[Show abstract][Hide abstract] ABSTRACT: We present calculations on the ratio between double and single ionization of helium by a strong laser pulse at a wavelength of 780 nm using the quantitative rescattering (QRS) model. According to this model, the yield for the doubly charged ion He2+ can be obtained by multiplying the returning electron wave packet (RWP) with the total cross sections (TCSs) for electron impact ionization and electron impact excitation of He+ in the singlet spin channel. The singlet constraint was imposed since the interaction of the helium atom with the laser and the recollision processes both preserve the total spin of the system. An R-matrix (close-coupling) code is used to obtain accurate TCSs, while the RWPs, according to the QRS, are calculated by the strong-field approximation for high-energy photoelectrons. The laser field, which lowers the required energy for the electron to escape from the nucleus at the time of recollision, is also taken into account. The simulated results are in good agreement with the measured He2+/He+ ratio over a broad range of laser intensities. The result demonstrates that the QRS approach based on the rescattering model is fully capable of quantitatively interpreting nonsequential double ionization processes.
Full-text · Article · Dec 2015 · Physical Review A
[Show abstract][Hide abstract] ABSTRACT: We investigate the physical mechanism of unipolar half-cycle pulses generation in resonant two-level media with inversion symmetry. The unipolar half-cycle pulse contains substantial nonzero dc or zero-frequency component in its Fourier spectrum of the electric field. Here the origin of zero-frequency component generation in inversion symmetric media driven by symmetric electric field is identified. We show that in the regime of extreme nonlinear optics, i.e. the Rabi frequency is comparable to or even larger than the carrier frequency of the laser pulse, the time evolution of the polarization can display obvious up-down asymmetric structure under certain conditions, which manifests in the zero-frequency component generation, and is responsible for the formation of unipolar half-cycle pulses in the course of pulse propagation.
No preview · Article · Oct 2015 · Laser Physics Letters
[Show abstract][Hide abstract] ABSTRACT: We investigate the soliton frequency shifts for few-cycle ultrashort laser pulses propagating through resonant media embedded within subwavelength structures, and we elucidate the underlying physics. Full-wave Maxwell–Bloch equations are solved numerically by using the finite-difference time-domain method. It is shown that both soliton blueshift and redshift can occur by changing the period of the structures. We found that the rereflected waves play an essential role in this process. When the pulse propagates through the periodic structures, the reflected waves can be rereflected back by the thin layers, which can further induce the controllable frequency shifts of the generated solitons. This suggests a way to tailor the light solitons over a large spectral range.
No preview · Article · May 2015 · Journal of optics
[Show abstract][Hide abstract] ABSTRACT: We investigate the carrier–envelope phase dependence of the unipolar half-cycle soliton pulse generation when few-cycle ultrashort laser pulses propagate in resonant asymmetric media. It is found that the half-cycle soliton pulse generation depends on both the permanent dipole moment and the electric field shape of the incident few-cycle ultrashort pulse. The irradiation of low-frequency electromagnetic waves superposes with the incident few-cycle ultrashort pulse, which leads to the generation of half-cycle solitons in quantum systems with broken inversion symmetry. Since the electric field shape of the incident few-cycle ultrashort laser pulse depends sensitively on the carrier–envelope phase, the generated half-cycle solitons depend on the carrier–envelope phase of the incident pulse.
[Show abstract][Hide abstract] ABSTRACT: We investigate the photoelectron momentum distribution of molecular-ion H2+driven by ultrashort intense circularly polarized laser pulses. Both numerical solutions of the time-dependent Schrödinger equation (TDSE) and a quasiclassical model indicate that the photoelectron holography (PH) with circularly polarized pulses can occur in molecule. It is demonstrated that the interference between the direct electron wave and rescattered electron wave from one core to its neighboring core induces the PH. Moreover, the results of the TDSE predict that there is a tilt angle between the interference pattern of the PH and the direction perpendicular to the molecular axis. Furthermore, the tilt angle is sensitively dependent on the wavelength of the driven circularly polarized pulse, which is confirmed by the quasiclassical calculations. The PH induced by circularly polarized laser pulses provides a tool to resolve the electron dynamics and explore the spatial information of molecular structures.
