Sub-cycle Oscillations in Virtual States Brought to Light

Understanding and controlling the dynamic evolution of electrons in matter is among the most fundamental goals of attosecond science. While the most exotic behaviors can be found in complex systems, fast electron dynamics can be studied at the fundamental level in atomic systems, using moderately intense (≲10(3) W/cm(2)) lasers to control the electronic structure in proof-of-principle experiments. Here, we probe the transient changes in the absorption of an isolated attosecond extreme ultraviolet (XUV) pulse by helium atoms in the presence of a delayed, few-cycle near infrared (NIR) laser pulse, which uncovers absorption structures corresponding to laser-induced “virtual” intermediate states in the two-color two-photon (XUV+NIR) and three-photon (XUV+NIR+NIR) absorption process. These previously unobserved absorption structures are modulated on half-cycle (~1.3 fs) and quarter-cycle (~0.6 fs) timescales, resulting from quantum optical interference in the laser-driven atom.