Dispersion Theory of Polaron in a Parabolic Quantum Dot at Finite Temperatures

By using the dispersion theory instead of the Fröhlich Hamiltonian, the polaron energy in a quantum dot with a parabolic confinement potential is investigated at finite temperatures. It is found that the self-trapping energy of the polaron decreases with the increasing temperature, and the temperature effect is more obvious in quantum dots with weaker confinement.     

Two-color laser wavelength effect on intense terahertz generation in air

Near-IR femtosecond lasers have been proposed to produce high-field terahertz radiation in the air via the laser-plasma interaction, but the physical mechanism still needs to be further explored. In this work, we theoretically investigate the effect of the two-color laser wavelength on the terahertz generation in the air based on a transient photocurrent model.We show that the long wavelength laser excitation can greatly enhance the terahertz amplitude for a given total laser intensity. Furthermore, we utilize a local current model to illustrate the enhancement mechanism. Our analysis shows that the terahertz amplitude is determined by the superposition of contributions from individual ionization events, and for the long wavelength laser excitation, the electron production concentrates in a few ionization events and acquires the larger drift velocities, which results in the stronger terahertz radiation generation. These results will be very helpful for understanding the terahertz generation process and optimizing the terahertz output.     

最优化“调控”区间分离密度泛函理论的研究进展

发展精确、高效的交换-关联泛函一直是密度泛函理论工作者所追求的神圣目标。传统密度泛函被证实在计算原子或分子体系的某些基态和激发态性能时存在困难,而且预测不具有普适性;另一方面,一些高水平方法如耦合簇（CC）理论和基于格林函数（G）和屏蔽库仑作用（W）近似的多体微扰理论（MBPT）,尽管相对精确但往往需要消耗昂贵的计算成本,因而其研究体系的尺寸和实用性受到了很大的限制。近年来,“最优化”调控区间分离泛函的发展在一定程度上使得上述问题得到改善,尤其是在消耗较少的计算成本前提下能够达到与高水平方法相媲美的预测精度,引起了越来越多的关注。本文首先简要回顾了密度泛函领域的理论背景,在区间分离密度泛函理论的基础上,重点介绍了最优化“调控”的概念;并且结合近期的理论工作对其在实际计算时的表现进行评价;最后,就最优化“调控”方法的前景和应用进行了展望。     

Achievement of Narrow-Band CARS Signal by Manipulating Broad-band Laser Spectrum

We theoretically demonstrate the achievement of narrow-band coherent anti-Stokes Raman scattering （CARS） signal by manipulating broad-band probe spectrum. The narrowing of the CARS signal depends on the spectrum bandwidth of the probe beam, and thus high-resolution CARS signal for a complicated quantum system can be obtained by the simple spectrum manipulation. Furthermore, the energy-level diagram for the complicated quantum system can also be labelled by measuring the CARS signal at a given frequency.     

Simulating resonance-mediated two-photon absorption enhancement in rare-earth ions by a rectangle phase modulation

Improving the up-conversion luminescence efficiency of rare-earth ions via the multi-photon absorption process is crucial in several related application areas. In this work, we theoretically propose a feasible scheme to enhance the resonance-mediated two-photon absorption in Er~(3+) ions by shaping the femtosecond laser field with a rectangle phase modulation. Our theoretical results show that the resonance-mediated two-photon absorption can be decomposed into the on-resonant and near-resonant parts, and the on-resonant part mainly comes from the contribution of laser central frequency components, while the near-resonant part mainly results from the excitation of low and high laser frequency components.So, the rectangle phase modulation can induce a constructive interference between the two parts by properly designing the modulation depth and width, and finally realizes the resonance-mediated two-photon absorption enhancement. Moreover, our results also show that the enhancement efficiency of resonance-mediated two-photon absorption depends on the laser pulse width(or laser spectral bandwidth), final state transition frequency, and intermediate and final state absorption bandwidths. The enhancement efficiency modulation can be attributed to the relative weight manipulation of on-resonant and near-resonant two-photon absorption in the whole excitation process. This study presents a clear physical insight for the quantum control of resonance-mediated two-photon absorption in the rare-earth ions, and there will be an important significance for improving the up-conversion luminescence efficiency of rare-earthions.     

