Stimulated collision induced processes in sodium vapor in the presence of helium

Stimulated resonant emission and stimulated Raman effects in sodium vapor in the presence of helium are investigated experimentally. The intensity dependence of these effects on the buffer gas and sodium vapor pressures, and on the intensity and detuning of the exciting radiation are obtained in experiment. Our experimental results agree with the most recent theoretical calculations.This work was carried out under the Polish Central Program of Fundamental Research CPBP 01.06     

Investigation of collisionally induced stimulated scattering in sodium vapor with temporal and spectral resolution

In this work we present fully time-resolved spectra of Amplified Spontaneous Emission (ASE) and Stimulated Raman Scattering (SRS) in sodium vapor. We observe that the ASE signal precedes the SRS emission, and the peak of the amplified Stokes pulse is delayed with respect to the peak of the laser pulse. The time evolution of the line widths of the ASE and Raman signals is also shown. We find that the line widths of the stimulated fluorescence component (ASE) and the stimulated Raman radiation change with time. Both spectra start as small narrow fluctuating lines, then the line widths reach maximum values and thereafter decrease with time.     

Laser-induced breakdown of krypton near five-photon ionization threshold

Selective excitation of laser-induced breakdown in krypton in the pressure range 1–4 bar and the spectral range 420–620 nm at a laser intensity of 10¹¹ W/cm² has been studied. It is shown that the breakdown of krypton can be induced selectively due to resonantly enhanced multiphoton ionization via excited atomic states. A number of four- and five-photon atomic resonances have been identified in breakdown excitation spectra. The role of different factors determining the selectivity of the breakdown has been studied. Possible analytical applications of a selective breakdown are discussed.     

Nonlinear Hanle effect in Cs vapor under strong laser excitation

We report results of a theoretical and experimental study of the ground state nonlinear Hanle effect under strong laser excitation. It is shown that besides the well-known zero-magnetic field suppression of absorption on F _g = F→F _e = F - 1 transitions caused by population trapping, an optical pumping induced enhanced absorption occurs on F _g = F→F _e = F + 1 transitions for small B-fields. The latter effect becomes more pronounced for high F values. The experiment with atomic vapor of Cs (D₂ line, F _g = 4) confirms an increase of the spectrally unresolved fluorescence yield at zero magnetic field and 600 mW/cm² laser intensity by 9% or 42%, when excitation occurs with linearly or circularly polarized light, respectively. The results of the experiment agree with numerical simulation studies using equations of motion for a density matrix. Received 24 November 2001 / Received in final form 25 March 2002 Published online 24 September 2002     

钾蒸汽中双光子激发产生2~3∏_g—α~3∑_u~+扩散带受激辐射

本文报道了在K_2-K中通过双光子共振激发原子或双光子激发分子产生2~3∏_(?)—a~3∑_u~+扩散带辐射的实验结果,并对激发机制以及扩散带辐射随激发能量和温度的变化进行了讨论.     

A novel-high energy pulse compression system: generation of multigigawatt sub-5-fs pulses

Received: 28 February 1997 / Revised version: 2 May 1997     

Photovoltaic solitons in two-photon photorefractive materials under open-circuit conditions

We present the evolution equation of one-dimensional spatial soliton in two-photon photorefractive media under open-circuit conditions. In the steady state regime, our solutions show that the dark and bright photovoltaic spatial solitons can be supported in two-photon photorefractive media under open-circuit conditions.     

A rich spectrum from potassium vapor on the 42 S 1/2−32 D 5/2 two-photon resonance

A rich emission spectrum in the wavelength range 300–600 nm, including 50 spectral lines, has been observed as a pulsed dye laser is tuned to the 4² S _1/2–3² D _5/2 two-photon resonance in potassium vapor. It is found that many kinds of mechanisms, including energy pooling, harmonic generation, quadrupole radiation, excitation transfer, and multiwave mixing etc., are involved in producing these lines. The process for each spectral line has been identified and is presented in tabular form.This project was supported by National Science Council of R.O.C. under the grant No. NSC 79-0208-M 009-21     

