共查询到20条相似文献，搜索用时 31 毫秒
1.
Shearer A Stappers B O'Connor P Golden A Strom R Redfern M Ryan O 《Science (New York, N.Y.)》2003,301(5632):493-495
We detected a correlation between optical and giant radio pulse emission from the Crab pulsar. Optical pulses coincident with the giant radio pulses were on average 3% brighter than those coincident with normal radio pulses. Combined with the lack of any other pulse profile changes, this result indicates that both the giant radio pulses and the increased optical emission are linked to an increase in the electron-positron plasma density. 相似文献
2.
Drescher M Hentschel M Kienberger R Tempea G Spielmann C Reider GA Corkum PB Krausz F 《Science (New York, N.Y.)》2001,291(5510):1923-1927
Single soft-x-ray pulses of approximately 90-electron volt (eV) photon energy are produced by high-order harmonic generation with 7-femtosecond (fs), 770-nanometer (1.6 eV) laser pulses and are characterized by photoionizing krypton in the presence of the driver laser pulse. By detecting photoelectrons ejected perpendicularly to the laser polarization, broadening of the photoelectron spectrum due to absorption and emission of laser photons is suppressed, permitting the observation of a laser-induced downshift of the energy spectrum with sub-laser-cycle resolution in a cross correlation measurement. We measure isolated x-ray pulses of 1.8 (+0.7/-1.2) fs in duration, which are shorter than the oscillation cycle of the driving laser light (2.6 fs). Our techniques for generation and measurement offer sub-femtosecond resolution over a wide range of x-ray wavelengths, paving the way to experimental attosecond science. Tracing atomic processes evolving faster than the exciting light field is within reach. 相似文献
3.
The thermal emission from Io's volcanic hot spots is linearly polarized. Infrared measurements at 4.76 micrometers show disk-integrated polarization as large as 1.6 percent. The degree and position angle of linear polarization vary with Io's rotation in a manner characteristic of emission from a small number of hot spots. A model incorporating three hot spots best fits the data. The largest of these hot spots lies to the northeast of Loki Patera, as mapped from Voyager, and the other spot on the trailing hemisphere is near Ra Patera. The hot spot on the leading hemisphere corresponds to no named feature on the Voyager maps. The value determined for the index of refraction of the emitting surface is a lower bound; it is similar to that of terrestrial basalts and is somewhat less than that of sulfur. 相似文献
4.
大黄鱼的发声及信号特性研究 总被引：3，自引：0，他引：3
对大黄鱼Pseudosciaena crocea觅食及产卵时的发声及信号特性进行了研究。结果表明:不同行为下大黄鱼发声信号的频谱特性一致,即约在800 Hz频率处有一个明显的谱峰;但不同行为下发声的时域信号差距较大,即觅食时大黄鱼的发声信号都是简单的单脉冲,脉冲间隔大多集中在1~30 ms,脉冲间隔比较小,而产卵时大黄鱼的发声信号则大部分是连续的双脉冲或三脉冲,只有极个别的单脉冲或多脉冲,脉冲间隔则集中在100~130 ms,脉冲间隔要大得多。 相似文献
5.
为明确偏振波谱光态光照参量对蝗虫趋偏响应效应的影响作用,获取蝗虫偏振诱导光场的技术特征,研制蝗虫偏振诱导光源,利用蝗虫偏光响应试验装置测试了蝗虫对紫、蓝部分偏光及线偏光的响应特征,以此优选波谱偏振光态矢量的作用模式,分析偏振波谱光态光照参量对蝗虫偏光响应机制的影响,探讨蝗虫偏光波谱矢量敏感模式变化的原因。结果表明,部分偏光中,紫波谱未引起而蓝波谱导致蝗虫视响应与视趋性敏感矢量发生改变;线偏光中,紫、蓝波谱均影响蝗虫趋偏响应效应的矢量敏感模式,且波谱相同,部分偏光矢量置向模式对蝗虫的操控诱导、视趋强度的调控性优于线偏光,而偏振矢量光态相同,紫波谱的作用效果强于蓝波谱。蝗虫趋偏响应效应与偏振波谱、偏振光态的异质作用效应有关,且线偏紫波谱120°左右置向矢量对蝗虫的诱导效果最强(94.5%),而紫波谱部分偏光左60°及右150°矢量对蝗虫视趋强度的作用效果最强(68.5%);部分偏光矢量置向模式的作用效果与波谱光致性视敏偏振效应有关,且蓝波谱中偏振度越高而光照度越低,矢量模式的操控诱导性越强,紫波谱中偏振度越低而光照度越强,矢量模式对蝗虫趋偏聚集程度的作用效果越强。从而,线偏与部分偏光不同矢量光照交替刺激模式可提高蝗虫趋偏响应敏感性。 相似文献
6.
