Competition between four-wave mixing and amplified spontaneous emission

There has recently been considerable interest in nonlinear optical interactions in which two different processes compete with one another. In this paper, we review some recent work in this area, and describe the results of our own investigation of competition between four-wave mixing and amplified spontaneous emission. We find that under two-photon excitation of the sodium 3d level, new optical frequency components can be generated either by amplified spontaneous emission at the 3d→3p transition frequency or by a resonantly enhanced four-wave mixing process. We have observed competition between these two processes, resulting in the suppression of amplified spontaneous emission. The transfer of population to the 3d level is inhibited by the destructive interference between two different pathways connecting the ground and upper levels.     

基于半导体光放大器四波混频原理的光采样

建立了可研究强超短光脉冲放大特性且包含自由载流子吸收、受激辐射、双光子吸收、光谱烧孔和超快非线性折射效应的半导体光放大器理论模型,用以建立脉冲四波混频模型,并进一步仿真了基于半导体光放大器的光采样过程,重点讨论了自由载流子吸收、双光子吸收效应对采样特性的影响。仿真结果与实验结果相符。     

Nonlinear optical techniques for plasma diagnostics

The application of various nonlinear optical laser spectroscopic techniques to plasma diagnostics are reported. The techniques discussed in this paper are two-photon laser induced fluorescence spectroscopy, double-resonant four-wave mixing, coherent anti-Stokes Raman-scattering (CARS), and a combination of emission spectroscopy and CARS. They are applied to measurements of atomic hydrogen densities, molecular temperature, chemical composition, electric field distributions, and vibrational population distribution. The basic principles are described and important aspects of the methods are discussed in context with application to various kinds of discharges at low and elevated pressures     

Ultrabroadband conical emission generated from the ultraviolet up to the far-infrared during the optical filamentation in air

Ultraviolet and infrared conical emissions were observed during the filamentation in air of powerful femtosecond laser pulses produced by a portable terawatt laser system. The broadband spectrum was measured from 200 nm up to 14 microm and covered the complete optical transmission window of the atmosphere. The angularly resolved spectrum showed some X-wave structure across the frequency range analyzed. However, we demonstrated that the strong conical emission observed in the mid- and far-infrared is mainly owing to the four-wave mixing between the pump pulse and its blueshifted conical emission.     

测量原子里德堡态的新方法——双光子共振非简并四波混频

本文介绍了一种测量原子里德堡态的新方法--具有中间态的双光子共振非简并四波混频,与现代实验方法不同,该方法是纯光学的方法,其光路简单,检测的是相干光,而不是电子或离子信号.当用窄带宽的激光时,该技术对里德堡能级的窄光谱结构可以获得消多普勒的分辨率.     

Enhanced four-wave mixing in mercury isotopes, prepared by stark-chirped rapid adiabatic passage

We demonstrate significant enhancement of four-wave mixing in coherently driven mercury isotopes to generate vacuum-ultraviolet radiation at 125 nm. The enhancement is accomplished by preparation of the mercury atoms in a state of maximum coherence, i.e. maximum nonlinear-optical polarization, driven by Stark-chirped rapid adiabatic passage (SCRAP). In this technique, a pump laser at 313 nm excites the two-photon transition between the ground state 6s²¹S₀ and the target state 7s ¹S₀ in mercury. A strong, off-resonant radiation field at 1064 nm generates dynamic Stark shifts. These Stark shifts serve to induce a rapid adiabatic passage process on the two-photon transition. During the process a coherent superposition of the two states is established, which enhances the nonlinear-optical polarization in the medium to the maximum possible value. The maximum coherence permits efficient four-wave mixing of a pump laser and an additional probe laser at 626 nm. The efficiency is further enhanced, as the SCRAP process allows to stimulate the complete set of different mercury isotopes to participate in the frequency conversion process. This enlarges the effective atomic density of the medium. Thus, we observe the generation of vacuum-ultraviolet radiation at 125 nm enhanced by more than one order of magnitude with respect to conventional frequency conversion. Parallel to the frequency conversion process, we monitored the evolution of the population in the medium by laser-induced fluorescence. These data demonstrate efficient coherent population transfer by SCRAP.     

