59 research outputs found
Exciton-polariton behaviour in bulk and polycrystalline ZnO
We report detailed reflectance studies of the exciton–polariton structure of thin film polycrystalline ZnO
and comparison with bulk crystal behaviour. Near-normal incidence reflectance spectra of these samples are
fitted using a two-band dielectric response function. Our data show that the reflectance data in polycrystalline
ZnO differ substantially from the bulk material, with Fabry–Perot oscillations at energies below the transverse A
exciton and above the longitudinal B exciton in the films. In the strong interaction regime between these energies no
evidence is seen of the normally rapid oscillations associated with the anomalous waves. We demonstrate that the
strong interaction of the damped exciton with the photon leads to polaritons in this region with substantial damping
such that the Fabry–Perot modes are eliminated. Good qualitative agreement is achieved between the model and
data. The importance of the polariton model in understanding the reflectance data of polycrystalline material is clearly
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Analysis on reflection spectra in strained ZnO thin films
Thin films of laser molecular-beam epitaxy grown ZnO films were studied with
respect to their optical properties. 4-K reflectivity was used to analyze
various samples grown at different biaxial in-plane strain. The spectra show
two structures at 3.37 eV corresponding to the A-free exciton transition and at
3.38 eV corresponding to the B-free exciton transition. Theoretical
reflectivity spectra were calculated using the spatial dispersion model. Thus,
the transverse energies, the longitudinal transversal splitting (ELT,), the
oscillator strengths, and the damping parameters were determined for both the
A- and B-free excitons of ZnO. As a rough trend, the strain dependence of the
energy E_LT for the A-excitons is characterized by a negatively-peaking
behavior with a minimum around the zero strain, while ELT for the B-excitons is
an increasing function of the strain field values.Comment: 4 pages, 2 figures, 1 table, conference: ICMAT2005 (Singapore), to
appear in an issue of J. Cryst. Growt
Strong and weak coupling limits in optics of quantum well excitons
A transition between the strong (coherent) and weak (incoherent) coupling
limits of resonant interaction between quantum well (QW) excitons and bulk
photons is analyzed and quantified as a function of the incoherent damping rate
caused by exciton-phonon and exciton-exciton scattering. For confined QW
polaritons, a second, anomalous, damping-induced dispersion branch arises and
develops with increasing damping. In this case, the strong-weak coupling
transition is attributed to a critical damping rate, when the intersection of
the normal and damping-induced dispersion branches occurs. For the radiative
states of QW excitons, i.e., for radiative QW polaritons, the transition is
described as a qualitative change of the photoluminescence spectrum at grazing
angles along the QW structure. Furthermore, we show that the radiative
corrections to the QW exciton states with in-plane wavevector approaching the
photon cone are universally scaled by an energy parameter rather than diverge.
The strong-weak coupling transition rates are also proportional to the same
energy parameter. The numerical evaluations are given for a GaAs single quantum
well with realistic parameters.Comment: Published in Physical Review B. 29 pages, 12 figure
Effect of polycrystallinity on the optical properties of highly oriented ZnO grown by pulsed laser deposition
We report the results of photoluminescence and reflectance measurements on highly c-axis oriented polycrystalline ZnO grown by pulsed laser deposition. The samples measured were grown under identical conditions and were annealed in-situ at various temperatures for 10-15 min. The band-edge photoluminescence spectra of the material altered considerably with an increase in grain size, with increased free exciton emission and observable excitonic structure in the reflectance spectra. The green band emission also increased with increasing grain size. A deformation potential analysis of the effect of strain on the exciton energy positions of the A- and B-excitons demonstrated that the experimental exciton energies could not be explained solely in terms of sample strain. We propose that electric fields in the samples due to charge trapping at grain boundaries are responsible for the additional perturbation of the excitons. This interpretation is supported by theoretical estimates of the exciton energy perturbation due to electric fields. The behaviour of the green band in the samples provides additional evidence in favour of our model
Long delays of light in ZnO caused by exciton-polariton propagation
We study the propagation of exciton-polaritons through bulk ZnO using time-resolved photoluminescence (PL) complemented by time-of-flight measurements of laser pulses. When the photon energy approaches donor bound exciton resonances, substantial time delays in PL light propagation are observed which reach up to 210 ps for a 0.55 mm thick crystal. By comparing results from time-of-flight measurements performed using PL light and laser pulses, the observed delay is shown to be due to the formation of exciton-polaritons and their spectral dispersion. It is also shown that the main contribution to the slow-down effect arises from free exciton-polaritons, whereas bound exciton-polaritons become important only in close vicinity to the corresponding resonances
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