Chaojin Zhang

Remote creation of coherent emissions in air with two-color ultrafast laser pulses

We experimentally demonstrate the generation of narrow-bandwidth emissions with excellent coherent properties at 391 and 428nm from N + (B 2 6 + (v 0 = 0) ! X 2 6 + g (v = 0,1)) inside a femtosecond filament in air by an orthogonally polarized two-color driver field (i.e. 800nm laser pulse and its second harmonic). The durations of the coherent emissions at 391 and 428nm are measured to be 2.4 and 7.8ps, respectively, both of which are much longer than the duration of the pump and its second harmonic pulses. Furthermore,

Gain dynamics of a free-space nitrogen laser pumped by circularly polarized femtosecond laser pulses

We experimentally demonstrate ultrafast dynamic of generation of the 337-nm nitrogen laser by injecting an external seed pulse into a femtosecond laser filament pumped by a circularly polarized laser pulse. In the pump-probe scheme, it is revealed that the population inversion between the C(3)Π(u) and B(3)Π(g) states of N(2) for the free-space 337-nm laser is firstly built up on the timescale of several picoseconds, followed by a relatively slow decay on the timescale of tens of picoseconds, depending on the nitrogen gas pressure. By measuring the intensities of 337-nm signal from nitrogen gas mixed with different concentrations of oxygen gas, it is also found that oxygen molecules have a significant quenching effect on the nitrogen laser signal. Our experimental observations agree with the picture of electron-impact excitation.

Identification of the physical mechanism of generation of coherent N 2 + emissions in air by femtosecond laser excitation

Recently, amplification of harmonic-seeded radiation generated through femtosecond laser filamentation in air has been observed, giving rise to coherent emissions at wavelengths corresponding to transitions between different vibrational levels of the electronic B(2)Σ(u)(+) and X(2)Σ(g)(+) states of molecular nitrogen ions [Phys. Rev. A. 84, 051802(R) (2011)]. Here, we carry out systematic investigations on its physical mechanism. Our experimental results do not support the speculation that such excellent coherent emissions could originate from nonlinear optical processes such as four-wave mixing or stimulated Raman scattering, leaving stimulated amplification of harmonic seed due to the population inversion generated in molecular nitrogen ions the most likely mechanism.

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.

Harmonic-seeded remote laser emissions in N 2 -Ar, N 2 -Xe and N 2 -Ne mixtures: a comparative study

We report on the investigation on harmonic-seeded remote laser emissions at 391 nm wavelength from strong-field ionized nitrogen molecules in three different gas mixtures, i.e., N₂-Ar, N₂-Xe and N₂-Ne. We observed a decrease in the remote laser intensity in the N₂-Xe mixture because of the decreased clamped intensity in the filament; whereas in the N₂-Ne mixture, the remote laser intensity slightly increases because of the increased clamped intensity within the filament. Remarkably, although the clamped intensity in the filament remains nearly unchanged in the N₂-Ar mixture because of the similar ionization potentials of N₂ and Ar, a significant enhancement of the lasing emission is realized in the N₂-Ar mixture. The enhancement is attributed to the stronger third harmonic seed, and longer gain medium due to the extended filament.

Femtosecond filamentation in argon and higher-order nonlinearities

We numerically investigate the femtosecond filamentation dynamics in argon by considering the higher-order nonlinearities measured by a recent experiment. Our result indicates that, instead of plasma, these higher-order nonlinearities can provide main defocusing effect as shown by a recent theoretical work. However, when the higher-order nonlinearity is so strong to provide a main defocusing effect, some phenomena inconsistent with previous experiments such as pulse splitting and pulse self-compression can appear. Therefore, we infer that the values of higher-order nonlinear coefficients are overestimated, and the plasma should be the defocusing mechanism at least in the argon filament.

Control of bandwidth and central wavelength of an enhanced extreme ultraviolet spectrum generated in shaped laser field

We theoretically investigate the enhancement of a selected spectral range in the high-order harmonic generation driven by the multi-color laser fields. The results show that the central wavelength of the enhanced narrow-bandwidth spectrum can be effectively controlled by adjusting the laser intensity and the time delay between laser pulses of different colors, due to the modified electron trajectory in the shaped laser field. Our approach can offer intense, bandwidth controllable single attosecond pulses for attosecond pump-probe experiments.

Generation of isolated attosecond pulses of sub-atomic-time durations with multi-cycle chirped polarization gating pulses

We theoretically investigate the X-ray supercontinuum generated by interaction of multi-cycle, chirped polarization gating pulses with the helium gas. It is shown that with this scheme, an isolated sub-50-attosecond pulse can be obtained straightforwardly without any phase compensation. Interestingly, if one selects an extremely broad spectral range near the high-order harmonic cutoff, an isolated and intense sub-24-attosecond pulse can be generated after phase compensation, which could be used to detect and control the electronic dynamics inside the atoms. Furthermore, it is found that the generation of such a broad and smooth X-ray supercontinuum is not so stringent on the selection of the simulated parameters, allowing for the experimental demonstration of this technique in the future.

Propagation effects in the generation process of high-order vortex harmonics

We numerically study the propagation of a Laguerre-Gaussian beam through polar molecular media via the exact solution of full-wave Maxwell-Bloch equations where the rotating-wave and slowly-varying-envelope approximations are not included. It is found that beyond the coexistence of odd-order and even-order vortex harmonics due to inversion asymmetry of the system, the light propagation effect results in the intensity enhancement of a high-order vortex harmonics. Moreover, the orbital momentum successfully transfers from the fundamental laser driver to the vortex harmonics which topological charger number is directly proportional to its order.

Ideal laser waveform construction for the generation of super-bright attosecond pulses

A new scheme is proposed to synthesize an ideal laser waveform via the superposition of few-cycle and half-cycle pulses with an optimized delay time or laser wavelengths and intensity matching. The numerical analysis confirms that this scheme is much more efficient for supporting the production of an isolated super-bright attosecond pulse, which provides a way in the final realization for the attosecond pump and attosecond probe techniques.