Exciton-polariton Luminescence Research Articles

Effects of Electron-Vibration Interaction in Polariton Luminescence: Non-Markovian Fano Resonances and Hot Luminescence.

We have developed a non-Markovian theory of the polariton luminescence taking the molecular vibrations into account. The calculations were performed in the polariton basis. We have shown that the frequency shift and the polariton spectral line broadening strongly depend on the frequency-dependent exciton contribution to the polariton. In the single-mode microcavity, our non-Markovian theory predicts the Fano resonances in the polariton luminescence and also narrowing of the spectral lines with the increase of the total number of molecules in the case of the intramolecular nature of the low-frequency vibrations. The theory enables us to consider a nonequilibrium (hot) exciton-polariton luminescence similar to the hot luminescence of molecules and crystals. This opens a way for its observation in organic-based nanodevices.

Surface-Radiative Modes and Longitudinal Excitons in the Spectra of Exciton–Polariton Luminescence

Low-temperature (T = 2 K) spectra of exciton–polariton luminescence in the vicinity of the exciton resonance frequency An = 1 of CdS crystals are studied with allowance for the damping ℏΓ of mechanical excitons. The results of numerical calculations of the partial and interference contributions of bulk and surface-radiative spectral modes in the geometry of the radiation of additional waves into a vacuum with s and p polarizations are analyzed. It is shown that the contributions of purely longitudinal excitons and their interference with polaritons of the upper dispersion branch in luminescence in the vicinity of longitudinal frequency ωL are small (∼10−30%). Nevertheless, these contributions should be taken into account to achieve a quantitative agreement with the experiment; they determine the formation of an additional AL. line under oblique radiation.

The Role of Radiative Surface Modes and Longitudinal Excitons in the Formation of Exciton-Polariton Luminescence Spectra of CdS-Type Crystals

The low-temperature (T = 2 K) exciton-polariton luminescence (EPL) spectra in the vicinity of the exciton-resonance frequency An=1 for CdS-type crystals have been theoretically and experimentally investigated with allowance for the mechanical exciton decay . The results of the numerical calculations of the partial and interference contributions of the bulk and radiative surface spectral modes to the EPL in the geometry of additional s- and p-polarized waves emitted into vacuum are analyzed. It is shown that the contributions of purely longitudinal excitons and their interference with polaritons of the upper dispersion branch near the longitudinal frequency ωL to the EPL are small (∼10% - 30%); nevertheless, they must be taken into account to obtain quantitative agreement with experimental data. Specifically these contributions are responsible for the formation of an additional line (along with the fundamental AT line) in the case of oblique incidence of radiation.

Polarization behavior of the exciton-polariton emission of ZnO-based microresonators

AbstractWe present the polarization behavior of the exciton-polariton luminescence of a ZnO-based all-oxide resonator. A splitting in the emission energy between the s- and p-polarized pho-toluminescence of the lower polariton branch was observed which increases with increasing emission angle. It is caused by the polarization behavior of the uncoupled cavity-photon mode, and reaches a maximum of about 5 meV at an emission angle near the bottleneck region. For lar-ger angles the energy splitting decreases. Additionally to the energy splitting, we observed dif-ferences in the photoluminescence intensity which we trace back to different occupation of the lower polariton branch for the two polarizations. Whereas for p-polarization a bottleneck effect is clearly observable, this effect is much weaker for s-polarization. These findings indicate that the relaxation of hot carriers into the bottleneck region is enhanced for the p-polarized photolumi-nescence compared to the s-polarized one. The differences between these two polarizations are most pronounced for a very large negative detuning and vanish with increasing detuning.

Polarized photoluminescence of exciton-polaritons in free-standing GaN

We report on the polarization properties of the exciton–polariton modes in GaN. The dispersion curves and the expected emission lineshape of polaritons for all polarization configurations are calculated taking into account the spatial dispersion and the simultaneous exciton–photon coupling of all optically active states. An experimental study of the exciton–polariton luminescence in a free-standing GaN layer is also performed. The spectra reveal a clear difference between the emissions polarized perpendicular and parallel to the c-axis of the crystal. The experimental results are discussed in terms of optical selection rules and population of the polariton states. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Triplet-to-Singlet Conversion in the Exciton System inβ-ZnP2- Anti-Stokes Exciton Luminescence -

