Quantum Mechanical Treatment of Stimulated Raman Cross Sections

Stimulated Raman scattering (SRS) has played an increasingly pivotal role in chemistry and photonics. Recently, understanding of light-molecule interaction during SRS was brought to a new quantitative level through the introduction of stimulated Raman cross section, $\sigma_{SRS}$. Measurements of Raman-active molecules have revealed interesting insights, and theoretical consideration has suggested an Einstein-coefficient-like relation between $\sigma_{SRS}$ and the commonly used spontaneous Raman cross sections, $\sigma_{Raman}$. However, the theoretical underpinning of $\sigma_{SRS}$ is not known. Herein we provide a full quantum mechanical treatment for $\sigma_{SRS}$, via both a semi-classical method and a quantum electrodynamic (QED) method. The resulting formula provides a rigorous theory to predict experimental outcome from first principles, and unveils key physical factors rendering $\sigma_{SRS}$ inherently strong response. Through this formula, we also confirm the validity of the Einstein-coefficient-like equation connecting $\sigma_{Raman}$ and $\sigma_{SRS}$ reported earlier, and discuss the inherent symmetry between all spontaneous and stimulated optical processes. Hence the present treatment shall deepen the fundamental understanding of the molecular response during SRS, and facilitate quantitative applications in various experiments.Comment: 19 pages, 2 figure

Polarized γ\gamma-photon beams produced by collision of two ultrarelativistic electron beams

Many studies have shown that high-energy $\gamma$-photon beams can be efficiently generated via nonlinear Compton scattering driven by laser pulses with intensities $> 10^{22}\rm{W/cm^2}$ recently available in laboratories. Here, we propose a laserless scheme to efficiently generate high-energy polarized $\gamma$-photon beams by collision of two ultrarelativistic electron beams. The self-generated field of a dense driving electron beam provides the strong deflection field for the other ultrarelativistic seeding electron beam. A QED Monte Carlo code based on the locally constant field approximation is employed to simulate the collision process, and the polarization properties of produced $\gamma$ photons are investigated. The simulation results and theoretical analysis indicate that the photon polarization, including both linear and circular polarizations, can be tuned by changing the initial polarization of the seeding beam. If an unpolarized seeding beam is used, linearly polarized photons with an average polarization of 55\% can be obtained. If the seeding beam is transversely (longitudinally) polarized, the linear (circular) polarization of photons above 3 GeV can reach 90\% (67\%), which is favorable for highly polarized, high-energy $\gamma$ photon sources.Comment: 12 pages, 8 figure

Theory for charge and orbital density-wave states in manganite La0.5_{0.5}Sr1.5_{1.5}MnO4_4

We investigate the high temperature phase of layered manganites, and demonstrate that the charge-orbital phase transition without magnetic order in La$_{0.5}$Sr$_{1.5}$MnO$_4$ can be understood in terms of the density wave instability. The orbital ordering is found to be induced by the nesting between segments of Fermi surface with different orbital characters. The simultaneous charge and orbital orderings are elaborated with a mean field theory. The ordered orbitals are shown to be $d_{x^2-y^2} \pm d_{3z^2-r^2}$.Comment: published versio

Radial Angular Momentum Transfer and Magnetic Barrier for Short-Type Gamma-Ray Burst Central Engine Activity

Soft extended emission (EE) following initial hard spikes up to 100 seconds was observed with {\em Swift}/BAT for about half of short-type gamma-ray bursts (SGRBs). This challenges the conversional central engine models of SGRBs, i.e., compact star merger models. In the framework of the black hole-neutron star merger models, we study the roles of the radial angular momentum transfer in the disk and the magnetic barrier around the black hole for the activity of SGRB central engines. We show that the radial angular momentum transfer may significantly prolong the lifetime of the accretion process and multiple episodes may be switched by the magnetic barrier. Our numerical calculations based on the models of the neutrino-dominated accretion flows suggest that the disk mass is critical for producing the observed EE. In case of the mass being $\sim 0.8M_{\odot}$, our model can reproduce the observed timescale and luminosity of both the main and EE episodes in a reasonable parameter set. The predicted luminosity of the EE component is lower than the observed EE with about one order of magnitude and the timescale is shorter than 20 seconds if the disk mass being $\sim 0.2M_{\odot}$. {\em Swift}/BAT-like instruments may be not sensitive enough to detect the EE component in this case. We argue that the EE component would be a probe for merger process and disk formation for compact star mergers.Comment: 9 pages, 3 figures, accepted for publication in Ap

Optical polarization rogue waves in fiber laser

A new kind of optical rogue waves, polarization rogue waves that appear with greatly deviated and unpredictable positions of polarization states, is identified in forming partially mode-locked fiber laser based on parametric frequency conversion