19 research outputs found
Excitonic Resonances in Coherent Anti-Stokes Raman Scattering from Single-Walled Carbon Nanotubes
In this work we investigate the role of exciton resonances in coherent anti-Stokes Raman scattering (er-CARS) in single walled carbon nanotubes (SWCNTs). We drive the nanotube system in simultaneous phonon and excitonic resonances, where we observe a superior enhancement by orders of magnitude exceeding non-resonant cases. We investigated the resonant effects in five chiralities and find that the er-CARS intensity varies drastically between different nanotube species. The experimental results are compared with a perturbation theory model. Finally, we show that such giant resonant non-linear signals enable rapid mapping and local heating of individualized CNTs, suggesting easy tracking of CNTs for future nanotoxology studies and therapeutic application in biological tissues
Excitonic Resonances in Coherent Anti-Stokes Raman Scattering from Single Wall Carbon Nanotubes
In this work we investigate the role of exciton resonances in coherent
anti-Stokes Raman scattering (er-CARS) in single walled carbon nanotubes
(SWCNTs). We drive the nanotube system in simultaneous phonon and excitonic
resonances, where we observe a superior enhancement by orders of magnitude
exceeding non-resonant cases. We investigated the resonant effects in five
chiralities and find that the er-CARS intensity varies drastically
between different nanotube species. The experimental results are compared with
a perturbation theory model. Finally, we show that such giant resonant
non-linear signals enable rapid mapping and local heating of individualized
CNTs, suggesting easy tracking of CNTs for future nanotoxology studies and
therapeutic application in biological tissues.Comment: 17 pages, 6 figure
Observation of Intra- and Inter-band Transitions in the Optical Response of Graphene
The optical conductivity of freely suspended graphene was examined under
non-equilibrium conditions using femtosecond pump-probe spectroscopy. We
observed a conductivity transient that varied strongly with the electronic
temperature, exhibiting a crossover from enhanced to decreased absorbance with
increasing pump fluence. The response arises from a combination of bleaching of
the inter-band transitions by Pauli blocking and induced absorption from the
intra-band transitions of the carriers. The latter dominates at low electronic
temperature, but, despite an increase in Drude scattering rate, is overwhelmed
by the former at high electronic temperature. The time-evolution of the optical
conductivity in all regimes can described in terms of a time-varying electronic
temperature.Comment: 10 pages (4 pages manuscript + Supplemental Info.
Observation of out-of-plane vibrations in few-layer graphene
We report the observation of layer breathing mode (LBM) vibrations in
few-layer graphene (FLG) samples of thickness from 2 to 6 layers, exhibiting
both Bernal (AB) and rhombohedral (ABC) stacking order. The LBM vibrations are
identified using a Raman combination band lying around 1720 cm-1. From double
resonance theory, we identify the feature as the LOZO' combination mode of the
out-of-plane LBM (ZO') and the in-plane longitudinal optical mode (LO). The
LOZO' Raman band is found to exhibit multiple peaks, with a unique line shape
for each layer thickness and stacking order. These complex line shapes of the
LOZO'-mode arise both from the material-dependent selection of different
phonons in the double-resonance Raman process and from the detailed structure
of the different branches of LBM in FLG.Comment: 17 pages, 7 figures, supplemental material include
Defect-Induced Supercollision Cooling of Photoexcited Carriers in Graphene
Defects play a fundamental role in
the energy relaxation of hot
photoexcited carriers in graphene, thus a complete understanding of
these processes are vital for improving the development of graphene
devices. Recently, it has been theoretically predicted and experimentally
demonstrated that defect-assisted acoustic phonon supercollision,
the collision between a carrier and both an acoustic phonon and a
defect, is an important energy relaxation process for carriers with
excess energy below the optical phonon emission. Here, we studied
samples with defects optically generated in a controlled manner to
experimentally probe the supercollision model as a function of the
defect density. We present pump and probe transient absorption measurements
showing that the decay time decreases as the density of defect increases
as predicted by the supercollision model