Generation of high-order harmonic continuum with two-hump-structure infrared laser pulse

We experimentally investigate the high-order harmonic generation in argon gas cell driven by a multi-cycle broadband infrared laser pulse from a tunable optical-parametric-amplifier （OPA） source. The generation of high-order harmonic continuum with the cut-off photon energy up to 110 eV is observed by tuning the chirp of the 800-nm laser pulse which pumps OPA source. The generation of harmonic continuum is understood in terms of the two-hump structure of the OPA output spectrum and the optimal relative phase of the two humps. The demonstrated scheme is of importance for the generation of extreme ultraviolet （XUV） continuum at higher photon energy region.     

Characterization of three-color CARS in a two-pulse broadband CARS spectrum

We demonstrate that a broadband coherent anti-Stokes Raman scattering (CARS) spectrum generated with a typical two-pulse scheme contains two distinct, significant signals: '2-color' CARS, where the pump and probe are provided by a narrowband pulse and the continuum pulse constitutes the Stokes light, and '3-color' CARS, where the pump and Stokes are provided by two different frequency components in the continuum pulse and the narrowband pulse serves as the probe. The CARS spectra from the two different mechanisms show distinct characteristics in Raman shift range, laser power dependence, and chirping dependence. We discuss the potential for a 3-color CARS signal to cover the fingerprint region with reduced photodamage of live cells. Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States.     

A scheme for the implementation of unconventional geometric phase gates with trapped ions

We propose a scheme for the realization of unconventional geometric two-qubit phase gates with two identical two-level ions, In the present scheme, the two ions are simultaneously illuminated by a standing-wave laser pulse with its pulse frequency being tuned to the ionic transition. The gate operation time can be much shorter, making the system robust against decoherence. In addition, we choose the appropriate experimental parameters to construct the geometric phase gate in one step, and thus avoid implementing the pure geometric single qubit operation.[第一段]     

Carrier-envelope phase fluctuations of amplified femtosecond pulses: characterization with a simple spatial interference setup

We demonstrate a direct and versatile scheme to determine the carrier-envelope phase fluctuations of tunable ultrashort optical pulses. The spatial interferogram between the high frequency components and the parametrically amplified and frequency doubled low frequency components of an octave broad white light continuum is measured for every single pulse. It directly reveals the carrier-envelope phase fluctuations of the pump pulses from the regenerative amplifier, as well as of the white light and the tunable pulses generated from it. PACS 42.25.Kb; 42.65.Yj; 42.65.Re     

Time-resolved investigations of induced absorption due to exciton-biexciton transitions in CuCl

The time-resolved induced absorption due to exciton-biexciton transition is observed in CuCl using an intense ultraviolet picosecond pulse excitation and a picosecond continuum as a probe beam. In the same experimental conditions, time-resolved luminescence is also recorded.     

Dynamic decoherence control of a solid-state nuclear-quadrupole qubit

We report on the application of a dynamic decoherence control pulse sequence on a nuclear-quadrupole transition in Pr3+:Y(2)SiO(5). Process tomography is used to analyze the effect of the pulse sequence. The pulse sequence was found to increase the decoherence time of the transition to over 30 seconds. Although the decoherence time was significantly increased, the population terms were found to rapidly decay on the application of the pulse sequence. The increase of this decay rate is attributed to inhomogeneity in the ensemble. Methods to circumvent this limit are discussed.     

Counterintuitive versus regular inversionless gain in a coherently prepared ladder scheme

In this research we study lasing without population inversion from a three-level atom interacting with two laser fields, in the ladder or cascade scheme. We investigate counterintuitive sequencing as well as regular sequencing of the time of laser fields application. In a counterintuitive sequence scheme a short probe pulse is introduced prior to the application of the coupling field, in contrast, to a standard sequence scheme, where both fields are introduced at the same time. The influence of varying the probe pulse width and time delay between the initiation of probe and coupling fields on transient probe gain is investigated. The calculations indicate the potential of producing gain without inversion via counterintuitive sequence scheme.     

Transitions to the Continuum: Three Different Approaches

We present three different derivations of the transition probabilities to the continuum. It is shown that calculations, performed as a direct application of the postulates of orthodox quantum mechanics (OQM), do not yield results consistent with experiments. Traditional treatments are summarized and criticized. The relation of the transitions to the continuum with the traditional quantum measurement problem is pointed out; we sum up and comment some contributions concerning this issue. It is shown that an approach based on the notion of spontaneous projections yields expressions of the transition probabilities similar to those obtained in the traditional way and new predictions which could be submitted to experimental tests.     