A new type of experiment was performed to investigate the effective nonradiative process causing the fast decay of the forbidden 1 s triplet exciton luminescence with polarization E // b in β-ZnP 2 . Excitation spectra of anti-Stokes 1 s singlet exciton luminescence with E // c have been measured by varying the excitation energy with E // b around the 1 s triplet exciton absorption peak which is located about 2 meV below the singlet exciton luminescence. A sharp peak of excitation efficiency has been confirmed at the triplet exciton resonance for the first time. Temperature dependence of the excitation peak is in accordance with the thermal behavior of the singlet exciton polariton luminescence both from lower- and upper-branches. This anti-Stokes acoustic phonon scattering of triplet exciton into singlet states is one of the main causes of the short lifetime of triplet exciton in β-ZnP 2 . Spin flip mechanism is briefly discussed.

Self-organized germanium quantum dots grown by molecular beam epitaxy on Si(100)

Under proper and critical growth conditions, very uniform Ge islands could be formed on Si(100) by using molecular beam epitaxy. The atomic force microscopic and cross-sectional transmission electron microscopic observations illustrate that the size and height uniformities of the islands are about ±3%. The Auger electron spectroscopic measurements show that the wetting layer is converted into a SiGe alloy layer due to its intermixing with the substrate during the growth. A very narrow peak with the full width at half maximum (FWHM) of 1.6 meV at the energy of 0.767 eV is observed at the temperature of 16 K. We attribute this peak to the free exciton longitudinal acoustic (LA) phonon replica originated from the Ge islands. As the temperature increases, this narrow peak splits into a doublet. This `camel-like' peak is temporarily explained as the exciton-polariton luminescence.

Time-resolved luminescence of exciton polaritons inPbI2

Exciton-polariton luminescence in ${\mathrm{PbI}}_{2}$ single crystals is investigated at 5 K using picosecond time-resolved spectroscopy. Decay curves of the luminescence of exciton polaritons are compared with those of the optical phonon sideband. The temporal behaviors are consistently analyzed by taking into account the diffusive motion of polaritons, and the decay-time constants are obtained as a function of the luminescence photon energy. The decay time changes from 50 ps around the longitudinal exciton energy up to 140 ps at the energy a little below the transverse exciton one. The change is ascribed to intraband relaxation of polaritons toward the bottleneck region.

Resonant Secondary Emission and Its Excitation Energy Dependence in Monoclinic Zinc Diphosphide

Resonant secondary emission of monoclinic zinc diphosphide (β-ZnP 2 ) has been investigated in detail under excitation into the energy range from the interband to the exciton-bands region. In addition to the 1LO line of 32.2 meV already reported, we have found more than 30 resonant Raman lines ranging from 9.4 meV to 58.9 meV by varying the excitation energy. These lines are observed distinctly when their scattered energies fall into the vicinity of the 1s exciton energy. It is confirmed that among phonons of 72 modes the LO phonon of 32.2 meV preferentially governs the relaxation processes of exciton polaritons in β-ZnP 2 . We have also observed a significant intensity decrease of the exciton polariton luminescence when the excitation energy is lowered across the 1s longitudinal exciton energy E L . A brief discussion is made in this connection.

Two-dimensional versus three-dimensional excitons in wide GaAs quantum wells

Excitons confined in high-quality GaAs/Al0.19Ga0.81As double heterostructures have been studied experimentally and theoretically with emphasis on phenomena associated with the transition from a two-dimensional exciton in a quantum well to a three-dimensional exciton in a thin film. Exciton luminescence and reflectance spectra are obtained for GaAs film widths between 750 and 5200 Å. Exciton polariton luminescence is observed for the 5200 Å sample. Highly resolved exciton peaks are obtained in the free-exciton luminescence spectra for both the 990 and 2010 Å samples. Magneto-exciton spectra of the 990 Å sample reveal an enhanced spin splitting of the ground-level exciton. Experimental results of both the 750 and the 990 Å samples are qualitatively explained by an effective-mass theory which considers mixing of a large number of quasi-two-dimensional excitons. The large spin-splitting of the 990 Å sample is a distinctive feature of a wide quantum well which can be explained by the well width dependence of exciton mixing. The comparison between the discrete luminescence spectrum of the 2010 Å sample and the reflectance spectrum with an exciton polariton interference pattern is presented. The quantization of the spatial dispersion contributes to the luminescence phenomena of the quasi-three-dimensional exciton which means that the exciton coherence length must be larger than 4020 Å.

Dynamical behavior of the exciton polariton in CuCl: Coherent propagation and momentum relaxation.

