Tomography of a high-purity narrowband photon from a transient atomic collective excitation

We demonstrate efficient heralded generation of high purity narrow-bandwidth single photons from a transient collective spin excitation in a hot atomic vapor cell. Employing optical homodyne tomography, we fully reconstruct the density matrix of the generated photon and observe a Wigner function reaching the zero value without correcting for any inefficiencies. The narrow bandwidth of the photon produced is accompanied by a high generation rate yielding a high spectral brightness. The source is, therefore, compatible with atomic-based quantum memories as well as other applications in light-atom interfacing. This Letter paves the way to preparing and measuring arbitrary superposition states of collective atomic excitations.     

光的受激放大

求解在激发作用下通过活性介质的光的输运方程与活性介质中的粒子反转数方程,我们得到在具有任意形式输入下的光子数与粒子反转数的时空分布函数的解析表达式;并以一个方形脉冲或一系列方形脉冲为例进行讨论。此外,我们还研究了强光注入laser对laser的稳定工作点的影响,以及在一定条件下通过微扰方法给出这时laser内的光子数与粒子反转数的形式解,对以红宝石laser为例所作的计算表明通过注入强光有可能成为加强巨脉冲的手段之     

Quantum entanglement of a large number of photons

A bipartite multiphoton entangled state is created through stimulated parametric down-conversion of strong laser pulses in a nonlinear crystal. It is shown how detectors that do not resolve the photon number can be used to analyze such multiphoton states. Entanglement of up to 12 photons is detected using both the positivity of the partially-transposed density matrix and a newly derived criteria. Furthermore, evidence is provided for entanglement of up to 100 photons. The multiparticle quantum state is such that even in the case of an overall photon collection and detection efficiency as low as a few percent, entanglement remains and can be detected.     

Mobile source of high-energy single-cycle terahertz pulses

The Teramobile laser facility was used to realize the first mobile source of high-power THz pulses. The source is based on a tilted-pulse-front pumping THz generation scheme optimized for application of terawatt laser pulses. Generation of 50-μJ single-cycle electromagnetic pulses centered at 0.19 THz with a repetition rate of 10 Hz was obtained for incoming 700-fs 120-mJ near-infrared laser pulses. The corresponding laser-to-THz photon conversion efficiency is approximately 100%.     

Spectral and temporal analysis of ultrashort ultraviolet pulses generated by high-intensity laser radiation in the atmosphere

We report direct spectral and temporal characterization of ultrashort ultraviolet (UV) pulses resulting from third-harmonic generation by high-intensity Ti: sapphire laser radiation in the atmosphere. The experimental technique implemented in this work enables the measurement of the pulse width and the chirp of the UV field, as well as the electron density of the plasma produced by laser pulses. The bandwidth of the third harmonic generated in our experiments by laser pulses with an initial pulse width of 45 fs supports transform-limited UV pulses as short as 30 fs.     

相干激光雷达中最大似然离散谱峰值估计及Monte Carlo仿真

相干测风激光雷达中一个核心的问题是从微弱的气溶胶后向散射信号中估计出风速。基于零均值复高斯随机过程协方差矩阵统计模型的后向散射信号,首先讨论了最大似然(ML)离散谱峰值(DSP)风速估计算法的克拉美-罗下界(CRLB)与由Fisher信息矩阵论得到的精确CRLB之间的关系。其次,对于ML DSP估计应用于相干测风激光雷达中协方差矩阵统计模型的后向散射信号时,使用计算机Monte Carlo仿真的方法研究了风速估计的概率密度函数。分别讨论了信噪比、激光脉冲累积发数和发射激光脉冲宽度对ML DSP风速估计性能的影响。计算仿真结果表明:ML DSP风速估计的CRLB低于精确的CRLB;在信噪比为-20dB,100发激光脉冲累积和信噪比为-30dB,10 000发激光脉冲累积条件下,ML DSP风速估计中"坏"的估计值所占的比例都为0,"好"的估计值的标准差分别为0.62m/s和0.50m/s。     

On the mechanism of the absorption of femtosecond laser pulses in the melting and ablation of Si and GaAs

The melting and ablation thresholds have been measured for Si and GaAs irradiated by 1240-nm femtosecond pulses of a chromium-forsterite laser, i.e., when the photon energy is lower than the bandgap width. A small difference of these quantities from the respective melting and ablation thresholds measured for the action of the 620-nm second-harmonic pulses with a photon energy higher than the bandgap width cannot be explained using available theoretical models. A new approach to the mechanism of the appearance of the electron-hole plasma and the formation of a thin, strongly absorbing surface layer in semiconductors irradiated by femtosecond laser pulses of the visible and infrared spectral ranges has been proposed on the basis of the avalanche mechanism of the filling of the conduction band.     

