Resonant third-harmonic generation a low-loss medium

The linear and nonlinear susceptibilities for resonant third-harmonic generation in an Mg-Kr system using the3s^{2}-3s3dtwo-photon resonance at 431 nm are presented. The Mg-Kr system is shown to be a suitable low-loss medium because of its small one-photon and two-photon absorption cross section. A detailed quantitative analysis up to a power conversion efficiency of1.2 cdot 10^{-4}was carried out. The phase-matching curves and the intensity dependence are in excellent agreement with a nonstationary modification of the theory of resonant third-harmonic generation in high density, phase-matched systems recently presented by Puell et al. [4].     

Pump depletion and saturation of two-photon resonant third-harmonic generation processes

Theoretical efficiency limitations for a two-photon resonant third-harmonic generation (THG) process are analyzed in a treatment which includes all nonlinear processes associated with the two-photon resonance assuming negligible material excitation. Results are presented for both collimated-beam and tight-focusing geometries. The theory is applied to experimental results for THG of CO2laser radiation in liquid CO-O2mixtures; good agreement is found for the observed saturation in the THG efficiency in a tight-focusing geometry.     

Quantum theory of third-harmonic generation in a gaseous system--Part I

We present a theoretical treatment for third-harmonic generation in gaseous systems by describing the atomic system as well as the radiation field quantum mechanically. The atomic system is represented by two energy levels. The effects of detuning in the two-photon interactions and the atomic motion on the third-harmonic generation are taken into account. The photon statistics of the third-harmonic field are obtained. The average photon number, second-order coherence, and output variance of the third-harmonic field are discussed. The above results show a nonlinear dependence on the detuning in the two-photon interactions and the Doppler parameter.     

Quantum theory of third-harmonic generation in a gaseous system--Part II

In Part I [1], we presented a quantum theory of third-harmonic generation (THG) under two-photon absorption in a two-level atomic medium. We studied the time evolution of the third-harmonic field and its coherence properties by neglecting the depletion of the pump power. In this paper we extend the theory by taking the depletion of pump power into account. The density matrix for the third-harmonic field is derived. The study of the growth rate of the density matrix in time shows the quantum mechanical effects more clearly. The effects of detuning in the two-photon interactions and atomic motion are pointed out. The results are compared with the earlier results and differences are brought out.     

Propagation analysis of forward degenerate four-wave mixing

We discuss the propagation problem associated with the forward four-wave degenerate mixing induced by the third-order non-linear susceptibilitychi_{(-omega; omega, omega, -omega)}^{(3)} = chi_{1}^{(3)} + ichi_{2}^{(3)}. Exact analytical solutions are presented of two-photon resonances, evidencing the influence of the Kerr effect on the efficiency of the process. Around a two-photon resonance, we solve the problem of the nearly parallel propagation of two beams undergoing auto and induced two-photon absorption (TPA). The resonant interaction is studied in detail and analytical solutions are provided with operational accuracy for current experimental conditions. We show by the way that in the two-beam two-photon experiments, when the phase matching condition is fulfilled, although TPA is governed bychi_{2}^{(3)}, the real part ofchi^{(3)}must be considered in a substantial way. All the above solutions are put in a form simple enough that the numerical computations can be performed on a programmable calculator.     

高功率激光在铯金属蒸气中产生三阶谐波

讨论了在铯（Ｃｓ）原子蒸气中的光学三阶谐波的产生。理论分析表明，铯的非线性极化率系数Ｘ￣（３）（３ω）在λ＝１．０７９０μｍ处存在双光子共振增强效应，给出了为达到位相匹配所需的温度。实验获得了１．０７９０μｍ光波作用于Ｃｓ所产生的三阶谐波谱线（３５９．７ｕｍ）的照片。     

Experimental and theoretical studies of third-harmonic generation in the chalcopyrite CdGeAs2

Experimental and theoretical studies of third-harmonic generation (THG) in the chalcopyrite semiconductor CdGeAs2are presented. The phase-matching configurations for THG are analyzed from the irreducible components point of view. A theory of the bound electron and free-carrier contribution to the third-order susceptibility is presented. The experimental results are given. The effective nonlinear coefficient for type-II THG is mainly due to the free-carrier contribution, and for a hole concentration of5 times 10^{16}/cm³it is measured to be(13 pm 6)10^{-11}ESU. The practical applications of THG in CdGeAs2are discussed.     