[Show abstract][Hide abstract] ABSTRACT: Recently, laser-induced electron diffraction has been used to image molecular structures with subangstrom precision and exposure times of a few femtoseconds by Blaga et al. [Nature (London) 483, 194 (2012)], who treated molecular ions as neutral molecules in the calculation of differential cross section (DCS) for elastic scattering of electrons by molecular ions. In this work, we investigate the long-range and short-range potential effects in elastic scattering of electrons with ions and atoms. It has been found that for incident electron energies above 100 eV and scattering angles greater than 40 degrees, the DCSs of long-range Coulomb potential scattering can be neglected and the DCSs for elastic scattering of electrons with atomic ions and neutral atoms agree well. Therefore, the validity of the treatment in Blaga et al. is verified.
No preview · Article · Nov 2013 · Physical Review A
[Show abstract][Hide abstract] ABSTRACT: Using a finite-difference time-domain method, we theoretically investigate the optical spectra of crossing perpendicular photonic crystal waveguides with quantum dots embedded in the central rod. The waveguides are designed so that the light mainly propagates along one direction and the cross talk is greatly reduced in the transverse direction. It is shown that when a quantum dot (QD) is resonant with the cavity, strong coupling can be observed via both the transmission and crosstalk spectrum. If the cavity is far off-resonant from the QD, both the cavity mode and the QD signal can be detected in the transverse direction since the laser field is greatly suppressed in this direction. This structure could have strong implications for resonant excitation and in-plane detection of QD optical spectroscopy.
[Show abstract][Hide abstract] ABSTRACT: The carrier-envelope phase (CEP)-dependent above-barrier ionization (ABI) has been investigated in order to probe the bound-state electron dynamics. It is found that when the system is initially prepared in the excited state, the ionization yield asymmetry between left and right sides can occur both in low-energy and high-energy parts of the photoelectron spectra. Moreover, in electron momentum map, a new interference effect along the direction perpendicular to the laser polarization is found. We show that this interference is related to the competition among different excited states. The interference effect is dependent on CEPs of few-cycle probe pulses, which can be used to trace the superposition information and control the electron wave packet of low excited states.
[Show abstract][Hide abstract] ABSTRACT: We report a new quantum path interference effect of electron trajectories in high-order harmonic generation (HHG) from two-center molecules, in which the interference minima are mainly located in the high-energy portion of HHG spectrum. The quantum calculations of the time-frequency analyses and the classical results of the electron trajectories demonstrate very good agreement and reveal that the positions of the interference minima are associated with the cutoff of various kinds of molecular electron trajectories. The interference fringes within a half optical cycle can be clearly seen in the time-frequency analysis spectrum. Moreover, the characteristics of both the HHG in frequency domain and the corresponding attosecond pulse generation in time domain permit tracing back the interference information of these electron trajectories. These interference phenomena offer new possibilities for getting insight into the attosecond electronic dynamics in molecules.
[Show abstract][Hide abstract] ABSTRACT: We investigate the local field effects in a ZnO dense medium. Our results show due to the local-field effects, the Lorentz shifts can be found in the reflected spectra driven by the few-cycle laser pulse. Moreover, the dynamic Lorentz shifts depend sensitively on the carrier-envelope phase (CEP) of the few-cycle laser pulse, which provides a useful means to obtain the CEP information by the frequency shifts.
[Show abstract][Hide abstract] ABSTRACT: We investigate the transmitted spectra of a few-cycle ultrashort pulse in an inversion-asymmetric medium with a permanent dipole moment (PDM). Our results show that even-order harmonics can be generated in this medium. Moreover, the generated even-order harmonics depend strongly on the carrier-envelope phase (CEP) of initial incident few-cycle ultrashort pulses. Physical analysis of the re-emitted spectra of the medium reveals that the CEP-dependent spectral effect is originated from the inversion-asymmetric structure and the corresponding PDM effects: two-photon transition dominates in the nonlinear process and further induces the generations of the even-order harmonics. Furthermore, the orientation relation between the electric field peak of the pulse and the PDM results in even-order harmonic generations depending on the CEP.
No preview · Article · Aug 2009 · Journal of Physics B Atomic Molecular and Optical Physics
[Show abstract][Hide abstract] ABSTRACT: We investigate the spectra of two-colour few-cycle pulses in a semiconductor quantum well (QW) with consideration of quantum size effects. Our results show that, in the few-cycle regime, the spectral peak at 4 omega(0) can be achieved even under the condition of a two-colour omega(0) - 3 omega(0) pulse combination. Moreover, when the quantum size is modified, the enlargement or suppression of the spectral peak at 4 omega(0) or 5 omega(0) can be selectively controlled by the relative phase of the two-colour few-cycle pulses.
