Acta Phys. Hung. B 26/3–4 (2006) 343–350 DOI: 10.1556/APH.26.2006.3–4.15 QUANTUM ELECTRONICS First Interferometric Measurement of the Atomic Dipole Phase in High-Order Harmonic Generation Angela Pirri, 1,@ Chiara Corsi, 1 Emiliano Sali, 1,2 Alessandra Tortora 1,3 and Marco Bellini 1,3 1 LENS, Via Nello Carrara 1, 50019 Sesto Fiorentino (FI), Italy 2 Dipartimento di Fisica, Universita di Firenze Via Sansone 1, 50019 Sesto Fiorentino (FI), Italy 3 Istituto Nazionale di Ottica Applicata Largo E. Fermi, 6, I-50125, Firenze, Italy @ Corresponding author; E-mail: pirri@lens.unifi.it Received 23 November 2006 Abstract. We report the interferometric measurement of the atomic dipole phase of high-order harmonics generated in noble gases. The contributions of both the main electronic quantum paths predicted by a simple semi-classical model are investigated for the first time in a direct and complete way. Keywords: atomic dipole phase, high-order harmonic generation, noble gases PACS: 42.65.Ky 1. Introduction High-order harmonic generation (HHG) is a highly nonlinear process which provides coherent radiation in the extreme ultraviolet (XUV). In order to use harmonic radiation for applications, the experimental parameters influencing the generation process must be accurately controlled. The coherence properties of harmonics are particularly important and they are deeply related to propagation effects in the dispersive gaseous medium as well as to single-atom effects. At low gas density and negligible ionization, harmonics in the plateau should show the desirable high degree of temporal [1,2] and spatial coherence [3]. However, an intrinsic phase modulation related to the single-atom response, the so-called atomic dipole phase [4], is always present. This effect is simply visualized in the framework of the semi-classical approach [5]. Here, only two main electron trajectories (during each optical half-cycle) are 1589-9535/ $ 20.00 c  2006 Akad´ emiai Kiad´o, Budapest