On the analysis of factors limiting third harmonic generation of Nd:YAlO<Subscript>3</Subscript> laser radiation in barium vapor

The efficiency of generation of the third harmonic (THG) of Nd:YAlO³ laser radiation in barium vapor is analyzed. The influence of the following factors is taken into consideration: the interference of transformed radiation in the case of THG via two channels ω+ω+ω→3ω and 2ω+2ω?ω →3ω, the Kerr effect, two-photon absorption, and the relationship between the energies of the first and second harmonics of radiation. It is shown that the Kerr effect, on the one hand, violates the phase matching conditions in the direct channel but, on the other hand, increases the conversion efficiency due to a defocusing action of the medium, which reduces the influence of the phase detuning between the pumping waves on the process of harmonic generation in the case of generation of difference frequencies. At the same time, the two-photon absorption leads to the depletion of one of the interacting waves (in this case, the second harmonic) and to the prevalence of third harmonic generation through the direct channel.     

Ultrahigh-order wave mixing in noncollinear high harmonic generation

We show that noncollinear high harmonic generation (HHG) can be fully understood in terms of nonlinear optical wave mixing. We demonstrate this by superposing on the fundamental ω1 field its second harmonic ω2 of variable intensity in a noncollinear geometry. It allows us to identify, by momentum conservation, each field's contribution (n1,n2) to the extreme ultraviolet emission at frequency Ω = n1ω1 + n2ω2. We observe that the photon (Ω) yield follows an n2 power law on the ω2 intensity, before saturation. It demonstrates that, although HHG is a highly nonperturbative process, a perturbation theory can still be developed around it.     

Fifth harmonic generation mechanisms in a two-species system

The fundamental mechanisms of high harmonic generation in a two-species system are examined. The “quasi-molecular” contribution is shown to be implicity included in the cascade interaction (ω + ω + ω → 3ω; 3ω + ω + ω → 5ω in the fifth harmonic case). This cascade process will normally be dominant if the two species are suitably selected.     

Nonlinear susceptibilities of vapors and solutions of organic dyes

Results of theoretical studies of laser and Kerr nonlinear susceptibilities of vapors and solutions of organic dyes using a series of polycyclic arenes as an example are presented. Nonlinear susceptibilities of the third χ⁽³⁾ (?3ω; ω, ω, ω) and the fifth χ⁽⁵⁾ (?3ω; ω, ω, ω, ω, ?ω) orders of a series of organic dyes responsible for third harmonic generation of Nd:YAG laser radiation are calculated within the context of the free electron model. Results of calculations of their Kerr third-order nonlinear susceptibilities χ⁽³⁾ (?ω; ω, ?ω, ω) and non-linear refractive indices n ₂ are presented. The calculation results are compared with experimental data on third harmonic generation in naphthalene vapors and with χ⁽³⁾ (?ω; ω, ?ω, ω) as well as n ₂ of paraterphenyl and naphthalene solutions.     

Surface wave second harmonic generation over a n-type semiconductor

The sharp gradient in the intensity of a surface wave over a simiconductor-vacuum interface gives rise to a strong ponderomotive force on electrons and thus generates a second harmonic wave. The power of the second harmonic shows a resonance at $\text{ω =}\text{ω}{}_{\mathrm{p}}\text{2}$ (ω and ω_p being the wave and plasma frequencies) and tends to a saturation value at very high values of ω_p.     

Picosecond measurements of the third order susceptibility tensor in liquids

Single frequency Jamin interferometry is used for observation of non-linear susceptibility tensor measurements in the picosecond range (25 ps). At low density, when avoiding the polarization state instabilities, the displacement of the fringes leads to the measurement of two components of the nonlinear susceptibility tensor. It is shown that X_xyyx (ω = ω + ω - ω) decreases when the pulse duration is reduced, whereas X_{xxxx (ω = ω + ω - ω)} remains constant. This is interpreted by the difference between the orientational and vibrational molecular contributions.     

Four-wave mixing in hollow photonic-crystal fibers

A radical enhancement of four-wave mixing in hollow-core photonic-crystal fibers is experimentally demonstrated. An enhancement ratio of about 800 relative to the regime of tight focusing is achieved for the four-wave mixing process 3ω=2ω+2ω?2ω, where ω and 2ω are the frequencies of fundamental radiation and the second harmonic of picosecond Nd:YAG laser pulses.     

Third harmonic generation in copolymer films with optically nonlinear N-substituted 4-nitroazobenzene in the side and main polymer chains

The third-order nonlinear optical susceptibility χ⁽³⁾(3ω; ω, ω, ω) of two types of copolymer films containing optically nonlinear N-substituted nitroazobenzene in the side (methacrylic series copolymers) and main (copolyimidoamido esters) polymer chains is investigated using the third harmonic generation method at a wavelength of 1.064 μm. __________ Translated from Fizika Tverdogo Tela, Vol. 44, No. 2, 2002, pp. 379–382. Original Russian Text Copyright ? 2002 by Lebedeva, Loretsyan, Ivanova, Romashkova, Lukoshkin, Kudryavtsev.     

