43,529 research outputs found
Identification of excitons, trions and biexcitons in single-layer WS2
Single-layer WS is a direct-gap semiconductor showing strong excitonic
photoluminescence features in the visible spectral range. Here, we present
temperature-dependent photoluminescence measurements on mechanically exfoliated
single-layer WS, revealing the existence of neutral and charged excitons at
low temperatures as well as at room temperature. By applying a gate voltage, we
can electrically control the ratio of excitons and trions and assert a residual
n-type doping of our samples. At high excitation densities and low
temperatures, an additional peak at energies below the trion dominates the
photoluminescence, which we identify as biexciton emission.Comment: 6 pages, 5 figure
The role of quantum-confined excitons vs defects in the visible luminescence of SiO2 films containing Ge nanocrystals
Synthesis of Ge nanocrystals in SiO2 films is carried out by precipitation from a supersaturated solid solution of Ge in SiO2 made by Ge ion implantation. The films exhibit strong room-temperature visible photoluminescence. The measured photoluminescence peak energy and lifetimes show poor correlations with nanocrystal size compared to calculations involving radiative recombination of quantum-confined excitons in Ge quantum dots. In addition, the photoluminescence spectra and lifetime measurements show only a weak temperature dependence. These observations strongly suggest that the observed visible luminescence in our samples is not due to the radiative recombination of quantum-confined excitons in Ge nanocrystals. Instead, observations of similar luminescence in Xe+ -implanted samples and reversible PL quenching by hydrogen or deuterium suggest that radiative defect centers in the SiO2 matrix are responsible for the observed luminescence
Lasing in Single Cadmium Sulfide Nanowire Optical Cavities
The mechanism of lasing in single cadmium sulfide (CdS) nanowire cavities was
elucidated by temperature-dependent and time-resolved photoluminescence (PL)
measurements. Temperature-dependent PL studies reveal rich spectral features
and show that an exciton-exciton interaction is critical to lasing up to 75 K,
while an exciton-phonon process dominates at higher temperatures. These
measurements together with temperature and intensity dependent life-time and
threshold studies suggest that lasing is due to formation of excitons, and
moreover, have implications for the design of efficient, low-threshold nanowire
lasers.Comment: 4 figure
Anisotropic interactions of a single spin and dark-spin spectroscopy in diamond
The nitrogen-vacancy (N-V) center in diamond is a promising atomic-scale
system for solid-state quantum information processing. Its spin-dependent
photoluminescence has enabled sensitive measurements on single N-V centers,
such as: electron spin resonance, Rabi oscillations, single-shot spin readout
and two-qubit operations with a nearby 13C nuclear spin. Furthermore, room
temperature spin coherence times as long as 58 microseconds have been reported
for N-V center ensembles. Here, we have developed an angle-resolved
magneto-photoluminescence microscopy apparatus to investigate the anisotropic
electron spin interactions of single N-V centers at room temperature. We
observe negative peaks in the photoluminescence as a function of both magnetic
field magnitude and angle that are explained by coherent spin precession and
anisotropic relaxation at spin level anti-crossings. In addition, precise field
alignment unmasks the resonant coupling to neighboring dark nitrogen spins that
are not otherwise detected by photoluminescence. The latter results demonstrate
a means of investigating small numbers of dark spins via a single bright spin
under ambient conditions.Comment: 13 pages, 4 figure
Mid-infrared emission and absorption in strained and relaxed direct bandgap GeSn semiconductors
By independently engineering strain and composition, this work demonstrates
and investigates direct band gap emission in the mid-infrared range from GeSn
layers grown on silicon. We extend the room-temperature emission wavelength
above ~4.0 {\mu}m upon post-growth strain relaxation in layers with uniform Sn
content of 17 at.%. The fundamental mechanisms governing the optical emission
are discussed based on temperature-dependent photoluminescence, absorption
measurements, and theoretical simulations. Regardless of strain and
composition, these analyses confirm that single-peak emission is always
observed in the probed temperature range of 4-300 K, ruling out defect- and
impurity-related emission. Moreover, carrier losses into thermally-activated
non-radiative recombination channels are found to be greatly minimized as a
result of strain relaxation. Absorption measurements validate the direct band
gap absorption in strained and relaxed samples at energies closely matching
photoluminescence data. These results highlight the strong potential of GeSn
semiconductors as versatile building blocks for scalable, compact, and
silicon-compatible mid-infrared photonics and quantum opto-electronics
Photoluminescence from In0.5Ga0.5As/GaP quantum dots coupled to photonic crystal cavities
We demonstrate room temperature visible wavelength photoluminescence from
In0.5Ga0.5As quantum dots embedded in a GaP membrane. Time-resolved above band
photoluminescence measurements of quantum dot emission show a biexpontential
decay with lifetimes of ~200 ps. We fabricate photonic crystal cavities which
provide enhanced outcoupling of quantum dot emission, allowing the observation
of narrow lines indicative of single quantum dot emission. This materials
system is compatible with monolithic integration on Si, and is promising for
high efficiency detection of single quantum dot emission as well as
optoelectronic devices emitting at visible wavelengths
Influence of an Sb doping layer in CIGS thin-film solar cells: a photoluminescence study
Sb doping of Cu(In,Ga)Se2 (CIGS) solar cells has been reported to exhibit a positive effect on the morphology of the absorber layer, offering a possibility to lower manufacturing cost by lowering the annealing temperatures during the CIGS deposition. In this work electron microscopy, energy-dispersive x-ray spectroscopy and photoluminescence experiments have been performed on cells deposited on soda-lime glass substrates, adding a thin Sb layer onto the Mo back contact prior to the CIGS absorber deposition. The defect structure of CIGS solar cells doped with Sb in this way has been investigated and is compared with that of undoped reference cells. The influence of substrate temperature during absorber growth has also been evaluated. For all samples the photoluminescence results can be explained by considering three donor–acceptor pair recombination processes involving the same defect pairs
VUV-Vis optical characterization of Tetraphenyl-butadiene films on glass and specular reflector substrates from room to liquid Argon temperature
The use of efficient wavelength-shifters from the vacuum-ultraviolet to the
photosensor's range of sensitivity is a key feature in detectors for Dark
Matter search and neutrino physics based on liquid argon scintillation
detection. Thin film of Tetraphenyl-butadiene (TPB) deposited onto the surface
delimiting the active volume of the detector and/or onto the photosensor
optical window is the most common solution in current and planned experiments.
Detector design and response can be evaluated and correctly simulated only when
the properties of the optical system in use (TPB film + substrate) are fully
understood. Characterization of the optical system requires specific, sometimes
sophisticated optical methodologies. In this paper the main features of TPB
coatings on different, commonly used substrates is reported, as a result of two
independent campaigns of measurements at the specialized optical metrology labs
of ENEA and University of Tor Vergata. Measured features include TPB emission
spectra with lineshape and relative intensity variation recorded as a function
of the film thickness and for the first time down to LAr temperature, as well
as optical reflectance and transmittance spectra of the TPB coated substrates
in the wavelength range of the TPB emission
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