resolved spectroscopy. The switching is triggered by polarized short pulses which create spin bullets of high polariton density. The spin packets travel with speeds of the order of 10(6) m/s due to the ballistic propagation and drift of exciton polaritons from high to low density areas. The speed is controlled by the angle of incidence of the excitation beams, which changes the polariton group velocity.

Micrometer‐Scale Photonic Circuit Components Based on Propagation of Exciton Polaritons in Organic Dye Nanofibers

A new approach to create micrometer-scale photonic circuits is developed by using the propagation of exciton polaritons in organic dye nanofibers. A microring resonator component with an I/O bus channel is fabricated by micromanipulation of the nanofibers. The component functions as a wavelength filter with a signal extinction of ∼5 dB for visible light.

Slowdown of light due to exciton-polariton propagation in ZnO

By employing time-of-flight spectroscopy, the group velocity of light propagating through bulk ZnO is demonstrated to dramatically decrease down to 2044 km/s when photon energy approaches the absorption edge of the material. The magnitude of this decrease is found to depend on light polarization. It is concluded that even though the slowdown is observed in the vicinity of donor bound exciton (BX) resonances, the effect is chiefly governed by dispersion of free exciton (FX) polaritons that propagate coherently via ballistic transport. Based on the experimentally determined spectral dependence of the polariton group velocity, the polariton dispersion is accurately determined.

Ultrafast decay of photoluminescence from high-density excitons inAlxGa1−xNmixed crystals: Diffusive propagation of exciton-polaritons

We report on the ultrafast decay dynamics of photoluminescence (PL) in highly excited ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}$ mixed crystals under exciton resonant excitation at low temperatures. When the excitation intensity is increased, the P-band emission appears due to exciton-exciton inelastic scattering processes. The PL intensity of the P band decays rapidly, with a much shorter decay time than the radiative recombination time of the excitons. We show that the ultrafast PL decay dynamics can be understood as due to the disorder-induced diffusive propagation of photonlike exciton-polaritons in ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}$ mixed crystals.

Polariton propagation in weak-confinement quantum wells

Exciton-polariton propagation in a quantum well, under center-of-mass quantization, is computed by a variational self-consistent microscopic theory. The Wannier exciton envelope functions basis set is given by the simple analytical model of D'Andrea and Del Sole [Phys. Rev. B 41, 1413 (1990)], based on pure states of the center-of-mass wave vector, free from fitting parameters and ``ad hoc'' (the so-called additional boundary conditions---ABCs) assumptions. In the present paper, the former analytical model is implemented in order to reproduce the center-of-mass quantization in a large range of quantum well thicknesses $(5{a}_{B}\ensuremath{\le}L\ensuremath{\le}\ensuremath{\infty})$. The role of the dynamical transition layer at the well/barrier interfaces is discussed at variance of the classical Pekar's dead layer and ABCs. The Wannier exciton eigenstates are computed and compared with various theoretical models with different degrees of accuracy. Exciton-polariton transmission spectra in large quantum wells $(L⪢{a}_{B})$ are computed and compared with experimental results of Schneider et al. [Phys. Rev. B 63, 045202 (2001)] in high-quality GaAs samples. The sound agreement between theory and experiment allows to unambiguously assign the exciton-polariton dips of the transmission spectrum to the pure states of the Wannier exciton center-of-mass quantization.

Fraction of a Millimeter Propagation of Exciton Polaritons in Photoexcited Nanofibers of Organic Dye

We report propagation of exciton polaritons (EPs) in photoexcited nanofibers of thiacyanine dye over a few hundred micrometers at room temperature. We determine the complex refractive index along the nanofibers by fluorescence microscopy measurements on single nanofibers and observe its anomalous behavior due to the EP effect. The longitudinal-transverse splitting energy (ΔE(L-T)) is evaluated to be ∼1 eV. The large ΔE(L-T) and waveguide function of the nanofibers allow a millimeter propagation of EPs at room temperature, which is hardly realized in other systems.

