73 research outputs found
Femtosecond Photoexcited Carrier Dynamics in Reduced Graphene Oxide Suspensions and Films
We report ultrafast response of femtosecond photoexcited carriers in single
layer reduced graphene oxide flakes suspended in water as well as few layer
thick film deposited on indium tin oxide coated glass plate using pump-probe
differential transmission spectroscopy at 790 nm. The carrier relaxation
dynamics has three components: ~200 fs, 1 to 2 ps, and ~25 ps, all of them
independent of pump fluence. It is seen that the second component (1 to 2 ps)
assigned to the lifetime of hot optical phonons is larger for graphene in
suspensions whereas other two time constants are the same for both the
suspension and the film. The value of third order nonlinear susceptibility
estimated from the pump-probe experiments is compared with that obtained from
the open aperture Z-scan results for the suspension.Comment: 4 pages, 4 figures, to appear in International Journal of Nanoscience
(IJN), 201
Ultrafast photoinduced enhancement of nonlinear optical response in 15-atom gold clusters on indium tin oxide conducting film
We show that the third order optical nonlinearity of 15-atom gold clusters is
significantly enhanced when in contact with indium tin oxide (ITO) conducting
film. Open and close aperture z-scan experiments together with non-degenerate
pump-probe differential transmission experiments were done using 80 fs laser
pulses centered at 395 nm and 790 nm on gold clusters encased inside
cyclodextrin cavities. We show that two photon absorption coefficient is
enhanced by an order of magnitude as compared to that when the clusters are on
pristine glass plate. The enhancement for the nonlinear optical refraction
coefficient is ~3 times. The photo-induced excited state absorption using
pump-probe experiments at pump wavelength of 395 nm and probe at 790 nm also
show an enhancement by an order of magnitude. These results attributed to the
excited state energy transfer in the coupled gold cluster-ITO system are
different from the enhancement seen so far in charge donor-acceptor complexes
and nanoparticle-conjugate polymer composites.Comment: To appear in Optics Express (2013);
http://dx.doi.org/10.1364/OE.21.00848
Acoustic and optical phonon dynamics from femtosecond time-resolved optical spectroscopy of superconducting iron pnictide Ca(Fe_0.944Co_0.056)_2As_2
We report temperature evolution of coherently excited acoustic and optical
phonon dynamics in superconducting iron pnictide single crystal
Ca(Fe_0.944Co_0.056)_2As_2 across the spin density wave transition at T_SDW ~
85 K and superconducting transition at T_SC ~20 K. Strain pulse propagation
model applied to the generation of the acoustic phonons yields the temperature
dependence of the optical constants, and longitudinal and transverse sound
velocities in the temperature range of 3.1 K to 300 K. The frequency and
dephasing times of the phonons show anomalous temperature dependence below T_SC
indicating a coupling of these low energy excitations with the Cooper-pair
quasiparticles. A maximum in the amplitude of the acoustic modes at T ~ 170 is
seen, attributed to spin fluctuations and strong spin-lattice coupling before
T_SDW.Comment: 6 pages, 4 figures (revised manuscript
Ultrafast quasiparticle dynamics in superconducting iron pnictide CaFe1.89Co0.11As2
Nonequilibrium quasiparticle relaxation dynamics is reported in
superconducting CaFe1.89Co0.11As2 single crystal using femtosecond
time-resolved pump-probe spectroscopy. The carrier dynamics reflects a
three-channel decay of laser deposited energy with characteristic time scales
varying from few hundreds of femtoseconds to order of few nanoseconds where the
amplitudes and time-constants of the individual electronic relaxation
components show significant changes in the vicinity of the spin density wave
(T_SDW ~ 85 K) and superconducting (T_SC ~ 20 K) phase transition temperatures.
The quasiparticles dynamics in the superconducting state reveals a charge gap
with reduced gap value of 2_0/k_BT_SC ~ 1.8. We have determined the
electron-phonon coupling constant \lemda to be ~ 0.14 from the temperature
dependent relaxation time in the normal state, a value close to those reported
for other types of pnictides. From the peculiar temperature-dependence of the
carrier dynamics in the intermediate temperature region between the
superconducting and spin density wave phase transitions, we infer a temperature
scale where the charge gap associated with the spin ordered phase is maximum
and closes on either side while approaching the two phase transition
temperatures.Comment: 6 pages, 4 figures (revised manuscript);
http://dx.doi.org/10.1016/j.ssc.2013.02.00
Ultrafast switching time and third order nonlinear coefficients of microwave treated single walled carbon nanotube suspensions
Microwave treated water soluble and amide functionalized single walled carbon
nanotubes have been investigated using femtosecond degenerate pump-probe and
nonlinear transmission experiments. The time resolved differential transmission
using 75 femtosecond pulse with the central wavelength of 790 nm shows a
bi-exponential ultrafast photo-bleaching with time constants of of 160 fs (130
fs) and 920 fs (300 fs) for water soluble (amide functionalized) nanotubes.
