Attosecond pulse trains generated using two color laser fields

We investigate the spectral and temporal structure of high harmonic emission from argon exposed to an infrared laser field and its second harmonic. For a wide range of generating conditions, trains of attosecond pulses with only one pulse per infrared cycle are generated. The synchronization necessary for producing such trains ensures that they have a stable pulse-to-pulse carrier envelope phase, unlike trains generated from one color fields, which have two pulses per cycle and a pi phase shift between consecutive pulses. Our experiment extends the generation of phase stabilized few cycle pulses to the extreme ultraviolet regime

Laserteknik berättar elektronernas dramatik

Attofysik handlar om att öppna ögonen för en värld som vi länge vetat fanns men fram till nu inte har kunnat se – elektronernas värld. En värld av ofattbart små och snabba förlopp och fenomen. Utvecklingen av lasertekniken har kommit så långt att vi faktiskt har möjlighet att studera den världen

Measurement and Control of Attosecond Light Fields

Attosecond pulses are used to study electron dynamics in atoms and molecules. In this thesis, schemes to control the generation of attosecond pulses and pulse-shaping techniques to compress the pulses are presented. Generation of attosecond pulses requires high intensity, which is reached by focusing a pulsed femtosecond laser. The emitted pulses come isolated or in an attosecond pulse train (APT), depending on the duration of the driving field. In several experiments, we have controlled the pulse repetition rate in the APT by adding the second harmonic to the driving field. An APT with one pulse per cycle of the driving field is then generated, instead of a train with two pulses per cycle, which is the case for a one-color field. A rather strong second harmonic changes the shape of the generating field, which leads to a tunable central photon energy of the attosecond pulses. With a short driving field an APT containing few pulses is generated. The spectrum of a short APT shows additional interference structures. In analogy with multi-slit interference, these structures are secondary maxima, positioned in between the principal maxima. The number of secondary maxima is related to the number of pulses in the APT. Attosecond pulses are emitted by a macroscopic medium. How the macroscopic conditions affect the pulse duration has also been studied. Directly after the generation the attosecond pulses have, in general, a relatively long pulse duration. Spectral filtering is important to shape the spectrum and compress the pulses. We have used thin transmission filters and multi-layer XUV-mirrors for filtering. We measured a pulse duration of 130 as, for attosecond pulses generated in Ne and filtered by Zr. Most schemes to characterize attosecond pulses are based on a cross-correlation with an IR field. We have used the RABITT (reconstruction of attosecond beating by interference of two-photon transitions) and the AC-streak camera techniques, capable of measuring different types of APT:s. Finally, attosecond pulses have been used in two application experiments: Momentum shearing interferometry; and the Quantum stroboscope, where electron scattering off the atomic potential was observed

Sub-cycle control of attosecond pulse generation using two-colour laser fields

Strong field laser-matter interaction is intrinsically a sub-cycle phenomenon, which is clearly illustrated by the generation of attosecond pulses through the high-order harmonic process. Therefore, to control strong field processes the structure of the field driving the generation has to be controlled on a sub-cycle level. One approach is to use phase stabilized few-cycle driving pulses and vary the carrier-envelope phase of these pulses; an alternative method is to use longer pulses and include the second harmonic to tailor the field structure

Transverse RF Deflecting Structures for the MAX IV LINAC

The MAX IV LINAC operates both as a full-energy injector for two electron storage rings, and as a driver for a Short Pulse Facility (SPF). Recently a conceptual design report for a Soft X-ray Laser (SXL) beamline at the end of the existing LINAC was started. For SPF and SXL operation, it is important to characterize beam parameters such as bunch profile, slice energy spread and slice emittance. For these measurements, two 3 m long transverse deflecting RF structures with a matching section are being developed. The structures are operating at S-band and have variable polarizations. When fed via a SLED pulse compressor, the two structures can generate a total integrated deflecting voltage higher than 100 MV which is sufficient for measurements with temporal resolutions down to 1 fs. This paper describes the initial RF design of the deflecting structures

Spectral shaping of attosecond pulses using two-colour laser fields

We use a strong two-colour laser field composed of the fundamental (800 nm) and the second harmonic (400 nm) of an infrared (IR) laser field to generate attosecond pulses with controlled spectral and temporal properties. With a second-harmonic intensity equal to 15% of the IR intensity the second-harmonic field is strong enough to significantly alter and control the electron trajectories in the generation process. This enables us to tune the central photon energy of the attosecond pulses by changing the phase difference between the IR and the second-harmonic fields. In the time domain the radiation is emitted as a sequence of pulses separated by a full IR cycle. We also perform calculations showing that the effect of even stronger second-harmonic fields leads to an extended tunable range under conditions that are experimentally feasible