Photonic quasicrystalline and aperiodic structures

We have theoretically studied propagation of exciton-polaritons in deterministic aperiodic multiple-quantum-well structures, particularly, in the Fibonacci and Thue-Morse chains. The attention is concentrated on the structures tuned to the resonant Bragg condition with two-dimensional quantum-well exciton. The superradiant or photonic-quasicrystal regimes are realized in these structures depending on the number of the wells. The developed theory based on the two-wave approximation allows one to describe analytically the exact transfer-matrix computations for transmittance and reflectance spectra in the whole frequency range except for a narrow region near the exciton resonance. In this region the optical spectra and the exciton-polariton dispersion demonstrate scaling invariance and self-similarity which can be interpreted in terms of the ``band-edge'' cycle of the trace map, in the case of Fibonacci structures, and in terms of zero reflection frequencies, in the case of Thue-Morse structures.

Exciton-polaritonic quasicrystalline and aperiodic structures

We have theoretically studied propagation of exciton-polaritons in deterministic aperiodic multiple-quantum-well structures, particularly, in the Fibonacci and Thue-Morse chains. The attention is concentrated on the structures tuned to the resonant Bragg condition with two-dimensional quantum-well exciton. Depending on the number of wells, the super-radiant either photonic-quasicrystal regimes are realized in these aperiodic structures. For moderate values of the exciton nonradiative damping rate $\ensuremath{\Gamma}$, the developed theory based on the two-wave approximation allows one to perceive and describe analytically the exact transfer-matrix computations for transmittance and reflectance spectra in the whole frequency range except for a narrow region near the exciton resonance ${\ensuremath{\omega}}_{0}$. In this region the optical spectra and the exciton-polariton dispersion demonstrate scaling invariance and self-similarity which can be interpreted in terms of the ``band-edge'' cycle of the trace map, in the case of Fibonacci structures, and in terms of zero reflection frequencies, in the case of Thue-Morse structures. With decreasing $\ensuremath{\Gamma}$, in the whole allowed polariton band the two-wave approximation stops to be valid, and a transition occurs from Bloch-like to localized states, with modes closer to ${\ensuremath{\omega}}_{0}$ becoming localized first.

Polarization beats in ballistic propagation of exciton-polaritons in microcavities

The emission from a microcavity in the strong coupling regime excited resonantly by linearly polarized light has been measured with spatial, directional, and temporal resolution. We observe ballistic propagation of the excited polaritons as a ring spreading in real space. The linear polarization degree P of the emission at the ring is found to oscillate as a function of the azimuthal angle θ and of time t according to P=sin(2θ)sin(ΩLTt∕2), where ΩLT is the longitudinal-transverse splitting of the exciton-polariton modes. This dependence holds for all investigated excitation energies corresponding to different values of ΩLT. The theoretical model assuming ballistic propagation of exciton polaritons in the specific cavity structure yields a detailed agreement with the experimental data. The observed polarization beats have the same nature as the recently predicted optical spin Hall effect.

Exciton-polariton confinement in Fibonacci quasiperiodic superlattice

We investigate the propagation of exciton-polaritons (bulk and surface modes) in quasiperiodic superlattices of Fibonacci type, truncated at z = 0, where z is defined as the growth axis. For our purposes, the Fibonacci structure can be realized experimentally by juxtaposing two basic building blocks A and B, following a Fibonacci sequential rule. Here A is the spatially dispersive medium modelled by a semiconductor from the nitride family (GaN, for instance), which alternates with a common dielectric medium B (sapphire). Our main aim is the investigation of the modified optical properties of the exciton polariton modes for a better understanding of the dynamics of the excitation in confined systems. The dispersion relation shows a bottleneck profile for the superlattice modes, whose behavior is similar to those found in the bulk crystal. The Cantor-like energy spectrum is also investigated, and scaling properties are shown for the Fibonacci quasiperiodic structure.

Nonlinear phase-resolved polariton spectroscopy in ZnSe epilayers

The influence of thermalized carriers on the exciton polariton propagation in a semiconductor ZnSe layer is investigated by reflection spectroscopy. Reflected laser pulses are characterized in amplitude and phase by using the method of spectral interferometry (SI). The complex coefficient of reflection is given for the low excitation density by a broadened Elliot formula. For increasing density scattering between the carriers and the laser-induced polarization is accounted by the solution of the semiconductor Bloch equations. In the spectral phase we find a delicate dependence of a π-jump at the heavy-hole resonance of the carrier density while the spectral amplitude practically remains the same.

