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Electron migration in molecules is the progenitor of chemical reactions and biological functions after light-matter interaction. Following this ultrafast dynamics, however, has been an enduring endeavor. Here we demonstrate that, by using machine learning algorithm to analyze high-order harmonics generated by two-color laser pulses, we are able to retrieve the complex amplitudes and phases of harmonics of single fixed-in-space molecules. These complex dipoles enable us to construct movies of laser-driven electron migration after tunnel ionization of N2 and CO2 molecules at time steps of 50 attoseconds. Moreover, the angular dependence of the migration dynamics is fully resolved. By examining the movies, we observe that electron holes do not just migrate along the laser polarization direction, but may swirl around the atom centers. Our result establishes a general scheme for studying ultrafast electron dynamics in molecules, paving a way for further advance in tracing and controlling photochemical reactions by femtosecond lasers. Seeing molecules in action under the influence of very short pulses is interesting. Here the authors demonstrate charge migration dynamics in molecules by using high harmonic spectroscopy and machine learning algorithm. [ABSTRACT FROM AUTHOR]

We report an experimental measurement of nonlinear absorption by using the near-infrared femtosecond laser and a crystalline MgO sample. We show that the nonlinear absorption inside MgO presents a four-fold modulation as a function of the crystal alignment to the laser polarization. In different intensities and wavelengths, we observe that the modulation amplitude is changed. We introduce a model based on the calculation of the carrier density to explain the phenomenon. It shows that the varied modulation is due to the variation of effective electron mass, which is related to the crystal structure. Our work makes a step forward in the application of nonlinear absorption in exploring solid structure. [ABSTRACT FROM AUTHOR]

By controlling the oxygen pressure, single-phase CuO and CuO thin films have been obtained on quartz substrates using a pulsed laser deposition technique. The structure properties and linear optical absorption of the films were characterized by X-ray diffraction and UV-VIS spectroscopy. By performing z-scan measurements using a femtosecond laser (800 nm, 50 fs), the real and imaginary parts of the third-order nonlinear susceptibility, Re χ and Im χ, of the films were determined. Both CuO and CuO films exhibited large optical nonlinearities, which is comparable to those in some representative semiconductor films such as ZnO and GaN films using femtosecond laser excitation. Compared with CuO films, the CuO films showed larger third-order nonlinear optical effects under off-resonance excitation. Furthermore, the mechanisms of the optical nonlinearities in CuO and CuO films are explained in the main text. It was suggested that the reasons of the difference in their nonlinear refractive effects may be related to the different electronic structure in CuO and CuO materials. [ABSTRACT FROM AUTHOR]