Observations and analysis of resonant laser ablation of GaAs

Laser ablation of solid GaAs samples has been studied using one tunable pulsed dye laser. At relatively low laser power, enhancements of up to several hundred times have been observed in the yield of resonantly ionised Ga using laser wavelengths corresponding to the atomic transition 4² P _1/2-4² D _3/2. The influences of laser power and target geometry, on the ion yield and spectral profile, are discussed. It is argued that the resonant excitation and ionisation processes occur in the gas phase of the atoms ablated from the sample surface, and the observed asymmetric spectral profile results from laser-induced collisional processes, e.g., optical collisions, under conditions of relatively high atomic density in the interaction region. Potential applications are foreseen for resonant laser ablation in trace analysis.     

The effect of the homogeneous broadening on the propagation of the light pulses

The propagation of resonant pulses in a two-level homogeneously broadened atomic medium is analyzed. The pulses with the duration comparable with the atomic coherence relaxation time are considered. The effects of the long pulse area stabilization and destabilization are reported. The numerical simulation is performed.     

Tracking individual electron trajectories in a high harmonic spectrum

The harmonic spectrum generated by a few-cycle laser pulse propagating through a gas jet is calculated. Two complementary atomic response models are used, one based on the time-dependent Schrödinger equation and the other on quantum orbits in which only a single pair of electron trajectories are taken into account. The role played by individual electron trajectories in determining particular features of the spectrum are considered and phase-matching maps are used to help understand their structure. A method based on this connection is proposed for diagnosing the carrier-envelope phase and the pulse duration of the incident laser field.     

Heating of cold cesium atoms by blue-detuned laser radiation

Received: 12 March 1997/Revised version: 30 July 1997     

Tunable picosecond infrared pulses generated by stimulated electronic Raman scattering of a mode-locked Ti:Sapphire laser in potassium vapor

A down-conversion to the mid-infrared region by using Stimulated Electronic Raman Scattering (SERS) in potassium vapor is described. The pump radiation is a frequency-doubled regeneratively amplified Ti:Sapphire laser with a pulse duration of 2 ps, pulse energy of 0.2 mJ, and repetition rate of 10 Hz. With the pumping frequency tuned around the potassium 4s-5p transition, nearly transform-limited infrared radiation tunable between 2.2 and 3.4 m has been generated with a peak infrared energy of 12 µJ, corresponding to a quantum efficiency of 17%, and with a pulse duration of 2 ps. The present tuning range could be extended by extending the tuning range of the pump laser. In comparison, intense infrared radiation of 90 µJ energy but with a very narrow tunability around 2.9 m has also been generated by SERS in barium vapor.     

Optical manipulation of third-harmonic generation via either one- or two-photon excitation in diarylethene-polymethylmethacrylate polymer thin films

Optical manipulation of third harmonic (TH) generation in diarylethene-polymethylmethacrylate (DE-PMMA) polymer thin films is obtained by either one- or two-photon excitation. TH intensity generated from a DE-PMMA polymer thin film decreases, when it is pumped by either 325 nm or 442 nm laser irradiation, which changes the molecular structure of DE molecules from an open-form (A form) to a closed-form (B form). TH intensity recovers to its original intensity level when all B form DE molecules return to an A form after being induced by either 532 nm or 1064 nm laser irradiation. The experimental results reveal that the second-order hyperpolarizability () of the A form molecules may be larger than that of the B form molecules. Moreover, TH output efficiency is independent of the angle between the pump and probe polarization directions. This result is attributed to the two-dimensional structure of DE molecules.This revised version was published online in March 2005. In the previous version, the published online date was missing     

Experimental investigation of non-linear selective reflection at the interface of Cs atomic vapor and quartz

We present an experimental study of non-linear selective reflection (SR) at a quartz–Cs-vapor interface in a V-type three-level scheme. The non-linear selective reflection at the Cs D₂ resonance line (⁶ S _1/2F=4→⁶ P _3/2) is monitored with and without pumping. The sub-Doppler reflection spectrum is observed and the effect of pumping on the signal of the selective reflection is investigated. The experimental result is in agreement with the theoretical calculation. Received: 16 April 2002 / Revised version: 12 June 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-351/701-1500, E-mail: zhaojm@sxu.edu.cn     