Propagation and Scattering of Exciton-Polaritons in a Graded Semiconductor Microcavity

Acceleration of the in-plane motion of photons inside the cavity wedge is a well known effect. Acceleration of exciton-polaritons in the same system seemed to be impossible because of the cavity selection rules. Nevertheless, we have observed it experimentally by means of time- and spatially-resolved reflection spectroscopy. The phenomenon is explained in terms of elastic scattering of the polaritons between optically forbidden and allowed states.

Theory of Propagation and Scattering of Exciton-Polaritons in Quantum Wells

A novel semi-classical theory of exciton-polaritons in quantum wells (QWs) is presented. Time-resolved coherent optical spectra of QWs are described by the light coupling with ensembles of localized quantum dot-like exciton states. The ratio between reflection and resonant Rayleigh scattering (RRS) intensity gives a direct measure of the density of these excitonic dots. Good agreement with experimental data for time-resolved angle-dependent reflection and RRS from a GaAs single QW is achieved.

PROPAGATION PROPERTIES OF HIGH-DENSITY EXCITON POLARITONS

We investigate the time evolution of the exciton wave packet by an interacting exciton polariton model. The analysis is based on the equation-of-motion for the expectation value of exciton polariton operator with respect to the macroscopic quantum state. We find that the nonlinear interaction between exciton polariton leads to the extraordinary fast propagation of high-density exciton polaritons. This result helps the proper understanding of the experiments on the unusually fast propagation of exciton polaritons in strongly excited BiI 3.

Polariton propagation in high quality semiconductors: Microscopic theory and experiment versus additional boundary conditions

Linear exciton-polariton propagation in semiconductors is analyzed using a microscopic theory. Numerical results are compared with various approximation schemes based on additional boundary conditions, and with phase-amplitude linear spectroscopy experiments in high-quality GaAs. A simultaneous description of the measured amplitude and phase of the transmitted electric field is only possible with the full model.

Propagation of Exciton-Polaritons in Nitride-Based Multiple Quantum Wells

We present a semi-classical theory of exciton–polariton effects in disordered multiple quantum well (MQW) structures in application to GaN/AlGaN MQWs. An original micro-model is proposed that allows to calculate all types of coherent optical spectra of MQWs. We show that GaN/AlGaN MQWs should exhibit pronounced propagation effects in the resonant Rayleigh scattering and the vertical motional narrowing effect in the time-resolved transmission. We predict polariton-induced oscillations in the time-resolved coherent optical spectra of GaN/AlGaN MQWs having a period of the order of hundreds of femtoseconds.

Propagation and Scattering of Exciton-Polaritons in Nitride-Based Multiple Quantum Wells

ABSTRACTWe show theoretically that disorder-induced scattering of light between « bright » and « dark » exciton-polariton modes governs the coherent optical response of GaN/AlGaN quantum wells (QWs) on a femtosecond scale. The incident light essentially excites the “bright” modes, they emit the most part of light radiatively during first few hundreds of femtoseconds. The rest of the coherent signal, i.e. the temporal behaviour on a scale from 1 to few picoseconds, comes from the “dark” modes, populated due to the elastic scattering processes in the system, and mixed so that they form two bulk-like polariton branches. The pico-second scale optical response of GaN/AlGaN QWs is essentially incoherent because of a very efficient scattering of exciton-polaritons by a structural disorder.

Propagation of exciton polaritons in inhomogeneous semiconductor films

A theoretical study of the propagation of short light pulses in semiconductor slabs containing excitons has been performed using the scattering-state technique and the steepest-descent method. These methods have allowed us to calculate numerically and to describe analytically the temporary behavior of the dielectric polarization inside the films as well as the time-resolved optical transmission spectra. The inhomogeneous broadening of exciton resonances has been taken into account within a local model. The appearance of a grating of the dielectric polarization in a semiconductor film illuminated by a short light pulse is predicted. This grating, which is due to interfering exciton polaritons, moves backward with respect to the light-propagation direction. We have shown that reabsorption and re-emission of photons by excitons in quantum wells and semiconductor films have a crucial effect on the coherent time-resolved spectra of these structures. The time-resolved transmission decay rate is found to depend on the thickness of semiconductor films with inhomogeneously broadened exciton resonances, which is not the case for a purely homogeneously broadened exciton. This fact, which follows from the energy dependence of the polariton damping in the former case, has also been explained in terms of multiple reabsorption--re-emission of photons by excitons.