Colloidal quantum rods of cadmium selenide (CdSe) exhibit linearly polarized emission. Empirical pseudopotential calculations predict that slightly elongated CdSe nanocrystals have polarized emission along the long axis, unlike spherical dots, which emit plane-polarized light. Single-molecule luminescence spectroscopy measurements on CdSe quantum rods with an aspect ratio between 1 and 30 confirm a sharp transition from nonpolarized to purely linearly polarized emission at an aspect ratio of 2. Linearly polarized luminescent chromophores are highly desirable in a variety of applications. 相似文献
7.
The pulse structure of the four known pulsars is given. The pulse is about 38 milliseconds for the two pulsars of longest period, and within the pulsewidth three subpulses typically appear. The pulsar of next longest period typically radiates two pulses separated about 23 milliseconds in time. The one short-period pulsar emits single pulses of constant shape. The first subpulses of all pulsars have nearly the same shape. The shape of the first subpulse agrees well with the pulse shape expected from a radio-emitting sphere which is excited by a spherically expanding disturbance, and in which the radio emission, once excited, decays exponentially. 相似文献
8.
基于多光束相干叠加法研究了线偏振光经过微平面腔的磁旋光相干透射特性,分析得到微腔透射光强、腔共振条件、总磁旋光角及共振透射光强,并讨论了检测灵敏度、相对灵敏度及检测角量程等旋光角检测特性,研究表明在单程旋光角为零的极限情况下,相干磁旋光系统的相对检测灵敏度与微腔的锐度系数成正比。 相似文献
9.
The Stokes parameters were measured as a function of frequency for the anonmalous 1665-megacycles-per-second OH emission originating near the thermal radio source W3. The emission is highly polarized, and the polarization parameters vary rapidly with frequency. The observed polarization can be described in terms of narrow, roughly Gaussian, emission features, all with uniform polarization but with several features overlapping without coherence near the center of the spectrum. Most of the individual features may be 100-percent polarized. Detailed examination of the brightest features suggest that they are not exactly Gaussian in shape. 相似文献
10.
A scheme is proposed wherein nuclear magnetic resonance (NMR) can be induced and monitored using only optical fields. In analogy to radio-frequency fields used in traditional NMR, circularly polarized light creates electron spins in semiconductors whose hyperfine coupling could tip nuclear moments. Time-resolved Faraday rotation experiments were performed in which the frequency of electron Larmor precession was used as a magnetometer of local magnetic fields experienced by electrons in n-type gallium arsenide. Electron spin excitation by a periodic optical pulse train appears not only to prepare a hyperpolarized nuclear moment but also to destroy it resonantly at magnetic fields proportional to the pulse frequency. This resonant behavior is in many ways supportive of a simple model of optically induced NMR, but a curious discrepancy between one of the observed frequencies and classic NMR values suggests that this phenomenon is more complex. 相似文献
11.
Observation of backward pulse propagation through a medium with a negative group velocity 总被引：1，自引：0，他引：1
Gehring GM Schweinsberg A Barsi C Kostinski N Boyd RW 《Science (New York, N.Y.)》2006,312(5775):895-897
The nature of pulse propagation through a material with a negative value of the group velocity has been mysterious, as simple models seem to predict that pulses will propagate "backward" through such a material. Using an erbium-doped optical fiber and measuring the time evolution of the pulse intensity at many points within the fiber, we demonstrate that the peak of the pulse does propagate backward inside the fiber, even though the energy flow is always in the forward direction. 相似文献
12.
