23 research outputs found
Spatiotemporal dynamics of photoexcited quasiparticles in two-dimensional crystals studied by ultrafast laser techniques
Layered materials in which atomic sheets are stacked together by weak van der Waals forces can be used to fabricate two-dimensional systems. They represent a diverse and rich, but largely unexplored, source of materials. Atomically-thin structures derived from these materials possess a number of interesting electrical, optical, and mechanical properties, and are attractive for new nanodevices. For their applications in semiconductor industry, it is necessary to understand the dynamics of photoexcited quasiparticles that occur on ultrafast time scales of less than one nanosecond. In this dissertation, I discuss ultrafast optical experimental techniques and results from various two-dimensional materials, which provide information about electronic dynamics. First, a second harmonic generation technique that can be used to find the crystalline orientation, thickness uniformity, layer stacking, and single-crystal domain size is discussed, with results presented on exfoliated and chemical vapor deposition MoS2 samples. Second, a third harmonic generation technique is discussed, which can be used to explore nonlinear optical properties of materials, and results are presented on graphene and few-layer graphite films. Third, a spatially resolved femtosecond pump-probe is described, which can be used to study hot carrier and photoexcited phonon dynamics and results are presented on Bi2 Se3 sample. Then, exciton dynamics in MoS2 and MoSe2 are explored by using transient absorption microscopy with a high spatiotemporal resolution. Finally, a polarization-resolved femtosecond transient absorption spectroscopy that can be used to study valley and spin dynamics is discussed, with results presented on monolayer, few-layer, and bulk MoSe2 samples
Exciton-exciton annihilation in MoSe2 monolayers
We investigate the excitonic dynamics in MoSe2 monolayer and bulk samples by
femtosecond transient absorption microscopy. Excitons are resonantly injected
by a 750-nm and 100-fs laser pulse, and are detected by a probe pulse tuned in
the range of 790 - 820 nm. We observe a strong density-dependent initial decay
of the exciton population in monolayers, which can be well described by the
exciton-exciton annihilation. Such a feature is not observed in the bulk under
comparable conditions. We also observe the saturated absorption induced by
exciton phase-space filling in both monolayers and the bulk, which indicates
their potential applications as saturable absorbers.Comment: 5 pages, 4 figure
Two-probe study of hot carriers in reduced graphene oxide
The energy relaxation of carriers in reduced graphene oxide thin films is
studied using optical pump-probe spectroscopy with two probes of different
colors. We measure the time difference between peaks of the carrier density at
each probing energy by measuring a time-resolved differential transmission and
find that the carrier density at the lower probing energy peaks later than that
at the higher probing energy. Also, we find that the peak time for the lower
probing energy shifts from about 92 to 37 fs after the higher probing energy
peak as the carrier density is increased from 1.5E12 to 3E13 per square
centimeter, while no noticeable shift is observed in that for the higher
probing energy. Assuming the carriers rapidly thermalize after excitation, this
indicates that the optical phonon emission time decreases from about 50 to
about 20 fs and the energy relaxation rate increases from 4 to 10 meV/fs. The
observed density dependence is inconsistent with the phonon bottleneck effect.Comment: 10 pages, 4 figure
Tightly bound excitons in monolayer WSe2
Exciton binding energy and excited states in monolayers of tungsten
diselenide (WSe2) are investigated using the combined linear absorption and
two-photon photoluminescence excitation spectroscopy. The exciton binding
energy is determined to be 0.37eV, which is about an order of magnitude larger
than that in III-V semiconductor quantum wells and renders the exciton excited
states observable even at room temperature. The exciton excitation spectrum
with both experimentally determined one- and two-photon active states is
distinct from the simple two-dimensional (2D) hydrogenic model. This result
reveals significantly reduced and nonlocal dielectric screening of Coulomb
interactions in 2D semiconductors. The observed large exciton binding energy
will also have a significant impact on next-generation photonics and
optoelectronics applications based on 2D atomic crystals.Comment: 19 pages, 4 figures, to appear in PR
Spatially resolved femtosecond pump-probe study of topological insulator Bi2Se3
Carrier and phonon dynamics in Bi2Se3 crystals are studied by a spatially
resolved ultrafast pump-probe technique. Pronounced oscillations in
differential reflection are observed with two distinct frequencies, and are
attributed to coherent optical and acoustic phonons, respectively. The rising
time of the signal indicates that the thermalization and energy relaxation of
hot carriers are both sub-ps in this material. We found that the thermalization
and relaxation time decreases with the carrier density. The expansion of the
differential reflection profile allows us to estimate an ambipolar carrier
diffusion coefficient on the order of 500 square centimeters per second. A
long-term slow expansion of the profile shows a thermal diffusion coefficient
of 1.2 square centimeters per second.Comment: 8 pages, 6 figure
Second harmonic microscopy of monolayer MoS2
We show that the lack of inversion symmetry in monolayer MoS2 allows strong
optical second harmonic generation. Second harmonic of an 810-nm pulse is
generated in a mechanically exfoliated monolayer, with a nonlinear
susceptibility on the order of 1E-7 m/V. The susceptibility reduces by a factor
of seven in trilayers, and by about two orders of magnitude in even layers. A
proof-of-principle second harmonic microscopy measurement is performed on
samples grown by chemical vapor deposition, which illustrates potential
applications of this effect in fast and non-invasive detection of crystalline
orientation, thickness uniformity, layer stacking, and single-crystal domain
size of atomically thin films of MoS2 and similar materials.Comment: 6 pages, 4 figure
Ionic-passivated FeS2 photocapacitors for energy conversion and storage
This is the publisher's version, also available electronically from http://pubs.rsc.org/en/Content/ArticleLanding/2013/CC/c3cc45088k#!divAbstrac