Generation of single dispersion precompensated 1-fs pulses by shaped-pulse optimized high-order stimulated Raman scattering

We propose and theoretically analyze a new approach for generating and shaping 1-fs pulses. It combines the ideas of strong-field molecular optics and optimal control to manipulate light generation in a pump-probe Raman regime. Flexible phase control over the generated spectrum of about 3 eV width is achieved by controlling the input pulses and maximizing the coherence of medium excitation by adiabatically aligning molecules in the medium with a specially shaped pump pulse. The generated pulse is optimized for an output window, precompensating for its dispersion to all orders.     

Excitation of He atoms in the strong-field tunneling regime

We report on strong-field excitation of He atoms with two interfering laser pulses of various polarization. The results are explained within the frustrated tunneling ionization model which describes multi-photon excitation in the strong-field tunneling regime.     

Formation of ground-state vibrational wave packets in intense ultrashort laser pulses

The formation of coherent vibrational wave packets in the electronic ground state of neutral molecules in intense ultrashort laser pulses and their subsequent detection by means of recently developed pump-probe experiments are discussed. The wave packet formation is due to the pronounced dependence of the strong-field ionization rate on the internuclear distance. This leads to a deformation of the initial wave function due to an internuclear-distance dependent depletion. The phenomenon is demonstrated with a time-dependent wave packet study for molecular hydrogen.     

激光方脉冲作用下强场自电离过程

本文研究了激光方脉冲作用下的强场自电离过程,从而推广了以往包含高阶离化的研究,讨论了基态布居和自电离共振线型在强场下受激光脉宽、光强的影响。结果表明,强场自电离过程及其中的高阶离化不仅与光强有关,而且还和原子与激光相互作用的时间紧密相关。     

Photo-induced superconductivity

Recent advances in laser technology have made it possible to generate of precisely shaped strong-field pulses at terahertz frequencies. These pulses are especially useful to selectively drive collective modes of solids, for example, to drive materials in a fashion similar to what done in the synthetic environment of optical lattices. One of the most interesting applications involves the creation of non-equilibrium phases with new emergent properties. Here, I discuss coherent control of the lattice to favour superconductivity at ‘ultra-high’ temperatures, sometimes far above the thermodynamic critical temperature T_c.     

Manipulating Photoelectron Distributions for Rare Gas Atoms Ionized with Polarized Femtosecond Laser Pulses

Above-threshold ionizations of rare gas atoms excited by polarized femtosecond laser pulses are investigated. The photoelectron momentum spectra are obtained applying the strong-field approximation (SFA) theory. It is found that, distribution of the emitted photoelectrons varies with different polarizations of laser pulses. We have interpreted the relationship between the observed distribution and the laser polarization taking advantage of tunneling ionization theory and simple-man model. The polarization sensitivity indicates that one can easily manipulate the photoelectron distribution by controlling the polarization of the exciting pulse.     

Multichannel selective femtosecond coherent control based on symmetry properties

We present and implement a new scheme for extended multichannel selective femtosecond coherent control based on symmetry properties of the excitation channels. Here, an atomic nonresonant two-photon absorption channel is coherently incorporated in a resonance-mediated (2+1) three-photon absorption channel. By proper pulse shaping, utilizing the invariance of the two-photon absorption to specific phase transformations of the pulse, the three-photon absorption is tuned independently over an order-of-magnitude yield range for any possible two-photon absorption yield. Noticeable is a set of "two-photon dark pulses" inducing widely tunable three-photon absorption.     

Coherent control of photofragment separation in the dissociative ionization of IBr

We have investigated coherent control of the dissociative ionization of IBr using phase-controlled two-color omega+2omega laser pulses with an intensity of 1.0 x 10(12) W/cm and a pulse duration of 130 fs. The directional asymmetries of the photofragment angular distributions showed oscillation behavior dependent on the relative phase difference between the omega and 2omega pulses. The phase dependencies of the directional asymmetries observed for iodine ions and bromine ions were out of phase with each other. This result shows that a phase-controlled omega+2omega optical field can produce molecular orientation in which the optical field discriminates between parallel and antiparallel configuration of molecules that have a permanent dipole.     

双共振激发多光子电离过程中电离光电子谱的量子相干调制

利用缀饰态和微扰理论,研究了双共振激发多光子电离过程中电离光电子谱的量子相干特性,讨论了强场作用下激发脉冲的面积和脉冲间的延迟对多光子电离光电子谱的影响.结果表明,脉冲面积和脉冲间的延迟对电离光电子谱有明显的调制作用.当第一个脉冲的面积和脉冲间的延迟选取合适时,实现了多光子电离光电子谱Autler-Townes分裂以及电离光电子谱中干涉条纹的控制,并且利用这一量子相干控制实现了粒子在两个缀饰态之间的选择性布居;第二个脉冲面积的变化不影响两个缀饰态上的粒子布居几率,但对电离光电子谱有着明显的调制作用.     

Femtosecond spectral electric field reconstruction using coherent transients

We have implemented a new approach for measuring the time-dependent intensity and phase of ultrashort optical pulses. It is based on the interaction between shaped pulses and atoms, leading to coherent transients.     

