An all non-optical method for accurately determining the pulse parameters of individual few-cycle laser shots is presented. By analyzing the "left" and "right" asymmetry of high-energy photoelectrons along the polarization axis using the recently developed quantitative rescattering theory, we show that the carrier-envelope phase, the pulse duration and the peak intensity of each single-shot pulse can be readily retrieved. Unlike optical measurements, this method provides the laser intensity in the interaction region directly.
The dissociation dynamics of diatomic molecules interacting with (near) optical laser pulses of different duration is investigated by an elaborate discussion of the electric field of the laser and by a direct solution of the time-dependent molecular Schrödinger equation. The systematic variation of the pulse duration from the electronic time scale (attoseconds) to the nuclear time scale (femtoseconds) shows that the employed few-cycle laser pulses lead to well-known but quite different dissociation mechanisms. A comparative calculation with a model pulse and an experimentally realized attoseco…
We investigate the dynamics of a pair of short laser pulse trains propagating in a medium consisting of three-level $Λ$-type atoms by numerically solving the Maxwell-Schrödinger equations for atoms and fields. By performing propagation calculations with different parameters, under conditions of electromagnetically induced transparency, we compare the propagation dynamics by a single pair of probe and coupling laser pulses and by probe and coupling laser pulse trains. We discuss the influence of the coupling pulse area, number of pulses, and detunings on the probe laser propagation and realizat…
We have observed the intensity fluctuations of the F=2 87Rb atom laser at low output coupling rate. Theoretically, we find that the atom loss of the condensate due to the output of atom laser leads to fluctuations of the laser pulses, which is inherent in all state changing out-coupling such as rf and Raman. Another reason leading to large fluctuations is the interference of output pulses.
We study the influence of the pulse duration on high harmonic generation (HHG) with exploring a wide laser-parameter region theoretically. Previous studies have showed that for high laser intensities near to the saturation ionization intensity, the HHG inversion efficiency is higher for shorter pulses since the ground-state depletion is weaker in the latter. Surprisingly, our simulations show this high efficiency also appears even for a moderate laser intensity at which the ionization is not strong. A classical effect relating to shorter travel distances of the rescattering electron in shorter…
The interaction of relativistically strong tailored laser pulses with an atomic system is considered. Due to a special tailoring of the laser pulse, the suppression of the relativistic drift of the ionized electron and a dramatic enhancement of the rescattering probability is shown to be achievable. The high harmonic generation rate in the relativistic regime is calculated and shown to be increased by several orders of magnitude compared to the case of conventional laser pulses. The energies of the revisiting electron at the atomic core can approach the MeV domain, thus rendering hard x-ray ha…
We propose a new laser cooling method for atomic species whose level structure makes traditional laser cooling difficult. For instance, laser cooling of hydrogen requires single-frequency vacuum-ultraviolet light, while multielectron atoms need single-frequency light at many widely separated frequencies. These restrictions can be eased by laser cooling on two-photon transitions with ultrafast pulse trains. Laser cooling of hydrogen, antihydrogen, and many other species appears feasible, and extension of the technique to molecules may be possible.
We join the tribute to Professor N.B. Delone in this memorial issue by presenting the results of new calculations on the effects of ellipticity on double ionization by short and strong near-optical laser pulses.
We have investigated the ionization of the H atom by intense few-cycle laser pulses, in particular the intra-pulse interference effects, and their dependence on the carrier-envelope phase (CEP) of the laser pulse. In the final momentum distribution of the continuum electrons the imprint of two types of intra-pulse interference effects can be observed, namely the temporal and spatial interference. During the spatial interference electronic wave packets emitted at the same time, but following different paths interfere leading to an interference pattern measurable in the electron spectra. This ca…
We report on the dark pulse emission of an all-normal dispersion erbium-doped fiber laser with a polarizer in cavity. We found experimentally that apart from the bright pulse emission, under appropriate conditions the fiber laser could also emit single or multiple dark pulses. Based on numerical simulations we interpret the dark pulse formation in the laser as a result of dark soliton shaping.
We report a novel optical pulse generation method for high-speed wavelength switching of amplified nanosecond (ns) laser pulses resonant to atomic transitions.Under free-running condition, a slave laser diode is blue-detuned with tens of GHz relative to the master laser. A ns pulse chain generated by modulating the continuous-wave master laser with a fiber-pigtailed electro-optical intensity modulator is injected into the slave laser diode to fast switch the slave laser's wavelength back and forth. The output beam of slave laser is filtered by a temperature-controlled etalon to get the amplifi…
Quantum-mechanical investigation is presented to study single attosecond pulse generation by gating high-order harmonic emission from H$_2^+$ molecule in intense laser pulses with time-dependent ellipticity. The high-order harmonic generation from H$_2^+$ molecule in superposition of a left and a right-hand circularly polarized Gaussian pulse is studied and the effect of time duration and carrier-envelope phase of laser field on single attosecond pulse generation is investigated. Using laser field formed by combination of a left and a right-hand elliptically polarized Gaussian pulse, the effec…
Nonsequential double ionization (NSDI) of helium in an intense few-cycle laser pulse is investigated by applying the three-dimensional semi-classical re-scattering method. It is found that the momentum distribution of He$^{2+}$ shows a single-double-single peak structure as the pulse intensity increases. According to the different mechanisms dominating the NSDI process, the laser intensity can be classified into three regimes where the momentum distribution of He$^{2+}$ exhibits different characteristics. In the relatively high intensity regime, an NSDI mechanism named the "laser-assisted coll…
We present direct time- and space- resolved measurements of the electron density of femtosecond laser pulse-induced plasma filaments. The dominant nonlinearity responsible for extended atmospheric filaments is shown to be field-induced rotation of air molecules.