Spectral phase distribution retrieval through coherent control of harmonic generation

The temporal intensity distribution of the third harmonic of a Ti:sapphire laser generated in Xe gas is fully reconstructed from its spectral phase and amplitude distributions. The spectral phases are retrieved by cross correlating the fundamental laser frequency field with that of the third harmonic, in a three laser versus one harmonic photon coupling scheme. The third harmonic spectral amplitude distribution is extracted from its field autocorrelation. The measured pulse duration is found to be in agreement with that expected from lowest order perturbation theory both for unstretched and chirped pulses.     

超短光脉冲波形对门模单光子探测的影响

采用两种不同的皮秒光脉冲波形进行了门模单光子探测实验.测量了单光子探测效率随探测器和超短脉冲光源之间的同步延迟细微调节时的变化曲线.结果表明：光脉冲波形直接影响光子到达时间与门脉冲时间窗口之间的精确同步和探测效率,使用其中一种脉宽较短的皮秒光脉冲时探测效率比使用另一种脉冲提高了约9%.     

On the Poisson approximation to photon distribution for faint lasers

The photon number statistics for attenuated faint laser pulses is quantitatively studied. It confirms that, even for a non-Poissonian laser source, after being attenuated into faint laser with ultra-low mean photon number, the photon number distribution would approximately be a Poisson distribution. The error of such an approximation is estimated, and numerical tests verify our theoretical analysis. This work lays a sound mathematical foundation for the well-known intuitive idea which has been widely used in quantum cryptography.     

Generation of femtosecond vacuum-ultraviolet pulses

High power femtosecond pulses in the Vacuum Ultra Violet (VUV) have been generated through the nonlinear interaction of femtosecond KrF pulses with xenon and argon gas. Under near resonant two photon excitation of xenon by a femtosecond KrF laser, parametric four wave mixing processes lead to VUV pulses at 147 and 108 nm with pulse energies in the 10 µJ range. Tuning is demonstrated by mixing the KrF pulse with a 500 fs dye laser pulse at 497 nm, resulting in 165 nm emission. In argon, a three photon resonance leads to third harmonic generation at 83 nm and micro joule level pulses near 127 nm generated by a six wave mixing process. Since the spectra of the VUV pulses show an ionization-induced blue shift with increasing KrF laser intensity, the VUV pulses can be shown to have temporal duration less than the pulse width (450 fs) of the KrF laser. Blue shifting of the third harmonic of the KrF laser in argon is dominated by a reduction in the neutral gas density rather than by an increase in the electron density.     

Modulation of a saturable absorber Q-switched laser by a train of picosecond pulses

In this experiment, the transmission of the saturable absorber of a mode-locked laser is modulated optically by a train of ultra-short pulses emitted by another laser, with some power reflected inside the cavity to initiate the emission. The laser threshold is only attained during the opening time of the modulator, and stimulated emission is prevented at any other time. Once pulses are formed, the photon density is sufficiently high for auto-modulating the dye.Q-switching of the laser is only realised if the difference between the time of a round-trip in the cavity and the period of modulation is less than ±17 psec. The emission is a series of pulses of 5.2 nsec period lasting for 60 nsec. The total energy in all pulses is 60 mJ for a neodymium doped glass rod of 8 mm diameter.We describe a method for measuring the noise of the series of pulses using a saturated photo-cell. We find a ratio of 100 between the energy of one pulse and the energy emitted between two pulses.     

Mev X rays and photoneutrons from femtosecond laser-produced plasmas

We demonstrate a novel method to monitor the total angular distribution of the spectrum of hard x-ray emission from a plasma generated with femtosecond laser pulses with an intensity of 5 x 10(18) W/cm2 on a solid target. Measured and calculated angular distributions of x rays show a pronounced anisotropy for MeV photon energies. We complemented the spectral information by demonstrating a (gamma,n) nuclear reaction with a tabletop laser system.     

kJ级宽带低相干激光驱动装置

激光等离子体相互作用的不稳定性将有望通过降低高功率激光装置输出光束的相干性得到大幅缓解。利用低相干光源作为种子源,采用钕玻璃放大介质,研制成功国际首台kJ级大带宽低相干激光装置,实现了带宽13 nm、能量960 J、脉宽3～10 ns可调,相干时间仅为300 fs的大能量光脉冲输出。输出脉冲光谱匀滑无纵模结构,且谱相位随机分布,可实现脉冲波形和光谱分布的无关联精密调控。该装置不仅成功演示验证了低相干激光驱动器的单元技术及系统集成技术,同时也为激光等离子体相互作用及高能量密度物理研究提供了全新的实验研究平台。     

