Dynamics of difference frequency radiation generation in the field of few-cycle laser pulse propagating in GaAs crystal with domain structure

We consider the process of generation of difference frequency in a GaAs crystal with periodic domain structure during propagation of a few-cycle laser pulse in the crystal in the regime of weakly expressed chromatic dispersion. Method of lines is used to obtain numerical solution to the system of coupled nonlinear differential equations in partial derivatives describing the evolution of the electric field of a few-cycle laser pulse in GaAs crystal both with periodic and aperiodic domain structure. It is shown that employing GaAs crystal with domain structure allows controlling the instantaneous frequency of broadband pulse radiation at difference frequency.     

Generation of a magnetic field in a weakly inhomogeneous plasma interacting with a short laser pulse

The generation of a quasistationary magnetic field in a plasma interacting with a weakly focused low-intensity short laser pulse has been studied. It has been shown that the magnetic field changes direction at times comparable with the free path time of effective electrons. Generation also occurs after the switching off of the short pulse and the maximum field is proportional to the duration of the pulse and is reached at times larger than the free path time of the suprathermal electrons.     

Phase-dependent effects of a few-cycle laser pulse

The Keldysh theory of above-threshold ionization (ATI) is applied to few-cycle laser pulses in order to explore the potential of a recently published new method to measure "carrier-envelope phase difference" phenomena. In this experiment, the carrier-envelope phase difference dependent left-right asymmetry of few-cycle ATI was measured and investigated with a correlation technique. Here, we explore spectral features of the asymmetry, present a theoretical analysis of the experiment, and establish a method to determine the duration of few-cycle pulses whose carrier-envelope phase differences are not controlled.     

Characterizing the polarity of a few-cycle infrared laser pulse

We demonstrate the characterization of polarity and carrier-envelope (CE) phase of a linearly polarized few-cycle infrared (IR) laser pulse (12?fs, 1.8???m) using Terahertz (THz) emission spectroscopy. Crossed polarizers electro-optic sampling geometry is applied to record temporal waveforms of far field THz emission from air filament driven by the few-cycle IR pulse. The THz waveforms are dependent on the carrier-envelope (CE) phase periodically. We confirm that the THz waveform and the intensity can be used jointly to determine the phase of a few-cycle laser field in laboratory frame.     

Determining the absolute carrier phase of a few-cycle laser pulse

In a strong laser field, electrons tunnel from an atom at a rate determined by the instantaneous field. If the pulse is only a few cycles in duration, the highly nonlinear nature of tunnel ionization ensures that the resultant electron wave packet is primarily formed in less than one period. Measuring the direction of above-threshold-ionization electrons produced by circularly polarized light provides a direct method of measuring the absolute carrier phase of a single pulse. The method is robust, surviving spatial and temporal integration as well as intensity fluctuations.     

Generation of a self-chirped few-cycle optical pulse in a FEL oscillator

We study the generation of a self-chirped optical pulse in a free-electron laser (FEL) oscillator. In a high-gain FEL oscillator, the frequency chirp is induced in the slippage region as a result of superradiant FEL resonance, and this time-frequency correlation evolves continuously into a few-cycle regime, if the optical cavity length is perfectly synchronized to the electron bunch interval. Numerical simulations based on the slowly evolving wave approximation and experimental results are presented.     

Propagation of a few-cycle laser pulse in a V-type three-level system

Propagation of a few-cycle laser pulse in a V-type three-level system (fine structure levels of rubidium) is investigated numerically. The full three-level Maxwell-Bloch equations without the rotating wave approximation and the standing slowly varying envelope approximation are solved by using a finite-difference time-domain method. It is shown that, when the usual unequal oscillator strengths are considered, self-induced transparency cannot be recovered and higher spectral components can be produced even for small-area pulses.     

Generation of XUV attosecond pulses in the process of atomic ionization by few-cycle laser radiation

Ionization of a model two-electron atom in the presence of a strong field of ultrashort laser pulses is investigated using the numerical integration of the nonstationary Schrödinger equation, which describes the dynamics of a quantum system in the presence of an electromagnetic wave. The features of two-electron ionization in the presence of one-and two-cycle pulses are analyzed. The suppression of double ionization in the presence of ultrashort laser pulses related to a finite-time interelectron energy exchange upon the laser action is demonstrated. The features of the generation of high-order harmonics and single XUV attosecond pulses are studied for the atomic ionization by few-cycle laser pulses. The parameters of the laser pulse are optimized for the effective generation of a single XUV attosecond pulse.     

