71 research outputs found
Tracking Berry curvature effect in molecular dynamics by ultrafast magnetic x-ray scattering
The spin-dependent Berry force is a genuine effect of Berry curvature in
molecular dynamics, which can dramatically result in spatial spin separation
and change of reaction pathways. However, the way to probe the effect of Berry
force remains challenging, because the time-reversal (TR) symmetry required for
opposite Berry forces conflicts with TR symmetry breaking spin alignment needed
to observe the effect, and the net effect could be transient for a molecular
wave packet. We demonstrate that in molecular photodissociation, the
dissociation rates can be different for molecules with opposite initial spin
directions due to Berry force. We showcase that the spatially separated spin
density, which is transiently induced by Berry force as the molecular wave
packet passes through conical intersection, can be reconstructed from the
circular dichroism (CD) of ultrafast non-resonant magnetic x-ray scattering
using free electron lasers
Structural Damage Identification Based on Rough Sets and Artificial Neural Network
This paper investigates potential applications of the rough sets (RS) theory and artificial neural network (ANN) method on structural damage detection. An information entropy based discretization algorithm in RS is applied for dimension reduction of the original damage database obtained from finite element analysis (FEA). The proposed approach is tested with a 14-bay steel truss model for structural damage detection. The experimental results show that the damage features can be extracted efficiently from the combined utilization of RS and ANN methods even the volume of measurement data is enormous and with uncertainties
Dynamic Alignment of C2H4 Investigated by Using Two Linearly Polarized Femtosecond Laser Pulses
We have studied multielectron ionization and Coulomb explosion of C2H4 irradiated by 110 fs, 800 nm laser pulses at an intensity of ∼1015 W/cm2. Strong anisotropic angular distributions were observed for the atomic ions Cn+(n = 1–3). Based on the results of two crossed linearly polarized laser pulses, we conclude that such anisotropic angular distributions result from dynamic alignment, in which the rising edge of the laser pulses aligns the neutral C2H4 molecules along the laser polarization direction. The angular distribution of the exploding fragments, therefore, reflects the degree of the alignment of molecules before ionization. Using the same femtosecond laser with intensity below the ionization threshold, the alignment of C2H4 molecules was also observed
Entangled X-ray Photon Pair Generation by Free Electron Lasers
Einstein, Podolsky and Rosen's prediction on incompleteness of quantum
mechanics was overturned by experimental tests on Bell's inequality that
confirmed the existence of quantum entanglement. In X-ray optics, entangled
photon pairs can be generated by X-ray parametric down conversion (XPDC), which
is limited by relatively low efficiency. Meanwhile, free electron laser (FEL)
has successfully lased at X-ray frequencies recently. However, FEL is usually
seen as a classical light source, and its quantum effects are considered minor
corrections to the classical theory. Here we investigate entangled X-ray photon
pair emissions in FEL. We establish a theory for coherently amplified entangled
photon pair emission from microbunched electron pulses in the undulator. We
also propose an experimental scheme for the observation of the entangled photon
pairs via energy and spatial correlation measurements. Such an entangled X-ray
photon pair source is of great importance in quantum optics and other X-ray
applications.Comment: 13 pages, 3 figure
Ramsey interferometry through coherent coupling and population transfer in N air laser
The laser-like coherent emission at 391nm from N gas irradiated by strong
800nm pump laser and weak 400nm seed laser is theoretically investigated.
Recent experimental observations are well simulated, including temporal
profile, optical gain and periodic modulation of the 391nm signal from N.