任意偏振双色相干场高次谐波

系统地研究了任意偏振双色相干场高次谐波，结果表明单个圆偏振光不能产生高次谐波，在适当的条件下，某些谐波可以部分或完全消失，而某些谐波可以得到加强，产生更高次谐波高转换效率的条件是双色线偏振场的结合。     

Non-adiabatic transitions for random processes in a two-level system

Non-adiabatic transitions in two-level systems are investigated theoretically for a random time dependence of ?ω, the energy difference, between the levels. We assumed that ω = ω(x) and the coordinate x = x(t) is a random function of time. Diffusion and Poisson processes (both homogeneous and with a source) for x(t) were assumed. The cases of linear crossing terms (ω = γx) and non-linear terms (ω = ω _e exp (- αx) + ω₀) were considered. Values of the non-adiabatic transition probability per unit time were obtained by perturbation theory for ω₁τ _c ? 1 where τ _c is the correlation time and ω₁ is the off-diagonal matrix element.     

光学超晶格中级联参量过程制备纠缠光子对

探索制备纠缠光子对的新途径是量子光学领域的一个研究热点.从理论上证明在准周期极化 的光学超晶格晶体中的级联参量过程能够产生纠缠光子对.分别利用脉冲和连续的532nm抽运 光在一块准周期极化的 LiTaO3晶体中同时实现非简并参量下转换（产生信号光与闲置光 ）与和频（闲置光与抽运光和频生成和频光）的级联参量过程，获得630nm的信号光与460nm 的和频光.指出信号光与和频光之间存在纠缠关联性质.该方案的优点是可产生短波长的纠缠 光子对. 关键词： 准相位匹配 级联参量过程 光量子纠缠态     

PuN基态和低激发态的分子结构与相对论有效原子实势

用Pu的三种不同相对论有效原子实势 (RECP)和密度泛函(B3LYP)方法对PuN基态和低激发态(4Σ+、6Σ+、8Σ+、10Σ+)的分子结构进行了计算,得到了相应的平衡几何构型和谐振频率.采用最小二乘法拟合得到了PuN分子的Murell-sorbie势能函数,在此基础上推导出完整的力常数和光谱数据,并与实验值作对比.结果表明:PuN的基态为X6Σ+,其余为低激发态;在三种RECP中,对于ωe的计算,60个中心电子的SDDRECP/B3LYP给出的结果与实验值符合得比较好(如X6Σ+:ωe=840.77 cm-1,ωeχe=5.73·"cm-1,De=5.880 6 eV,Re=0.176 3 nm,Be=0.408 4 cm-1,αe=3.3E-03 cm-1).计算还给出了相应的电荷布居、自旋密度、电偶极矩和能量特征等系列分子的性质.     

Field-resolved characterization of femtosecond electromagnetic pulses with 400 THz bandwidth

We propose and demonstrate an ultrabroadband concept to characterize amplitude and phase changes of femtosecond pulses. The radiation is superimposed with the first subharmonic spectral components from the same laser source. This effective ω/2ω pulse pair induces a coherently controlled charge current in a time-integrating semiconductor detector. An interferometric variation of the time delay between the harmonically related components then reveals the electric field of the 2ω part. This method is realized with the second harmonic of a compact Er:fiber source centered at 390 THz and a GaAs-based detector. Most strikingly, it is sensitive to ～π/20 phase changes and can be utilized to analyze femtojoule pulses.     

Optical second harmonic generation induced by picosecond terahertz pulses in centrosymmetric antiferromagnet NiO

Optical second harmonic generation at the photon energy of 2?ω = 2eV in the model centrosymmetric antiferromagnet NiO irradiated with picosecond terahertz pulses (0.4–2.5 THz) at room temperature is detected. The analysis of experimental results shows that induced optical second harmonic generation at the moment of the impact of a terahertz pulse arises through the electric dipole mechanism of the interaction of the electric field of a pump pulse with the electron subsystem of NiO. Temporal changes in optical second harmonic generation during 7 ps after the action of the pulse are also of an electric dipole origin and are determined by the effects of propagation of the terahertz pulse in a NiO platelet. Coherent oscillations of spins at the antiferromagnetic resonance frequency induced by the magnetic component of the terahertz pulse induce a relatively weak modulation of magnetic dipole optical second harmonic generation.     

Quantum chemical investigation on the structure and first hyperpolarizability for N‐substituted [n]cyclacene

