973 research outputs found
Optical alignment and polarization conversion of neutral exciton spin in individual InAs/GaAs quantum dots
We investigate exciton spin memory in individual InAs/GaAs self-assembled
quantum dots via optical alignment and conversion of exciton polarization in a
magnetic field. Quasiresonant phonon-assisted excitation is successfully
employed to define the initial spin polarization of neutral excitons. The
conservation of the linear polarization generated along the bright exciton
eigenaxes of up to 90% and the conversion from circular- to linear polarization
of up to 47% both demonstrate a very long spin relaxation time with respect to
the radiative lifetime. Results are quantitatively compared with a model of
pseudo-spin 1/2 including heavy-to-light hole mixing.Comment: 5 pages, 3 figure
Optical manipulation of a single Mn spin in a CdTe-based quantum dot
A system of two coupled CdTe quantum dots, one of them containing a single Mn
ion, was studied in continuous wave and modulated photoluminescence,
photoluminescence excitation, and photon correlation experiments. Optical
writing of information in the spin state of the Mn ion has been demonstrated,
using orientation of the Mn spin by spin-polarized carriers transferred from
the neighbor quantum dot. Mn spin orientation time values from 20 ns to 100 ns
were measured, depending on the excitation power. Storage time of the
information in the Mn spin was found to be enhanced by application of a static
magnetic field of 1 T, reaching hundreds of microseconds in the dark. Simple
rate equation models were found to describe correctly static and dynamical
properties of the system.Comment: 4 pages, 3 figure
Microphotoluminescence study of disorder in ferromagnetic (Cd,Mn)Te quantum well
Microphotoluminescence mapping experiments were performed on a modulation
doped (Cd,Mn)Te quantum well exhibiting carrier induced ferromagnetism. The
zero field splitting that reveals the presence of a spontaneous magnetization
in the low-temperature phase, is measured locally; its fluctuations are
compared to those of the spin content and of the carrier density, also measured
spectroscopically in the same run. We show that the fluctuations of the carrier
density are the main mechanism responsible for the fluctuations of the
spontaneous magnetization in the ferromagnetic phase, while those of the Mn
spin density have no detectable effect at this scale of observation.Comment: 4 pages, 3 figure
Femtosecond study of the interplay between excitons, trions, and carriers in (Cd,Mn)Te quantum wells
We present an absorption study of the neutral and positively charged exciton
(trion) under the influence of a femtosecond, circularly polarized, resonant
pump pulse. Three populations are involved: free holes, excitons, and trions,
all exhibiting transient spin polarization. In particular, a polarization of
the hole gas is created by the formation of trions. The evolution of these
populations is studied, including the spin flip and trion formation processes.
The contributions of several mechanisms to intensity changes are evaluated,
including phase space filling and spin-dependent screening. We propose a new
explanation of the oscillator strength stealing phenomena observed in p-doped
quantum wells, based on the screening of neutral excitons by charge carriers.
We have also found that binding heavy holes into charged excitons excludes them
from the interaction with the rest of the system, so that oscillator strength
stealing is partially blockedComment: 4 pages, 4 figure
p-Type doping of II-VI heterostructures from surface states: application to ferromagnetic CdMnTe quantum wells
We present a study of p-type doping of CdTe and CdMnTe quantum
wells from surface states. We show that this method is as efficient as usual
modulation doping with nitrogen acceptors, and leads to hole densities
exceeding cm. Surface doping was successfully applied
to obtain carrier-induced ferromagnetism in a CdMnTe quantum well.