Polarization-dependent colored conical emission in a quadratically nonlinear medium

Both supercontinuum conical emission (SCCE) and blue-green conical emission (CE) by means of second harmonic generation (SHG) were observed alternately in a β-barium borate (BBO) crystal induced by femtosecond laser pulses with vertical polarization state. Three theoretical models were analyzed to interpret the angular beam of SCCE. The experimental conical angles with different wavelengths in BBO crystal and BK-7 glass can be explained well by nonlinear X-wave model and Cerenkov type phase matching model other than four-wave mixing (FWM) model.     

Conical four-wave mixing in sodium vapour excited by femtosecond laser pulses

Broadband (sometimes exceeding 1500 cm^-1) red-shifted (with respect to the sodium 3S–3P transition frequency) conical emission has been observed with the pump wavelength tuned in the range between 540 and 589 nm. Such broadband emission was attributed to the generation and amplification of light at the Rabi sideband frequencies in the field of intense femtosecond laser pulses. It has been shown that the cone angle of the emitted radiation is determined by the process of four-wave mixing under the conditions of longitudinal (Cherenkov-type) phase matching. PACS 42.65.Ky; 42.65.Re; 42.50.Hz     

引入量子干涉的双光子共振非简并四波混频

研究了级联四能级系统中双光子共振非简并四波混频(NFWM)由于加入耦合光场而产生的量子干涉效应，发现在强耦合场作用下NFWM频谱呈现AT劈裂，它反映的是两个缀饰态的能级.量子干涉还使NFWM信号被抑制或增强.此方法牵涉到三光子共振，因此可以成为研究原子或分子高激发态的有效光谱学工具. 关键词： 四波混频 量子干涉     

Quantum interference in atomic vapor observed by four-wave mixing with incoherent light

Pairs of pulses from an incoherent source are used to investigate the time-resolved four-wave mixing response of atomic rubidium when a two-photon resonance is involved in the nonlinear process. By varying the relative polarization of the pulse pairs, we are able to select the quantum pathways and clearly distinguish optical and quantum interferences.     

锂蒸气中双光子泵浦的六波、四波混频和受激辐射

本文报道锂蒸气中由双光子泵浦所产生的位于紫外、可见和红外区的相干和受激辐射。包括:由一特殊六波混频过程产生的在1.03μm附近的可调谐相干辐射,由锂分子双光子离解或原子串级跃迁产生的波长为812.6nm的受激辐射,以及由四波混频产生的位于323.3nm和670.8nm的相干辐射。 关键词：     

A reliable cw Lyman-α laser source for future cooling of antihydrogen

We demonstrate a reliable continuous-wave (cw) laser source at the 1 S–2 P transition in (anti)hydrogen at 121.56 nm (Lyman-α) based on four-wave sum-frequency mixing in mercury. A two-photon resonance in the four-wave mixing scheme is essential for a powerful cw Lyman-α source and is well investigated.     

汞蒸汽中双光子共振四波和频产生125nm的相干辐射

利用汞原子6~1S_0—7~1S_0双光子共振四波和频产生了125nm的相干辐射.对双光子共振增强效应、双光子共振克尔(Kerr)效应进行了研究.讨论了平面波和聚焦情况下最佳相位匹配的条件.实验中还观测到了四波参量振荡和四波差频的信号输出.     