Constructive interference via collision—aided radiative excitation

We have studied theoretically constructive interference via collision-aided radiative excitation in an open four-level system using a density matrix approach. The four-level system consists of a Λ-type three-level quantum-beat configuration driven by one laser field and a fourth level coupled by a vacuum mode. It is shown that through the incoherent process (collision), coherence between widely-separated doublets and subsequent constructive interference can be realized. We analyse the effects of the collision-induced coherent and incoherent decay rates, laser intensity, and energy separation of the doublets on the interference. Meanwhile, the constructive interference between the two transition pathways 3P_1/2-4D and 3P_3/2-4D via equal-frequency hybrid excitation and collision-aided radiative excitation in an Na₂-Na system is experimentally observed. A good agreement between the theoretical and experimental results is obtained.     

High-Resolution Selective Excitation of Resonance-Enhanced Multiphoton-Ionization Photoelectron Spectroscopy by Shaping Femtosecond Laser Pulses

Femtosecond laser-induced resonance-enhanced multiphoton-ionization photoelectron spectroscopy （REMPI-PS） is faced with two drawbacks of low spectral resolution and poor selective excitation due to the broad spectral bandwidth. We propose a scheme to obtain a high-resolution selective excitation of （2＋1） REMPI-PS by com- bining 7r and cosinusoidal phase modulation. Our theoretical results indicate that the （2＋1） REMPI-PS signals related to neighboring excited states can be differentiated from their indistinguishable photoelectron spectra by the 7r phase modulation, and then their selective excitation can be realized by supplementally adding the cosi- nusoidal phase modulation. Furthermore, the physical mechanism of the high-resolution selective excitation of （2＋1） REMPI-PS is explained by considering the two-photon power spectrum.     

Optimization of High-Order Harmonic by Genetic Algorithm for the Chirp and Phase of Few-Cycle Pulses

The brightness of a particular harmonic order is optimized for the chirp and initial phase of the laser pulse by genetic algorithm. The influences of the chirp and initial phase of the excitation pulse on the harmonic spectra are discussed in terms of the semi-classical model including the propagation effects. The results indicate that the harmonic intensity and cutoff have strong dependence on the chirp of the laser pulse, but slightly on its initial phase. The high-order harmonics can be enhanced by the optimal laser pulse and its cutoff can be tuned by optimization of the chirp and initial phase of the laser pulse.     

Plasma-induced spatiotemporal modulation of propagating femtosecond pulses

The dynamics of the femtosecond pulse propagation in a plasma channel is investigated by the pump-probe longitudinal diffractometry and second harmonic generation frequency-resolved optical gating (SHG-FROG) technique. The spatial characteristics, corresponding to the electronic density and the size of the channel, can be observed by the recorded ring pattern, and the spectral and temporal characteristics are recorded by the SHG-FROG traces. The spatiotemporal characteristics will help us to better understand the dynamics of the plasma induced by the femtosecond pulse and the femtosecond pulse propagating in the plasma channel.     

Carrier－Envelop Phase－Dependent Effect on Photoelectron Angular Distribution in Single－Cycle Laser Pulses ＊

Using a nonperturbative scattering theory, We study the photoelectron angular distributions (PADs) of Kr atoms irradiated by an infinite sequence of intense single-cycle pulses of circular polarization. We demonstrate the inversion asymmetry of PADs and the dependence of PADs on the carrier-envelop phase of the single-cycle pulses. The inversion asymmetry is caused by the interference between transition channels where the different channels are characterized by different combinations of absorbed-photon numbers in the ionization process. Our results provide a possible method to determine the value of carrier-envelop phase by the detected PADs.