利用非均匀正交激光选择谐波发射短量子路径

理论研究了谐波发射的量子路径在非均匀正交激光场下的调控机制.结果表明,在适当的双色激光偏振角和非均匀参数的组合下,不仅谐波截止能量有明显增大;谐波发射的短量子路径可以被单独选择出来对谐波连续区起到贡献作用.并且,该方案在长脉宽激光下依然适用.最后,利用谐波连续区可以获得脉宽在50 as的孤立阿秒脉冲.     

Energy-based coupling of smooth particle hydrodynamics and molecular dynamics with thermal fluctuations

We propose a thermodynamically consistent and energy conserving coupling scheme between the atomistic and the continuum domain. The coupling scheme links the two domains using the DPDE (Dissipative Particle Dynamics at constant Energy) thermostat and is designed to handle strong temperature gradients across the atomistic/continuum domain interface. The fundamentally different definitions of temperature in the continuum and atomistic domain – internal energy and heat capacity versus particle velocity – are accounted for in a straightforward and conceptually intuitive way by the DPDE thermostat. We verify the here proposed scheme using a fluid, which is simultaneously represented as a continuum using Smooth Particle Hydrodynamics, and as an atomistically resolved liquid using Molecular Dynamics. In the case of equilibrium contact between both domains, we show that the correct microscopic equilibrium properties of the atomistic fluid are obtained. As an example of a strong non-equilibrium situation, we consider the propagation of a steady shock-wave from the continuum domain into the atomistic domain, and show that the coupling scheme conserves both energy and shock-wave dynamics.     

Motional Quantum-State Engineering and Implementation of a Quantum Phase-Gate for Multiple Trapped Ions

We propose a scheme to generate a superposition of motional coherent states with arbitrary coefficients on a line in phase space and implement a quantum controlled phase-gate for multiple trapped ions with a single standing-wave laser pulse whose carrier frequency is tuned to the ions transition. In the scheme each ion does not need to be exactly positioned at the node of the standing wave, which is very important from viewpoint of experiment. Furthermore, our scheme may allow the generation of a superposition of coherent states with large mean phonon number for a large number of trapped ions in a fast way by choosing suitable laser intensity. We show that it can also be used to generate maximally entangled states of multiple trapped ions.     

Motional Quantum-State Engineering and Implementation of a Quantum Phase-Gate for Multiple Trapped Ions

We propose a scheme to generate a superposition of motional coherent states with arbitrary coefficients on a line in phase space and implement a quantum controlled phase-gate for multiple trapped ions with a single standing-wave laser pulse whose carrier frequency is tuned to the ions transition. In the scheme each ion does not need to be exactly positioned at the node of the standing wave, which is very important from viewpoint of experiment, Furthermore, our scheme may allow the generation of a superposition of coherent states with large mean phonon number for a large number of trapped ions in a fast way by choosing suitable laser intensity. We show that it can also be used to generate maximally entangled states of multiple trapped ions.     

Counterintuitive Gaussian pulse sequence in three-level ladder scheme

In a recent publication we have studied counterintuitive pulse sequence applied to a three level ladder scheme, utilizing rectangular-shaped pulses. In this research we investigate amplification without population inversion in a three level ladder system interacting with two electromagnetic fields, namely, the probe and coupling fields. We take both probe and drive fields in the form of Gaussian shaped pulses. In a counterintuitive sequence scheme, the short probe pulse is introduced prior to the application of the coupling field, in contrast to a regular sequence scheme, where both fields are introduced at the same time. The influence of varying the probe pulse width and time delay between the initiation times of the probe and coupling fields on transient probe gain is investigated. It is found that the system exhibits a kind of memory about previously applied weak probe pulse. This may allow for detection of the past event by applying a strong drive pulse in the future.     

Analysis of time phase of characteristic radiation in plasma induced by laser ablating aluminum

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12-fs pulses from a continuous-wave-pumped 200-nJ Ti:sapphire amplifier at a variable repetition rate as high as 4 MHz

We demonstrate a novel compact femtosecond Ti:sapphire laser system operating at repetition rates from 10 kHz to 4 MHz. The scheme is based on the combination of a broadband cavity-dumped oscillator and a double-pass Ti:sapphire amplifier pumped by a low-noise cw solid-state laser. Amplified pulses with an extremely smooth spectrum, a duration of only 12 fs, and less than 0.25% rms fluctuation are generated in a beam with M2 < 1.2. A maximum pulse energy of 210 nJ and an average output power of as much as 720 mW are achieved. This output energy is sufficient to generate a stable continuum in a sapphire disk.     

