33 research outputs found
Electron-hole correlation effects in the emission of light from quantum wires
We present a self-consistent treatment of the electron-hole correlations in
optically excited quantum wires within the ladder approximation, and using a
contact potential interaction. The limitations of the ladder approximation to
the excitonic low-density region are largely overcome by the introduction of
higher order correlations through self consistency. We show relevance of these
correlations in the low-temperature emission, even for high density relevant in
lasing, when large gain replaces excitonic absorption.Comment: 4 paes 3 figure
Gain in a quantum wire laser of high uniformity
A multi-quantum wire laser operating in the 1-D ground state has been
achieved in a very high uniformity structure that shows free exciton emission
with unprecedented narrow width and low lasing threshold. Under optical pumping
the spontaneous emission evolves from a sharp free exciton peak to a
red-shifted broad band. The lasing photon energy occurs about 5 meV below the
free exciton. The observed shift excludes free excitons in lasing and our
results show that Coulomb interactions in the 1-D electron-hole system shift
the spontaneous emission and play significant roles in laser gain.Comment: 4 pages, 4 figures, prepared by RevTe
Role of bound pairs in the optical properties of highly excited semiconductors: a self consistent ladder approximation approach
Presence of bound pairs (excitons) in a low-temperature electron-hole plasma
is accounted for by including correlation between fermions at the ladder level.
Using a simplified one-dimensional model with on-site Coulomb interaction, we
calculate the one-particle self-energies, chemical potential, and optical
response. The results are compared to those obtained in the Born approximation,
which does not account for bound pairs. In the self-consistent ladder
approximation the self-energy and spectral function show a characteristic
correlation peak at the exciton energy for low temperature and density. In this
regime the Born approximation overestimates the chemical potential. Provided
the appropriate vertex correction in the interaction with the photon is
included, both ladder and Born approximations reproduce the excitonic and free
pair optical absorption at low density, and the disappearance of the exciton
absorption peak at larger density. However, lineshapes and energy shifts with
density of the absorption and photoluminescence peaks are drastically
different. In particular, the photoluminescence emission peak is much more
stable in the ladder approximation. At low temperature and density a sizeable
optical gain is produced in both approximations just below the excitonic peak,
however this gain shows unphysical features in the Born approximation. We
conclude that at low density and temperature it is fundamental to take into
account the existence of bound pairs in the electron-hole plasma for the
calculation of its optical and thermodynamic properties. Other approximations
that fail to do so are intrinsically unphysical in this regime, and for example
are not suitable to address the problem of excitonic lasing.Comment: 14 pages, 12 figure
Mott transition from a diluted exciton gas to a dense electron-hole plasma in a single V-shaped quantum wire
We report on the study of many-body interactions in a single high quality
V-shaped quantum wire by means of continuous and time-resolved
microphotoluminescence. The transition from a weakly interacting exciton gas
when the carrier density n is less than 10^5 cm^-1 (i.e. n aX < 0.1, with aX
the exciton Bohr radius), to a dense electron-hole plasma (n > 10^6 cm^-1, i.e.
n aX > 1) is systematically followed in the system as the carrier density is
increased. We show that this transition occurs gradually : the free carriers
first coexist with excitons for n aX > 0.1, then the electron-hole plasma
becomes degenerate at n aX = 0.8. We also show that the non-linear effects are
strongly related to the kind of disorder and localization properties in the
structure especially in the low density regime.Comment: 5 figure
Critical media education in Malaysia: A challenge to vocational-orientation
This article examines the pedagogic vacuum in media education in Malaysia where critical inquiry has been made subservient to the acquisition of technical skills. It suggests that vocational oriented communication courses should engage in constructing alternative representations, meaning and values so that what is experienced by the students and the community is a mutual educative process of participatory communication
Dis-empowering women on Malaysian TV
While women are encountering profound changes in their lives, the role and responsibility of television is critical in facilitating women’s voices, perspectives and issues. An analysis of women’s programs in daytime television in Malaysia reveals that far from promoting active citizenry by raising the level of public discussion, Malaysian television is weak and ineffectual, lacking clear political focus. Women’s concerns are not taken seriously and television fails to contribute to women’s empowerment in society
Coulomb correlation and band gap renormalization at high carrier densities in quantum wires
We have studied the luminescence of narrow quantum wires at photoexcitation densities of up to similar to 3 x 10(6) cm(-1). We show that even at these densities, which are well above the expected Mott density of 8 x 10(5) cm(-1), excitonic recombination dominates over other recombination channels in stark contrast with the behavior of quantum wells and bulk structures at equivalent densities. As we observe no significant shift in the peak energy with density, an upper limit to the band gap renormalization can be set