Effects of the carrier-envelope phase of chirped laser pulses in the multiphoton ionization regime

By solving the time-dependent Schr?dinger equation for the Cs atom, we find that, as long as the spectral bandwidth is sufficiently broad, the asymmetry of photoelectron ejection is strongly phase dependent and persists even when the chirped pulse duration becomes more than several cycles. The asymmetry survives even after the angle integration over the hemisphere, implying that the detection efficiency can be significantly improved. This counterintuitive and robust finding provides a simple way to measure the phase for both transform-limited and chirped pulses.     

Phase-dependent excitation and ionization in the multiphoton ionization regime

We theoretically study the dependence of atomic excitation and ionization on the carrier envelope phase of few-cycle laser pulses in the multiphoton ionization regime. Our theoretical results for the hydrogen atom based on the solution of the 3D time-dependent Schr?dinger equation show that the strong phase dependence can be seen in not only total ionization, but also bound-state population under the weak laser intensity regime.     

Observation of carrier-envelope phase phenomena in the multi-optical-cycle regime

So far the role of the carrier-envelope phase of a light pulse has been clearly experimentally evidenced only in the sub-6-fs temporal regime. Here we show, both experimentally and theoretically, the influence of the carrier-envelope phase of a multi-optical-cycle light pulse on high-order harmonic generation. For the first time, we demonstrate that the short and long electron quantum paths contributing to harmonic generation are influenced in a different way by the pulse carrier-envelope phase.     

Comparison study of atomic and molecular single ionization in the multiphoton ionization regime

In this Letter, we report, for the first time in the multiphoton ionization regime, a comparison study of single-electron ionization of diatomic molecules versus rare gas atoms with virtually the same ionization potentials. In comparing N2+ to Ar+, a higher ion signal is seen in N2+ compared to Ar+ for linear polarization but the difference vanishes in circularly polarized light. In comparing O2+ to Xe+, we observe a suppression in O2+ compared to Xe+ for both linear and circular polarization but this suppression exhibits an intensity dependence; i.e., there is little suppression for O2+ at the lowest intensity range, but the suppression becomes increasingly stronger as the laser intensity increases. The multielectron screening model is used to discuss possible mechanisms of this intensity dependent suppression in O2+ in the multiphoton ionization regime.     

Improved carrier-envelope phase locking of intense few-cycle laser pulses using above-threshold ionization

A robust nonoptical carrier-envelope phase (CEP) locking feedback loop, which utilizes a measurement of the left-right asymmetry in the above-threshold ionization (ATI) of Xe, is implemented, resulting in a significant improvement over the standard slow-loop f-to-2f technique. This technique utilizes the floating average of a real-time, every-single-shot CEP measurement to stabilize the CEP of few-cycle laser pulses generated by a standard Ti:sapphire chirped-pulse amplified laser system using a hollow-core fiber and chirped mirror compression scheme. With this typical commercially available laser system and the stereographic ATI method, we are able to improve short-term (minutes) CEP stability after a hollow-core fiber from 450 to 290 mrad rms and long-term (hours) stability from 480 to 370 mrad rms.     

Single-atom effects in high-order harmonic generation: role of carrier-envelope phase in the few-optical-cycle regime

Recently the role of the carrier-envelope phase of few-optical-cycle light pulses on high-order harmonic generation has been experimentally observed. By using a three-dimensional nonadiabatic propagation model, we show that the observed carrier-envelope phase dependence of the spectral characteristics of the harmonic radiation is essentially a single-atom effect. Nonadiabatic single-atom response has been calculated in the framework of the saddle-point approximation. PACS 42.65.Ky; 42.65.Re; 42.50.Hz     

Symmetry of carrier-envelope phase difference effects in strong-field, few-cycle ionization of atoms and molecules

In few-cycle pulses, the exact value of the carrier-envelope phase difference (CEPD) has a pronounced influence on the ionization dynamics of atoms and molecules. We show that, for atoms in circularly polarized light, a change in the CEPD is mapped uniquely to an overall rotation of the system, and results for arbitrary CEPD are obtained by rotation of the results from a single calculation with fixed CEPD. For molecules, this is true only for linear molecules aligned parallel with the propagation direction of the field. The effects of CEPD are classified as geometric or nongeometric. The observations are exemplified by strong-field calculations on hydrogen.     