Dynamic control of the entanglement in the presence of the time-varying field

The interactions between a two-level atom and a field with a time-varying frequency have been investigated. The two typical cases, the frequency of the field varying with time in the forms of sine and rectangle, have been considered. The dynamic behavior of the atom-field entanglement is investigated numerically as function of time. A number of novel phenomena are discovered and discussed. The sine modulation of the field frequency can help to realize and stabilize the degree of the atom-field entanglement at high level. The influence of the rectangular frequency modulated field on the atom-field entanglement indicates that the entanglement is more sensitive to the detuning than the correlation between different Rabi oscillations. Not only the sine field frequency modulation but also the rectangular field frequency modulation is favorable to improve, enhance and stabilize the degree of the atom-field entanglement.     

Optimal control of one- and two-photon transitions with shaped femtosecond pulses and feedback

This paper reports two pump–probe experiments in sodium where dynamically tailored ultrashort pulses from a Ti:Sapphire-pumped optical parametric amplifier were employed. The first study focuses on the one-photon Na(3s→3p) transition to derive sensitive criteria which judge the performance of a frequency-domain pulse shaper using a spatial light modulator. On the basis of the interpretation, follow-up experiments are suggested to test their cogency. The second experiment uses coherent quantum control by placing an appropriate phase distribution on the incident beam to enhance or cancel the transition probability in the nonresonant two-photon process Na(3s→→5s). Ignorant of the “ideal” phase function, an evolutionary algorithm which uses a feedback derived from the experiment performs the optimization and produces the desired bright or dark pulses within a few minutes. Attention is given to the role of resonant 3s→3p transitions excited by the spectral wings of the pump pulse. Different parametrizations of the phase distribution have been examined. Two of these produced solutions which had not previously been predicted by theory still meet the objective of the experiment. The study represents the first successful application of a feedback-organized self-learning algorithm to the design of dark pulses. Received: 3 November 1999 / Published online: 5 July 2000     

Detection of oxygen atoms in a high pressure surfatron microwave discharge via two-photon four-wave mixing

3 P_J) oxygen atoms are detected in a high pressure surface wave discharge driven at 2.45 GHz via two-photon resonant degenerate four-wave mixing spectroscopy (TP-DFWM) in the forward folded BOX configuration. The nonlinear optical signal provides a direct measure of the relative oxygen atom concentration with high spatial resolution inside the discharge up to pressures of 1 bar and without distortions due to linear absorption or saturation effects. The axial distribution of oxygen atom concentration is observed to depend sensitively on total pressure and gas flux. For total gas pressures up to 600 mbar both in case of a mixture of 10% O₂/He and 10% O₂/Ar the concentration of oxygen atoms increases linearly with pressure. At higher pressures an increase with larger slope is observed for 10% O₂/Ar, while the concentration remains constant for 10% O₂/He. This is interpreted by an increase of the three-body recombination rate in O₂/He mixtures. Received: 26 July 1996/Revised version: 15 November 1996     

Production of a highly polarized sodium atomic beam

Two experimental techniques for preparing atoms in selected states were combined in order to obtain a highly polarized sodium beam: Firstly state selection by an inhomogeneous magnetic field and secondly optical pumping with laser light. This results in a dominating population of the 3s ² S _1/2 ground state levelF=2M _F=+2 (or alternativelyF=2M _F=−2) corresponding to high electron as well as nuclear spin polarization. Polarization values of 0.85±0.05 were easily obtained. The sign of the polarization can be reversed by changing the light polarization. The method can also be applied to other atoms. In addition, it is demonstrated that the optical pumping process allows a determination of the spin-selectivity of hexapole magnets.     

Quantum Entropy Controlling in the Damping Jaynes-Cummings Model

The quantum entropy of the damping Jaynes-Cummings model is investigated in different decay coefficients under detuning conditions. The results indicate that the larger the decay coefficient is, the more quickly the entropy decays. The detuning of the atom and field frequencies reduce the entanglement maximum in short time regions, but delays the damping process of the entanglement. The sine modulation enhances the entanglement in short time regions.