349 research outputs found
On a biphononic origin of the 1125 cm^(-1) absorption band in cuprous oxide
We report on the IR spectroscopic studies in both reflection (50-900 cm^{-1})
and transmission (900-3000 cm^{-1}) mode of the vibration spectrum of the
cuprous oxide. A detailed analysis based on a comparison of the temperature
dependences of the absorption band at 1125 cm^{-1} and of IR and Raman active
fundamental vibrations results in assignment of the former to a biphonon.Comment: 5 pages, 5 figures (to appear in Phys.Lett. A
Evidence for As lattice location and Ge bound exciton luminescence in ZnO implanted with 73As and 73Ge
The results of photoluminescence (PL) measurements performed on high quality single crystal ZnO implanted
with radioactive 73Ga and 73As, both of which decay to stable 73Ge, are presented. Identical effects are observed in the two cases, with a sharp line at 3.3225(5) eV found to grow in intensity in accordance with the growth rate of the Ge daughter atom populations. On the strength of the well-established result that Ga occupies Zn sites, we conclude from the identical outcomes for 73Ga and 73As implantations that implanted As also preferentially occupies Zn sites. This result supports the findings of others that As preferentially occupies the Zn rather than the O site in ZnO. The thermal quenching energy of the 3.3225(5) eV line is found to be only 2.9(1) meV in contrast to its large spectral shift of 53.4(1) meV with respect to the lowest energy free exciton. The PL is attributed to exciton recombination at neutral Ge double donors on Zn sites involving transitions that leave the donor in an excited state
First-order spatial coherence of excitons in planar nanostructures: a k-filtering effect
We propose and analyze a k-filtering effect which gives rise to the drastic
difference between the actual spatial coherence length of quasi-two-dimensional
(quasi-2D) excitons or microcavity (MC) polaritons in planar nanostructures and
that inferred from far-field optical measurements. The effect originates from
the conservation of in-plane wavevector k in the optical decay of the particles
in outgoing bulk photons. The k-filtering effect explains the large coherence
lengths recently observed for indirect excitons in coupled quantum wells (QWs),
but is less pronounced for MC polaritons at low temperatures, T<10K
Random Scattering by Atomic Density Fluctuations in Optical Lattices
We investigate hitherto unexplored regimes of probe scattering by atoms
trapped in optical lattices: weak scattering by effectively random atomic
density distributions and multiple scattering by arbitrary atomic
distributions. Both regimes are predicted to exhibit a universal semicircular
scattering lineshape for large density fluctuations, which depend on
temperature and quantum statistics.Comment: 4 pages, 2 figure
Blueshifts of the emission energy in type-II quantum dot and quantum ring nanostructures
We have studied the ensemble photoluminescence (PL) of 11 GaSb/GaAs quantum dot/ring (QD/QR) samples over ≥5 orders of magnitude of laser power. All samples exhibit a blueshift of PL energy, ΔE, with increasing excitation power, as expected for type-II structures. It is often assumed that this blueshift is due to band-bending at the type-II interface. However, for a sample where charge-state sub-peaks are observed within the PL emission, it is unequivocally shown that the blueshift due to capacitive charging is an order of magnitude larger than the band bending contribution. Moreover, the size of the blueshift and its linear dependence on occupancy predicted by a simple capacitive model are faithfully replicated in the data. In contrast, when QD/QR emission intensity, I, is used to infer QD/QR occupancy, n, via the bimolecular recombination approximation (I ∝ n 2), exponents, x, in Δ E ∝ I x are consistently lower than expected, and strongly sample dependent. We conclude that the exponent x cannot be used to differentiate between capacitive charging and band bending as the origin of the blueshift in type-II QD/QRs, because the bimolecular recombination is not applicable to type-II QD/QRs
Response theory for time-resolved second-harmonic generation and two-photon photoemission
A unified response theory for the time-resolved nonlinear light generation
and two-photon photoemission (2PPE) from metal surfaces is presented. The
theory allows to describe the dependence of the nonlinear optical response and
the photoelectron yield, respectively, on the time dependence of the exciting
light field. Quantum-mechanical interference effects affect the results
significantly. Contributions to 2PPE due to the optical nonlinearity of the