五能级原子开放系统中的多波混频共存特性

研究了受强缀饰场作用的五能级原子系统所产生的多波混频共存特性.研究发现,通过调节激光束方向,沿同一方向出射的八波混频、双缀饰四波混频和单缀饰六波混频信号因共用能级的原子相干产生而出现竞争现象,通过控制缀饰场减弱四波混频和六波混频信号强度,可以达到增强八波混频信号的目的.还对级联和并联两种不同缀饰方式下双缀饰四波混频过程的差别进行了详细分析. 关键词： 双缀饰四波混频 单缀饰六波混频 八波混频 抑制增强     

Interaction of atomic wave packets with four-wave mixing: detection of rubidium and potassium wave packets by coherent ultraviolet emission

The observation of an atomic wave packet by use of a coherent, nonlinear-optical process is reported. Wave packets formed in K or Rb vapor by two-photon excitation of ns and (n-2)dstates (n=8 for K; n=11 , 12 for Rb) with red (~620-nm) , 80-100-fs pulses were detected by four-wave mixing in pump-probe experiments. The temporal behavior of the wave packet is observed by monitoring the coherent UV radiation generated near the alkali mp(2)P? (2)S(1/2) (7相似文献   

Observations of four-wave mixing processes in barium vapour

Emission at fifty discrete wavelengths is observed when Ba I is excited by two laser beams, a dye laser tuned to the λ 7911 Å intercombination line, 6s²¹S₀-6s6p ³P₁ and a ruby laser. The wavelengths range from 2312 Å to 8027 Å. Most of the emission lines can be attributed to four-wave mixing processes in barium.     

Reducing the first-order Doppler shift in a Sagnac interferometer

We demonstrate a technique to reduce first-order Doppler shifts in crossed atomic/molecular and laser beam setups by aligning two counterpropagating laser beams as part of a Sagnac interferometer. Interference fringes on the exit port of the interferometer reveal minute deviations from perfect antiparallelism. Residual Doppler shifts of this method scale with the ratio v/(4d) of the typical atomic/molecular velocity v and the laser beam diameter d. The method is implemented for precision frequency calibration studies at deep-UV wavelengths, both in one- and two-photon excitation schemes: the 6s(2) --> 6s30p(3/2)J=1 line in Yb at 199 nm and the 4p(6) --> 4p(5)p[1/2](0) transition in Kr at lambda=212 nm. The achieved precision of 6 x 10(-10) is limited by the characteristics of the laser system.     

Generation of multi-color stimulated raman emission from two-color linearly or circularly polarized pump beams

A laser beam at two different frequencies separated by 587 cm^–1 is focused into pressurized hydrogen (rotational transition energy, 587 cm^–1) to generate multi-color stimulated Raman emission. The polarization state and the pulse energy are measured for each generated emission line using linearly and circularly polarized pump beams. The effect of the polarization is discussed by using a parameter characterizing the polarizability of hydrogen and a conservation rule for the angular momentum in four-wave mixing. Many rotational lines are generated with linearly polarized pump beams through a four-wave mixing process. This is in striking contrast to the results obtained by using a single-color circularly polarized pump beam which generates only one rotational line through a conventional stimulated Raman process.     

Observation of interference between four-wave mixing and six-wave mixing

We report generation of four-wave mixing enhanced by electromagnetically induced transparency and optical pumping in a ladder-type atomic system. When two pumping laser beams are used to form a conjugate small-angle static grating, both four-wave and six-wave mixing processes are shown to exist at the same time. Interference between these two nonlinear wave-mixing signals is experimentally demonstrated.     

阿秒和频相位共轭极化拍频的不对称色锁二阶相干控制

研究了级联三能级系统中孪生色锁随机光场的相位共轭阿秒和频极化拍频(ASPB)不对称新效应。从理论上分析了自关联单光子简并四波混频(DFWM)和双光子非简并四波混频(NDFWM)信号辐射场与物质间不对等效应、混合型太赫兹失谐振荡特性,及双光子非简并共振四波混频信号的延时不对称性;同时也分析了由干涉计两臂不平衡色散效应引起的阿秒和频极化拍频信号的延时不对称性,即阿秒和频极化拍频信号的最大值相对于零延时有一个小偏移,结果表明该不对称性并这不影响阿秒和频极化拍频零差探测两偶极禁戒跃迁能级分裂的总体精度。阿秒和频极化拍频作为一种消除多普勒增宽的超快调制过程,从理论上说它可以扩展到任何两偶极禁戒跃迁能级系统。