Zhaoyan Zhou

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.

Theoretical study of terahertz generation from atoms and aligned molecules driven by two-color laser fields

We study the generation of terahertz (THz) radiation from atoms and molecules driven by an ultrashort fundamental laser and its second harmonic field by solving time-dependent Schrödinger equation. The simulations show that the initial wave-packet and its subsequent acceleration in the laser field, and rescattering with long-range Coulomb potential play key roles on THz generation. We also present the dependence of the optimal phase delay and yield of terahertz radiation on the laser intensity, wavelength, duration, the ratio of two-color laser components, and against the molecular alignment.

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.

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.

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.

Single-scan coherent detection with enhanced time resolution for arbitrarily polarized terahertz wave

We present an enhanced scheme of polarization-sensitive THz-ABCD which can provide about twice broader bandwidth than the conventional method. In our experiment using a 26 fs laser pulse, compared with 0.3~40 THz in the conventional scheme, bandwidth coverage from 0.3 to 80 THz has been achieved in the resolution-enhanced scheme. It also should have to be noted the terahertz source may also restrict the detection bandwidth. Employing the polarizationsensitive technology, the polarization in time domain, as well as the field amplitude, can be achieved with just one single scan.

Dynamics of highly charged ions in ultraintense laser fields

The dynamics of highly charged ions in a strong laser field are investigated by solving the two-dimensional time-dependent Dirac equation. Relativistic effects are discussed for the hydrogen-like ions with nuclear charges of Z=4, 24 and 79 by considering the changes in the electron trajectory and coherent emission spectrum with changes in the frequency and intensity of the laser field. With increasing Z, the relative field strength between the ionic core and laser fields changes and this results in different characteristics of the dynamical electronic trajectory as well as the corresponding emission spectrum. For an infrared laser field of strength E=8.0 au, the tunneling ionization can hardly be seen for the hydrogen-like ions of Z=24 and only the resonant multiphoton emission spectra below the ionization threshold are predicted. The above threshold harmonics need more intense laser fields or a higher photon energy.

Terahertz radiation from free-free transitions of an atom in strong superposing laser fields

Terahertz (THz) emissions of an isolated atom in an ultrashort (100 fs) laser field are simulated by solving the time-dependent Schrödinger equation. From numerical calculations with one- and three-dimensional hydrogen atom models and a short-range shallow potential model, it can be concluded that continuum THz emissions occur more readily following transitions involving intermediate states above rather than those well below the ionization threshold of the system. Line-shaped THz emissions from transitions between high-lying Rydberg states are also found. The models are also used to describe the observed enhanced terahertz emissions with a superposed second-order harmonic laser field or a spatially constant electric field. The dependence of the THz field strength on the intensities of the fundamental laser field and the superposed field is also examined. Strong field approximation is extended to analyze the general features of THz emissions resulting from continuum free-free transitions of an electron in strong laser fields. These calculations contribute to an understanding of the THz emission processes when strong laser fields interact with atomic and molecular systems that have larger ionization potentials and where multi-photon processes are involved in order to generate THz emissions effectively.