Doubly resonant four-photon interactions in Cesium vapor

Four-photon interactions involving both a two-photon resonance with the9d^{2}D_{3/2}level and a one-photon resonance with an np²P level (n = 7-10) are investigated in detail. Gain, gain saturation, gain threshold, and forward-to-backward generation ratios are measured. Comparison with numerical estimates indicates that9d^{2}D_{3/2}tonp^{2}Plasing is the dominant gain mechanism but that a doubly resonant four-photon parametric oscillation makes a significant contribution. Gain saturation is dominated by population of np²P levels. Ionization is important, especially at higher pump powers.     

Coherent VUV generation by resonantly-enhanced nonlinear frequency mixing in Cd vapor

Broadly tunable coherent VUV radiation has been generated by two-photon resonant sum-frequency mixing in Cd vapor, using frequency-doubled Nd:YAG laser-pumped dye lasers and the three two-photon resonances5^{1}S_{0} - 5^{1}D_{2}, - 7^{1}S_{0}, and-6^{1}D_{2}. The generated VUV having no spectral structure covers a wavelength range of 119.6-136.8 nm, and the VUV generation efficiency has been measured as a function of wavelength. Besides the tunable VUV, additional intense VUV lines were observed in a spectral range of 145.3-171.1 nm, which were generated by four-wave parametric and second-harmonic processes. The generation mechanisms of these VUV lines are discussed.     

Infrared image upconversion using two-photon resonant optical four-wave mixing in alkali metal vapors

We present a theoretical and experimental study of IR image upconversion using two-photon resonant optical four-wave mixing in alkali metal vapors. A closed form solution for the upconversion efficiency which includes Doppler broadening of the two-photon transition and a finite bandwidth pump laser is given. Experimental results for two types of image upconverters using Cs are presented. The first upconverts 2.94 μm to 4558 Å with a power efficiency of 32 percent and1.6 times 10^{4}resolvable spots. The second is IR tunable, but of lower efficiency and resolution. Finally, the optimized characteristics of the Cs upconverter are compared to crystal upconverters and to the requirements of various applications.     

Hg蒸气中双光子共振四波和频产生XUV相干光的研究

我们以Ｈｇ蒸气为非线性光学介质，用毛细列阵做输出窗口，成功地得到了波长小于１００ｎｍ的可调谐ＸＵＶ相干辐射。本文深入研究了毛细列阵作为输出窗产生ＸＵＶ相干辐射的可行性，以Ｈｇ的６＾１Ｄ、６＾３Ｄ为共振能级的双光子共振三次谐波过程产生ＸＵＶ可调谐相干辐射的机制；以及影响ＸＵＶ光产生效率的因素，诸如光学斯塔克效应，介质对泵光及产生光的吸收，不同非线性过程的相互竞争等，燕在此基础上提出改进方案。     

The influence of a second resonance on the saturationcharacteristics of the four-wave sum mixing

Experimental results are presented, related to the saturation mechanisms of the four-wave sum mixing (4-WSM) generation in Hg when a double resonance enhancement is employed, that is, a two photon resonance to the δ¹S₀ state followed by the δ¹S₀-6p'³D₁ one photon coupling. This coupling is shown to alter the saturation characteristics of the four-wave sum mixing radiation by affecting the two-photon excitation to the δ¹S₀ state, reduce the ionization of the Hg atoms by the incoming radiation and create an absorption window for the generated radiation in a region near the autoionizing state by modifying the linear susceptibility     

Frequency up-conversion to the VUV in Hg vapor

The efficient generation (up to 1 percent, 10 kW) of tunable, narrow-band radiation in the spectral ranges 1219-1221 Å, 1228- 1235 Å, and 1247-1255 Å, using two-photon resonant, four-wave sum mixing in Hg vapor, is reported, using both the planewave and the tight focusing situations. Oscillator strengths are derived from the planewave experiments for the6s^{1}S_{0}-9P, 10p, and11p^{1}P_{1}transitions with an accuracy of 3 percent.     

Infrared third-harmonic generation in molecular gases

We report the first observation of third-harmonic generation (THG) in molecular gases. Using a CO2TEA laser and choosing molecules with vibrational transitions that contribute to a resonance enhancement of the third-order susceptibility tensor we have studied absolute conversion efficiencies, spectral response, and pressure dependences for THG in SF6, BCl3, and CO and compared the results to theoretical predictions.     

