Sun Zhen-Rong

Experimental observation of constructive quantum interference via hybrid excitation in an Na2-Na system

We report on an experimental observation of constructive quantum interference between two atomic transition pathways 3P1/2-5S (or 4D) and 3P3/2-5S (or 4D) via equal-frequency hybrid excitation and collision-aided radiative excitation in an Na2-Na system. Through the special excitation processes, constructive quantum interference is realized for a Λ-type quantum-beat configuration with a widely-spaced doublet (about 17 cm-1) in the excited sodium atom driven by one laser. The dependences of quantum interference on the argon buffer gas pressure and sample temperature are also examined.

Effect of Time-Dependent Ionization on Propagation of a Few-Cycle Circularly Polarized Laser Pulse in Two-Level Medium

Influence of multiphoton ionization on the propagation and spectrum of few-cycle circularly (elliptically) polarized laser pulses in an open two-level medium (two-level plus continuum model) is investigated based on the conventional two-level model proposed by Slavcheva and Hess (Phys. Rev. A 72 (2005) 053804), and the propagation dynamics of an arbitrary elliptically polarized laser pulse is reduced into that of right and left circularly polarized laser pulses. When the laser intensity is high enough to cause ionization, there are significant impacts of ionization on the pulse reshaping and on the higher order spectral components, and the impacts for the open two-level model are different from those for the closed two-level model.

The photoluminescence of ZnSe bulk single crystals excited by femtosecond pulse

This paper reports on the photoluminescence spectra of ZnSe single crystal with trace chlorine excited by the femtosecond laser pulse. Three emission bands, including second-harmonic-generation, two-photon-excited peak and a broad band at 500–700nm, were detected. The thermal strain induced by femtosecond pulse strongly influences the photoluminescence of ZnSe crystal. The corresponding strain in ZnSe crystal is estimated to be about 8.8 ×10−3 at room temperature. The zinc-vacancy, as the main point defect induced by femtosecond pulse, is successfully used to interpret the broad emission at 500–700nm. The research shows that self-activated luminescence possesses the recombination mechanism of donor–vacancy pair, and it is also influenced by a few selenium defects and the temperature. The rapid decrease in photoluminescence intensity of two-photon-excited fluorescence and second-harmonic generation emission at lower temperature is attributed to the fact that more point defects result in the thermal activation of the two-photo-absorption energy converting to the stronger recombination emission of chlorine–zinc vacancy in 500–700nm. The experimental results indicate that the femtosecond exciting photoluminescence shows a completely different emission mechanism to that of He–Cd exciting luminescence in ZnSe single crystal. The femtosecond laser exhibits a higher sensitive to the impurity in crystal materials, which can be recommended as an efficient way to estimate the trace impurity in high quality crystals.

Coherent Phase Control of Multiphoton Ionization in Three-Level Ladder-Type System

We present the theoretical investigation of photoelectron spectroscopy resulting from the strong field induced multiphoton ionization in a typical three-level ladder-style system. Our theoretical results show that the photo-electron spectral structure can be alternatively steered by spectral phase modulation. This physical mechanism for strong field quantum control is explicitly exploited by the time-dependent dressed state population. It is concluded that the phase-shaped laser pulses can be used to selectively manipulate the multiphoton ionization process in complicated quantum systems.

Controlling field-free molecular orientation with combined single- and dual-color laser pulses

We propose a scheme to achieve the field-free molecular orientation around the half rotational periods with the combination of single- and dual-color laser pulses. We show that the molecular orientation can be obtained and controlled by precisely controlling the time delay between the two laser pulses. Furthermore, we discuss the effect of the laser intensity and pulse duration of the single-color laser pulse on the molecular orientation created by the dual-color laser pulse, and show that the molecular orientation depends on the change in the alignment degree between the odd and even rotational wave-packet contributions created by the single-color laser pulse.

Selective excitation and suppression of coherent anti-Stokes Raman scattering by shaping femtosecond pulses

Femtosecond coherent anti-Stokes Raman scattering (CARS) suffers from poor selectivity between neighbouring Raman levels due to the large bandwidth of the femtosecond pulses. This paper provides a new method to realize the selective excitation and suppression of femtosecond CARS by manipulating both the probe and pump (or Stokes) spectra. These theoretical results indicate that the CARS signals between neighbouring Raman levels are differentiated from their indistinguishable femtosecond CARS spectra by tailoring the probe spectrum, and then their selective excitation and suppression can be realized by supplementally manipulating the pump (or Stokes) spectrum with the π spectral phase step.

Quantum coherent control of two-photon transitions by square phase-modulation

A femtosecond laser pulse can be tailored to control the two-photon transitions using the ultra-fast pulse-shaping technique. This paper theoretically and experimentally demonstrates that two-photon transitions in molecular system with broad absorption line can be effectively controlled by square phase-modulation in frequency domain, and the influence of all parameters characterizing the square phase-modulation on two-photon transitions is systemically investigated and discussed. The obtained results have potential application in nonlinear spectroscopy and molecular physics.

Linear and third-order nonlinear optical properties for the heptamethine cyanine chromophore H-aggregates thin film

The thin film of a heptamethine cyanine chromophore HC was prepared by spin-coating technique. Its surface morphology and linear optical property were characterized by atomic force microscopy (AFM) and UV-visible absorption spectroscopy. The results show that HC molecules are arranged in a well-ordered H-aggregate type. The third-order nonlinear optical properties of the spin-coating film were also measured by degenerate four-wave mixing (DFWM) measurement. Enhanced third-order nonlinear susceptibility can be attributed to molecular aggregation effects, and the corresponding mechanism was dealt with by collective electronic oscillator (CEO) approach.

Effects of spontaneously induced coherence on absorption of a ladder-type atom

This paper investigates the effects of spontaneously induced coherence on absorption properties in a nearly equispaced three-level ladder-type system driven by two coherent fields. It find that the absorption properties of this system with the probe field applied on the lower transition can be significantly modified if this coherence is optimized. In the case of small spontaneous decay rate in the upper excited state, it finds that such coherence does not destroy the electromagnetically induced transparency (EIT). Nevertheless, the absorption peak on both sides of zero detuning and the linewidth of absorption line become larger and narrower than those in the case corresponding to the effects of spontaneously induced coherence; while in the case of large decay rate, it finds that, instead of EIT with low resonant absorption, a sharp absorption peak at resonance appears. That is, electromagnetically induced absorption in the nearly equispaced ladder-type system can occur due to such coherent effects.

Optimal Feedback Control of Two-Photon Fluorescence Based on Genetic Algorithm

An optimal feedback control of two-photon fluorescence in the ethanol solution of 4-dicyanomethylene-2-methyl-6-p-dimethyl-amiiiostryryl-4H-pyran (DCM) using pulse-shaping technique based on genetic algorithm is demonstrated experimentally. The two-photon fluorescence of the DCM ethanol solution is enhanced in intensity of about 23%. The second harmonic generation frequency-resolved optical gating (SHG-FROG) trace indicates that the effective population transfer arises from the positively chirped pulse. The experimental results appear the potential applications of coherent control to the complicated molecular system.