124 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 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
Power optimized programmable embedded controller
Now a days, power has become a primary consideration in hardware design, and
is critical in computer systems especially for portable devices with high
performance and more functionality. Clock-gating is the most common technique
used for reducing processor's power. In this work clock gating technique is
applied to optimize the power of fully programmable Embedded Controller (PEC)
employing RISC architecture. The CPU designed supports i) smart instruction
set, ii) I/O port, UART iii) on-chip clocking to provide a range of frequencies
, iv) RISC as well as controller concepts. The whole design is captured using
VHDL and is implemented on FPGA chip using Xilinx .The architecture and clock
gating technique together is found to reduce the power consumption by 33.33% of
total power consumed by this chip.Comment: 11 pages,11 figures,International Journal Publicatio
Graphene analogue BCN: femtosecond nonlinear optical susceptibility and hot carrier dynamics
Third-order nonlinear absorption and refraction coefficients of a few-layer
boron carbon nitride (BCN) and reduced graphene oxide (RGO) suspensions have
been measured at 3.2 eV in the femtosecond regime. Optical limiting behavior is
exhibited by BCN as compared to saturable absorption in RGO. Nondegenerate
time-resolved differential transmissions from BCN and RGO show different
relaxation times. These differences in the optical nonlinearity and carrier
dynamics are discussed in the light of semiconducting electronic band structure
of BCN vis-\`a-vis the Dirac linear band structure of graphene.Comment: 18 pages, 5 figure
Ultrafast electron dynamics and cubic optical nonlinearity of free standing thin film of double walled carbon nanotubes
Ultrafast degenerate pump-probe experiments performed on a free standing film
of double walled carbon nanotubes near the first metallic transition energy of
outer tube show ultrafast () photobleaching followed by a photo-induced
absorption with a slow relaxation of . Femtosecond closed and open
aperture z-scan experiments carried out at the same excitation energy show
saturation absorption and negative cubic nonlinearity. From these measurements,
real and imaginary part of the third order nonlinear susceptibility are
estimated to be Re( and
Im(.Comment: 8 pages, 3 figure
Ultrafast Dynamics of Gold Nanorods: Tuning between Photo-Bleaching and Photo-Induced Absorption
We report ultrafast electron dynamics in gold nanorods investigated using 80
fs laser pulses centered at 1.57 eV. Five types of nanorod colloidal
suspensions in water having their longitudinal surface plasmon peak (ELSP) on
either side of the laser photon energy (EL) have been studied. For ELSP>EL,
photo-induced absorption with single decay time constant is observed. On the
other hand, for ELSP<EL, photo-bleaching is observed having biexponential decay
dynamics; the faster one between 1-3 ps and slower one between 7ps to 22ps both
of them increasing almost linearly with the difference |EL-ELSP|. These time
constants increase linearly with the pump intensity. Simulations have been
carried out to understand the interplay between photo-bleaching and
photoinduced absorption.Comment: 4 pages, 4 figures, To appear in International Journal of
Nanoscience, 201
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