Open and closed aperture z-scans show saturation absorption and positive
(negative) nonlinear refraction for water soluble (amide functionalized)
nanotubes. Two photon absorption coefficient,beata ~250 cm/GW (650 cm/GW) and
nonlinear index, gamma ~ 15 cm^2/pW (-30 cm^2/pW) are obtained from the
theoretical fit in the saturation limit to the data for two types of nanotubes.Comment: 15 pages, 4 figure
Large-amplitude chirped coherent phonons in tellurium mediated by ultrafast photoexcited carrier diffusion
We report femtosecond time-resolved reflectivity measurements of coherent
phonons in tellurium performed over a wide range of temperatures (3K to 296K)
and pump laser intensities. A totally symmetric A coherent phonon at 3.6
THz responsible for the oscillations in the reflectivity data is observed to be
strongly positively chirped (i.e, phonon time period decreases at longer
pump-probe delay times) with increasing photoexcited carrier density, more so
at lower temperatures. We show for the first time that the temperature
dependence of the coherent phonon frequency is anomalous (i.e, increasing with
increasing temperature) at high photoexcited carrier density due to
electron-phonon interaction. At the highest photoexcited carrier density of
1.4 10cm and the sample temperature of 3K, the
lattice displacement of the coherent phonon mode is estimated to be as high as
0.24 \AA. Numerical simulations based on coupled effects of optical
absorption and carrier diffusion reveal that the diffusion of carriers
dominates the non-oscillatory electronic part of the time-resolved
reflectivity. Finally, using the pump-probe experiments at low carrier density
of 6 10 cm, we separate the phonon anharmonicity to
obtain the electron-phonon coupling contribution to the phonon frequency and
linewidth.Comment: 22 pages, 6 figures, submitted to PR
Femtosecond carrier dynamics and saturable absorption in graphene suspensions
Nonlinear optical properties and carrier relaxation dynamics in graphene,
suspended in three different solvents, are investigated using femtosecond (80
fs pulses) Z-scan and degenerate pumpprobe spectroscopy at 790 nm. The results
demonstrate saturable absorption property of graphene with a nonlinear
absorption coefficient, , of ~2 to 9x10^-8 cm/W. Two distinct time scales
associated with the relaxation of photoexcited carriers, a fast one in the
range of 130-330 fs (related to carrier-carrier scattering) followed by a
slower one in 3.5-4.9 ps range (associated with carrier-phonon scattering) are
observed.Comment: 3 pages, 2 figures, 2 table
Large nonlinear absorption and refraction coefficients of carbon nanotubes estimated from femtosecond Z-scan measurements
Nonlinear transmission of 80 and 140 femtosecond pulsed light with wavelength through single walled carbon nanotubes suspended in water
containing sodium dodecyl sulphate is studied. Pulse-width independent
saturation absorption and negative cubic nonlinearity are observed,
respectively, in open and closed aperture Z-scan experiments. The theoretical
expressions derived to analyze the z-dependent transmission in the saturable
limit require two photon absorption coefficient and a
nonlinear index to fit the data.Comment: 10 pages, 2 figures. Accepted and to appear in Applied Physics
Letter
Gemella Morbillorum as a Cause of Septic Shock.
The gram-positive bacterium Gemella morbillorum has been recovered from patients with endocarditis but has rarely been associated with acute fulminant infections. We describe two children with a rapid onset of septic shock, which was fatal in one, following infection with this organism. G. morbillorum is a commensal organism of the upper respiratory tract; it gained access to the bloodstreams in these patients, and bacteremia occurred. A clinical drawback is that the initial colonial morphology of this organism leads to presumptive identification as a viridans streptococcus, an organism not commonly associated with septic shock syndrome. Resistance of G. morbillorum to penicillin appears to be common; therefore, initial empirical combination therapy (a beta-lactam agent and an aminoglycoside) or vancomycin treatment should be considered
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