108 research outputs found
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
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
Structured air lasing of N2+
Structured light has attracted great interest in scientific and technical
fields. Here, we demonstrate the first generation of structured air lasing in
N2+ driven by 800 nm femtosecond laser pulses. By focusing a vortex pump beam
at 800 nm in N2 gas, we generate a vortex superfluorescent radiation of N2+ at
391 nm, which carries the same photon orbital angular momentum as the pump
beam. With the injection of a Gaussian seed beam at 391 nm, the coherent
radiation is amplified, but the vorticity is unchanged. A new physical
mechanism is revealed in the vortex N2+ superfluorescent radiation: the vortex
pump beam transfers the spatial spiral phase into the N2+ gain medium, and the
Gaussian seed beam picks up the spatial spiral phase and is then amplified into
a vortex beam. Moreover, when we employ a pump beam with a cylindrical vector
mode, the Gaussian seed beam is correspondingly amplified into a cylindrical
vector beam. Surprisingly, the spatial polarization state of the amplified
radiation is identical to that of the vector pump beam regardless of whether
the Gaussian seed beam is linearly, elliptically, or circularly polarized.
Solving three-dimensional coupled wave equations, we show how a Gaussian beam
becomes a cylindrical vector beam in a cylindrically symmetric gain medium.
This study provides a novel approach to generating structured light via N2+ air
lasing.Comment: 18 pages, 5 figures, 3 equation
Prompt-to-afterglow transition of optical emission in a long gamma-ray burst consistent with a fireball
Long gamma-ray bursts (GRBs), which signify the end-life collapsing of very
massive stars, are produced by extremely relativistic jets colliding into
circumstellar medium. Huge energy is released both in the first few seconds,
namely the internal dissipation phase that powers prompt emissions, and in the
subsequent self-similar jet-deceleration phase that produces afterglows
observed in broad-band electromagnetic spectrum. However, prompt optical
emissions of GRBs have been rarely detected, seriously limiting our
understanding of the transition between the two phases. Here we report
detection of prompt optical emissions from a gamma-ray burst (i.e. GRB 201223A)
using a dedicated telescope array with a high temporal resolution and a wide
time coverage. The early phase coincident with prompt {\gamma}-ray emissions
show a luminosity in great excess with respect to the extrapolation of
{\gamma}-rays, while the later luminosity bump is consistent with onset of the
afterglow. The clearly detected transition allows us to differentiate physical
processes contributing to early optical emissions and to diagnose the
composition of the jetComment: Authors' version of article published in Nature Astronomy, see their
website for official versio
An asymmetric upwind flow, Yellow Sea Warm Current : 1. New observations in the western Yellow Sea
Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 116 (2011): C04026, doi:10.1029/2010JC006513.The winter water mass along the Yellow Sea Trough (YST), especially on the western side of the trough, is considerably warmer and saltier than the ambient shelf water mass. This observed tongue-shape hydrographic feature implies the existence of a winter along-trough and onshore current, often referred to as the Yellow Sea Warm Current (YSWC). However, the YSWC has not been confirmed by direct current measurements and therefore skepticism remains regarding its existence. Some studies suggest that the presence of the warm water could be due to frontal instability, eddies, or synoptic scale wind bursts. It is noted that in situ observations used in most previous studies were from the central and eastern sides of the YST even though it is known that the warm water core is more pronounced along the western side. Data from the western side have been scarce. Here we present a set of newly available Chinese observations, including some from a coordinated effort involving three Chinese vessels in the western YST during the 2006–2007 winter. The data show unambiguously the existence of the warm current on the western side of YST. Both the current and hydrography observations indicate a dominant barotropic structure of YSWC. The westward deviation of YSWC axis is particularly obvious to the south of 35°N and is clearly associated with an onshore movement of warm water. To the north of 35°N, the YSWC flows along the bathymetry with slightly downslope movement. We conclude that the barotropic current is mainly responsible for the warm water intrusion, while the Ekman and baroclinic currents play an important but secondary role. These observations help fill an observational gap and establish a more complete view of the YSWC.The authors have been supported by China’s
National Basic Research Priorities Programmer (2007CB411804 and
2005CB422303), the Ministry of Education’s 111 Project (B07036), the Program
for New Century Excellent Talents in University (NECT‐07‐0781),
and the China National Science Fundation (40976004, 40921004. and
40930844). J.Y. is supported by the U.S. National Science Foundation and
the Woods Hole Oceanographic Institution’s Coastal Ocean Institute
Structural, thermal, in vitro degradation and cytocompatibility properties of P2O5-B2O3-CaO-MgO-Na2O-Fe2O3 glasses
Borophosphate glasses with compositions of (48 − x)P2O5-(12 + x)B2O3-14CaO-20MgO-1Na2O-5Fe2O3 (where x = 0, 3, 8 mol%) were prepared via a melt-quenching process. The effects of replacing P2O5 with B2O3 on the structural, thermal, degradation properties and cytocompatibility were investigated. Fourier transform infrared (FTIR) spectroscopy analysis confirmed the existence of BO3 triangular units and BO4 tetrahedral units within all the glasses with an increase of B/P ratio from 0.25 to 0.5. The BO4 units within the glass structure were observed to cause an increase in density (ρ) as well as glass transition (Tg) temperature and to decrease the crystallisation temperature (Tc). A decrease in thermal stability which indicated by process window was also observed in the case of substitution of P2O5 with B2O3. Degradation analysis of the glasses indicated that the dissolution rate increased with the addition of B2O3. The decrease in the thermal stability and chemical durability were attributed to the increase of BO3 units, which could increase crystallisation tendency and be easily hydrolysed by solution. The effect of boron addition on the cytocompatibility of the glasses was analysed using Alamar Blue and alkaline phosphatase (ALP) assays and DNA quantification. MG63 osteosarcoma cells cultured in direct contact with the glass samples surface for 14 days showed better cytocompatibility, compared to the tissue culture plastic (TCP) control group. In summary, the glass formulation with 12 mol% B2O3 presented the best cytocompatibility and thermal stability, thus could be considered for continuous fibre fabrication in future research and downstream activities
What Makes It Findable?: An Exploration on User Search Behavior and the Findability of Technical Documentation
This paper was presented at the Invited Panel session “Technical Communication in China”. Findability is one of the most important qualitative factors of websites. With the rapid growth in navigation complexity and in number of technical documentations in help centers, whether users can easily locate the target document could directly determine the information retrieval task outcome. Providing users with a fine guide to target documents and then helping them find solutions to their problems is the most important function of a help center. Investigation on user search behavior data and perceived findability of documentation has to be done in order to further apply website log data to predicting user subjective assessment. In this paper we analyze the correlation between subjective document findability, subjective task complexity, and user search behavior. We found several search behavior metrics which significantly correlate with the two subjective measures above
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