Scattering of a few-cycle laser pulse by a single relativistic electron

The scattering of a single relativistic electron with few-cycle plane wave laser pulse with intensity of about $I=1.38\times 10^{14}\,\text{ W/cm }^{2}$ is theoretically and numerically analyzed in the linear regime, and the radiated energy spectra of electron shows that zeptosecond X-ray pulses can be supported. The influences of the initial carrier-envelope phase offset $\varphi _0$ of the incident few-cycle laser pulses are studied, and the results demonstrate that a single zeptosecond pulse can be produced from scattering by using a single-cycle laser pulse with fixed initial carrier-envelope phase offset $\varphi _0 =\pi /2$ . It is discovered that the influence of the initial carrier-envelope phase $\varphi _0$ on the spectrum of the radiation is apparent for low and high frequency of the spectrum, but there is no influence of the central part of the spectrum.     

Observation of pulse trapping in a near-zero dispersion regime

We report pulse trapping in passively mode-locked fiber lasers operating in a near-zero dispersion regime. Two polarization components of vector pulses have different central wavelengths while copropagating as a unit in fiber lasers. The vector pulses exhibit smooth Gaussian spectral profile without any sidebands, qualitatively distinct from those observed in net-anomalous and net-normal fiber lasers. Numerical simulations suggest that the pulse trapping depends not only on fiber birefringence and cavity dispersion but also on saturable absorption effect. The experimental observations are in good agreement with the numerical simulations.     

Spectral broadening of a few-cycle laser pulse propagating in fused quartz

Results of theoretical study of nonlinear propagation of a few-cycle laser pulse in a fused quartz are presented. With use of the finite-difference technique we have integrated numerically the system of the nonlinear Maxwell equations describing the propagation of a linearly polarized pulse at the central wavelength in the range of zero dispersion (λ₀ = 1.27 μm) and duration 20 fs. The evolution of the laser pulse spectrum is calculated and the dependences of the shift of the spectral peak on both the strength of electric field and medium length are obtained.     

Generation of GW radiation pulses from a VUV free-electron laser operating in the femtosecond regime

Experimental results are presented from vacuum-ultraviolet free-electron laser (FEL) operating in the self-amplified spontaneous emission (SASE) mode. The generation of ultrashort radiation pulses became possible due to specific tailoring of the bunch charge distribution. A complete characterization of the linear and nonlinear modes of the SASE FEL operation was performed. At saturation the FEL produces ultrashort pulses (30-100 fs FWHM) with a peak radiation power in the GW level and with full transverse coherence. The wavelength was tuned in the range of 95-105 nm.     

Dynamics of molecular alignment steered by a few-cycle terahertz laser pulse

The field-free alignment of molecule ClCN is investigated by using a terahertz few-cycle pulse (THz FCP) based on the time-dependent density matrix theory. It is shown that a high degree of molecular alignment can be obtained by changing the matching number of the THz FCPs in the adiabatic regime and the non-adiabatic regime. The matching number can affect both the maximum value of the alignment and the time at which it is achieved. It is also found that a higher degree of alignment can be achieved by using the THz FCP at lower intensity and there exists an optimal threshold of molecular alignment with the increase of the field amplitude. Also found is the frequency sensitive region in which the degree of maximum alignment can be enhanced greatly by modulating the center frequencies of different THz FCPs. The investigation demonstrates that comparing with a THz single-cycle pulse, a better result of the field-free alignment can be created by a THz FCP at a constant rotational temperature of molecule.     

Dynamics of the strong field of a few-cycle optical pulse in a dielectric medium

The dependence of the conditions for the dominance of different physical factors in the self-action of few-cycle optical pulses in dielectrics on the intensity, duration, and spectrum of radiation has been theoretically analyzed. It is shown that the larger the pulse width and the central wavelength, the stronger the effect of plasma nonlinearity. For example, for a quartz glass in the field of pulses with a duration of 10 fs and a central wavelength of 780 nm, this nonlinearity mechanism is dominant at intensities exceeding 3 × 10¹³ W cm^?2.     