Warwick JW Pearce JB Evans DR Carr TD Schauble JJ Alexander JK Kaiser ML Desch MD Pedersen M Lecacheux A Daigne G Boischot A Barrow CH 《Science (New York, N.Y.)》1981,212(4491):239-243
The Voyager 1 planetary radio astronomy experiment detected two distinct kinds of radio emissions from Saturn. The first, Saturn kilometric radiation, is strongly polarized, bursty, tightly correlated with Saturn's rotation, and exhibits complex dynamic spectral features somewhat reminiscent of those in Jupiter's radio emission. It appears in radio frequencies below about 1.2 megahertz. The second kind of radio emission, Saturn electrostatic discharge, is unpolarized, extremely impulsive, loosely correlated with Saturn's rotation, and very broadband, appearing throughout the observing range of the experiment (20.4 kilohertz to 40.2 megahertz). Its sources appear to lie in the planetary rings. 相似文献
13.
Martín F Fernández J Havermeier T Foucar L Weber T Kreidi K Schöffler M Schmidt L Jahnke T Jagutzki O Czasch A Benis EP Osipov T Landers AL Belkacem A Prior MH Schmidt-Böcking H Cocke CL Dörner R 《Science (New York, N.Y.)》2007,315(5812):629-633
H2, the smallest and most abundant molecule in the universe, has a perfectly symmetric ground state. What does it take to break this symmetry? We found that the inversion symmetry can be broken by absorption of a linearly polarized photon, which itself has inversion symmetry. In particular, the emission of a photoelectron with subsequent dissociation of the remaining H+2 fragment shows no symmetry with respect to the ionic H+ and neutral H atomic fragments. This lack of symmetry results from the entanglement between symmetric and antisymmetric H+2 states that is caused by autoionization. The mechanisms behind this symmetry breaking are general for all molecules. 相似文献
14.
Human simple reaction times and magnitude estimates of taste intensity increased as the duration of 500-millimolar sodium chloride or 2-millimolar saccharin sodium pulses lengthened from 100 to 1000 milliseconds. Responses to "What was the taste?" ranged from 94 to 100 percent "sweet" for saccharin and 68 to 83 percent "salty" for salt across all pulse durations when both substances were randomized with water pulses. 相似文献
15.
Direct observation of the femtosecond excited-state cis-trans isomerization in bacteriorhodopsin 总被引：12，自引：0，他引：12
Femtosecond optical measurement techniques have been used to study the primary photoprocesses in the light-driven transmembrane proton pump bacteriorhodopsin. Light-adapted bacteriorhodopsin was excited with a 60-femtosecond pump pulse at 618 nanometers, and the transient absorption spectra from 560 to 710 nanometers were recorded from -50 to 1000 femtoseconds by means of 6-femtosecond probe pulses. By 60 femtoseconds, a broad transient hole appeared in the absorption spectrum whose amplitude remained constant for about 200 femtoseconds. Stimulated emission in the 660- to 710-nanometer region and excited-state absorption in the 560- to 580-nanometer region appeared promptly and then shifted and decayed from 0 to approximately 150 femtoseconds. These spectral features provide a direct observation of the 13-trans to 13-cis torsional isomerization of the retinal chromophore on the excited-state potential surface. Absorption due to the primary ground-state photoproduct J appears with a time constant of approximately 500 femtoseconds. 相似文献
16.
