Light-induced states in He atoms were characterized using attosecond transient absorption spectroscopy. A 400 as pulse covering the 20-24 eV spectral range serves as the probe pulse, and the effect of a few-cycle near infrared pulse (12 fs, 780 nm) on the absorption spectrum is measured as a function of time delay and near-infrared intensities varying from (5.0 ± 2) × 1010 to (1 ± 0.4) × 1013 W/cm2. Light-induced states resulting from near-infrared coupling of 1s2p to 1s2s, 1s3d, and 1s3s states are observed. Absorption features that likely result from coupling of 1s3p to 1s4s, 1s4d, 1s5s, and 1s5d states are also observed. The light-induced states with the smallest detunings (1s3d and 1s3s) from the dressing frequency may shift to higher frequencies as the dressing intensity is increased. © 2013, Taylor & Francis.

The development of attosecond technology is one of the most significant recent achievements in the field of ultrafast optics; it opens up new frontiers in atomic and molecular spectroscopy and dynamics. A unique attosecond pumpprobe apparatus using a compact Mach-Zehnder interferometer is developed. The interferometer system is compact (∼290 cm²) and completely located outside of the vacuum chamber. The location reduces the mechanical vibration from vacuum components such as turbopumps and roughing pumps. The stability of the interferometer is ∼50 as RMS over 24 hours, stabilized with an active feedback loop. The pump and probe fields can be easily altered to incorporate multiple colors. In the interferometer, double optical gating optics are arranged to generate isolated attosecond pulses with a supercontinuum spectrum. The frequencies of the attosecond pulses can be selected to be in the extreme ultraviolet (XUV) region (25–55 eV, 140 as) or the vacuum ultraviolet (VUV) region (15–24 eV, ∼400 as) by metal filters. Furthermore, the near infrared probe field (1.65 eV) can be upconverted to the ultraviolet (3.1 eV). The frequency tunability in the XUV and VUV is critical for selecting excited states of target atoms and molecules.

Recording the transmitted spectrum of a weak attosecond pulse through a medium, while a strong femtosecond pulse copropagates at variable delay, probes the strong-field dynamics of atoms, molecules, and solids. Usually, the interpretation of these measurements is based on the assumption of a thin medium. Here, the propagation through a macroscopic medium of helium atoms in the region of fully allowed resonances is investigated both theoretically and experimentally. The propagation has dramatic effects on the transient spectrum even at relatively low pressures (50 mbar) and short propagation lengths (1 mm). The absorption does not evolve monotonically with the product of propagation distance and pressure, but regions with characteristics of Lorentz line shapes and characteristics of Fano line shapes alternate. Criteria are deduced to estimate whether macroscopic effects can be neglected or not in a transient absorption experiment. Furthermore, the theory in the limit of single-atom response yields a general equation for Lorentz- and Fano-type line shapes at variable pulse delay. © 2013 American Physical Society.