Petrissa Eckle

Generation of intense few-cycle laser pulses through filamentation – parameter dependence

Intense few-cycle laser pulses as short as 5.1 fs are generated though self-filamentation in a noble gas atmosphere. We study the dependence of the laser pulse fidelity on the driving pulse profile and chirp as well as on the gas parameters, quantify their pointing stability and spatial quality.

5.1 fs pulses generated by filamentation and carrier envelope phase stability analysis

Intense 5.1 fs CEO (carrier envelope offset) phase stable pulses were generated through two-fold filamentation in a noble gas at atmospheric pressure. The preservation of the CEO phase during the filamentation process was investigated. We show that generating these short pulses using filaments is not detrimental for the CEO phase stabilization, and that the more than one octave-spanning spectrum intrinsically generated by the process is feasible, and offers certain benefits, for direct use in single shot f–2f spectral interferometry.

Spectral signature of short attosecond pulse trains.

We report experimental measurements of high-order harmonic spectra generated in Ar using a carrier-envelope-offset (CEO) stabilized 12 fs, 800 nm laser field and a fraction (less than 10%) of its second harmonic. Additional spectral peaks are observed between the harmonic peaks, which are due to interferences between multiple pulses in the train. The position of these peaks varies with the CEO and their number is directly related to the number of pulses in the train. An analytical model, as well as numerical simulations, support our interpretation.

Semiclassical model for attosecond angular streaking.

Attosecond angular streaking is a new technique to achieve unsurpassed time accuracy of only a few attoseconds. Recently this has been successfully used to set an upper limit on the electron tunneling delay time in strong laser field ionization. The measurement technique can be modeled with either the time-dependent Schrödinger equation (TDSE) or a more simple semiclassical approach that describes the process in two steps in analogy to the three-step model in high harmonic generation (HHG): step one is the tunnel ionization and step two is the classical motion in the strong laser field. Here we describe in detail a semiclassical model which is based on the ADK theory for the tunneling step, with subsequent classical propagation of the electron in the laser field. We take into account different ellipticities of the laser field and a possible wavelength-dependent ellipticity that is typically observed for pulses in the two-optical-cycle regime. This semiclassical model shows excellent agreement with the experimental result.

The Attoclock: A Novel Ultrafast Measurement Technique with Attosecond Time Resolution

The recent progress of the ultrafast laser technology enables to capture and control electrons dynamics, which is the key to understand how energy and charge are transported not only in atoms but also in more complex solid-state and molecular systems. This task calls for the development of novel measurement techniques with attosecond time resolution. The “attoclock” is a relatively simple method, which provides attosecond time resolution without the explicit need of attosecond pulses. In this chapter we review the details of this powerful experimental technique, which was employed in the recent years to investigate electron tunneling dynamics and to study the electron kinematics in strong field single and double ionization.

Semiclassical model for attosecond angular streaking: errata

A misprint occurred in our publication [Opt. Express 18, 17640 (2010)] in the second equation. The misprint is corrected here.

Attosecond Angular Streaking and Tunneling Delay Time in Strong Laser Field Ionization

We use attosecond angular streaking to place an intensity-averaged upper limit of 12 attoseconds on the tunneling delay time in strong field ionization of a helium atom. This is much shorter than the Keldysh time.

Laser induced tunneling ionization in less than 12 attoseconds measured by attosecond angular streaking

It is typically assumed that electrons can escape from atoms through tunneling when exposed to strong laser fields, but the timing of the process has been controversial, and far too rapid to probe in detail. We have used attosecond angular streaking [1] to place an upper limit of 34 attoseconds and an intensity-averaged upper limit of 12 attoseconds on the tunneling delay time in strong field ionization of a helium atom in the non-adiabatic tunneling regime [2]. This is the fastest process that has ever been measured. Our experimental results give a strong indication that there is no real tunneling delay time, which is also confirmed with numerical simulations using the time-dependent Schrödiger equation [3].

Attosecond angular streaking: an ideal technique to measure an electron tunneling time?

We used attosecond angular streaking to measure attosecond ionization dynamics in the non-adiabatic tunneling regime of helium using slightly elliptically polarized 5.9 fs pulses with a peak intensity ranging from 2.3 to 3.5 x 1014 W/cm2 (corresponding to a Keldysh parameter variation of 1.45 to 1.17). With our technique we could demonstrate intensityindependent “instantaneous” ionization with an accuracy of 50 as. Numerical simulations based on the time-dependent Schrodinger equation confirm such ionization behavior with no distinct electron wave packets. This implies that we would not expect a tunneling time or multi-photon ionization delay in the ionization dynamics

Ionization of ar with circularly polarized 5.5-fs pulses for the determination of CEO phase

The ionization rate of Ar with circularly polarized ultrashort laser pulses shows a spatial dependence attributable to the carrier-envelope phase of the pulse. A numerical simulation shows similar characteristics.