Carrier-envelope phase-dependent above-threshold ionization of atoms observed with multi-cycle laser pulses

Abstract

Technological advances in ultrashort high-power lasers have led to the development of a new field of science called strong-field physics. Above-threshold ionization (ATI), an intriguing process of strong-field ionization, has revealed electron dynamics in ionization processes and atomic structures. Electrons from ATI consist of peaks separated by the photon energy in a spectrum after overcoming the ionization threshold with excessive absorption of photons. As the pulse duration becomes shorter, the phase of the electric field, specifically carrier-envelope phase (CEP), turns out to be an important parameter that affects outcomes in ionization processes. ATI in few-cycle pulses showed electrons with higher kinetic energy are more sensitive to the change of phase of the pulses. Among various schemes for CEP measurement, ATI is one of the prospective candidates for the measurement as well as its stabilization. This technique, however, is not applicable when the pulse duration is longer than 5 optical cycles due to its poor accuracy above the limit and restriction on the intensity to produce the high-energy plateau electrons sensitive to CEP. To complete a scenario that falls into a general description of CEP effects regardless of pulse duration we pursued experimental parameters, especially for multi-cycle laser pulses. The experimental setups are described as follows: CEP of linearly polarized 30-fs laser pulses were stabilized using the direct locking technique. A gaseous xenon atoms was used for ATI measurement. Then a home-built velocity-map imaging spectrometer was operated to measure the position and angle of the emitted electrons via ATI. The laser intensity was constrained below $10^{14}{\ \rm W/cm^{2}}$ to ensure non-negligible contribution from multi-photon ionization. CEP-dependent modulation appeared clearly as a pattern of linear slopes in an asymmetry map, as a function of photoelectron energy and CEP. The period of the pattern was determined to be ...