Orbital perspective on high-harmonic generation from solids.

High-harmonic generation in solids allows probing and controlling electron dynamics in crystals on few femtosecond timescales, paving the way to lightwave electronics. In the spatial domain, recent advances in the real-space interpretation of high-harmonic emission in solids allows imaging the field-free, static, potential of the valence electrons with picometer resolution. The combination of such extreme spatial and temporal resolutions to measure and control strong-field dynamics in solids at the atomic scale is poised to unlock a new frontier of lightwave electronics.

Role of Spin-Orbit Coupling in High-Order Harmonic Generation Revealed by Supercycle Rydberg Trajectories.

High-harmonic generation is typically thought of as a sub-laser-cycle process, with the electron's excursion in the continuum lasting a fraction of the optical cycle. However, it was recently suggested that long-lived Rydberg states can play a particularly important role in high harmonic generation by atoms driven by the combination of the counterrotating circularly polarized fundamental light field and its second harmonic. Here we report direct experimental evidence of very long and stable Rydberg trajectories contributing to high-harmonic generation in such fields.

High-harmonic generation in metallic titanium nitride.

High-harmonic generation is a cornerstone of nonlinear optics. It has been demonstrated in dielectrics, semiconductors, semi-metals, plasmas, and gases, but, until now, not in metals. Here we report high harmonics of 800-nm-wavelength light irradiating metallic titanium nitride film.

Chiral high-harmonic generation and spectroscopy on solid surfaces using polarization-tailored strong fields.

Strong-field methods in solids enable new strategies for ultrafast nonlinear spectroscopy and provide all-optical insights into the electronic properties of condensed matter in reciprocal and real space. Additionally, solid-state media offers unprecedented possibilities to control high-harmonic generation using modified targets or tailored excitation fields. Here we merge these important points and demonstrate circularly-polarized high-harmonic generation with polarization-matched excitation fields for spectroscopy of chiral electronic properties at surfaces.

Threshold photodissociation dynamics of NO studied by time-resolved cold target recoil ion momentum spectroscopy.

We study the near-threshold photodissociation dynamics of NO by a kinematically complete femtosecond pump-probe scheme using a cold target recoil ion momentum spectrometer. We excite NO to the optically bright ÃB state with a 400 nm pulse and probe the ensuing dynamics via strong field single and double ionization with a 25 fs, 800 nm pulse. The pump spectrum spans the NO(XΠ) + O(P) dissociation channel threshold, and therefore, following internal conversion, excited NO is energetically prepared both "above threshold" (dissociating) and "below threshold" (nondissociating).

Molecular Frame Reconstruction Using Time-Domain Photoionization Interferometry.

Photoionization of molecular species is, essentially, a multipath interferometer with both experimentally controllable and intrinsic molecular characteristics. In this work, XUV photoionization of impulsively aligned molecular targets (N_{2}) is used to provide a time-domain route to "complete" photoionization experiments, in which the rotational wave packet controls the geometric part of the photoionization interferometer. The data obtained is sufficient to determine the magnitudes and phases of the ionization matrix elements for all observed channels, and to reconstruct molecular frame interferograms from lab frame measurements.

Tailored semiconductors for high-harmonic optoelectronics.

The advent of high-harmonic generation in gases 30 years ago set the foundation for attosecond science and facilitated ultrafast spectroscopy in atoms, molecules, and solids. We explore high-harmonic generation in the solid state by means of nanostructured and ion-implanted semiconductors. We use wavelength-selective microscopic imaging to map enhanced harmonic emission and show that the generation medium and the driving field can be locally tailored in solids by modifying the chemical composition and morphology.

Revealing the Cooper minimum of N2 by molecular frame high-harmonic spectroscopy.

Molecular frame high-harmonic spectra of aligned N2 molecules reveal a Cooper-like minimum. By deconvolving the laboratory frame alignment distribution, what was previously thought to be a maximum of emission along the molecular axis is found to be maxima at 35 degrees off axis, with a spectral minimum on axis. Both of these features are supported by photoionization calculations that underline the relationship between high-harmonic spectroscopy and photoionization measurements.

Conical intersection dynamics in NO2 probed by homodyne high-harmonic spectroscopy.

Conical intersections play a crucial role in the chemistry of most polyatomic molecules, ranging from the simplest bimolecular reactions to the photostability of DNA. The real-time study of the associated electronic dynamics poses a major challenge to the latest techniques of ultrafast measurement. We show that high-harmonic spectroscopy reveals oscillations in the electronic character that occur in nitrogen dioxide when a photoexcited wave packet crosses a conical intersection.

High-harmonic homodyne detection of the ultrafast dissociation of Br2 molecules.

We report the time-resolved observation of the photodissociation of Br2 using high-harmonic generation (HHG) as a probe. The simultaneous measurement of the high-harmonic and ion yields shows that high harmonics generated by the electronically excited state interfere with harmonics generated by the ground state. The resulting homodyne effect provides a high sensitivity to the excited state dynamics.

Controlling the interference of multiple molecular orbitals in high-harmonic generation.

We demonstrate a new method to investigate the origin of spectral structures in high-harmonic generation. We report detailed measurements of high-harmonic spectra in aligned nitrogen and carbon dioxide molecules. Varying the wavelength and intensity of the generating laser field, we show that the minimum in aligned N2 molecules is nearly unaffected, whereas the minimum in aligned CO2 molecules shifts over more than 15 eV.

Following a chemical reaction using high-harmonic interferometry.

The study of chemical reactions on the molecular (femtosecond) timescale typically uses pump laser pulses to excite molecules and subsequent probe pulses to interrogate them. The ultrashort pump pulse can excite only a small fraction of molecules, and the probe wavelength must be carefully chosen to discriminate between excited and unexcited molecules. The past decade has seen the emergence of new methods that are also aimed at imaging chemical reactions as they occur, based on X-ray diffraction, electron diffraction or laser-induced recollision--with spectral selection not available for any of these new methods.

Frequency-resolved high-harmonic wavefront characterization.

We introduce and demonstrate a novel concept of frequency-resolved wavefront characterization. Our approach is particularly suitable for high-harmonic, extreme-UV (XUV) and soft X-ray radiation. The concept is based on an analysis of radiation diffracted from a slit scanned in front of a flat-field XUV spectrometer.