A quantum dot single-photon turnstile device

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.     

White-light filaments for atmospheric analysis

Most long-path remote spectroscopic studies of the atmosphere rely on ambient light or narrow-band lasers. High-power femtosecond laser pulses have been found to propagate in the atmosphere as dynamically self-guided filaments that emit in a continuum from the ultraviolet to the infrared. This white light exhibits a directional behavior with enhanced backward scattering and was detected from an altitude of more than 20 kilometers. This light source opens the way to white-light and nonlinear light detection and ranging applications for atmospheric trace-gas remote sensing or remote identification of aerosols. Air ionization inside the filaments also opens promising perspectives for laser-induced condensation and lightning control. The mobile femtosecond-terawatt laser system, Teramobile, has been constructed to study these applications.     

Ultrafast lasers. Strobe light breaks the attosecond barrier

On page 1689 of this issue, physicists report creating the fastest strobe light ever made, with individual pulses lasting just 220 attoseconds, or 220 billionths of a billionth of a second. These unimaginably short pulses are the first to be confirmed as breaking the attosecond barrier, a goal of high-speed-laser researchers for nearly a decade. Such pulses may one day serve as an ultrafast camera, allowing researchers to freeze action and perhaps to spot the gyrations of individual electrons whirling around an atomic nucleus.     

Attosecond synchronization of high-harmonic soft x-rays

Subfemtosecond light pulses can be obtained by superposing several high harmonics of an intense laser pulse. Provided that the harmonics are emitted simultaneously, increasing their number should result in shorter pulses. However, we found that the high harmonics were not synchronized on an attosecond time scale, thus setting a lower limit to the achievable x-ray pulse duration. We showed that the synchronization could be improved considerably by controlling the underlying ultrafast electron dynamics, to provide pulses of 130 attoseconds in duration. We discuss the possibility of achieving even shorter pulses, which would allow us to track fast electron processes in matter.     

Nonlinear property of the visual system at fusion

The response of the visual system to intermittent stimulation at rates above the fusion point is generally considered to be independent of frequency, that is, linear with respect to time-average luminance. However, trains of 1000 light pulses per second alternated successively with trains of 500 light pulses per second may be perceived as flickering even though the time-average luminance is the same in each train.     

温室植物生产用人工光源研究进展

光是植物光合作用的能量源。温室植物体中的叶绿素a和叶绿素b分别吸收600～700nm的红光和400～520nm的蓝光进行光合作用。光同时是植物形态形成的信号源。600～700nm的红光能降低植物体内赤霉素的质量分数,从而减小节间长度和植株高度;而700～800nm的远红光能提高体内赤霉素的质量分数,从而增加节间长度和植株高度。基于温室植物对光的需求,分析了目前使用的温室传统人工光源的特性和存在的问题。发光二极管（LED）光源较传统人工光源具有明显的优势,但受限于单颗提供的光照度有限,必须组合使用才能满足温室植物正常生长发育对光的需求。尽管LED光源已成功应用于组培苗栽培,但组培室中组培苗的受光特性完全不同于温室作物,所以针对温室应用的LED成套光源有必要进一步研制。参32     

Photoelectric and Spectroscopic Observations Related to a Possible Optical Counterpart for Pulsar CP 1919+21

Spectroscopic observations of the two stars near the pulsar CP 1919+21 are not sufficiently conclusive to permit an identification of either object with the source of the radio pulses. However, our most extensive series of photometric observations of a region of sky near the radio source position, which region includes the brighter of the two stars, suggests an approximately sinusoidal variation. It is significant that the period of the variation is double the period of the radio pulsations.     

X-ray pulses approaching the attosecond frontier

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.     

Single-cycle nonlinear optics

Nonlinear optics plays a central role in the advancement of optical science and laser-based technologies. We report on the confinement of the nonlinear interaction of light with matter to a single wave cycle and demonstrate its utility for time-resolved and strong-field science. The electric field of 3.3-femtosecond, 0.72-micron laser pulses with a controlled and measured waveform ionizes atoms near the crests of the central wave cycle, with ionization being virtually switched off outside this interval. Isolated sub-100-attosecond pulses of extreme ultraviolet light (photon energy approximately 80 electron volts), containing approximately 0.5 nanojoule of energy, emerge from the interaction with a conversion efficiency of approximately 10(-6). These tools enable the study of the precision control of electron motion with light fields and electron-electron interactions with a resolution approaching the atomic unit of time ( approximately 24 attoseconds).     

