Carrier-Envelope Phase Effects of a Single Attosecond Pulse in Two-Color Photoionization Candong Liu, 1,2 Maurizio Reduzzi, 1 Andrea Trabattoni, 1 Anumula Sunilkumar, 1 Antoine Dubrouil, 1 Francesca Calegari, 3 Mauro Nisoli, 1,3 and Giuseppe Sansone 1,3, * 1 Dipartimento di Fisica, Politecnico Piazza Leonardo da Vinci 32, 20133 Milano, Italy 2 State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China 3 CNR-IFN Politecnico Piazza Leonardo da Vinci 32, 20133 Milano Italy (Received 6 March 2013; published 16 September 2013) The attosecond streak camera method is usually implemented to characterize the temporal phase and amplitude of isolated attosecond pulses produced by high-order harmonic generation. This approach, however, does not provide any information about the carrier-envelope phase of the attosecond pulses. We demonstrate that the photoelectron spectra generated by an attosecond waveform and an intense synchronized infrared field are sensitive to the electric field of the attosecond pulse. The dependence on the carrier-envelope phase of the attosecond pulse is understood in terms of the coherent superposition of two photoelectron wave packets. This effect suggests an experimentally feasible method for complete reconstruction of attosecond waveforms. DOI: 10.1103/PhysRevLett.111.123901 PACS numbers: 42.65.Re, 42.55.Vc The generation and characterization of isolated attosec- ond pulses have strongly benefited from the development of carrier-envelope-phase (CEP) stable, few-cycle, intense, IR driving pulses [1,2]. The first evidence of isolated atto- second waveforms was reported in 2001 [3], measuring the photoelectron energy distribution emitted by the combina- tion of a subfemtosecond extreme ultraviolet (XUV) pulse and an intense IR ultrashort pulse. In the experiment, electrons were released with a velocity vðt 0 Þ (t 0 is the ionization instant) by absorption of a single XUV photon and then accelerated (decelerated) by the synchronized IR field. The final momentum p depends on the value of the vector potential of the IR field, A IR ðtÞ, at the release instant: p ¼ vðt 0 Þ A IR ðt 0 Þ (atomic units are used throughout unless otherwise indicated). Changing the rela- tive delay between the XUV and IR fields, the photoelec- tron momentum distribution is shifted according to the oscillation of the vector potential [4]. This technique, known as attosecond streak camera [5,6], allows one to retrieve information about the IR and XUV pulses. Implementing reconstruction algorithms, such as the frequency-resolved-optical gating for complete reconstruc- tion of attosecond bursts (FROG-CRAB) [7], the complete IR field [8] and the temporal envelope and phase of the attosecond electron wave packet released by the XUV pulse can be reconstructed. This distribution directly maps the temporal evolution of the isolated attosecond pulse [9,10]. Even though the attosecond streak camera has allowed the demonstration of single-cycle attosecond waveform [9], it does not provide information about its CEP. The FROG-CRAB method, and other techniques proposed for the characterization of attosecond pulses [11], allow one to retrieve the XUV spectral phase within an arbitrary constant, which leaves the CEP undefined in the time domain. In the few-cycle regime the electric field is strongly affected by the precise value of the CEP, calling for methods able to give access to this parameter. The photoelectron angular distribution emitted by intense XUV (I XUV  10 14 W=cm 2 ) pulses was investigated in Refs. [12,13], evidencing a CEP-dependent asymmetry along the laser polarization. In this unexplored regime, the strong-field electron dynamics is driven by the pre- cise electric field of the attosecond pulse rather than by its intensity profile. These intensities, however, pose a challenge for current attosecond technology. The inves- tigation of CEP-induced effects is required for advancing our capability to precisely shape attosecond waveforms. As the precise control of infrared laser electric fields paved the way to the generation and application of atto- second pulses, the subcycle control of the laser-matter interaction at high frequencies will open new perspec- tives for the generation of even shorter electric field pulses, exploiting nonlinear processes in the XUV and x-ray energy range. In this work we demonstrate that information on the CEP of isolated attosecond pulses can be obtained by measuring the photoelectron spectra emitted by the combination of a XUV attosecond field and a CEP-stable IR pulse with an intensity higher than the one typically used for the temporal characterization of attosecond waveforms. We investigate the dependence of the photoelectron distribution on the CEP of the XUV pulse, showing that above-treshold ionization (ATI) electrons can be used as a reference to highlight variations in the CEP of the attosecond pulse. Our theoretical approach is based on the solution of the time-dependent Schro¨dinger equation in the single- active electron approximation. The equation describing PRL 111, 123901 (2013) PHYSICAL REVIEW LETTERS week ending 20 SEPTEMBER 2013 0031-9007= 13=111(12)=123901(5) 123901-1 Ó 2013 American Physical Society