Our calculation sheds light on the long standing controversy on whether
population inversion is indispensable for the optical gain. We demonstrate the
Ramsey interference fringes of the emission intensity at 391nm formed by
additionally injecting another 800nm pump or 400nm seed, which are well
explained by the coherent modulation of transition dipole moment and population
between the - states as well as the
- states. This study provides versatile
possibilities for the coherent control of air laser.Comment: 5 pages, 5 figure
Controlling the polarization of nitrogen ion lasing
Air lasing provides a promising technique to remotely produce coherent
radiation in the atmosphere and attracts continuous attention. However, the
polarization properties of N2+ lasing with seeding has not been understood
since it was discovered ten years ago, in which the behaviors appear disordered
and confusing. Here, we performed an experimental and theoretical investigation
on the polarization properties of N2+ lasing and successfully revealed its
underlying physical mechanism. We found that the optical gain is anisotropic
owing to the permanent alignment of N2+ induced by the preferential ionization
of the pump light. As a result, the polarization of N2+ lasing tends to align
with that of the pump light after amplification, which becomes more pronounced
with increasing amplification factor. Based on the permanent alignment of N2+,
we built a theoretical model that analytically interpreted and numerically
reproduced the experimental observations, which points out the key factors for
controlling the polarization of N2+ lasing.Comment: 12 pages, 4 figure
Population Redistribution among Multiple Electronic States of Molecular Nitrogen Ions in Strong Laser Fields
We carry out a combined theoretical and experimental investigation on the
population distributions in the ground and excited states of tunnel ionized N2
molecules at various driver wavelengths in the near- and mid-infrared range.
Our results reveal that efficient couplings (i.e., population exchanges)
between the ground state and the excited states occur in strong laser fields.
The couplings result in the population inversion between the ground and the
excited states at the wavelengths near 800 nm, which is verified by our
experiment by observing the amplification of a seed at ~391 nm. The result
provides insight into the mechanism of free-space nitrogen ion lasers generated
in remote air with strong femtosecond laser pulses.Comment: 18 pages, 4 figure
Amplification of light pulses with orbital angular momentum (OAM) in nitrogen ions lasing
Nitrogen ions pumped by intense femtosecond laser pulses give rise to optical
amplification in the ultraviolet range. Here, we demonstrated that a seed light
pulse carrying orbital angular momentum (OAM) can be significantly amplified in
nitrogen plasma excited by a Gaussian femtosecond laser pulse. With the
topological charge of +1 and -1, we observed an energy amplification of the
seed light pulse by two orders of magnitude, while the amplified pulse carries
the same OAM as the incident seed pulse. Moreover, we show that a spatial
misalignment of the plasma amplifier with the OAM seed beam leads to an
amplified emission of Gaussian mode without OAM, due to the special spatial
profile of the OAM seed pulse that presents a donut-shaped intensity
distribution. Utilizing this misalignment, we can implement an optical switch
that toggles the output signal between Gaussian mode and OAM mode. This work
not only certifies the phase transfer from the seed light to the amplified
signal, but also highlights the important role of spatial overlap of the
donut-shaped seed beam with the gain region of the nitrogen plasma for the
achievement of OAM beam amplification.Comment: 10 pages, 7 figure
High-Power and Ultralong-Life Aqueous Zinc-Ion Hybrid Capacitors Based on Pseudocapacitive Charge Storage
© 2019, © 2019, The Author(s). Rechargeable aqueous zinc-ion hybrid capacitors and zinc-ion batteries are promising safe energy storage systems. In this study, amorphous RuO2·H2O for the first time was employed to achieve fast and ultralong-life Zn2+ storage based on a pseudocapacitive storage mechanism. In the RuO2·H2O||Zn zinc-ion hybrid capacitors with Zn(CF3SO3)2 aqueous electrolyte, the RuO2·H2O cathode can reversibly store Zn2+ in a voltage window of 0.4–1.6 V (vs. Zn/Zn2+), delivering a high discharge capacity of 122 mAh g−1. In particular, the zinc-ion hybrid capacitors can be rapidly charged/discharged within 36 s with a very high power density of 16.74 kW kg−1 and a high energy density of 82 Wh kg−1. Besides, the zinc-ion hybrid capacitors demonstrate an ultralong cycle life (over 10,000 charge/discharge cycles). The kinetic analysis elucidates that the ultrafast Zn2+ storage in the RuO2·H2O cathode originates from redox pseudocapacitive reactions. This work could greatly facilitate the development of high-power and safe electrochemical energy storage.[Figure not available: see fulltext.]
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