F. Frank

Characterization of high-intensity sub-4-fs laser pulses using spatially encoded spectral shearing interferometry

We report on the full amplitude and phase characterization of high-intensity few-cycle laser pulses generated in a single-stage hollow core fiber system with subsequent compression by ultrabroadband chirped mirrors. We use a spatially-encoded arrangement (SEA) spectral phase interferometry for direct electric field reconstruction (SPIDER) with spectral filters for ancilla generation to characterize the sub-4 fs pulses with spatial resolution.

Invited review article: technology for attosecond science.

We describe a complete technological system at Imperial College London for Attosecond Science studies. The system comprises a few-cycle, carrier envelope phase stabilized laser source which delivers sub 4 fs pulses to a vibration-isolated attosecond vacuum beamline. The beamline is used for the generation of isolated attosecond pulses in the extreme ultraviolet (XUV) at kilohertz repetition rates through laser-driven high harmonic generation in gas targets. The beamline incorporates: interferometers for producing pulse sequences for pump-probe studies; the facility to spectrally and spatially filter the harmonic radiation; an in-line spatially resolving XUV spectrometer; and a photoelectron spectroscopy chamber in which attosecond streaking is used to characterize the attosecond pulses. We discuss the technology and techniques behind the development of our complete system and summarize its performance. This versatile apparatus has enabled a number of new experimental investigations which we briefly describe.

Sub-4-fs laser pulse characterization by spatially resolved spectral shearing interferometry and attosecond streaking

We demonstrate the generation of high-energy sub-2-cycle laser pulses generated through hollow core fibre pulse compression. We demonstrate their full characterization with two independent methods. For all-optical characterization in amplitude and spectral phase, we employ spatially encoded arrangement spectral phase interferometry for direct electric-field reconstruction using spectrally filtered ancilla beams to characterize the sub-4-fs pulses with spatial resolution. For field-sensitive pulse characterization, we generate isolated attosecond pulses around 93 eV. The attosecond pulse as well as the infrared few-cycle pulse is characterized in amplitude and phase using the frequency resolved optical gating for complete reconstruction of the attosecond bursts technique. We find good agreement between the two methods.

High-order harmonic generation in graphite plasma plumes using ultrashort laser pulses: a systematic analysis of harmonic radiation and plasma conditions

High-order harmonic generation in graphite-ablated plasmas was systematically studied using ultrashort (3.5 and 30 fs) laser pulses. We observed the efficient frequency conversion of 3.5 fs Ti:sapphire laser pulses in the range of 15-26 eV. Stabilization of the harmonic yield at a 1 kHz pulse repetition rate was accomplished using a rotating graphite target. We also show the results of harmonic generation in carbon plasma using 1300 nm, 40 ps pulses, which allowed the extension of the harmonic cutoff while maintaining a comparable conversion efficiency to the case of 780 nm driving radiation. The time-of-flight mass spectrometric analysis of the plasma components and the scanning electron microscopy of plasma debris under optimal conditions for harmonic generation suggest the presence of small carbon clusters (C10-C30 )i n the plasma plume at the moment of femtosecond pulse propagation, which further aggregate on nearby substrates. We present the results of plasma spectroscopy obtained under unoptimized plasma conditions that elucidate the reduction in harmonic signal. We also present calculations of plasma concentration under different excitation conditions of the ablated graphite target. (Some figures may appear in colour only in the online journal)

Stable generation of high-order harmonics of femtosecond laser radiation from laser produced plasma plumes at 1 kHz pulse repetition rate.

We present a method for the creation of stable weakly ionized plasmas from laser ablation of solid targets using a 1 kHz pulse repetition rate laser, which can be used for stable high-order harmonic generation from plasma plumes. The plasma plumes were generated from cylindrical rotating targets. Without target rotation the intensity of harmonics in the 40-80 nm range drops by more than one order of magnitude during less than 10(3) shots, while, with rotation of the target at typically 30 revolutions per minute, stable emission of high-order harmonics from aluminum plasma plumes with variation of less than 10% was maintained for >10(6) laser shots.

Numerical simulation of attosecond nanoplasmonic streaking

The characterization of the temporal profile of plasmonic fields is important both from the fundamental point of view and for potential applications in ultrafast nanoplasmonics. It has been proposed by Stockman et al (2007 Nat. Photonics 1 539) that the plasmonic electric field can be directly measured by the attosecond streaking technique; however, streaking from nanoplasmonic fields differs from streaking in the gas phase because of the field localization on the nanoscale. To understand streaking in this new regime, we have performed numerical simulations of attosecond streaking from fields localized in nanoantennas. In this paper, we present simulated streaked spectra for realistic experimental conditions and discuss the plasmonic field reconstruction from these spectra. We show that under certain circumstances when spatial averaging is included, a robust electric field reconstruction is possible.

Enhancement of high harmonics from plasmas using two-color pump and chirp variation of 1 kHz Ti:sapphire laser pulses

We have investigated resonance effects in high-order harmonic generation (HHG) within laser-produced plasmas. We demonstrate a significantly improved harmonic yield by using two-color pump-induced enhancement and a 1 kHz pulse repetition rate. Together with an increased HHG output, the even harmonics in the cutoff region were enhanced with respect to odd harmonics. We report the observation of a resonance-induced growth in intensity of 20th harmonic in silver plasma (2×), 26th harmonic in vanadium plasma (4×), and 28th harmonic in chromium plasma (5×).

Isolated sub-fs XUV pulse generation in Mn plasma ablation

We report studies of high-order harmonic generation in laser-produced manganese plasmas using sub-4-fs drive laser pulses. The measured spectra exhibit resonant enhancement of a small spectral region of about 2.5 eV width around the 31st harmonic (~50eV). The intensity contrast relative to the directly adjacent harmonics exceeds one order of magnitude. This finding is in sharp contrast to the results reported previously for multi-cycle laser pulses [Physical Review A 76, 023831 (2007)]. Theoretical modelling suggests that the enhanced harmonic emission forms an isolated sub-femtosecond pulse.

Lateral shearing interferometry of high-harmonic wavefronts

We present a technique for frequency-resolved wavefront characterization of high harmonics based on lateral shearing interferometry. Tilted replicas of the driving laser pulse are produced by a Mach-Zehnder interferometer, producing separate focii in the target. The interference of the resulting harmonics on a flat-field extreme ultraviolet spectrometer yields the spatial phase derivative. A comprehensive set of spatial profiles, resolved by harmonic order, validate the technique and reveal the interplay of single-atom and macroscopic effects.

Enhancement of high harmonics generated by field steering of electrons in a two-color orthogonally polarized laser field

We demonstrate enhancement by 1 order of magnitude of the high-order harmonics generated in argon by combining a fundamental field at 1300 nm (10(14) W cm(-2)) and its orthogonally polarized second harmonic at 650 nm (2 × 10(13) W cm(-2)) and by controlling the relative phase between them. This extends earlier work by ensuring that the main effect is the combined field steering the electron trajectory with negligible contribution from multiphoton effects compared to the previous schemes with 800/400 nm fields. We access a broad energy range of harmonics (from 20 eV to 80 eV) at a low laser intensity (far below the ionization saturation limit) and observe deep modulation of the harmonic yield with a period of π in the relative phase. Strong field theoretical analysis reveals that this is principally due to the steering of the recolliding electron wave packet by the two-color field. Our modeling also shows that the atto chirp can be controlled, leading to production of shorter pulses.