Isolated ultrashort attosecond pulse generation from a coherent superposition state in a few-cycle chirped laser pulse

We theoretically demonstrate an efficient method for producing an isolated ultrashort attosecond pulse by high-order harmonic generation in an intense few-cycle chirped laser pulse. Our simulation calculations show that the harmonic spectrum reveals an ultrabroad extreme ultraviolet supercontinuum when the initial state is prepared in a coherent superposition of the ground and first excited states. It is also shown that the method can enhance the short quantum path and eliminate the long one, and then an isolated 23-attosecond pulse is produced. By properly adjusting the chirp parameter, a clean single attosecond pulse as short as 11 attoseconds is directly obtained.     

Effects of higher-order dispersion on pulse splitting in the normal dispersion regime

By solving numerically the extended nonlinear Schrödinger equation we investigate the influence of higher-order dispersion effects on the propagation of optical pulses in the normal dispersion regime in a highly nonlinear fiber. Already a small amount of third-order dispersion can lead to a pulse-breakup above a certain pulse power. The splitting is followed by an expansion of the spectrum towards longer wavelengths without any impact of Raman scattering. The transfer of energy to longer wavelengths strongly depends on the dispersion profile of the fiber.     

啁啾激光与半周期脉冲形成的组合场驱动原子产生单个阿秒脉冲

提出了由波长为800 nm、脉冲宽度为5 fs的啁啾激光与半周期脉冲形成组合场,并利用这种组合场驱动一维模型氦原子获得单个阿秒脉冲. 通过数值求解一维氦原子的含时薛定谔方程,发现氦原子在组合场驱动下高次谐波谱的截止位置可以扩展到I_p+21.6U_p. 对第二平台区域不同范围内高次谐波的叠加都能得到单个阿秒脉冲,最短可达37 as,特别是对平台区域的前端进行叠加不仅能够得到较短的单个阿秒脉冲,而且与截止位置附近高次谐波构造的阿秒脉冲相比,强度提高了3个数量级. 关键词： 啁啾激光场 半周期脉冲 高次谐波 阿秒脉冲     

Generation of 8.5-nJ pulse compensation-free from all-fiber dispersion Yb-doped laser

We report the generation of an 8.5-nJ chirped pulse from a mode-locked all-fiber Yb-doped laser. Mode- locking is achieved through nonlinear polarization evolution （NPE） along with spectral filtering. The laser delivers 135 mW of average output power with positively chirped 10.9-ps pulses. The pulse repetition rate is 15.9 MHz, which results in an energy of 8.5 nJ per pulse. The externally dechirped pulse duration is 223 fs, and the pulse energy is 6 n J, which corresponds to the peak power of -27 kW.     

Generation of 8.5-nJ pulse from all-fiber dispersion compensation-free Yb-doped laser

We report the generation of an 8.5-nJ chirped pulse from a mode-locked all-fiber Yb-doped laser.Modelocking is achieved through nonlinear polarization evolution(NPE) along with spectral filtering.The laser delivers 135 mW of average output power with positively chirped 10.9-ps pulses.The pulse repetition rate is 15.9 MHz,which results in an energy of 8.5 nJ per pulse.The externally dechirped pulse duration is 223 fs,and the pulse energy is 6 nJ,which corresponds to the peak power of～27 kW.     

Generation of femtosecond radiation at 211 nm by femtosecond pulse upconversion in the field of a picosecond pulse

We show that, in the case of sum-frequency mixing, one can alleviate group-velocity mismatch between IR and UV pulses by choosing different pulse widths, thus extending the interaction length of ultrashort pulses within nonlinear crystals. By fifth-harmonic generation with a Nd:glass laser, we demonstrate efficient frequency upconversion of 195-fs 264-nm pulses under the envelope of 0.9-ps 1055-nm pulses in beta-barium borate crystal, yielding <270-fs pulses with energy of up to 110muJ at 211 nm.