Light emission from porous silicon single and multiple cavities

Experimental and theoretical techniques are used to examine the effects of microstructuring on the optical properties of multilayer, single and multiple microcavity structures fabricated from porous silicon. Measurements of the reflectivity and photoluminescence spectra of three multilayer samples are presented. The results are modelled using a transfer matrix technique including a negative absorption term to represent the effect of spontaneous emission which gives luminescence. The emitted light is strongly controlled by the optical modes of the structures and very good agreement is observed between theory and experiment.     

Dynamics of coherent and incoherent emission from an artificial atom in a 1D space

We study dynamics of a two-level superconducting quantum system, analogous to a natural atom in an open space, by measuring the evolution of its coherent and incoherent emission. The emitted waves containing full information about the states of the artificial atom are efficiently collected due to strong atom-transmission-line coupling. This allows us to do simultaneous measurements of all the quantum state projections and perform a full characterization of the system. We derive coherence times and extract the two-time correlation function from the dynamics of the coherent emission.     

A computational sensorimotor model of bat echolocation

A computational sensorimotor model of target capture behavior by the echolocating bat, Eptesicus fuscus, was developed to understand the detection, localization, tracking, and interception of insect prey in a biological sonar system. This model incorporated acoustics, target localization processes, flight aerodynamics, and target capture planning to produce model trajectories replicating those observed in behavioral insect capture trials. Estimates of target range were based on echo delay, azimuth on the relative intensity of the echo at the two ears, and elevation on the spectral pattern of the sonar return in a match/mismatch process. Flapping flight aerodynamics was used to produce realistic model trajectories. Localization in all three spatial dimensions proved necessary to control target tracking and interception for an adequate model of insect capture behavior by echolocating bats. Target capture using maneuvering flight was generally successful when the model's path was controlled by a planning process that made use of an anticipatory internal simulation, while simple homing was successful only for targets directly ahead of the model bat.     

3D reconstruction of nanocrystalline particles from a single projection

This paper describes an approach to retrieve the three-dimensional atomic structure of a nanocrystalline particle from the reconstructed electron exit wave function in a single projection direction. The method employs wave propagation to determine the local exit surface of each atomic column together with its mass. The exit wave in between colums is used as internal calibration so as to remove the background noise and improve the precision to the level of single atom sensitivity. The validity of the approach is tested with exit wave functions of a gold wedge reconstructed from simulated images containing different levels of noise.     

Separating overlapping click trains originating from multiple individuals in echolocation recordings

Recordings of the acoustic activity of free-swimming groups of echolocating dolphins increase the likelihood of collecting overlapping click trains, originating from multiple individuals, in the same set of data. In order to evaluate the click properties of each individual based on such recordings it is necessary to identify which clicks originate from which animal. This paper suggests a computationally efficient strategy to separate overlapping click trains originating from multiple free-swimming bottlenose dolphins, enabling echolocation analysis at an individual level on several animals. This technique is based on sequential matching of the frequency spectra of successive clicks. The clicks are grouped together as individual click trains if the correlation coefficients between clicks are higher than a pre-set threshold level. The robustness of the algorithm is tested by adding artificially generated white Gaussian noise and comparing the results with other comparable commonly used methods based on inter-click intervals, centroid frequencies, and amplitude levels. The described method is applicable to a variety of experimental and observational contexts, e.g., those regarding echolocation development of calves, the hypothesized acoustic "etiquette" among dolphins when investigating the same object, and the possible occurrence of eavesdropping in large dolphin pods.     

一种面向空间非合作目标位姿测量应用的三维点云滤波算法

针对激光位姿敏感器获得的原始点云有噪声和直接参与解算消耗星上计算资源过大问题,给出一种适用于空间非合作目标位姿测量的点云滤波和特征提取算法。应用仿真的方法分别验证了算法滤除空间随机噪声和点云降采样的有效性,验证了特征点对目标位姿变化和高斯测量噪声的鲁棒性。在非合作目标绕飞、抵近、捕获全物理试验平台上,以扫描激光位姿敏感器获得的原始点云数据为输入,验证了算法在实际空间目标位姿测量中的性能。试验结果表明,该算法实现了原始点云93.1%的降采样,节省了92.9%的位姿解算时间,可有效提升星上数据处理的效率和姿态解算的实时性。     

