Lixin He

Spectrally resolved spatiotemporal features of quantum paths in high-order harmonic generation

We experimentally disentangle the contributions of different quantum paths in high-order-harmonic generation (HHG) from the spectrally and spatially resolved harmonic spectra. By adjusting the laser intensity and focusing position, we simultaneously observe the spectrum splitting, frequency shift, and intensity-dependent modulation of harmonic yields both for the short and long paths. Based on the simulations, we discriminate the physical mechanisms of the intensity-dependent modulation of HHG due to the quantum path interference and macroscopic interference effects. Moreover, it is shown that the atomic dipole phases of different quantum paths are encoded in the frequency shift. In turn, it enables us to retrieve the atomic dipole phases and the temporal chirps of different quantum paths from the measured harmonic spectra. This result gives an informative mapping of spatiotemporal and spectral features of quantum paths in HHG.

Attosecond Probing of Nuclear Dynamics with Trajectory-Resolved High-Harmonic Spectroscopy

We report attosecond-scale probing of the laser-induced dynamics in molecules. We apply the method of high-harmonic spectroscopy, where laser-driven recolliding electrons on various trajectories record the motion of their parent ion. Based on the transient phase-matching mechanism of high-order harmonic generation, short and long trajectories contributing to the same harmonic order are distinguishable in both the spatial and frequency domains, giving rise to a one-to-one map between time and photon energy for each trajectory. The short and long trajectories in H_{2} and D_{2} are used simultaneously to retrieve the nuclear dynamics on the attosecond and ångström scale. Compared to using only short trajectories, this extends the temporal range of the measurement to one optical cycle. The experiment is also applied to methane and ammonia molecules.

Coulomb-corrected molecular orbital tomography of nitrogen.

High-order harmonic generation (HHG) from aligned molecules has provided a promising way to probe the molecular orbital with an Ångström resolution. This method, usually called molecular orbital tomography (MOT) replies on a simple assumption of the plane-wave approximation (PW), which has long been questioned due to that PW approximation is known to be valid in the keV energy region. However, the photon energy is usually no more than 100 eV in HHG. In this work, we experimentally reconstruct the highest occupied molecular orbital (HOMO) of nitrogen (N2) by using a Coulomb-corrected MOT (CCMOT) method. In our scheme, the molecular continuum states are described by a Coulomb wave function instead of the PW approximation. With CCMOT, the reconstructed orbital is demonstrated to agree well with the theoretical prediction and retain the main features of the HOMO of N2. Compared to the PW approximation method, the CCMOT shows a significant improvement in eliminating the artificial structures caused by PW approximation.

Shaped multi-cycle two-color laser field for generating an intense isolated XUV pulse toward 100 attoseconds.

The isolated attosecond pulse (IAP) generated from high-order harmonic (HH) radiation has been established as an important technique for the ultrafast optics over past decade. The applications of IAP in ultrafast processes can be greatly extended by further developing the high-intensity IAP. Here, we theoretically propose to shape a two-color field by performing peak amplitude-wavelength analysis. It is found that a 240-as IAP can be generated even without carrier envelop phase (CEP) stabilization using a 25 fs/800 nm fundamental field and a relative weak 25 fs/1330 nm control field, which enables us to markedly relax the requirements of the driving laser fields both in pulse duration and CEP control. On the other hand, if the CEPs of driving laser fields are stabilized, a 65-eV broadband continual harmonic, supporting a 81-as IAP, can be directly produced with the optimized intensity ratio of 0.866 and control wavelength of 1400 nm. Moreover, the propagation effect of two-color field on the macroscopic build-up of HH for generating a high-energy IAP is discussed. We found that the method of phase match still works for the efficient continuous harmonic generation as long as the ionization level and the pressure of gas medium are kept low enough. Since the phase-matched short IAP can be generated with our shaped two-color scheme in combination with a relaxed requirement of driving laser fields, the commercial available high-energy laser source with a loosely focused geometry is promising for scaling up the energy of IAP, showing the potential for the realization of IAP with high focused intensity toward 100 attoseconds.