The observed temperature dependence of photoluminescence spectra, and the
critical temperature, correspond well to those previously reported for
ferromagnetic quantum wells doped with nitrogen.Comment: 4 figure
Correlated Photon Emission from a Single II-VI Quantum Dot
We report correlation and cross-correlation measurements of photons emitted
under continuous wave excitation by a single II-VI quantum dot (QD) grown by
molecular-beam epitaxy. A standard technique of microphotoluminescence combined
with an ultrafast photon correlation set-up allowed us to see an antibunching
effect on photons emitted by excitons recombining in a single CdTe/ZnTe QD, as
well as cross-correlation within the biexciton ()-exciton ()
radiative cascade from the same dot. Fast microchannel plate photomultipliers
and a time-correlated single photon module gave us an overall temporal
resolution of 140 ps better than the typical exciton lifetime in II-VI QDs of
about 250ps.Comment: 4 pages, 3 figures, to appear in Appl. Phys. Let
Investigating the content of microplastics and other extraneous particles in Polish bottled water
Bottled water has enjoyed a global increase in popularity since it is generally perceived to be superior in quality to tap water and necessary when tap water is non-potable. As a result, ensuring that it meets the requisite quality standards is of vital importance. This work aims to examine the content of solid particles, including microplastics, in bottled water available in Polish stores. The second aspect is the preliminary determination of the influence of the water gassing process, together with thermal and light factors, on the content of particles in the water. The number of particles was counted by colour and shape, with the number ranging from 87 to 188 per litre of water; on average, there were 136 ±32 particles per litre of water, demonstrating that water from disposable plastic bottles is contaminated with various substances. The difference in the number of particles may be due to the origin of the waters, the processes they were subjected to prior to bottling, the properties of the bottles as packaging, and the conditions and length of storage and transport. Additional Fourier-transform infrared spectroscopy (FT-IR) analysis confirmed that about 75% of the particles were polymers, and 50% of them were plastics. Particularly alarming is the fact that the bottled waters mostly contained microplastic particles (MP) of smaller sizes, the kind which is recognized as being the most dangerous to human health. In the study, most particles were in the form of irregular shapes, which may indicate that they come from the destruction of waste or plastic products. This is also indicated by the domination of colourless particles. More particles were found in waters exposed to high and low temperatures than in waters stored at room temperature, potentially indicating that storage conditions for drinking water are important. Taking into account the results obtained, increasing attention should be paid to the health risks posed by such microplastics and there is a clear need to introduce legal regulations on the matter. The lack of any legal guidelines or unified standards in the field of MP research means that the results are not always representative, and it also makes it difficult to compare the results from different studies
Influence of s,p-d and s-p exchange couplings on exciton splitting in (Zn,Mn)O
This work presents results of near-band gap magnetooptical studies on
(Zn,Mn)O epitaxial layers. We observe excitonic transitions in reflectivity and
photoluminescence, that shift towards higher energies when the Mn concentration
increases and split nonlinearly under the magnetic field. Excitonic shifts are
determined by the s,p-d exchange coupling to magnetic ions, by the
electron-hole s-p exchange, and the spin-orbit interactions. A quantitative
description of the magnetoreflectivity findings indicates that the free
excitons A and B are associated with the Gamma_7 and Gamma_9 valence bands,
respectively, the order reversed as compared to wurtzite GaN. Furthermore, our
results show that the magnitude of the giant exciton splittings, specific to
dilute magnetic semiconductors, is unusual: the magnetoreflectivity data is
described by an effective exchange energy N_0(beta-alpha)=+0.2+/-0.1 eV, what
points to small and positive N_0 beta. It is shown that both the increase of
the gap with x and the small positive value of the exchange energy N_0 beta
corroborate recent theory describing the exchange splitting of the valence band
in a non-perturbative way, suitable for the case of a strong p-d hybridization.Comment: 8 pages, 8 figure
Excitonic giant Zeeman effect in GaN:Mn^3+
We describe a direct observation of the excitonic giant Zeeman splitting in
(Ga,Mn)N, a wide-gap III-V diluted magnetic semiconductor. Reflectivity and
absorption spectra measured at low temperatures display the A and B excitons,
with a shift under magnetic field due to s,p-d exchange interactions. Using an
excitonic model, we determine the difference of exchange integrals between
Mn^3+ and free carriers in GaN, N_0(alpha-beta)=-1.2 +/- 0.2 eV. Assuming a
reasonable value of alpha, this implies a positive sign of beta which
corresponds to a rarely observed ferromagnetic interaction between the magnetic
ions and the holes.Comment: 4 pages, 4 figure
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