Single photon emission from a single molecule

Photon emission from a single molecule at room temperature exhibits nonclassical features. Continuous wave fluorescence excitation provides antibunching in the emitted photons sequence as a signature of the property to only emit one photon at a time. A short pulsed excitation can then produce single photons on demand, with an overall quantum efficiency up to 4.5% in our experimental setup. Direct measurement of the Mandel parameter Q(T) for an observation period of duration T follows a subpoissonian statistics on short time scale and superpoissonian statistics on longer time scale. The latter is attributed to blinking in the fluorescence due to the occurence of a metastable molecular triplet state. To cite this article: F. Treussart et al., C. R. Physique 3 (2002) 501–508.     

A full 3D numerical model of the industrial electrostatic coating process for moving targets

In this paper, a full three dimensional FLUENT numerical model of the electrostatic coating process as used in the automotive industry is presented and selected parametric studies on the numerical model have been conducted. The model incorporates a moving mesh capability in order to simulate movements of the target. The target follows piecewise linear motion and the entire area of the target has been coated in multiple passes. All the dominant mechanical and electrical phenomena were taken into account and cases with both neutral and charged particles were examined. It was confirmed that the transfer efficiency increases when the electrical forces are present in the model and that the coating uniformity improves with the movement of the target. The deposition pattern corresponds to the motion of the target and gradually builds up in a cumulative manner. Due to the aggregation effect associated with the target movement, the deposition uniformity is dramatically improved.     

Evaluation of the echolocation model for range estimation of multiple closely spaced objects

Experimental evidence indicates that bats can use frequency-modulated echolocation to identify objects with an accuracy of less than 1 μs. However, when modeling this process, it is difficult to estimate the delay times of multiple closely spaced objects by analyzing the echo spectrum, because the sequence of delay separations cannot be determined without information on the temporal changes in the interference patterns of the echoes. To extract the temporal changes, Gaussian chirplets with a carrier frequency compatible with bat emission sweep rates are introduced. The delay time for object 1 (T(1)) is estimated from the echo spectrum around the onset time. The T(2) is obtained by adding the T(1) to the delay separation between objects 1 and 2. Further objects are located in sequence by this procedure. Here echoes were measured from single and multiple objects at a low signal-to-noise ratio. It was confirmed that the delay time for a single object could be estimated with an accuracy of about 1.3 μs. The range accuracy was less than 6 μs when the frequency bandwidth was less than 10 kHz. The delay time for multiple closely spaced objects could be estimated with a high range resolution by extracting the interference pattern.     

Stimulated emission from a single excited atom in a waveguide

We study stimulated emission from an excited two-level atom coupled to a waveguide containing an incident single-photon pulse. We show that the strong photon correlation, as induced by the atom, plays a very important role in stimulated emission. Additionally, the temporal duration of the incident photon pulse is shown to have a marked effect on stimulated emission and atomic lifetime.     

Spontaneous two-photon emission from a single quantum dot

Spontaneous two-photon emission from a solid-state single quantum emitter is observed. We investigated photoluminescence from the neutral biexciton in a single semiconductor quantum dot coupled with a high Q photonic crystal nanocavity. When the cavity is resonant to the half energy of the biexciton, the strong vacuum field in the cavity inspires the biexciton to simultaneously emit two photons into the mode, resulting in clear emission enhancement of the mode. Meanwhile, the suppression of other single photon emission from the biexciton was observed, as the two-photon emission process becomes faster than the others at the resonance.     

