Few-femtosecond extreme-ultraviolet pulses fully reconstructed by a ptychographic technique.

Matteo Lucchini,Mauro Nisoli,Nicola Fabris,Sandro De Silvestri,Mario Murari,Luca Poletto,Fabio Frassetto,Giacinto D Lucarelli,Andrea Trabattoni

doi:10.1364/oe.26.006771

Abstract

Sub-10-fs pulses tunable in the extreme-ultraviolet (XUV) spectral region are particularly important in many research fields: from atomic and molecular spectroscopy to the study of relaxation processes in solids and transition phase processes, from holography to free-electron laser injection. A crucial prerequisite for all applications is the accurate measurement of the temporal characteristics of these pulses. To fulfill this purpose, many phase retrieval algorithms have been successfully applied to reconstruct XUV attosecond pulses. Nevertheless, their extension to XUV femtosecond pulses is not trivial and has never been investigated/reported so far. We demonstrate that ultrashort XUV pulses, produced by high-order harmonic generation, spectrally filtered by a time-delay compensated monochromator, can be fully characterized, in terms of temporal intensity and phase, by employing the ptychographic reconstruction technique while other common reconstruction algorithms fail. This allows us to report on the generation and complete temporal characterization of XUV pulses with duration down to 5 fs, which constitute the shortest XUV pulse ever achieved via a time-delay compensated monochromator.

Highlights

The process of high-order harmonic generation (HHG) in gas is at the center of active experimental and theoretical investigation since its discovery and it is at the heart of the emergence of attosecond science [1,2,3]
In the previous section we demonstrated with theoretical simulations that extended ptychographic iterative engine (ePIE) can correctly reconstruct the temporal properties of HHs
We have demonstrated that ultrashort XUV pulses produced by HHG and spectrally selected by a monochromator can be temporally characterized by using the FROG CRAB technique in combination with the extended ptychographic engine, where other standard reconstruction algorithm and the cross-correlation formula would fail

Summary

Introduction

The process of high-order harmonic generation (HHG) in gas is at the center of active experimental and theoretical investigation since its discovery and it is at the heart of the emergence of attosecond science [1,2,3]. Remarkable examples are time- and angular-resolved photoemission spectroscopy (tr-ARPES) [7,8,9] or femtosecond pump-probe spectroscopy of core and deep valence levels [10,11,12,13,14] For all these applications, a proper measurement of the temporal characteristics of the harmonic radiation is required. Due to the limited flux, one possibility is to combine the extreme-ultraviolet (XUV) radiation with a portion of the generating infrared (IR) pulse as it is done in the frequency-resolved optical gating for the complete reconstruction of attosecond bursts (FROG CRAB) technique [15]. The narrower bandwidth associated with a single harmonic and the associated loss of sub-cycle resolution, reduce the level of information redundancy in a FROG CRAB trace For this reason the most commonly used reconstruction algorithms cannot be applied to few-femtosecond XUV pulses. In the last section we present a simplified cross-correlation model, which intuitively explains the physical origin of the main features of the measured spectrograms

Single harmonic spectrogram

Ptychographic reconstruction

Experimental results

Simple model

Conclusions