Experimental efforts toward the detection of the nonperturbative strong-field regime of the Breit-Wheeler pair creation process plan to combine incoherent sources of GeV $γ$ quanta and the coherent fields of tightly focussed optical laser pulses. This endeavour calls for a theoretical understanding of how the pair yields depend on the applied laser field profile. We provide estimates for the number of produced pairs in a setup where the high-energy radiation is generated via bremsstrahlung. Attention is paid to the role of the transversal and longitudinal focussing of the laser field, along wi…
With the development of laser technology, pulse length enters the optical cycle regime and hence the interaction time between laser pulse and atoms becomes prominent. We investigate this problem in this Letter through the photoelectron spectrum of hydrogen atom in few-cycle xuv laser pulses. By solving one-dimensional time-dependent Schrödinger equation, we find that due to the insufficient interaction time, the electron can not gain enough energy from optical field when escaping the bind of the nuclear and then the abnormality appears in the photoelectron spectrum: the peak of photoelectron s…
We use classical electron ensembles and the aligned-electron approximation to examine the effect of laser pulse duration on the dynamics of strong-field double ionization. We cover the range of intensities $10^{14}-10^{16} W/cm^2$ for the laser wavelength 780 nm. The classical scenario suggests that the highest rate of recollision occurs early in the pulse and promotes double ionization production in few-cycle pulses. In addition, the purely classical ensemble calculation predicts an exponentially decreasing recollision rate with each subsequent half cycle. We confirm the exponential behavior …
The creation of electron-positron pairs by the strong-field Breit-Wheeler process in intense short laser pulses is investigated in the framework of laser-dressed quantum electrodynamics. Regarding laser field parameters in the multiphoton regime, special attention is brought to the energy spectrum of the created particles, which can be reproduced and explained by means of an intuitive model. The model is based on the probabilities of multiphoton events driven by the spectral components of the laser pulse. It allows, in particular, to identify interferences between different pair production cha…
We study the Compton scattering of x-rays off electrons that are driven by a relativistically intense short optical laser pulse. The frequency spectrum of the laser-assisted Compton radiation shows a broad plateau in the vicinity of the laser-free Compton line due to a nonlinear mixing between x-ray and laser photons. Special emphasis is placed on how the shape of the short assisting laser pulse affects the spectrum of the scattered x-rays. In particular, we observe sharp peak structures in the plateau region, whose number and locations are highly sensitive to the laser pulse shape. These stru…
High-order harmonic generation (HHG) is investigated for H$_2^+$ and its isotopomers under seven- and ten-cycle trapezoidal laser pulses at 800 nm wavelength and $I$=4$\times 10^{14}$ W$/$cm$^2$ intensity. We solved numerically full-dimensional electronic time-dependent Schrödinger equation with and without the Born-Oppenheimer approximation. We show that the HHG at the falling edge of a trapezoidal laser pulse can result in redshift and complexity on the total HHG spectrum which can be removed by considering different laser pulse duration and nuclear motion not possible for sin$^2$ and Gaussi…
We propose a mechanism to produce a superposition of atomic and molecular states by a train of ultrashort laser pulses combined with weak control fields. By adjusting the repetition rate of the pump pulses and the intensity of the coupling laser, one can suppress a transition while simultaneously enhancing the desired transitions. As an example, various superpositions of vibrational states of the $K_2$ molecule are shown.
We investigate, through simulation, the evolution of polarization states during atmospheric propagation of high power, ultrashort laser pulses. A delayed rotational response model handling arbitrary, transverse polarization couples both the amplitude and phase of the polarization states. We find that, while circularly and linearly polarized pulses maintain their polarization, elliptically polarized pulses become depolarized due to energy equilibration between left and right circularly polarized states. The depolarization can be detrimental to remote radiation generation schemes and obscures ti…
We show that a strong laser pulse combined with a strong x-ray pulse can be employed in a detection scheme for characterizing high-energy $γ$-ray pulses down to the zeptosecond timescale. The scheme employs streak imaging technique built upon the high-energy process of electron-positron pair production in vacuum through the collision of a test pulse with intense laser pulses. The role of quantum radiation reaction in multiphoton Compton scattering process and limitations imposed by it on the detection scheme are examined.
A recent communication [Opt. Commun. doi:10.1016/j.optcom.2010.06.076 (2010)] presents experimental results in which dark pulses are observed in a dispersion-managed (DM) net-anomalous dispersion fiber laser. Disagreement on the formation mechanism proposed in this communication, we would like to indicate a more accurate explanation in order to clarify some potential misunderstanding on dark pulses in fiber lasers.