单个病毒的X射线3D成像

自从1957 年确立肌红蛋白的结构以来,研究人员利用X射线晶体学确定了数以万计蛋白、核酸和其他生物分子的结构,并由此展开了分子功能研究。这种方法要求被研究的分子必须可以形成晶体,当X射线通过晶体时会发生衍射,由此可获得结构信息。但是,很多重要的分子无法结晶,因此不能用这种方法来研究。这就是X射线晶体学的“阿喀琉斯之踵”。科学家们正在寻找重建分子结构的新方案：他们希望只需要将单分子注入到自由电子激光(XFEL)的强激光束中即可通过X 射线衍射信息重建分子结构。由瑞典Uppsala 大学Hajdu 领导的研究小组报道了一项关键进展：如果利用随机取向的单个病毒获得大量的衍射图样,那么就有可能重建病毒的三维结构。     

An echolocation model for the restoration of an acoustic image from a single-emission echo

Bats can form a fine acoustic image of an object using frequency-modulated echolocation sound. The acoustic image is an impulse response, known as a reflected-intensity distribution, which is composed of amplitude and phase spectra over a range of frequencies. However, bats detect only the amplitude spectrum due to the low-time resolution of their peripheral auditory system, and the frequency range of emission is restricted. It is therefore necessary to restore the acoustic image from limited information. The amplitude spectrum varies with the changes in the configuration of the reflected-intensity distribution, while the phase spectrum varies with the changes in its configuration and location. Here, by introducing some reasonable constraints, a method is proposed for restoring an acoustic image from the echo. The configuration is extrapolated from the amplitude spectrum of the restricted frequency range by using the continuity condition of the amplitude spectrum at the minimum frequency of the emission and the minimum phase condition. The determination of the location requires extracting the amplitude spectra, which vary with its location. For this purpose, the Gaussian chirplets with a carrier frequency compatible with bat emission sweep rates were used. The location is estimated from the temporal changes of the amplitude spectra.     

Electromagnetically induced transparency from a single atom in free space

In this Letter, we report an absorption spectroscopy experiment and the observation of electromagnetically induced transparency from a single trapped atom. We focus a weak and narrow band Gaussian light beam onto an optically cooled 138Ba+ ion using a high numerical aperture lens. Extinction of this beam is observed with measured values of up to 1.35%. We demonstrate electromagnetically induced transparency of the ion by tuning a strong control beam over a two-photon resonance in a three-level Λ-type system. The probe beam extinction is inhibited by more than 75% due to population trapping.     

临近空间单粒子串扰的解析模型

随着尺寸不断缩减, 串扰对单粒子效应的影响越来越重要. 为了量化串扰效应对单粒子瞬态(SET)的影响, 基于SET等效电路和互连线的6 结点模型, 利用所定义的导纳的四种规则, 简化计算, 推导了单粒子串扰(SEC)的解析模型. 通过求导和泰勒公式, 导出了串扰电压峰值的表达式. 仿真结果显示, 该解析模型与SPICE电路的一致性较好, 平均相对误差为2.51%, 最大误差为5.11%.     

3D black holes on a 2-brane in 4D Minkowski space

We investigate three-dimensional black hole solutions in the realm of pure and new massive gravity in 2+1 dimensions induced on a 2-brane embedded in a flat four-dimensional spacetime. There is no cosmological constant neither on the brane nor on the four-dimensional bulk. Only gravitational fields are turned on and we indeed find vacuum solutions as black holes in 2+1 dimensions even in the absence of any cosmological solution. There is a crossover scale that controls how far the three- or four-dimensional gravity manifests on the 2-brane. Our solutions also indicate that local BTZ and SdS₃ solutions can flow to local four-dimensional Schwarzschild-like black holes, as one probes from small to large distances, which is clearly a higher dimensional manifestation on the 2-brane. This is similar to the DGP scenario where the effects of extra dimensions for large probed distances along the brane manifest.     

Single-mode spontaneous emission from a single quantum dot in a three-dimensional microcavity

The spontaneous emission from an isolated semiconductor quantum dot state has been coupled with high efficiency to a single, polarization-degenerate cavity mode. The InAs quantum dot is epitaxially formed and embedded in a planar epitaxial microcavity, which is processed into a post of submicron diameter. The single quantum dot spontaneous emission lifetime is reduced from the noncavity value of 1.3 ns to 280 ps, resulting in a single-mode spontaneous emission coupling efficiency of 78%.     

Comparison of pure and combined search strategies for single and multiple targets

We address the generic problem of random search for a point-like target on a line. Using the measures of search reliability and efficiency to quantify the random search quality, we compare Brownian search with Lévy search based on long-tailed jump length distributions. We then compare these results with a search process combined of two different long-tailed jump length distributions. Moreover, we study the case of multiple targets located by a Lévy searcher.