Temporal compression of nanosecond laser pulses using coherent control

We present a study of the temporal compression of nanosecond laser pulses resulting from the coherent control peculiarities of the propagation dynamics in a regime of electromagnetically induced transparency. We describe the general theoretical framework and discuss the crucial conditions required in order to experimentally realize such a temporal compression scheme. A proof-of-concept experimental realization of a scheme of this type in a sample of hot sodium vapors is currently being implemented: we describe in detail the experimental setup designed for this purpose.     

Temporal coherent control of superfluorescent pulses

We demonstrate that collective atomic interferences can be investigated by measuring the superfluorescence (SF) time delay. A pair of broadband (≈20 nm), ultrashort (≈80 fs), collinear pulses with a variable delay coherently excites rubidium (Rb) atoms. The generated superfluorescent pulses at 420 nm on the cascade transition are recorded by a picosecond streak camera. Both intensity and SF time delay of the 420 nm pulse are altered as the delay between input pulses varies. In particular, the SF time delay of the normalized 420 nm pulse exhibits oscillations with different periods. This can be understood in terms of atomic and quantum interferences due to two possible two-photon excitation pathways through the intermediate levels (Rb D-lines).     

Temporal self-action and compression of intense ultrashort laser pulses in hollow photonic-crystal waveguides

Hollow-core waveguides with a periodic (photonic-crystal) cladding are shown to allow efficient temporal compression of high-intensity ultrashort laser pulses and formation of megawatt soliton-like features in the regime of robust isolated guided modes. We numerically analyze the temporal envelope evolution and spectral transformation of the light field in air-guided modes of gas-filled hollow coaxial periodic Bragg waveguides. Based on this analysis, we define optimal compression regimes, permitting high compression ratios (of about six) and high compression efficiencies (up to 73%) to be achieved for microjoule laser pulses with an initial pulse length of 80–400 fs.     

Terahertz emission in tenuous gases irradiated by ultrashort laser pulses

Mechanism of terahertz (THz) pulse generation in gases irradiated by ultrashort laser pulses is investigated theoretically. Quasi-static transverse currents produced by laser field ionization of gases and the longitudinal modulation in formed plasmas are responsible for the THz emission at the electron plasma frequency, as demonstrated by particle-in-cell simulations including field ionization. The THz field amplitude scaling with the laser amplitude within a large range is also discussed.     

Terahertz emission in tenuous gases irradiated by ultrashort laser pulses

Mechanism of terahertz (THz) pulse generation in gases irradiated by ultrashort laser pulses is investigated theoretically. Quasi-static transverse currents produced by laser field ionization of gases and the longitudinal modulation in formed plasmas are responsible for the THz emission at the electron plasma frequency, as demonstrated by particle-in-cell simulations including field ionization. The THz field amplitude scaling with the laser amplitude within a large range is also discussed.     

Ultrafast coherent population transfer driven by two few-cycle laser pulses

We investigate ultrafast coherent population transfer driven by few-cycle pump and Stokes laser pulses in the Λ-type three-level system with the stimulated Raman adiabatic passage technique beyond the rotating-wave approximation. In contrast to the case with the rotating wave approximation, the most efficient population transfer may be realized without the satisfaction of the one-photon resonances or two-photon resonance and the transfer efficiency depends critically on the Rabi frequencies and initial optical phases of the two laser fields when the peak Rabi frequencies are much larger than the respective transition frequencies. Moreover, complete and robust population transfer can still be obtained with the variations of the Rabi frequencies, pulse durations, and one-photon or two-photon detuning in a moderate range, though a considerable transient population may reside in the excited state. These abnormal behaviors result from the counterrotating terms, which are not taken into account in the traditional rotating wave approximation.     

Symmetries and asymmetries in coherent atomic excitation by chirped laser pulses

We analyze the effects of laser-induced Stark shift and irreversible population loss on the technique of chirped-frequency adiabatic passage, and the ensuing symmetries and asymmetries in the ionization and fluorescence signals. We find that the properties of the detection signal depend critically on the fashion in which it is collected: for example, the post-pulse populations of the ground and excited states, and the ionization signal collected during the excitation, possess different symmetry properties with respect to the frequency chirp rate and the static frequency detuning. We illustrate these features with two exactly soluble analytic models, which describe simultaneous excitation and ionization of a two-state quantum system, as it typically occurs in atomic excitation with femtosecond laser pulses. We find that the ionization signal may exhibit unexpected oscillations and derive the conditions for maximizing their contrast.     

Subfemtosecond compression of periodic laser pulses

It is shown that a recently proposed technique of molecular modulation can be used to compress mode-locked laser pulses into the subfemtosecond domain. The method relies on driving a molecular oscillation with a periodic pulse train. The coherent molecular motion, in turn, modulates the laser frequency and allows pulse compression by group-velocity dispersion of the same molecular medium.     

Tiled-grating compression of multiterawatt laser pulses

High-energy petawatt lasers require large diffraction gratings for pulse compression. As an alternative to meter-sized gratings, we demonstrate the capability of a tiled-grating system to compress multiterawatt subpicosecond laser pulses. Using a 100 TW-class Nd:glass chirped-pulse amplification laser facility, we report on the performance of a two-grating mosaic to compress high-energy pulses to 2.5 J, 450 fs (5.5 TW) in air with a beam size of 50 mm and energy transmission of 63%. Stability of the grating mosaic alignment was realized by use of an accurate nanopositioning optomechanical system. The output Gaussian spectrum was preserved from grating-gap spectral clipping and was free of modulation.     