Six N‐substituted [n]cyclacene (n = 5, 6, 7,…,10) molecules were designed to study the relationship between the structure and first hyperpolarizability. Their static first hyperpolarizabilities (β₀) were obtained by MP2/6‐31 + g(d) level. Two interesting relationships between the β₀ value and the structure have been found: (1) The β₀ value increases with the increase of the number n when n is odd: 3155 ([5]cyclacene) < 48,905 ([7]cyclacene) < < 393,444 ([9]cyclacene), and when n is even: 357,620 ([6]cyclacene) < 618,608 ([8]cyclacene) < 3,513,644 a.u. ([10]cyclacene). (2) The β₀ values (in the range of 357,620 ~ 3,513,644 a.u.) of the N‐substituted [n]cyclacene (when n is odd) are much larger (in the range of 3155~393,444 a.u.) than that of the N‐substituted [n]cyclacene (when n is even). Furthermore, their frequency‐dependent β (?2ω; ω, ω) and β (?ω; ω, 0) (ω = 0.005, 0.01, and 0.0239 a.u.) were also estimated by Møller–Plesset perturbation/6‐31 + g(d) level. Among the frequency‐dependent β (ω), [10]cyclacene has the largest β (?ω; ω, 0) and β (?2ω; ω, ω) to be 1.2 × 10⁸ (ω = 0.01) and 2.9 × 10⁷ a.u. (ω = 0.005 a.u.), which are much larger than the static β₀ = 3.5 × 10⁶ a.u. by 34 and 8 times. Our present work may offer a new idea in the design of high‐performance tubiform nonlinear optical materials. Copyright © 2011 John Wiley & Sons, Ltd.     

Dependence of third-order nonlinear susceptibility on strain induced piezoelectric field in InxGa1-xN/GaN quantum well

The third-order susceptibility of InxGa1-xN/GaN quantum well (QW) has been investigated by taking into account the strain-induced piezoelectric (PZ) field, and the effective-mass SchrSdinger equation is solved numerically. It is shovn that the third-order susceptibility for third harmonic generation (THG)of InxGa1-x,N/GaN QW is related to indium content in QW and the intensity of the PZ field. The characteristics ofX(3)THG(-3ω,(3) ω, ω,ω) as the function of the wavelength of incident beam, well width and indium content, have been analyzed.     

Study of the process e+e-→π+π-π0π0 at energies √s< 1 GeV

In an experiment with the Spherical Neutral Detector at VEPP-2M collider the cross section of the process e+e-→π+π-π0π0 was measured. At energies √s < 920 MeV this cross section was measured for the first time. The energy dependence of the cross section is well discribed by the vector dominance model with contributions from ρ, ρ' ρ", mesons. The decay probability ρ→π+π-π0π0was found to be Bρ = (1.60±0.74±0.18)×10-5. The upper limit for the decay ω→π+π-π0π0 was improved by two orders of magnitude compared to the previous measurements and is Bω < 2 × 10-4 at 90% confidence level.     

Nonlinear Raman scattering of photons by a channeled particle

Channeled particles are characterized by the discrete spectrum of bound transverse motion. The interaction of photons with channeled particles in single crystals can be accompanied by energy transitions between the levels of transverse motion of the channeled particle. The Raman scattering of photons at a quasibound channeled particle leads to the appearance of a combination of frequencies: the incident radiation frequency ω₀ and the frequency Δω_{m, n}, i.e., ω = ω₀ ± Δω_m,n where Δω_m,n = 2Δε_m,nγ²; Δε_{m, n} is the energy of the transition between quantum states (m and n) of the transverse motion of the channeled particle; and γ = E/mc² is the Lorentz factor of the channeled particle. The appearance of a violet satellite (the anti-Stokes component) in the Raman scattering spectrum is analyzed. The three-photon Raman-type transition, which is the process of the simultaneous absorption of two photons with the frequency ω₀ with the emission of a photon with the frequency ωs = 2ω₀ ± 2Δε_m,nγ², is considered. The conditions for resonance observation during the formation of the second harmonic (ω = 2ω₀) are discussed.     

相位失配法实现倍频稳定输出的模拟分析

 对BBO晶体Ⅰ类相位匹配倍频过程和KTP晶体XY平面内的Ⅱ类相位匹配倍频过程进行数值模拟，结果表明：Ⅰ类和Ⅱ类相位匹配倍频过程，都可通过引入适量的相位失配量实现稳定倍频输出；随着相位失配量的增大，实现稳定倍频输出所需的晶体长度都变小，倍频稳定输出起伏变小，倍频转换效率有不同程度的下降；相对于XY平面内的Ⅱ类相位匹配倍频过程，BBO晶体的Ⅰ类相位匹配倍频过程对相位匹配偏离角更敏感，且稳定输出时倍频转换效率更低。     

1+1维和2+1维空间上定向聚合问题的数值研究

对1+1和2+1维空间上定向聚合问题的数值模拟结果显示,任意有限温度下的横向涨落和自由能涨落在聚合尺度t较大时都将趋于零温度时强耦合下的结果:Δx∝t^ν和ΔF∝t^ω(d=1+1时,ν=2/3,ω=1/3;d=2+1时,ν≈0.6,ω≈0.2).有限温度下,由于1+1维和2+1维空间上的比热C(T,t)∝t和1+1维空间上的熵涨落ΔS∝t^1/2,1+1维空间上的系综能量涨落和内能涨落以及2+1维空间上的系综能量涨落均趋于t^1/2而远强于自由能涨落.在2+1维空间上,定向聚合问题有发生相变的迹象:当熵涨落达到其最大值时,单位聚合尺度的熵涨落和内能涨落在聚合尺度t→∞时,可能会由低温下趋于有限值(零温度除外)转变为高温下趋于零 关键词： 定向聚合 表面生长现象 Kardar-Parisi-Zhang方程