113 research outputs found
Visualizing the Local Optical Response of Semiconducting Carbon Nanotubes to DNA-Wrapping
We studied the local optical response of semiconducting single-walled carbon nanotubes to wrapping by DNA segments using high resolution tip-enhanced near-field microscopy. Photoluminescence (PL) near-field images of single nanotubes reveal large DNA-wrapping-induced red shifts of the exciton energy that are two times higher than indicated by spatially averaging confocal microscopy. Near-field PL spectra taken along nanotubes feature two distinct PL bands resulting from DNA-wrapped and unwrapped nanotube segments. The transition between the two energy levels occurs on a length scale smaller than our spatial resolution of about 15 nm
Exciton decay dynamics in individual carbon nanotubes at room temperature
We studied the exciton decay dynamics of individual semiconducting single-walled carbon nanotubes at room temperature using time-resolved photoluminescence spectroscopy. The photoluminescence decay from nanotubes of the same (n,m) type follows a single exponential decay function, however, with lifetimes varying between about 1 and 40 ps from nanotube to nanotube. A correlation between broad photoluminescence spectra and short lifetimes was found and explained by defects promoting both nonradiative decay and vibronic dephasing
Nitrogen-Functionalized Graphene Nanoflakes (GNFs:N): Tunable Photoluminescence and Electronic Structures
This study investigates the strong photoluminescence (PL) and X-ray excited
optical luminescence observed in nitrogen-functionalized 2D graphene nanoflakes
(GNFs:N), which arise from the significantly enhanced density of states in the
region of {\pi} states and the gap between {\pi} and {\pi}* states. The
increase in the number of the sp2 clusters in the form of pyridine-like N-C,
graphite-N-like, and the C=O bonding and the resonant energy transfer from the
N and O atoms to the sp2 clusters were found to be responsible for the blue
shift and the enhancement of the main PL emission feature. The enhanced PL is
strongly related to the induced changes of the electronic structures and
bonding properties, which were revealed by the X-ray absorption near-edge
structure, X-ray emission spectroscopy, and resonance inelastic X-ray
scattering. The study demonstrates that PL emission can be tailored through
appropriate tuning of the nitrogen and oxygen contents in GNFs and pave the way
for new optoelectronic devices.Comment: 8 pages, 6 figures (including toc figure
Multiwavelength study of extreme variability in LEDA 1154204: A changing-look event in a type 1.9 Seyfert
Context. Multiwavelength studies of transients in actively accreting
supermassive black holes have revealed that large-amplitude variability is
frequently linked to significant changes in the optical spectra -- a phenomenon
referred to as changing-look AGN (CLAGN).
Aim. In 2020, the Zwicky Transient Facility detected a transient flaring
event in the type-1.9 AGN 6dFGS~gJ042838.8-000040, wherein a sharp increase in
magnitude of 0.55 and 0.3 in the - and -bands, respectively,
occurred over 40 days. Spectrum Roentgen Gamma (SRG)/eROSITA also
observed the object in X-rays as part of its all-sky survey, but only after the
flare had started decaying.
Methods. We performed a three-year, multiwavelength follow-up campaign of the
source to track its spectral and temporal characteristics. This campaign
included multiple ground-based facilities for optical spectroscopic monitoring
and space-based observatories including \textit{XMM-Newton} and \textit{Swift}
for X-ray and UV observations.
Results. An optical spectrum taken immediately after the peak revealed a
changing-look event wherein the source had transitioned from type 1.9 to 1,
with the appearance of a double-peaked broad H line and a blue
continuum, both absent in an archival spectrum from 2005. The X-ray emission
exhibits dramatic flux variation: a factor of 17, but with no spectral
evolution, as the power-law photon index remained 1.9. There is no
evidence of a soft X-ray excess. Overall the object exhibits no apparent
signatures of a tidal disruption event.
Conclusions. The transient event was likely triggered by a disk instability
in a pre-existing accretion flow, culminating in the observed multi-wavelength
variability and CLAGN event.Comment: 34 pages, 24 figures, Submitted to Astronomy & Astrophysic
Discovery of the luminous X-ray ignition eRASSt J234402.9352640; I. Tidal disruption event or a rapid increase in accretion in an active galactic nucleus?
In November 2020, a new, bright object, eRASSt J234402.9352640, was
discovered in the second all-sky survey of SRG/eROSITA. The object brightened
by a factor of at least 150 in 0.2--2.0 keV flux compared to an upper limit
found six months previous, reaching an observed peak of erg cm s. The X-ray ignition is associated with
a galaxy at , making the peak luminosity log=. Around the time of the rise in
X-ray flux, the nucleus of the galaxy brightened by approximately 3 mag. in
optical photometry, after correcting for the host. We present data from Swift,
XMM-Newton, and NICER, which reveal a very soft spectrum as well as strong
0.2--2.0 keV flux variability on multiple timescales. Optical spectra taken in
the weeks after the ignition event show a blue continuum with broad, asymmetric
Balmer emission lines, and high-ionisation ([OIII]4959,5007)
and low-ionisation ([NII]6585, [SII]6716,6731) narrow
emission lines. Following the peak in the optical light curve, the X-ray, UV,
and optical photometry all show a rapid decline. The X-ray light curve shows a
decrease in luminosity of 0.45 over 33 days and the UV shows a drop of
0.35. eRASSt J234402.9352640 also shows a brightening in the
mid-infrared, likely powered by a dust echo of the luminous ignition. We find
no evidence in Fermi-LAT -ray data for jet-like emission. The event
displays characteristics of a tidal disruption event (TDE) as well as of an
active galactic nucleus (AGN), complicating its classification. Based on the
softness of the X-ray spectrum, the presence of high-ionisation optical
emission lines, and the likely infrared echo, we find that a TDE within a
turned-off AGN best matches our observations.Comment: 25 pages, 17 figures, 9 tables, Accepted for publication in A&
Graphene Photonics and Optoelectronics
The richness of optical and electronic properties of graphene attracts
enormous interest. Graphene has high mobility and optical transparency, in
addition to flexibility, robustness and environmental stability. So far, the
main focus has been on fundamental physics and electronic devices. However, we
believe its true potential to be in photonics and optoelectronics, where the
combination of its unique optical and electronic properties can be fully
exploited, even in the absence of a bandgap, and the linear dispersion of the
Dirac electrons enables ultra-wide-band tunability. The rise of graphene in
photonics and optoelectronics is shown by several recent results, ranging from
solar cells and light emitting devices, to touch screens, photodetectors and
ultrafast lasers. Here we review the state of the art in this emerging field.Comment: Review Nature Photonics, in pres
Graphene Mode-Locked Ultrafast Laser
Graphene is at the center of a significant research effort. Near-ballistic
transport at room temperature and high mobility make it a potential material
for nanoelectronics. Its electronic and mechanical properties are also ideal
for micro and nanomechanical systems, thin-film transistors and transparent and
conductive composites and electrodes. Here we exploit the optoelectronic
properties of graphene to realize an ultrafast laser. A graphene-polymer
composite is fabricated using wet-chemistry techniques. Pauli blocking
following intense illumination results in saturable absorption, independent of
wavelength. This is used to passively mode-lock an Erbium-doped fibre laser
working at 1559nm, with a 5.24nm spectral bandwidth and ~460fs pulse duration,
paving the way to graphene-based photonics
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