XiaoJun Liu

Generation of a coherent x ray in the water window region at 1 kHz repetition rate using a mid-infrared pump source

We demonstrate the generation of a coherent x ray in the water window region in a gas cell filled with neon gas using a wavelength-tunable mid-IR femtosecond laser operating at 1 kHz repetition rate. The cutoff energy and conversion efficiency of the water window x ray can be optimized by tuning gas pressure as well as the focal position.

Alignment-Dependent Fluorescence Emission Induced by Tunnel Ionization of Carbon Dioxide from Lower-Lying Orbitals

The study of the ionization process of molecules in an intense infrared laser field is of paramount interest in strong-field physics and constitutes the foundation of imaging of molecular valence orbitals and attosecond science. We show measurement of alignment-dependent ionization probabilities of the lower-lying orbitals of the molecules by experimentally detecting the alignment dependence of fluorescence emission from tunnel ionized carbon dioxide molecules. The experimental measurements are compared with the theoretical calculations of the strong field approximation and molecular Ammosov-Delone-Krainov models. Our results demonstrate the feasibility of an all-optical approach for probing the ionization dynamics of lower-lying orbitals of molecules, which is until now still difficult to achieve by other techniques. Moreover, the deviation between the experimental and theoretical results indicates the incompleteness of current theoretical models for describing strong field ionization of molecules.

Enhanced third harmonic generation in a silicon metasurface using trapped mode

We experimentally demonstrate enhanced third harmonic generation (THG) using a silicon metasurface, which is consist of symmetric spindle-shape nanoparticle array. Relying on the trapped mode supported by the high quality factor all-dielectric metasurface, the conversion efficiency of THG is about 300 times larger than that of bulk silicon slab. The maximum extinction ratio of the intensity of THG reaches about 25 dB by tuning the polarization of incident light. The simulation results agree with the experimental performances.

Laser-sub-cycle two-dimensional electron-momentum mapping using orthogonal two-color fields

We study laser-sub-cycle control over electron trajectories concomitantly in space and time using orthogonally polarized two-color laser fields. We compare experimental photoelectron spectra of neon recorded by coincidence momentum imaging with photoelectron spectra obtained by semiclassical and numerical solutions of the time-dependent Schrodinger equation. We find that a resolution of a quarter optical cycle in the photoelectron trajectories can be achieved. It is shown that depending on their sub-cycle birth time the trajectories of photoelectrons are affected differently by the ions Coulomb field.

Wavelength scaling of elliptical-polarization dependence of high-order harmonic generation.

We report on an experimental study on the wavelength-scaling law of elliptical-polarization dependence of high-order harmonic generation. In the elliptical-polarization dependence measurements, we keep an identical peak intensity for pump pulses with carrier wavelength ranging from 800 nm to the mid-IR region, and the harmonic emission near the cutoff region is selected for comparison. We find that the experimental measured wavelength scaling of elliptical-polarization dependence could be well fitted with a function (epsilon(1/2) proportional to lambda(-1)) predicted by semiclassical model.

Attosecond interference induced by Coulomb-field-driven transverse backward-scattering electron wave packets

A universal interference structure is found in the photoelectron momentum distribution of atoms in intense infrared laser field. Theoretical analysis shows that this structure can be attributed to a form of Coulomb-field-driven backward-scattering photoelectrons in the direction perpendicular to the laser field, in contrast to the conventional rescattering along the laser polarization direction. This transverse backward-scattering process is closely related to a family of photoelectrons initially ionized within a time interval of less than 200 as around the crest of the laser electric field. Those electrons, acquiring near-zero return energy in the laser field, will be pulled back solely by the ionic Coulomb field and backscattered in the transverse direction. Moreover, this rescattering process mainly occurs at the first or second return time, giving rise to different phases of the photoelectrons. The interference between these photoelectrons leads to unique curved interference fringes which are observable for most current intense field experiments, opening another way to record the electron dynamics in atoms and molecules on a time scale much shorter than an optical cycle.

Ultrafast Excited-State Dynamics of 6-Azauracil Studied by Femtosecond Transient Absorption Spectroscopy

The excited-state dynamics of 6-azauracil in different solvents have been studied using femtosecond transient absorption spectroscopy. The molecule is populated to the S2 state with a pump pulse at 264 nm. Broad-band white light continuum which covers from 320 to 600 nm is used as the probe. With a global fitting analysis of the measured transient spectra, three decay time constants, i.e., <0.3, 5.2 ± 0.1, and >1000 ps, are directly obtained in the solvent of acetonitrile. These newly observed lifetime constants are important in clarifying its decay dynamics as well as in providing a criterion for the ultrafast dynamics simulations in 6-azauracil using quantum chemical theories. In combination with previous theoretical works, the main decay channel is proposed: the initially populated S2 decays to S1 through internal conversion in <0.3 ps, followed by an intersystem crossing from S1 to T1 in 5.2 ± 0.1 ps. The >1000 ps component is due to the decay of the T1 state. A comparison of the excited-state dynamics in different solvents reveals that the decay from S1 to T1 shows a clear dependence on the polarity of the solvents. With higher polarity, the S1 excited state decays faster. This observation is in line with the prediction by Etinski et al. [ Phys. Chem. Chem. Phys. 2010 , 12 , 15665 - 15671 ], where a blue-shift of the T1 state potential energy surface leading to an increase of the intersystem crossing rate was proposed. With the new information obtained in the present measurement, a clearer picture of the decay dynamics of 6-azauracil on the S2 excited state is provided.

Scaled-energy spectroscopy of strontium atoms in an electric field

Strontium atoms in the ground state were excited to higher \MI\= 1 Rydberg states while the laser wavelength (transition energy E) and the electric field strength (F) were scanned simultaneously to keep the scaled energy E = E/rootF constant. Spectra were recorded for epsilon = -3.00, -2.50 and -1.88, The experimental Fourier-transformed recurrence spectra were compared with the hydrogenic closed-orbit calculations. The positions of the experimental recurrence peaks coincide well with the theory, but the strengths of the peaks do not, indicating a strong core effect.

Photoionization microscopy of the hydrogen atom in parallel electric and magnetic fields

In photoionization microscopy experiments, an atom in an electric field is ionized by a laser with sharply defined frequency, the electron is drawn toward a position-sensitive detector, and the current is measured as a function of position. Multiple classical paths lead from the atom to any point in the classically allowed region on the detector, and waves traveling along these paths produce an interference pattern. If a magnetic field is added parallel to the electric field, trajectories become chaotic. There is an infinite set of different families of trajectories, leading to an extremely complicated interference patterns on the detector. We present calculations predicting the kind of structure that will be seen in experiments.

Angle-resolved conical emission spectra from filamentation in a solid with an Airy pattern and a Gaussian laser beam.

Filamentation dynamics in fused silica are investigated using an Airy pattern and a Gaussian laser beam. The angle-resolved conical emission spectra are measured and compared with the predictions of several models. Our experimental observations are consistent with the X-waves model in both cases. This indicates that both laser beams spontaneously evolve into nonlinear X-waves and suggests a universal evolution of filaments in fused silica, regardless of the initial laser beam profile.