多纵模短腔染料激光放大前后光谱的比较

实验观测了短腔染料激光器输出的多纵模激光及其经一级染料放大的激光光谱，比较了光谱特性。在一定情况下，短腔染料激光器的多纵模激光经放大器放大可产生一个至几个新纵模，新纵模与短腔染料激光器输出的纵模有相似的频率间隔、线宽及频率牵引等特征。新纵模产生属于三阶四光子混频（即四波混频）为主导的非线性光学效应。存在频率牵引表明，增益介质中的多波混频存在频率失配，在这一四波混频中光子能量并不守恒     

Waveform-controlled near-single-cycle milli-joule laser pulses generate sub-10 nm extreme ultraviolet continua

We demonstrate the generation of waveform-controlled laser pulses with 1?mJ pulse energy and a full-width-half-maximum duration of ～4 fs, therefore lasting less than two cycles of the electric field oscillating at their carrier frequency. The laser source is carrier-envelope-phase stabilized and used as the backbone of a kHz repetition rate source of high-harmonic continua with unprecedented flux at photon energies between 100 and 200?eV (corresponding to a wavelength range between 12-6?nm respectively). In combination we use these tools for the complete temporal characterization of the laser pulses via attosecond streaking spectroscopy.     

Spectral broadening of femtosecond pulses in an nonlinear optical fiber amplifier

A promising method for the generation of a supercontinuum with a high spectral power density based on the spectral broadening of ultrashort pulses in a fiber amplifier is considered. The advantage of the method, as compared to the conventional way of the supercontinuum generation in a microstructure fiber, is a lower pulse spectral broadening rate, which allows one to achieve higher SC spectral power densities. The initial stage of the supercontinuum generation in an ytterbium fiber amplifier (a fiber core diameter of 7 μm) with side pumping from an array of laser diodes with a total power of 8 W at a wavelength of 976 nm is experimentally studied. Yb:KYW laser pulses with a duration of 250 fs, a central wavelength of 1046 nm, and an average power of 150 mW have been supplied to the input of the amplifier. In this case spectrally broadened radiation with an average spectral power density of higher than 65 mW/nm and a spectrum width of 50 nm has been obtained.     

Entanglement formation and violation of Bell's inequality with a semiconductor single photon source

We report the generation of polarization-entangled photons, using a quantum dot single photon source, linear optics, and photodetectors. Two photons created independently are observed to violate Bell's inequality. The density matrix describing the polarization state of the postselected photon pairs is reconstructed and agrees well with a simple model predicting the quality of entanglement from the known parameters of the single photon source. Our scheme provides a method to create no more than one entangled photon pair per cycle after postselection, a feature useful to enhance quantum cryptography protocols based on shared entanglement.     

Measurement of Carrier-Envelope-Phase shifts using spectral interferometry with a broad frequency laser source

We investigate and analyze in detail an approach to measure Carrier Envelope Phase shifts by using spectral interferometry. The method which uses a broadband laser source is applied to the characterization of an Electro Optic Carrier Envelope Phase shifter and leads to quick and accurate measurements on a large frequency domain. The shifter is inserted in one arm of a Mach–Zehnder interferometer illuminated by a wide frequency spectrum coherent source. By carefully analyzing the observed fringes, we successfully derived, with a very good accuracy, the main useful parameters of the CEP shifter on the whole investigated wavelength spectrum. The laser source which delivers pulses with a Gaussian spatial distribution and a nearly flat spatial phase does not need to be CEP stabilized and no control of the spectral phase of the pulses is necessary. This has specific advantages compared to other methods.     

A novel femtosecond-laser formation of CdS nanocrystallites in zirconia matrices

A novel method for direct laser writing of two-dimensional cadmium sulfide (CdS) semiconductor nanoparticle microstructures is reported. A two photon or a higher-order multiphoton absorption process, originating from femtosecond laser pulses, was used to decompose CdS precursors dispersed in a zirconia thin film previously dip-coated on a glass substrate. The kinetics of nanoparticle formation as a function of laser power were monitored in situ by photoluminescence spectroscopy. Raman spectroscopy was also performed to characterize the structural changes of the zirconia matrix under irradiation and to verify the formation of CdS nanoparticles. Results show that CdS nanoparticles were formed by two-photon absorption (TPA) with or without the help of an additional carbazole photoinitiator.