Exciton-polariton luminescence spectra in CuCl single crystals of high purity are investigated at 2 K using picosecond time-resolved spectroscopy with high temporal and spectral resolutions. The excitation density is kept in the weak-excitation regime. Experimental results are analyzed in the ``polariton-picture'' framework for the exciton-photon system. Under excitation well above the lowest exciton energy, the intraband relaxation and the bottleneck effect of the polaritons are clearly manifested in the 2-LO-phonon replica of the exciton luminescence. On the other hand, pulsed responses are found in the polariton resonant luminescence near the transverse-exciton energy. The coherent propagation of polaritons and their radiative escape from the crystal surface produce the pulsed responses in the luminescence while they propagate to and fro between the front and the rear surfaces of the slab sample. They propagate for a distance of more than 20 \ensuremath{\mu}m. The momentum-relaxation probability per unit time is evaluated as a function of the energy from the intensity ratio of successive pulsed responses of the resonant luminescence. It varies from ${10}^{9}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ near the transverse-exciton energy up to ${10}^{10}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ near the longitudinal-exciton energy. The sample dependence of the polariton luminescence spectra is also elucidated by the competitive relation between the lifetime, propagation time, and the intraband relaxation time of the polaritons. The discrepancy between the transmission spectrum calculated from the momentum-relaxation probability and that obtained experimentally is discussed. This discrepancy is associated with the strong effect that very fine surface roughness may have on the transmission spectrum.

Polariton Luminescence in Monoclinic ZnP2Crystal

Exciton polariton luminescence and its temperature dependence have been investigated in monoclinic ZnP 2 crystals. The energies of transverse and longitudinal excitons ( E T and E L ) are determined by a fit of the reflection spectrum to a simple polariton model. In the vicinity of the 1 s exciton, two emission bands are observed at energies E T and E L . They are attributed to the luminescence from lower and upper branch polaritons, respectively. In particular, the lower polariton luminescence band exhibits a well-defined doublet structure at 2 K. Both components are separated from each other by about 1 meV with comparable intensities and the same polarization dependence. With increasing temperature, the higher-energy component decreases rapidly as compared to the lower-energy component. The origin of these emission components is discussed.

Reabsorption kinetics of free- and bound-exciton luminescence in high-purity n-GaAs

Reabsorption kinetics of free- and bound-exciton luminescence have been studied in high-purity n-GaAs at 4.2 K, by a sensitive detection method using the dual laser beams. As well as the large reabsorption effects of the (D o,X) line, the reabsorption anomaly of the exciton-polariton luminescence just above the bottleneck region has been found for the first time. The reabsorption properties significantly depend upon the sample treatments.

Excitonic Surface Polaritons in Luminescence from ZnTe Crystals

AbstractThe form and structure of reflection and exciton–polariton luminescence spectra of ZnTe crystals are studied in the region of the ground (n = 1) exciton state. The longitudinal–transverse splitting magnitude ΔELT is determined from the shape of the reflection spectra. A detected doublet structure of an emission band from the lower polariton branch is associated with the k‐linear term. The evolution of bulk and surface polariton luminescence spectra versus temperature and wave length of the exciting light is investigated.

Time Characteristics of Secondary Emissions in ZnTe under Band-to-Band Excitation

Transient behavior of secondary emission intensity has been measured near the fundamental absorption edge of ZnTe by means of a time-correlated single photon counting method under the excitation by subnanosecond light pulses from a CW mode-locked Ar + ion laser. The obtained time responses of bound-exciton luminescence lines, exciton-polariton luminescence bands and a Raman-like 2 L O line are analyzed. The dynamics of excitonic polaritons derived from the result are compared with a theoretical calculation, and the mechanisms of the appearance of the Raman-like L O lines and the polariton luminescence bands are discussed.

Polariton effects in luminescence from ZnTe crystals: surface and bulk polaritons

The exciton-polariton luminescence spectra of the ZnTe cystals are investigated in the region of the ground ( n= 1) exciton state. In addition to emission of the both branches of bulk polaritons, luminescence of surface polaritons is first revealed. The temperature dependence of luminescence of the bulk and surface polaritons is considered.

Lifetimes of exciton-polariton luminescence of anthracene crystal at low temperature

The lifetimes of anthracene crystal luminescence have been measured at 1.6K The wavelength dependence of the lifetime has been determined near the threshold for electronic excitation. The results are discussed in terms of the exciton-polariton model of luminescence.