surface region are derived and shown to be relevant close to a plasmon
resonance. The interplay between pulse shape, relaxation times of excited
electrons, and band structure is analyzed directly in the time domain. While
our theory works for arbitrary pulse shapes, we mainly focus on the case of two
pulses of the same mean frequency. Difficulties in extracting relaxation rates
from pump-probe experiments are discussed, for example due to the effect of
detuning of intermediate states on the interference. The theory also allows to
determine the range of validity of the optical Bloch equations and of
semiclassical rate equations, respectively. Finally, we discuss how collective
plasma excitations affect the nonlinear optical response and 2PPE.Comment: 27 pages, including 11 figures, version as publishe
White Electroluminescence Using ZnO Nanotubes/GaN Heterostructure Light-Emitting Diode
We report the fabrication of heterostructure white light–emitting diode (LED) comprised of n-ZnO nanotubes (NTs) aqueous chemically synthesized on p-GaN substrate. Room temperature electroluminescence (EL) of the LED demonstrates strong broadband white emission spectrum consisting of predominating peak centred at 560 nm and relatively weak violet–blue emission peak at 450 nm under forward bias. The broadband EL emission covering the whole visible spectrum has been attributed to the large surface area and high surface states of ZnO NTs produced during the etching process. In addition, comparison of the EL emission colour quality shows that ZnO nanotubes have much better quality than that of the ZnO nanorods. The colour-rendering index of the white light obtained from the nanotubes was 87, while the nanorods-based LED emit yellowish colour
Quantum fluids of light
This article reviews recent theoretical and experimental advances in the
fundamental understanding and active control of quantum fluids of light in
nonlinear optical systems. In presence of effective photon-photon interactions
induced by the optical nonlinearity of the medium, a many-photon system can
behave collectively as a quantum fluid with a number of novel features stemming
from its intrinsically non-equilibrium nature. We present a rich variety of
photon hydrodynamical effects that have been recently observed, from the
superfluid flow around a defect at low speeds, to the appearance of a
Mach-Cherenkov cone in a supersonic flow, to the hydrodynamic formation of
topological excitations such as quantized vortices and dark solitons at the
surface of large impenetrable obstacles. While our review is mostly focused on
a class of semiconductor systems that have been extensively studied in recent
years (namely planar semiconductor microcavities in the strong light-matter
coupling regime having cavity polaritons as elementary excitations), the very
concept of quantum fluids of light applies to a broad spectrum of systems,
ranging from bulk nonlinear crystals, to atomic clouds embedded in optical
fibers and cavities, to photonic crystal cavities, to superconducting quantum
circuits based on Josephson junctions. The conclusive part of our article is
devoted to a review of the exciting perspectives to achieve strongly correlated
photon gases. In particular, we present different mechanisms to obtain
efficient photon blockade, we discuss the novel quantum phases that are
expected to appear in arrays of strongly nonlinear cavities, and we point out
the rich phenomenology offered by the implementation of artificial gauge fields
for photons.Comment: Accepted for publication on Rev. Mod. Phys. (in press, 2012
Electrodynamics of Correlated Electron Materials
We review studies of the electromagnetic response of various classes of
correlated electron materials including transition metal oxides, organic and
molecular conductors, intermetallic compounds with - and -electrons as
well as magnetic semiconductors. Optical inquiry into correlations in all these
diverse systems is enabled by experimental access to the fundamental
characteristics of an ensemble of electrons including their self-energy and
kinetic energy. Steady-state spectroscopy carried out over a broad range of
frequencies from microwaves to UV light and fast optics time-resolved
techniques provide complimentary prospectives on correlations. Because the
theoretical understanding of strong correlations is still evolving, the review
is focused on the analysis of the universal trends that are emerging out of a
large body of experimental data augmented where possible with insights from
numerical studies.Comment: 78 pages, 55 figures, 984 reference
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