Zhihui Lü

Synchronizing terahertz wave generation with attosecond bursts.

We perform a joint measurement of terahertz waves and high-harmonics generated from argon atoms driven by a fundamental laser pulse and its second harmonic. By correlating their dependence on the phase delay between the two pulses, we determine the generation of THz waves in tens of attoseconds precision. Compared with simulations and models, we find that the laser-assisted soft collision of the electron wave packet with the atomic core plays a key role. It is demonstrated that the rescattering process, being indispensable in high-harmonic generation processes, dominates THz wave generation as well in a more elaborate way. The new finding might be helpful for the full characterization of the rescattering dynamics.

Polarization-sensitive air-biased-coherent-detection for terahertz wave

Employing an orientation-modulated bias field, a polarization-sensitive scheme for terahertz air-biased-coherent-detection (THz-ABCD) is presented to directly measure the amplitude and polarization angle of THz field in the time domain. It can provide all characteristics of arbitrarily polarized THz wave with one single-scan measurement. Measuring convenience, broad bandwidth, and high angular resolution have been achieved. Polarization-sensitive THz time-domain spectroscopy can surely be developed based on this technology. Many other applications in the THz spectral region are also believed.

Joint Measurements of Terahertz Wave Generation and High-Harmonic Generation from Aligned Nitrogen Molecules Reveal Angle-Resolved Molecular Structures.

We report the synchronized measurements of terahertz wave generation and high-harmonic generation from aligned nitrogen molecules in dual-color laser fields. Both yields are found to be alignment dependent, showing the importance of molecular structures in the generation processes. By calibrating the angular ionization rates with the terahertz yields, we present a new way of retrieving the angular differential photoionization cross section (PICS) from the harmonic signals which avoids specific model calculations or separate measurements of the alignment-dependent ionization rates. The measured PICS is found to be consistent with theoretical predications, although some discrepancies exist. This all-optical method provides a new alternative for investigating molecular structures.

Attosecond synchronization of terahertz wave and high-harmonics

Argon atom ionizing in the ultrafast two-colour field has been used to investigate the physical mechanism of terahertz (THz) generation. We measured simultaneously pulse energies of THz and high-harmonics as the phase delay between the fundamental and its second harmonic was varied. The optimal phase delay of THz generation is determined according to the inherent attochirp of the emitted high-harmonics. The dynamic analysis of the tunnelling electron wave packet driven by the Coulomb–laser coupling shows that laser-assisted soft collisions of the electron wave packet with the atomic core play a key role. It is demonstrated that the rescattering process, being indispensable in high-harmonic generation processes, dominates THz wave generation as well in a more elaborate way.

In situ spatial mapping of Gouy phase slip with terahertz generation in two-color field

We establish a one-to-one mapping between the local phase slip and the spatial position near the focus by scanning a thin jet along the propagation direction of laser beams. The measurement shows that the optimal phase of terahertz can be utilized to characterize in situ the spatially dependent relative phase of the two-color field. We also investigate the role of the Gouy phase shift on terahertz generation from two-color laser-induced plasma. The result is of critical importance for phase-dependent applications of two-color laser-field, including high-order harmonic and terahertz generation.

Enhancement of terahertz radiation by using circularly polarized two-color laser fields

Terahertz radiation from tunneling ionization of gaseous atoms and molecules in the two-color laser fields with various polarizations has been investigated. We experimentally demonstrate that the efficiency of terahertz emission in the circularly polarized laser fields with the same helicity is 5 times higher than that with linearly polarized two-color femtosecond pulses in high laser intensity. By solving time-dependent Schrodinger equation, this enhancement is well explained based on the analysis of electron tunneling ionization and subsequent dynamics.

Coherently controlled terahertz source for a time domain spectroscopy system via injection current in bulk ZnSe

We have observed terahertz generation via injection current induced by harmonically related two-color beams in an unbiased ZnSe bulk at room temperature using a femtosecond Ti:sapphire oscillator. The terahertz intensity is just several times smaller than that obtained via optical rectification and further enhancements are believed possible. Experimental results demonstrate that the terahertz radiation is mainly attributed to the transition from the split-off band. This conclusion provides a novel approach to effectively generate a broadband and coherently controlled terahertz radiation, which leads to practical applications of terahertz radiation via this mechanism.

Characterization of domain distributions by second harmonic generation in ferroelectrics

Domain orientations and their volume ratios in ferroelectrics are recognized as a compelling topic recently for domain switching dynamics and domain stability in devices application. Here, an optimized second harmonic generation method has been explored for ferroelectric domain characterization. Combing a unique theoretical model with azimuth-polarization-dependent second harmonic generation response, the complex domain components and their distributions can be rigidly determined in ferroelectric thin films. Using the proposed model, the domain structures of rhombohedral BiFeO3 films with 71° and 109° domain wall, and, tetragonal BiFeO3, Pb(Zr0.2Ti0.8)O3, and BaTiO3 ferroelectric thin films are analyzed and the corresponding polarization variants are determined. This work could provide a powerful and all-optical method to track and evaluate the evolution of ferroelectric domains in the ferroelectric-based devices.Ferroelectrics: optical method characterizes domain structuresAn optimized optical method is developed to determine the ferroelectric domain structure and their distributions in several ferroelectric films. A team led by Xiangli Zhong and Yichun Zhou from Xiangtan University, and Jianmin Yuan from National University of Defense Technology in China, combined optimized second harmonic generation (SHG) measurements and theoretical analysis to determine the polarization components of domains and their distribution in rhobohedral BiFeO3, tetragonal BiFeO3, Pb(Zr0.2Ti0.8)O3 and BaTiO3 thin films. They first collect azimuth-polarization-dependent SHG signals with respect to both the polarization angle of the incident beam and the azimuth angle of sample relative to the beam. Then they fit the SHG signals with a theoretical model with good agreement. This work provides an effective and all-optical method to determine the domain structures and distributions for ferroelectric thin films.

High-harmonic and terahertz wave spectroscopy (HATS) for aligned molecules

We present the experimental and theoretical details of our recent published letter [Phys. Rev. Lett. 115. 123002] on synchronized high-harmonic and terahertz-wave spectroscopy (HATS) from nonadiabatically aligned nitrogen molecules in dual-color laser fields. Associating the alignment-angle dependent terahertz wave generation with the synchronizing high-harmonic signal, the angular differential photoionization cross section (PICS) for molecules can be reconstructed, and the minima of the angle on PICS show great convergence between the theoretical predictions and the experimental deduced results. We also show the optimal relative phase between the dual-color laser fields for terahertz wave generation dose not change with the alignment angle at a precision of

Surface-Step-Terrace Tuned Second-Order Nonlinear Optical Coefficients of Epitaxial Ferroelectric BaTiO3 Films

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