Gouy phase shift for annular beam profiles in attosecond experiments.

F. Schlaepfer,M. Lucchini,M. Volkov,U. Keller,L. Gallmann,L. Kasmi,A. Ludwig

doi:10.1364/oe.25.003646

F. Schlaepfer, M. Lucchini + Show 5 more

Open Access

https://doi.org/10.1364/oe.25.003646

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Journal: Optics Express	Publication Date: Feb 10, 2017
Citations: 13	License type: cc-by

Abstract

Attosecond pump-probe measurements are typically performed by combining attosecond pulses with more intense femtosecond, phase-locked infrared (IR) pulses because of the low average photon flux of attosecond light sources based on high-harmonic generation (HHG). Furthermore, the strong absorption of materials at the extreme ultraviolet (XUV) wavelengths of the attosecond pulses typically prevents the use of transmissive optics. As a result, pump and probe beams are typically recombined geometrically with a center-hole mirror that reflects the larger IR beam and transmits the smaller XUV, which leads to an annular beam profile of the IR. This modification of the IR beam can affect the pump-probe measurements because the propagation that follows the reflection on the center-hole mirror can strongly deviate from that of an ideal Gaussian beam. Here we present a detailed experimental study of the Gouy phase of an annular IR beam across the focus using a two-foci attosecond beamline and the RABBITT (reconstruction of attosecond beating by interference of two-photon transitions) technique. Our measurements show a Gouy phase shift of the truncated beam as large as 2π and a corresponding rate of 50 as/mm time delay change across the focus in a RABBITT measurement. These results are essential for attosecond pump-probe experiments that compare measurements of spatially separated targets.

Highlights

To date most attosecond pump-probe measurements combine extreme-ultraviolet (XUV) attosecond pulses with a few-femtosecond phase-locked infrared (IR) pulse [1,2]
A very common geometrical beam recombination scheme is to use a mirror with a center hole, for which the smaller XUV beam is transmitted through the center, while the larger IR beam is reflected on the outer mirror part
To extract Φ2q (z) from the experimental traces we looked at the product between the Fourier transform of a particular SB signal in the first chamber [Fig. 3(c)], SB2(1q) (ω), and the complex conjugate of the same quantity extracted from the second time-of-flight spectrometer (ToF) spectrum

Summary

Introduction

To date most attosecond pump-probe measurements combine extreme-ultraviolet (XUV) attosecond pulses with a few-femtosecond phase-locked infrared (IR) pulse [1,2]. A very common geometrical beam recombination scheme is to use a mirror with a center hole, for which the smaller XUV beam is transmitted through the center, while the larger IR beam is reflected on the outer mirror part This produces an annular IR beam profile with a considerably different Gouy phase compared to an ideal Gaussian beam [24]. The combined collinearly propagating beams after the center-hole mirror are focused with a toroidal mirror into the first reference target interaction chamber with a neon gas target and a time-of-flight spectrometer (ToF). As we will see later, this point is in full agreement with the position where the Gouy phase goes through zero

Phase measurement

Equivalent photoionization delays

Findings

Conclusion