A variety of organisms have evolved mechanisms to detect and respond to light, in which the response is mediated by protein structural changes following photon absorption. The initial step is often the photo-isomerization of a conjugated chromophore. Isomerization occurs on ultrafast timescales, and is substantially influenced by the chromophore environment. Here we identify structural changes associated with the earliest steps in the trans to cis isomerization of the chromophore in photoactive yellow protein. Femtosecond, hard X-ray pulses emitted by the Linac Coherent Light Source were used to conduct time-resolved serial femtosecond crystallography on PYP microcrystals over the time range from 100 femtoseconds to 3 picoseconds to determine the structural dynamics of the photoisomerization reaction.

We report on the observation of enhanced high harmonics from a carbon plasma using sub-5-fs laser pulses. The efficiency of harmonic generation in the range of 14-25 eV was up to five times higher in the case of a plasma medium (graphite ablation) compared with gas (argon) under similar experimental conditions. The harmonic enhancement can be attributed to the presence of carbon nanoparticles in the ablation plumes.

We report studies of high-order harmonic generation in laser-produced manganese plasmas using sub-4-fs drive laser pulses. The measured spectra exhibit resonant enhancement of a small spectral region of about 2.5 eV width around the 31st harmonic (~50eV). The intensity contrast relative to the directly adjacent harmonics exceeds one order of magnitude. This finding is in sharp contrast to the results reported previously for multi-cycle laser pulses [Physical Review A 76, 023831 (2007)]. Theoretical modelling suggests that the enhanced harmonic emission forms an isolated sub-femtosecond pulse.

We study high-order harmonic generation from a laser-produced tin plasma using 780 and 390 nm, 40 fs, 1 kHz pulses. Varying the chirp of the laser radiation, we observe variation of the harmonic frequency and intensity. The enhanced 16th and 17th harmonics are attributed to the influence of the strong ionic transitions of tin. Theoretical calculations of the photoabsorption spectra reveal the decisive role of Sn II and Sn III transitions in the enhancement of single harmonics. Varying the fundamental frequency we study numerically the spectral width where the harmonic is enhanced. We find, both theoretically and experimentally, that this width is wider than the one of the corresponding resonance in the photoabsorption spectrum. The resonance line modification can be explained by the Stark shift of the autoionizing state in the laser field, as well as the photoionization of this state.

We present a method for the creation of stable weakly ionized plasmas from laser ablation of solid targets using a 1 kHz pulse repetition rate laser, which can be used for stable high-order harmonic generation from plasma plumes. The plasma plumes were generated from cylindrical rotating targets. Without target rotation the intensity of harmonics in the 40-80 nm range drops by more than one order of magnitude during less than 10(3) shots, while, with rotation of the target at typically 30 revolutions per minute, stable emission of high-order harmonics from aluminum plasma plumes with variation of less than 10% was maintained for >10(6) laser shots.

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We investigate how short and long electron trajectory contributions to high harmonic emission and their interferences give access to intra-molecular dynamics. In the case of unaligned molecules, we show experimental evidences that the long trajectory signature is more dependent upon the molecule than the short one, providing a high sensitivity to cation nuclear dynamics within 100's of as to few fs. Using theoretical approaches based on Strong Field Approximation and Time Dependent Schödinger Equation, we examine how quantum path interferences encode electronic motion whilst molecules are aligned. We show that the interferences are dependent on channels superposition and upon which ionisation channel is involved. In particular, quantum path interferences encodes electronic migration signature while coupling between channels is allowed by the laser field. Hence, molecular quantum path interferences is a promising method for Attosecond Spectroscopy, allowing the resolution of ultra-fast charge migration in molecules after ionisation in a self-referenced manner.

We present the results of experimental and theoretical studies of high-order harmonic generation (HHG) in plasmas containing fullerenes using pulses of different duration and wavelength. We find that the harmonic cutoff is extended in the case of few-cycle pulses (3.5 fs, 29th harmonic) compared to longer laser pulses (40 fs, 25th harmonic) at the same intensity. Our studies also include HHG in fullerenes using 1300 and 780 nm multicycle (35 and 40 fs) pulses. For 1300 nm pulses, an extension of the harmonic cutoff to the 41st order was obtained, with a decrease in conversion efficiency that is consistent with theoretical predictions based on wave packet spreading for single atoms. Theoretical calculations of fullerene harmonic spectra using the single active electron approximation were carried out for both few-cycle and multicycle pulses.