Saturation of resonant third-harmonic generation due to self-defocusing and a redistribution of the population densities

Using a high-power excimer laser, pumped dye laser third-harmonic generation in strontium was investigated around several two-photon resonances for input intensities which are well above the onset of saturation. The dominant role of self-defocusing and of the time-dependent redistribution of the population densities is demonstrated. Strong stimulated emission on several transitions branching off from the excited state of the two-photon resonance and several competing parametric processes have been observed. Good agreement is obtained with a theoretical model which accounts for the redistribution of the population densities and which shows the importance of the ac Stark effect for the central structure of the high-intensity conversion profiles.     

Design of a multi-petawatt optical parametric chirped pulseamplifier for the iodine laser ASTERIX IV

A theoretical design is presented for the generation of peak powers up to ~5 PW and focused intensities higher than 10²³ Wcm^-2 with pulse duration ~20 fs using an optical parametric chirped pulse amplifier (OPCPA) pumped by the high-power iodine laser Asterix IV. These values represent an improvement to the existing Asterix laser by a factor of ~1000 and at least LOO 000 for the output peak power and focused intensity, respectively. The OPCPA system consists of two LBO preamplifiers and a KDP power amplifier and is pumped by the laser third-harmonic output (438 nm, 500 ps, 500 J). Numerical simulations show that saturation within the OPCPA can be effectively used to counteract the bandwidth narrowing effects brought about by the temporal variation of the pump pulse intensity. The results demonstrate that the OPCPA represents a powerful technique for the generation of ultrahigh powers with iodine lasers at a level currently not achievable with any other approach, and proves that the iodine system is particularly suitable as an OPCPA pump source     

Design and Application for PV Generation System Using a Soft-Switching Boost Converter With SARC

In order to improve the efficiency of energy conversion for a photovoltaic (PV) system, a soft-switching boost converter using a simple auxiliary resonant circuit, which is composed of an auxiliary switch, a diode, a resonant inductor, and a resonant capacitor, is adopted in this paper. The conventional boost converter decreases the efficiency because of hard switching, which generates losses when the switches are turned on/off. During this interval, all switches in the adopted circuit perform zero-current switching by the resonant inductor at turn-on, and zero-voltage switching by the resonant capacitor at turn-off. This switching pattern can reduce the switching losses, voltage and current stress of the switching device. Moreover, it is very easy to control. In this paper, we have analyzed the operational principles of the adopted soft-switching boost converter, and it is designed for PV generation system. Simulation and experimental results are presented to confirm the theoretical analysis.      

Tunable VUV radiation generated by two-photon resonant frequency mixing in xenon

Efficient two-photon resonant frequency conversion of powerful pulsed dye laser radiation generates in Xe intense VUV light atomega_{VUV} = 2omega_{UV} - omega_{D}. The frequency ωUVis tuned to the 5p-6p [frac{5}{2},2] two-photon resonance (lambda_{UV} = 255.9nm). For dye laser radiation ωDatlambda_{D} = 300-760nm ωVUVis tunable in the spectral range oflambda_{VUV} = 155-220nm. The resonantly enhanced conversion efficiency is typically 0.05-0.2 percent; the generated VUV pulse powerP_{VUV} = 0.5-1kW. The experimental results provide detailed information on conversion limiting saturation phenomena and their dependence on the input power and the detuning of ωUVfrom the two-photon resonance.     

双光子共振激发的电离复合产生受激电子辐射

文中报道利用双光子共振激发锂原子到4s态,获得了多条受激电子辐射和相干辐射,同时观察到了高于4s能级的nd,ns态(n≥4)到2p态的跃迁,以及从连续态到3s态的辐射。     

钠蒸气中基于Na-Na碰撞能量转移的光泵级联辐射

本文报道了钠蒸气中基于Na-Na近共振碰撞能量转移而产生的光泵受激辐射。当双光子共振激发4d能级时,可探测到起始于4f能级的受激辐射;而当进行4f能级的偶极禁戒双光子共振激发时,则可产生起始于4f能级的受激辐射及其跟随的级联受激辐射。文中对有关过程进行了讨论。