附加谐波脉冲生成强的39阿秒孤立脉冲

采用多周期800 nm激光组合它的27次谐波脉冲生成超短、宽频的孤立阿秒脉冲.研究表明,脉宽为1 fs的27次谐波脉冲可以有效地控制电子动力学过程.将其加入到单色激光场的特定时域,可以控制电离概率在半个光学周期内迅速提升,使得谐波的转化效率相对于单色场情形增强4个数量级,并实现单一的短量子路径选择,出现频宽为108 eV的超连续辐射谱,叠加该连续谱140次到210次谐波获得了脉宽为39as的强、短孤立脉冲.与文献[7]中采用5 fs/800 nm激光附加紫外阿秒脉冲的方法相比,该方案生成的连续谱频带增宽一 关键词： 组合激光脉冲 连续辐射谱 阿秒脉冲     

Demonstration of transient gain x-ray lasers near 20nm for nickellike yttrium, zirconium, niobium, and molybdenum

We demonstrate strong lasing on the Ni-like 4d(1)S(0)?4p(1)P(1) transition at 18.9, 20.3, 22.0, and 24.0 nm for Mo, Nb, Zr, and Y ions, respectively, using the transient collisional excitation scheme. Approximately 5 J of laser energy in a combination of a 600-ps pulse and a 1-ps pulse from the Compact Multipulse Terawatt (COMET) tabletop laser system is used to irradiate slab targets of these materials. Small-signal gains of 17-26cm (-1) are determined on the 4d?4p transition, with overall gain-length products gL of 11-12. Lasing is observed and gain is measured on the 4f(1)P(1)?4d(1)P(1) transition, which is pumped by collisional excitation combined with self-photopumping, for what is to our knowledge the first time.     

Coherent control multiphoton processes in semiconductor saturable Bragg reflector with freezing phase algorithm

A freezing phase concept has been proposed for adaptive coherent control with a femtosecond pulse shaper. We applied the scheme to investigate multiphoton processes in InAs quantum dot saturable Bragg reflector (SBR). The optical transition of InAs quantum dots can be revealed in the spectral phase sensitivity plot of second harmonic signal. We also achieved a three-time increase in image contrast on regions with photoluminescent wavelength differing only 18 nm by using coherent control nonlinear optical microscopy. Our results suggest the new freezing phase scheme to be useful for various investigations which require fast and reliable complete-field characterization and coherent control on one setup. PACS 42.65.Re; 42.50.Md; 78.67.Hc     

Ultrabroadband continuum generated in air (down to 230 nm) using ultrashort and intense laser pulses

We present results on supercontinuum generation extended up to 230 nm in air during the propagation of a powerful femtosecond laser pulse. The broad supercontinuum generated in air is contributed by self-phase modulation and self-steepening of the fundamental laser pulse, the third-harmonic pulse and their interaction. In particular, the strong interaction between the fundamental and the third-harmonic pulses leads to broad and efficient continuum generation of the third-harmonic pulse itself. The spectrum of the third-harmonic generated in air extends over several tens of nm and overlaps with the shorter wavelength extent of the fundamental continuum. PACS 42.65.Ky; 42.65.Jx; 52.35.Mw     

Continuum generation of the third-harmonic pulse generated by an intense femtosecond IR laser pulse in air

The continuum generation by intense femtosecond IR laser pulses focused in air including the effect of third-harmonic generation is investigated. We have used a theoretical model that includes the full spatio-temporal dynamics of both the fundamental and the third-harmonic pulses. Results of our numerical calculations show that a two-color filamentation effect occurs, in which the third-harmonic conversion efficiency remains almost constant over the whole filament length. It is found that this effect is rather independent of the wavelength of the input beam and the focal geometry. During the filamentation process the third-harmonic pulse itself generates a broad continuum, which can even overlap with the continuum of the fundamental pulse for the longer pump wavelengths. In consequence, the continuum generation generated by intense IR laser pulses is further extended into the UV. PACS 42.65.Jx; 42.65.Ky; 52.35.Mw