Popmintchev T Chen MC Popmintchev D Arpin P Brown S Alisauskas S Andriukaitis G Balciunas T Mücke OD Pugzlys A Baltuska A Shim B Schrauth SE Gaeta A Hernández-García C Plaja L Becker A Jaron-Becker A Murnane MM Kapteyn HC 《Science (New York, N.Y.)》2012,336(6086):1287-1291
High-harmonic generation (HHG) traditionally combines ~100 near-infrared laser photons to generate bright, phase-matched, extreme ultraviolet beams when the emission from many atoms adds constructively. Here, we show that by guiding a mid-infrared femtosecond laser in a high-pressure gas, ultrahigh harmonics can be generated, up to orders greater than 5000, that emerge as a bright supercontinuum that spans the entire electromagnetic spectrum from the ultraviolet to more than 1.6 kilo-electron volts, allowing, in principle, the generation of pulses as short as 2.5 attoseconds. The multiatmosphere gas pressures required for bright, phase-matched emission also support laser beam self-confinement, further enhancing the x-ray yield. Finally, the x-ray beam exhibits high spatial coherence, even though at high gas density the recolliding electrons responsible for HHG encounter other atoms during the emission process. 相似文献
17.
Michler P Kiraz A Becher C Schoenfeld WV Petroff PM Zhang L Hu E Imamoglu A 《Science (New York, N.Y.)》2000,290(5500):2282-2285
Quantum communication relies on the availability of light pulses with strong quantum correlations among photons. An example of such an optical source is a single-photon pulse with a vanishing probability for detecting two or more photons. Using pulsed laser excitation of a single quantum dot, a single-photon turnstile device that generates a train of single-photon pulses was demonstrated. For a spectrally isolated quantum dot, nearly 100% of the excitation pulses lead to emission of a single photon, yielding an ideal single-photon source. 相似文献
18.
Readhead AC Myers ST Pearson TJ Sievers JL Mason BS Contaldi CR Bond JR Bustos R Altamirano P Achermann C Bronfman L Carlstrom JE Cartwright JK Casassus S Dickinson C Holzapfel WL Kovac JM Leitch EM May J Padin S Pogosyan D Pospieszalski M Pryke C Reeves R Shepherd MC Torres S 《Science (New York, N.Y.)》2004,306(5697):836-844
Polarization observations of the cosmic microwave background with the Cosmic Background Imager from September 2002 to May 2004 provide a significant detection of the E-mode polarization and reveal an angular power spectrum of polarized emission showing peaks and valleys that are shifted in phase by half a cycle relative to those of the total intensity spectrum. This key agreement between the phase of the observed polarization spectrum and that predicted on the basis of the total intensity spectrum provides support for the standard model of cosmology, in which dark matter and dark energy are the dominant constituents, the geometry is close to flat, and primordial density fluctuations are predominantly adiabatic with a matter power spectrum commensurate with inflationary cosmological models. 相似文献
19.
利用方波脉冲电场增强乳酸菌细胞通透性,通过将乳酸菌细胞离心,制成浓度为108cfu.mL-1菌悬液,采用不同方波电脉冲条件对其进行处理。结果表明,在0.5 mol.L-1蔗糖的等渗脉冲介质中,电场强度2.25kV.cm-1,脉冲宽度15μs,脉冲数5,间隔时间2 s,此时,乳酸菌细胞的存活率平均为78.24%,细胞内平均荧光强度为5 480。表明脉冲电场可有效增强乳酸菌细胞通透性。 相似文献
20.
Cronyn WM 《Science (New York, N.Y.)》1970,168(3938):1453-1455
Angular scattering in the interstellar medium results in multipath dispersion which can amount to more than one pulse period for pulsars of short period and high dispersion measure. The dispersion, if operative, imposes on the pulsation flux a cutoff inversely proportional to the fourth power of the observing wavelength. The low-frequency pulse shape of pulsar NP0532 suggests that this pulsar is subject to such scattering and that the observed low-frequency cutoff in the apparent spectrum is not an intrinsic property of the pulsar. In fact, there is evidence that NP0532 may be identified as the compact, low-frequency source in the Crab Nebula and that the pulsar may radiate in accordance with its high-frequency spectrum down to frequencies as low as 10 megahertz, although the periodic time variations are suppressed by the scattering below 100 megahertz. 相似文献