Frequency Dependence of Polarization of Pulsar CP 0328

The circularly polarized emission from the pulsar CP 0328 has an approximately flat spectrum in the 1-megahertz band centered at 113.6 megahertz, whereas the linearly polarized emission varies with frequency and from pulse to pulse. A simple model for the source that has a constant Faraday rotation measure fits some of the linearly polarized spectra observed for individual pulses, but changes in the rotation measure of as much as 30 radians per square meter are required between adjacent pulses. The simple model does not fit the average spectrum of the linearly polarized emission, although the average spectrum had the same form on two nights.     

JETLAG resets the Drosophila circadian clock by promoting light-induced degradation of TIMELESS

Organisms ranging from bacteria to humans synchronize their internal clocks to daily cycles of light and dark. Photic entrainment of the Drosophila clock is mediated by proteasomal degradation of the clock protein TIMELESS (TIM). We have identified mutations in jetlag-a gene coding for an F-box protein with leucine-rich repeats-that result in reduced light sensitivity of the circadian clock. Mutant flies show rhythmic behavior in constant light, reduced phase shifts in response to light pulses, and reduced light-dependent degradation of TIM. Expression of JET along with the circadian photoreceptor cryptochrome (CRY) in cultured S2R cells confers light-dependent degradation onto TIM, thereby reconstituting the acute response + of the circadian clock to light in a cell culture system. Our results suggest that JET is essential for resetting the clock by transmitting light signals from CRY to TIM.     

光照与物镜距离对采集玉米籽粒图像影响的研究

设计了具有主光源尧辅助光源,物镜距离可调的玉米籽粒图像采集实验箱,应用数码相机采集实验箱载物台上玉米籽粒的图像。以玉米籽粒形状特征、颜色特征为对象,分析同一物镜距离下辅助光源的放置方式与强度对图像采集的影响,光源确定的情况下物镜距离对图像采集的影响。结果表明：闪光灯留下的光斑影响特征提取,辅助光源平行和垂直放置尧物镜距离在26~31 cm范围内较佳。     

Melanopsin (Opn4) requirement for normal light-induced circadian phase shifting

The master circadian oscillator in the hypothalamic suprachiasmatic nucleus is entrained to the day/night cycle by retinal photoreceptors. Melanopsin (Opn4), an opsin-based photopigment, is a primary candidate for photoreceptor-mediated entrainment. To investigate the functional role of melanopsin in light resetting of the oscillator, we generated melanopsin-null mice (Opn4-/-). These mice entrain to a light/dark cycle and do not exhibit any overt defect in circadian activity rhythms under constant darkness. However, they display severely attenuated phase resetting in response to brief pulses of monochromatic light, highlighting the critical role of melanopsin in circadian photoentrainment in mammals.     

Generation of an artificial aurora

On 26 January 1969 an Aerobee 350 rocket was fired from Wallops Island, Virginia, carrying an electron accelerator. Above 230 kilometers the electron guns put out a beam of 0.5 ampere of 10 kev electrons in pulses of 1-second duration aimed downward along the earth's magnetic field lines. The interaction of electron beam with the atmosphere at an altitude of about 100 kilometers generated enough light so that the auroral rays produced could be photographed on the ground by television camera systems.     

Attosecond control and measurement: lightwave electronics

Electrons emit light, carry electric current, and bind atoms together to form molecules. Insight into and control of their atomic-scale motion are the key to understanding the functioning of biological systems, developing efficient sources of x-ray light, and speeding up electronics. Capturing and steering this electron motion require attosecond resolution and control, respectively (1 attosecond = 10(-18) seconds). A recent revolution in technology has afforded these capabilities: Controlled light waves can steer electrons inside and around atoms, marking the birth of lightwave electronics. Isolated attosecond pulses, well reproduced and fully characterized, demonstrate the power of the new technology. Controlled few-cycle light waves and synchronized attosecond pulses constitute its key tools. We review the current state of lightwave electronics and highlight some future directions.     