强激光场原子隧道电离的研究新进展

光与物质的相互作用一直是科学的主旋律之一.随着超强超短激光技术的快速发展,如今人们可以研究单个原子的内部世界,并调控光与电子的相互作用,从而实现了对原子内电子的超快动力学过程的探索.强激光诱导的原子隧道电离是众多强场物理现象的基石,具有重要的研究意义,也是研究前沿的热点之一.综述了强场原子隧道电离的最新研究进展,基于隧...     

Experimental ionization of atomic hydrogen with few-cycle pulses

We present experimental data on strong-field ionization of atomic hydrogen by few-cycle laser pulses. We obtain quantitative agreement at the 10% level between the data and an ab initio simulation over a wide range of laser intensities and electron energies.     

Coherent Control of High Harmonic Generation Driven by Metal Nanotip Photoemission

Steering ultrafast electron dynamics with well-controlled laser fields is very important for generation of intense supercontinuum radiation. It can be achieved through coherent control of the symmetry of the interaction between strong-field laser fields and a metal nanotip. We employ a scheme of two-color laser pulses combined with a weak static field to realize the control of a single quantum path to generate high harmonic generation from a single solid-state nanoemitter. Moreover, a smooth and ultrabroad supercontinuum in the extreme ultraviolet region is obtained, which can produce a single attosecond pulse. Our findings are beneficial for efficient generation of isolated sub-100 as XUV pulses from solid-state sources.     

Background-free coherent Raman spectroscopy by detecting the spectral phase of molecular vibrations

We propose and demonstrate a new approach to subtracting high nonresonant background in coherent anti-Stokes Raman scattering spectroscopy. The method is based on the retrieval of the spectral phase of molecular vibrations using the technique of frequency-resolved optical gating of Raman scattering. In the presence of high nonresonant background the retrieved phase corresponds directly to the background-free spectrum of the coherent Raman response.     

Femtosecond phase-and-polarization control for background-free coherent anti-Stokes Raman spectroscopy

Phase-and-polarization coherent control is applied to control the nonlinear response of a quantum system. We use it to obtain high-resolution background-free single-pulse coherent anti-Stokes Raman spectra. The ability to control both the spectral phase and the spectral polarization enables measurement of a specific off-diagonal susceptibility tensor element while exploiting the different spectral response of the resonant Raman signal and the nonresonant background to achieve maximal background suppression.     

Measurement of the phase of few-cycle laser pulses

For the shortest pulses generated to date, the amplitude of the electromagnetic wave changes almost as rapidly as the field oscillates. The temporal variation of the field, which directly governs strong-field interactions, therefore depends on whether the maximum of the pulse amplitude coincides with that of the wave cycle or not, i.e., on the phase of the field with respect to the pulse envelope. It is demonstrated that the direction of electron emission from photoionized atoms can be controlled by varying the phase of the field, providing for the first time a tool for its accurate determination. Directing fast electron emission to the right or to the left with the light phase constitutes a new kind of coherent control.     

Strong-field atomic phase matching

We interpret a learning-control experiment with the goal of optimizing multiphoton population transfer in atomic sodium in the strong-field limit. Despite multiple experimental constraints, a learning algorithm discovers optimal pulses that can be understood in terms of a simple dynamic picture of the atom-field interaction. We show that the shaped pulses counteract the dynamic Stark-induced stimulated emission that would otherwise impede the efficient use of a pi pulse to invert a multiphoton transition.     

Multiphoton ionization of molecular hydrogen by single-cycle pulses

Multiphoton ionization, first observed by N.B. Delone in 1965, has been a subject of intense studies ever since. In this paper we consider multiphoton ionization of molecules in the limit of subopticalcycle pulse duration. Moreover, we study the regime where the molecules are first prepared in a coherent vibrational superposition state, and then are subjected to sub-cycle laser pulses synchronized with respect to the phase of the coherent molecular motion. The present approach is based on the Keldysh formalism, which assumes that the final free electron’s state is much more sensitive to the pulse than the bound initial wavefunction [1]. We find that the ionization rate depends not only on the sub-cycle shape of the laser pulses, but also on the time delay between the arrival of pulses and molecular motion.     

On the complex momentum spectra of photoelectrons formed from H<Subscript>2</Subscript> in a strong laser field

We have experimentally investigated the ionization of molecular hydrogen in a strong, linearly polarized laser field. The angularly-resolved photoelectron spectra of H₂ were determined as a function of laser intensity at 800 nm, using pulses of 100 fs duration. A substantial portion of the molecular photoelectron spectra mimics those of atomic hydrogen. We provide qualitative interpretations for this and other features in the spectra. The very high resolution of our data will pose a stringent test on the validity of strong-field theories.     

Interaction of argon clusters with intense VUV-laser radiation: the role of electronic structure in the energy-deposition process

The response of Ar clusters to intense vacuum-ultraviolet pulses is investigated with photoion spec-troscopy. By varying the laser wavelength, the initial excitation was either tuned to absorption bands of surface or bulk atoms of clusters. Multiple ionization is observed, which leads to Coulomb explosion. The efficiency of resonant 2-photon ionization for initial bulk and surface excitation is compared with that of the nonresonant process at different laser intensities. The specific electronic structure of clusters plays almost no role in the explosion dynamics at a peak intensity larger than 1.8 x 10(12) W/cm(2). The inner ionization of atoms for resonant and nonresonant excitation is then saturated and the energy deposition is mainly controlled by the plasma heating rate. Molecular dynamics simulations indicate that standard collisional heating cannot fully account for the strong energy absorption.