Few-cycle Ultrashort Laser Pulses Research Articles

Soliton frequency shifts in subwavelength structures

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.

Bragg reflection in a quantum periodic structure

We investigate the reflected field for few-cycle ultra-short laser pulses propagating through resonant media embedded within wavelength-scale structures. Full-wave Maxwell–Bloch equations are solved numerically by using the finite-difference time-domain method. The results show that the spectral feature of the reflected spectrum is determined by the Bragg reflection condition, and that the periodic structure of a dense atomic system can be regarded as a one-dimensional photonic crystal and even as a highly reflective multilayer film. Our study explains the suppression of the frequency shifts in the reflected spectrum based on the Bragg reflection theory and provides a method to control the frequency and frequency intervals of the spectral spikes in the reflected spectrum.

Carrier–envelope phase dependence of a half-cycle soliton generation in asymmetric media

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.

Ionization of atoms by ultrashort UV laser pulses - Iterative solution of the time dependent Schrödinger equation

The ionization of the hydrogen atom by few-cycle ultrashort laser pulses is studied using classical and quantum mechanical models. The reliability of these models for various laser pulse parameters is discussed. Our attention is focused on the intra-pulse interference effects [1] and its dependence on the laser pulse parameters.

Characterization of broadband few-cycle laser pulses with the d-scan technique

We present an analysis and demonstration of few-cycle ultrashort laser pulse characterization using second-harmonic dispersion scans and numerical phase retrieval algorithms. The sensitivity and robustness of this technique with respect to noise, measurement bandwidth and complexity of the measured pulses is discussed through numerical examples and experimental results. Using this technique, we successfully demonstrate the characterization of few-cycle pulses with complex and structured spectra generated from a broadband ultrafast laser oscillator and a high-energy hollow fiber compressor.

Supercontinuum pulse measurement by molecular alignment based cross-correlation frequency resolved optical gating

We demonstrate that complex supercontinuum and few-cycle ultrashort laser pulses can be fully characterized by using a cross-correlation frequency-resolved optical gating with molecular alignment induced birefringence functioned as a gate. The temporal envelope and phase of the broadband supercontinuum pulse are retrieved by the principal component generalized projection algorithm. This technique shows advantages without phase-matching constraint that may limit the measurable spectral bandwidth, experimental robustness in operating through the whole transparent spectral region of the molecular gases, and intensity sensitivity to measure weak pulses which is inherited from the intrinsic linear process in recording the molecular birefringence induced polarization spectroscopy. Experimental measurements of a few-cycle pulse in the visible region of 525-725 nm confirm that the molecular alignment gating supports a full field characterization of the ultrashort pulse around 10 fs in duration.

Phase control of higher spectral components in the presence of a static electric field

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.

Generation of intense extreme supercontinuum radiation via resonant propagation effects

Confinement of electromagnetic energy into a single well-controlled oscillation of light is very important for generation of intense supercontinuum radiation. We find that the pulse breakup of few-cycle ultrashort laser pulses via resonant propagation effects can achieve this aim. By extracting such pulses and then focusing them to drive the He atoms, about 200 eV intense supercontinuum radiation can be generated, which is capable of supporting similar to 20 attosecond isolated pulse generation.

Carrier-Envelope Phase Dependence of Few-Cycle Ultrashort Laser Pulse Propagation in a Polar Molecule Medium

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.

Polarization evolution of a few-cycle ultrashort laser pulse propagating in a degenerate three-level medium

The propagation of an arbitrary polarized few-cycle ultrashort laser pulse in a degenerate three-level medium is investigated by using an iterative predictor-corrector finite-difference time-domain method. It is found that the polarization evolution of the ultrashort laser pulse is dependent not only on the initial atomic coherence of the medium but also on the polarization condition of the incident laser pulse. When the initial effective area is equal to $2\ensuremath{\pi}$, complete linear-to-circular and circular-to-linear polarization conversion of few-cycle ultrashort laser pulses can be achieved due to the quantum interference effects between the two different transition paths.

Propagation of an arbitrary elliptically polarized few-cycle ultrashort laser pulse in resonant two-level quantum systems

We investigate the propagation of an arbitrary elliptically polarized few-cycle ultrashort laser pulse in resonant two-level quantum systems using an iterative predictor-corrector finite-difference time-domain method. It is shown that when the initial effective area is equal to $2\ensuremath{\pi}$, the effective area will remain invariant during the course of propagation, and a complete Rabi oscillation can be achieved. However, for an elliptically polarized few-cycle ultrashort laser pulse, polarization conversion can occur. Eventually, the laser pulse will evolve into two separate circularly polarized laser pulses with opposite helicities.

Single attosecond pulse generation by tightly focused laser beam-electron interaction

The nonlinear Thomson scattering by an electron in a focused circularly polarized laser beam is investigated and a novel scheme for single attosecond pulse generation is proposed.Through computer simulation we find that by using a tightly focused laser beam,the ratio of the highest peak pulse and the second highest peak pulse of the radiation which is defined as the signal-to-noise ratio can be efficiently increased, such that a single attosencond pulse rather than a train of attosecond pulses is obtained. The pulse duration of the radiation decreases and the signal-to-noise ratio increases with increasing laser intensity.Simultaneously,the signal-to-noise ratio can be also improved as the beam-waist decreases.We find that a single attosencond pulse can be generated by using a few-cycle ultrashort laser pulse as well.

Effect of time-dependent ionization on the propagation of a few-cycle laser pulse in a two-level medium

We investigate the influence of ionization on the propagation and spectral effects of a few-cycle ultrashort laser pulse in a two-level medium. It is found that when the fractional ionization is weak, the production of higher spectral components makes no difference. However, when the two states are essentially depleted before the peak of the laser pulse, the impact of ionization on the higher spectral components is very significant.

Photon absorption of conduction band electronsand impact ionization under irradiation of few-cycle ultrashort laser pulses

We discuss the limitation of classical Drude Model, solve the kinematics equation for electrons in the few-cycle laser field, and calculate the photon absorpti on of conduction-band electrons(CBE) and impact ionization under irradiation of few-cycle ultrashort laser pulses. The effects of laser intensity, carrier-en velope-phase on impact ionization are discussed.