Monitoring ultrafast vibrational dynamics of isotopic molecules with frequency modulation of high-order harmonics

Molecules constituted by different isotopes are different in vibrational modes, making it possible to elucidate the mechanism of a chemical reaction via the kinetic isotope effect. However, the real-time observation of the vibrational motion of isotopic nuclei in molecules is still challenging due to its ultrashort time scale. Here we demonstrate a method to monitor the nuclear vibration of isotopic molecules with the frequency modulation of high-order harmonic generation (HHG) during the laser-molecule interaction. In the proof-of-principle experiment, we report a red shift in HHG from H2 and D2. The red shift is ascribed to dominant HHG from the stretched isotopic molecules at the trailing edge of the laser pulse. By utilizing the observed frequency shift, the laser-driven nuclear vibrations of H2 and D2 are retrieved. These findings pave an accessible route toward monitoring the ultrafast nuclear dynamics and even tracing a chemical reaction in real time.Previous studies on high harmonic generation from molecules have been used to identify the spectral properties and orbital contributions. Here the authors measure the isotopic effects in the energy shift of the HHG spectra caused by the nuclear motion of the molecules.

Ultra-broadband water window supercontinuum generation with high efficiency in a three-color laser field

We propose an efficient method to generate an ultra-broadband supercontinuum by using a three-color field synthesized by a 1600 nm driving pulse and two weak (3200 nm and 400 nm) control pulses. The results show that the acceleration and ionization steps in the HHG process are simultaneously controlled in our scheme, which can not only broaden the bandwidth but also enhance the harmonic yields of the generated spectrum. Thus a high-efficiency supercontinuum with the photon energies ranging from 205 eV to 480 eV is observed. Based on our 3D propagation simulations, we show that the short quantum path of the supercontinuum is well phase-matched and can be selected under suitable propagation conditions. Thereby, the modulation due to quantum path interference is largely reduced and intense isolated sub-100 as pulses with tunable central wavelengths are directly obtained within water window spectral region.

High-order harmonic generation from Rydberg atoms in inhomogeneous fields

We theoretically investigate the high-order harmonic generation (HHG) from Rydberg atoms considering the spatial inhomogeneity of the driving field. It is found that in the inhomogeneous field, the effect of the cutoff extension in the harmonic spectrum from Rydberg atoms can be extended to multi-cycle regime, while in the homogeneous field case, the extension of the harmonic cutoff is limited to the few-cycle regime (less than two optical cycles). The underlying physics of the cutoff extension from Rydberg atoms in the inhomogeneous field is analyzed based on the classical and quantum-mechanical models. Furthermore, by optimizing the field inhomogeneity, the electron dynamics can be well controlled to generate a smooth supercontinuum in the extended spectral region. This can support the efficient generation of isolated attosecond pulses in Rydberg atoms from multi-cycle laser fields.

Method for direct observation of Bloch oscillations in semiconductors

We propose a scheme for real-time observations of Bloch oscillations in semiconductors using time-resolved band gap emission spectroscopy. By solving the time-dependent Schrödinger equation, we find one remarkable band gap emission besides the normal high harmonics generated in the interaction of a mid-infrared laser pulse and a semiconductor. It is shown that the band gap emission yield is directly connected to the population in the conduction band (CB). By adopting a pump-probe scheme, the time-dependent population in the CB, that is the dynamical Bloch oscillation, can be probed by measuring the band gap emission signal versus pump-probe delay. We also present a model based on accelerated Bloch states to explain the time-resolved measurement of dynamical Bloch oscillation.

Resonance enhanced high-order harmonic generation in H2+ by two sequential laser pulses

We investigate high-order harmonic generation in H2+ by using two sequential laser pulses, which consist of a 800-nm pump pulse and a time-delayed 1600-nm probe pulse. Based on the solution of the time-dependent Schrödinger equation, we demonstrate that the harmonic cutoff in our two-pulse scheme is significantly extended compared to that in the 1600-nm probe pulse alone. Meanwhile, the harmonic efficiency is enhanced by 2-3 orders of magnitude due to charge-resonance-enhanced ionization steered by the 800-nm pump pulse. By using a probe pulse with longer wavelength, our scheme can be used for efficient high harmonic generation in the water window region. In addition, the influence of the intensity of the pump pulse and the relative time delay of the two laser pulses on the harmonic generation are also investigated.

Probing molecular structure and dynamics with high harmonic spectroscopy

High harmonic generation has offered new opportunities for probing molecular structures and dynamics. Here we demonstrate to reconstruct molecular orbital with a Coulomb-corrected tomography method and probe the nuclear dynamics with spectral shift.