Nonlinear effects on the carrier-envelope phase

Calculations, using the Maxwell equations, are presented of nonlinear effects on the carrier-envelope phase for pulse durations in the two-optical-cycle range for pulses propagating in sapphire. Initially, with increasing pulse intensity the carrier-envelope phase increases monotonically. However, at higher intensities there is a turnabout in the direction of the phase change and a large change in the phase that is caused by a warping of the envelope from strong nonlinear effects. The Kerr nonlinear response occurs on a time scale of approximately 1 fs and gives significantly different results from those obtained by assuming an instantaneous response.     

Long-term carrier-envelope phase coherence

The carrier-envelope phase of the pulse train emitted by a 10-fs mode-locked laser has been stabilized such that carrier-envelope phase coherence is maintained for at least 150 s (measurement limited). The phase coherence time was measured independently of the feedback loop.     

Role of the carrier-envelope phase in laser filamentation

We numerically study the influence of the initial carrier-envelope phase (CEP) on the filamentation of ultrashort laser pulses in noble gas. Emphasis is put on the CEP-induced changes of pulses that reach their clamping intensity during near-cycle self-compression. In other propagation regimes, the CEP does not significantly alter the pulse evolution. Our results indicate that third-harmonic generation, compared to plasma generation, is dominant in driving these changes. Finally, the stability of the filament CEP against shot-to-shot fluctuations is examined.     

General theory of carrier-envelope phase effects

We present a general framework for understanding carrier-envelope phase (CEP) effects in a quantum system interacting with an intense, short laser pulse. We establish a simple connection between the CEP and the wave function that can be exploited to obtain the full CEP dependence of an observable given the wave function at a single CEP. Within this framework, all CEP effects are interpreted as interference between different photon amplitudes which, in turn, can be used to put limits on the pulse lengths and intensities required to see significant CEP effects.     

Resonant multiphoton ionization of Li

We present calculations of three photon ionization of Li produced by 3-ps laser pulses within a single electron local model potential using the dressed state picture. The laser frequency ranges from 15 000 to 18 400 cm⁻¹. We have found that the measured ionization signal as a function of photon frequency results from ionization processes in a region where the laser intensity is not homogeneous. We assume a Gaussian shape for the light pulse in the interaction volume. Our results are shown to be in good agreement with experiment. We propose that free electrons are submitted to a ponderomotif potential to interpret experimental results. This revised version was published online in August 2006 with corrections to the Cover Date.     

Determining the phase-energy coupling coefficient in carrier-envelope phase measurements

For f-to-2f interferometers based on white-light generation in sapphire plates, the accuracy of the carrier-envelope (CE) phase measurement and stabilization is affected by the laser energy fluctuation. The coupling coefficient between the CE phase and the laser energy has been determined by modulating the pulse energy in an in-loop f-to-2f interferometer while measuring the CE phase variation with an out-loop interferometer. When the total spectral phase measured by the in-loop interferometer was locked, a 1% change in laser energy caused a 160 mrad shift in the CE phase of the output pulses.     

Effects of carrier-envelope phase differences of few-optical-cycle light pulses in single-shot high-order-harmonic spectra

For the first time single-shot harmonic spectra generated by few-optical-cycle pulses have been measured. Clear carrier-envelope phase effects have been observed in the cutoff harmonic spectral structure. Results have been interpreted in terms of the nonadiabatic single-atom response of the nonlinear medium excited by few-optical-cycle pulses.     

Effects of the laser pulse transverse spatial distribution on multiphoton ionization processes

In this paper we study the influence of the transverse spatial distribution of the laser beam on the estimate of the photoionization cross section for the specific case of two-step photoionization (TSPI) of benzene. In particular, we discuss the sensitivity of the inferred cross section on the multimode transverse operation of the dye laser oscillator.     

Electron and multiphoton ionization of xenon

A method for determining photoionization cross sections based on comparison of the characteristics of electron- and photon-impact processes is described, discussed, and demonstrated to have many advantageous features.