72,799 research outputs found
Ultrafast and octave-spanning optical nonlinearities from strongly phase-mismatched cascaded interactions
Cascaded nonlinearities have attracted much interest, but ultrafast
applications have been seriously hampered by the simultaneous requirements of
being near phase-matching and having ultrafast femtosecond response times. Here
we show that in strongly phase-mismatched nonlinear frequency conversion
crystals the pump pulse can experience a large and extremely broadband
self-defocusing cascaded Kerr-like nonlinearity. The large cascaded
nonlinearity is ensured through interaction with the largest quadratic tensor
element in the crystal, and the strong phase-mismatch ensures an ultrafast
nonlinear response with an octave-spanning bandwidth. We verify this
experimentally by showing few-cycle soliton compression with noncritical
cascaded second-harmonic generation: Energetic 47 fs infrared pulses are
compressed in a just 1-mm long bulk lithium niobate crystal to 17 fs (under 4
optical cycles) with 80% efficiency, and upon further propagation an
octave-spanning supercontinuum is observed. Such ultrafast cascading is
expected to occur for a broad range of pump wavelengths spanning the near- and
mid-IR using standard nonlinear crystals.Comment: resubmitted, revised version, accepted for Phys. Rev. Let
Satellite estimates of wide-range suspended sediment concentrations in Changjiang (Yangtze) estuary using MERIS data
The Changjiang (Yangtze) estuarine and coastal waters are characterized by suspended sediments over a wide range of concentrations from 20 to 2,500 mg l-1. Suspended sediment plays important roles in the estuarine and coastal system and environment. Previous algorithms for satellite estimates of suspended sediment concentration (SSC) showed a great limitation in that only low to moderate concentrations (up to 50 mg l-1) could be reliably estimated. In this study, we developed a semi-empirical radiative transfer (SERT) model with physically based empirical coefficients to estimate SSC from MERIS data over turbid waters with a much wider range of SSC. The model was based on the Kubelka–Munk two-stream approximation of radiative transfer theory and calibrated using datasets from in situ measurements and outdoor controlled tank experiments. The results show that the sensitivity and saturation level of remote-sensing reflectance to SSC are dependent on wavelengths and SSC levels. Therefore, the SERT model, coupled with a multi-conditional algorithm scheme adapted to satellite retrieval of wide-range SSC, was proposed. Results suggest that this method is more effective and accurate in the estimation of SSC over turbid water
Tunable Quantum Fluctuation-Controlled Coherent Spin Dynamics
Temporal evolution of a macroscopic condensate of ultra cold atoms is usually
driven by mean field potentials, either due to scattering between atoms or due
to coupling to external fields; and coherent quantum dynamics have been
observed in various cold-atom experiments. In this article, we report results
of studies of a class of quantum spin dynamics which are purely driven by zero
point quantum fluctuations of spin collective coordinates. Unlike the usual
mean-field coherent dynamics, quantum fluctuation-controlled spin dynamics or
QFCSD studied here are very sensitive to variation of quantum fluctuations and
can be tuned by four to five order of magnitude using optical lattices. They
have unique dependence on optical lattice potential depths and quadratic Zeeman
fields. QFCSD can be potentially used to calibrate quantum fluctuations and
investigate correlated fluctuations and various universal scaling properties
near quantum critical points.Comment: 14 pages, 12 figures included; including detailed discussions on
thermal effects, trapping potentials and spin exchange losses. (To appear in
PRA
Quantum fluctuation-induced uniaxial and biaxial spin nematics
It is shown that zero point quantum fluctuations (ZPQFs) completely lift the
accidental continuous degeneracy that is found in mean field analysis of
quantum spin nematic phases of hyperfine spin 2 cold atoms. The result is two
distinct ground states which have higher symmetries: a uniaxial spin nematic
and a biaxial spin nematic with dihedral symmetry . There is a novel
first order quantum phase transition between the two phases as atomic
scattering lengths are varied. We find that the ground state of atoms
should be a uniaxial spin nematic. We note that the energy barrier between the
phases could be observable in dynamical experiments.Comment: 4 pages, 2 figures included; published versio
Determinants of immediate price impacts at the trade level in an emerging order-driven market
The common wisdom argues that, in general, large trades cause large price
changes, while small trades cause small price changes. However, for extremely
large price changes, the trade size and news play a minor role, while the
liquidity (especially price gaps on the limit order book) is a more influencing
factor. Hence, there might be other influencing factors of immediate price
impacts of trades. In this paper, through mechanical analysis of price
variations before and after a trade of arbitrary size, we identify that the
trade size, the bid-ask spread, the price gaps and the outstanding volumes at
the bid and ask sides of the limit order book have impacts on the changes of
prices. We propose two regression models to investigate the influences of these
microscopic factors on the price impact of buyer-initiated partially filled
trades, seller-initiated partially filled trades, buyer-initiated filled
trades, and seller-initiated filled trades. We find that they have
quantitatively similar explanation powers and these factors can account for up
to 44% of the price impacts. Large trade sizes, wide bid-ask spreads, high
liquidity at the same side and low liquidity at the opposite side will cause a
large price impact. We also find that the liquidity at the opposite side has a
more influencing impact than the liquidity at the same side. Our results shed
new lights on the determinants of immediate price impacts.Comment: 21 IOP tex pages including 5 figures and 5 tables. Accepted for
publication in New Journal of Physic
Melt-growth dynamics in CdTe crystals
We use a new, quantum-mechanics-based bond-order potential (BOP) to reveal
melt-growth dynamics and fine-scale defect formation mechanisms in CdTe
crystals. Previous molecular dynamics simulations of semiconductors have shown
qualitatively incorrect behavior due to the lack of an interatomic potential
capable of predicting both crystalline growth and property trends of many
transitional structures encountered during the melt crystal
transformation. Here we demonstrate successful molecular dynamics simulations
of melt-growth in CdTe using a BOP that significantly improves over other
potentials on property trends of different phases. Our simulations result in a
detailed understanding of defect formation during the melt-growth process.
Equally important, we show that the new BOP enables defect formation mechanisms
to be studied at a scale level comparable to empirical molecular dynamics
simulation methods with a fidelity level approaching quantum-mechanical method
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