Light pulses that shift rhythms induce gene expression in the suprachiasmatic nucleus

Lighting cycles synchronize (entrain) mammalian circadian rhythms by altering activity of cells in the suprachiasmatic nucleus (SCN) of the hypothalamus, a circadian pacemaker. Exposure of hamsters and rats to light pulses at those phases of the circadian rhythm during which light can shift the rhythm caused increased immunoreactivity for the product of the immediate-early gene c-fos in cells in the region of the SCN that receives retinal fibers. Light pulses also increased messenger RNA for the Fos protein and for the immediate-early protein NGFI-A in the rat SCN. Similar increases in mRNA for NGFI-A were seen in the SCN of hamsters. Thus cells in this portion of the SCN undergo alterations in gene expression in response to retinal illumination, but only at times in the circadian cycle when light is capable of influencing entrainment.     

Suppressing drosophila circadian rhythm with dim light

Drosophila larvae were reared and allowed to pupate in continuous bright white light. The pupae were then transferred to a much dimmer blue light. In continuous blue light of intensity below 0.001 erg per square centimeter per second, adult flies emerged in pulses 24.7 hours apart, each pulse occupying about 6 hours. But in continuous light of intensity exceeding 0.1 erg per square centimeter per second, they emerged at a steady rate. This intensity range from effective darkness to effective light is roughly from starlight to moonlight. Inside this range, the emergence peaks broaden for about a week with little change of period.     

Atomic memory for correlated photon states

We experimentally demonstrate emission of two quantum-mechanically correlated light pulses with a time delay that is coherently controlled via temporal storage of photonic states in an ensemble of rubidium atoms. The experiment is based on Raman scattering, which produces correlated pairs of spin-flipped atoms and photons, followed by coherent conversion of the atomic states into a different photon beam after a controllable delay. This resonant nonlinear optical process is a promising technique for potential applications in quantum communication.     

Effects of light-emitting diodes on tissue culture plantlets and seedlings of rice(Oryza sativa L.)

Light-emitting diodes(LEDs) are a new light source with low energy consumption and high photoelectric conversion efficiency, and they can satisfy the energy-saving needs of plant culture systems. However, the effects of LED light sources on rice tissue culture and rice seedling cultivation are poorly understood. This study aimed to evaluate the effects of LEDs on the growth of tissue culture plantlets and seedlings of the rice(Oryza sativa L.) cultivar Nipponbare. The best light source for rice tissue culture was different from that for rice seedling cultivation. Blue(B) LED light was the most appropriate light for rice tissue culture. Under a B LED light, the time required for callus proliferation, differentiation and regeneration was the shortest, and the frequency of plantlet initiation, differentiation and regeneration was the highest. A blue:red(B:R)=1:1 LED light facilitated the growth of rice seedlings and produced the highest chlorophyll and carotenoid contents and photosynthetic rates in the rice seedlings. Abundant photosynthetic products were more effectively generated in the rice seedlings under the B:R=1:1 LED and R LED lights than under the B LED light. B LED light is the most appropriate light for rice tissue culture plantlets and can be used as an alternative light source for rice tissue culture, and B:R=1:1 LED light facilitated the cultivation of robust rice seedlings and can be used as the primary light source for rice factory seedling cultivation.     

光质调控蔬菜作物生长和形态研究进展

光是植物生长发育的重要能量源和信息源,光质则作为光环境中的关键影响因子通过光受体传导途径调节植物的整个生命周期。随着光生物学研究的不断深入和低成本高光效的人工光源的不断研发,光质调控技术已逐渐运用到蔬菜作物的生产过程。从光质调控植物光合作用和光形态建成的机理、及其在蔬菜生产中的应用等方面综述了光质对蔬菜作物生长和形态调控方面的研究进展。较详细介绍了光质对植物生长过程的调控机理,在芽苗菜、绿叶类蔬菜、茄果类蔬菜和食用菌类等作物的影响的研究成果,以及光质研究提高了蔬菜生产的经济效益。阐述了人工光环境下蔬菜作物栽培可供参考的光质配制方案,并提出仍需进一步研究光质对蔬菜各生育期的作用及综合光强和光周期进行试验,以期完善不同蔬菜作物光配方。