No preview · Article · May 2009 · Journal of Physics B Atomic Molecular and Optical Physics
[Show abstract][Hide abstract] ABSTRACT: The carrier-envelope phase (CEP)–dependent spectral effect of the reflected few-cycle ultrashort pulse from a dense medium is investigated. It is shown that when adding static electric field, a pronounced CEP dependence of the reflected spectrum can be obtained. Moreover, the period of the CEP-dependent signal becomes 2π because the inversion symmetry for laser-matter interaction is broken, which allows for extraction of the CEP information in both sign and amplitude.
No preview · Article · Apr 2009 · Physical Review A
[Show abstract][Hide abstract] ABSTRACT: We investigate the higher spectral component generations driven by a few-cycle laser pulse in a dense medium when a static electric field is present. Our results show that, when assisted by a static electric field, the dependence of the transmitted laser spectrum on the carrier-envelope phase (CEP) is significantly increased. Continuum and distinct peaks can be achieved by controlling the CEP of the few-cycle ultrashort laser pulse. Such a strong variation is due to the fact that the presence of the static electric field modifies the waveform of the combined electric field, which further affects the spectral distribution of the generated higher spectral components.
No preview · Article · Feb 2009 · Journal of Physics B Atomic Molecular and Optical Physics
[Show abstract][Hide abstract] ABSTRACT: We theoretically propose a method of generating a single sub-100 attosecond (as) pulse with a two-colour time-gating laser field. The field is synthesized by an 8 fs/800 nm (three optical cycles) pulse and a 24 fs/2400 nm (three optical cycles) pulse with an optimal time delay between them. In our simulation, we obtain a supercontinuum with an extremely broad spectrum of 150 eV and generate an isolated attosecond pulse with 96 as pulse duration without any dispersion compensation.
Full-text · Article · May 2008 · Journal of Physics B Atomic Molecular and Optical Physics
[Show abstract][Hide abstract] ABSTRACT: We investigate the carrier-wave Rabi flopping effects in an asymmetric semiparabolic semiconductor quantum well (QW) with few-cycle pulse. It is found that higher spectral components of few-cycle ultrashort pulses in the semiparabolic QW depend crucially on the carrier-envelope phase (CEP) of the few-cycle ultrashort pulses: continuum and distinct peaks can be achieved by controlling the CEP. Our results demonstrate that by adjusting the CEP of few-cycle ultrashort pulses, the intersubband dynamics in the asymmetric semiparabolic QW can be controlled in an ultrashort timescale with moderate laser intensity.
[Show abstract][Hide abstract] ABSTRACT: We theoretically demonstrate the generation of extreme ultraviolet supercontinua in an orthogonally polarized two-color few-cycle laser field. We show that the ionized electrons can be driven back to their parent ion after traveling along curved trajectories in a plane perpendicular to the beam propagation direction, giving rise to a train of attosecond pulses at different polarization angles. A single isolated attosecond pulse can be obtained by blocking the low-order high harmonics, which contribute to the formation of the satellite pulses.
[Show abstract][Hide abstract] ABSTRACT: We theoretically investigate carrier-envelope phase dependence of few-cycle ultrashort laser pulse propagation in a polar molecule medium. Our results show that a soliton pulse can be generated during the two-photon resonant propagation of few-cycle pulse in the polar molecule medium. Moreover, the main features of the soliton pulse, such as pulse duration and intensity, depend crucially on the carrier-envelope phase of the incident pulse, which could be utilized to determine the carrier-envelope phase of a few-cycle ultrashort laser pulse from a mode-locked oscillator.
No preview · Article · Oct 2007 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: By controlling the initial coherent preparation of the medium and the effective area of the incident ultrashort laser pulse, we demonstrate that highly efficient linear-to-circular and circular-to-linear light polarization conversion can be realized in a degenerate three-level medium due to the quantum interference effects between the two different transition paths.
[Show abstract][Hide abstract] ABSTRACT: We theoretically investigate the high-order harmonic generation in a helium atom with a two-color optical field synthesized by an intense 6 fs pulse at 800 nm and a relatively weak 21.3 fs pulse at 400 nm. When the frequency-doubled pulse is properly time shifted with respect to the fundamental pulse, an ultrabroad extreme ultraviolet supercontinuum spectrum with a 148 eV spectral width can be generated which directly creates an isolated 65 as pulse even without phase compensation. We explain this extraordinary phenomenon by analyzing maximum electron kinetic energies at different return times.
Full-text · Article · Jun 2007 · Physical Review Letters