不同气氛下飞秒激光诱导硅表面微结构

利用钛宝石飞秒激光脉冲对单晶硅在SF6、空气和真空环境中进行了累积脉冲辐照,研究了硅表面微结构的演化。在SF6气氛中,在激光辐照的初始阶段,硅表面形成了１维的波纹结构,随着辐照脉冲数的增加,波纹结构演化成了２维凹凸结构。累积600个脉冲后,硅表面产生了准规则排列且具有大纵横比的锥形尖峰结构。该结构呈现高度相对较低、锥形尖端小球不明显的特征,分析认为主要与环境气压的大小有关。对比空气、SF6和真空中的微结构发现,尖峰的数密度依次减小;SF6中形成的尖峰高度最大,其次为真空,再次为空气。研究结果表明,真空、SF6和空气3种环境下微结构的形成及表面形貌主要由激光烧蚀、化学刻蚀和氧化决定。     

Tunable ultrashort laser pulses generated through filamentation in gases

Tunable and stable ultrashort laser pulses in the visible spectrum are generated with high efficiency by four-wave mixing process during the filamentation of near-infrared and infrared laser pulses in gases. It is shown that these tunable ultrashort pulses have a very low energy fluctuation and an excellent mode quality due to the processes of intensity clamping and self-filtering in the filament.     

Controllable optical delay of wideband laser pulses by means of coherent effects

We report the first experimental observation of controllable temporal delay of wideband laser pulses (input spectral bandwidth of 3.3 GHz) using electromagnetically induced transparency. We obtain this result with limited temporal distortion of our pulses and excellent values of the delay-bandwidth product. Our experimental results are in agreement with a theoretical analysis.     

Stability theory of coherent laser pulses in an inverted medium

Analytic steady-state solutions describing pulse propagation in a homogenously broadened laser amplifier are known. In the present paper we show the stability of these solutions.     

Small rare gas clusters in XUV laser pulses

Semi-classical molecular dynamics simulations of small rare gas clusters in short laser pulses of 100 nm wavelength were performed. For comparison, the cluster response to 800 nm laser pulses was investigated as well. The inner ionization dynamics of multi-electron atoms inside the cluster was treated explicitly. The simulation results underpin the belief that at XUV (extreme ultraviolet) wavelengths collisions play an important role in the energy absorption. The generation of the surprisingly high charge states of Xe atoms inside clusters, as they were observed in a free-electron laser experiment at DESY, Hamburg, Germany by Wabnitz et al. is due to the reduced ionization potential of atoms inside charged clusters, the ionization ignition mechanism, and collisions. PACS 36.40.Gk; 36.40.Vz; 31.15.Gy     

High-energy femtosecond laser pulse compression in single- and multi-ionization regime of rare gases: experiment versus theory

We report experimental and numerical results on the post-compression of 40 fs duration pulses down to 10 fs at high energy level (multi-mJ). The spectral broadening is achieved through the self-phase modulation resulting from optical-field-ionization of different noble gases (He, Ne, Ar) by the 40-fs laser pulse propagating in a low-pressure gas-filled hollow capillary. We discuss the influence of the multi-ionization dynamics, through the gas dependence, on the laser energy carried by the capillary, as well as on the duration and temporal shape of the post-compressed pulses. In all the different experimental conditions investigated in this article (pressures and gases used), the experimental data is in good agreement with the numerical results from a three-dimension propagation code. Through this study, we demonstrate the robustness of the proposed post-compression technique with regard to multi-ionization, indicating that it can be used on a large intensity range by judiciously choosing the gas.     

Fully coherent x-ray pulses from a regenerative-amplifier free-electron laser

We propose and analyze a regenerative-amplifier free-electron laser (FEL) to produce fully coherent, hard x-ray pulses. The method makes use of narrow-bandwidth Bragg crystals to form an x-ray feedback loop around a relatively short undulator. Self-amplified spontaneous emission (SASE) from the leading electron bunch in a bunch train is spectrally filtered by the Bragg reflectors and is brought back to the beginning of the undulator to interact repeatedly with subsequent bunches in the bunch train. The FEL interaction with these short bunches regeneratively amplifies the radiation intensity and broadens its spectrum, allowing for effective transmission of the x rays outside the crystal bandwidth. The spectral brightness of these x-ray pulses is about 2 to 3 orders of magnitude higher than that from a single-pass SASE FEL.     

On the nature of coherent phonons generated by ultrashort laser pulses in single-crystal antimony

Reflectivity oscillations generated by A_1g coherent phonons in an antimony single crystal have been studied by a method involving pumping and probing by femtosecond laser pulses, which was complemented by spectral filtration of the signal. An analysis of the spectrally resolved signal showed that not only the integrated intensity but also the spectrum of the probe pulse are functions of the delay time between the pumping and probing and oscillate between the Stokes and anti-Stokes components at the optical-phonon frequency. A comparison of the integrated lattice excitation relaxation dynamics with the spectrally resolved lattice excitation relaxation dynamics revealed new facets in the nature and generation mechanism of coherent phonons.     

Broadband coherent light generation in a Raman-active crystal driven by two-color femtosecond laser pulses

We demonstrate broadband light generation by focusing two-color ultrashort laser pulses into a Raman-active crystal, lead tungstate (PbWO(4)). As many as 20 anti-Stokes and 2 Stokes fields are generated due to strong near-resonant excitation of a Raman transition. The generated spectrum extends from the infrared, through the visible region, to the ultraviolet, and it consists of discrete spatially separated sidebands. Our measurements confirm good mutual spatial and temporal coherence among the generated fields and open possibilities for synthesis of subfemtosecond light waveforms.