Self-demodulation of acoustic pulses in partially water-saturated river sand

Laboratory experiments on self-demodulation of high-frequency acoustic pulses in partially water-saturated river sand are reported. Based on experimentally revealed dependences of demodulated pulse parameters (propagation velocity, amplitude, and duration) on the static pressure and on the amplitude of the primary pulses, an equation of state for this medium is proposed. Parameters of this equation are derived from the comparison of theoretical predictions and experimental results.     

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.     

Submillimeter wave generation by second harmonic operation of tunable gyrotrons

The gyrotrons developed at the University of Sydney are tunable sources of millimetre and submillimetre radiation. The generation of submillimetre wavelengths requires operation at the second harmonic of the electron cyclotron frequency, or higher. Our latest gyrotron, GYROTRON IVA, has successfully achieved second harmonic operation and obtained frequencies up to 590 GHz (=0.51 mm). The experimental results and conditions for second harmonic operation will be presented. The design of a new gyrotron, GYROTRON V, which is especially optimised for the second harmonic, will also be included.     

二倍频晶体温升分布对输出光场分布的影响

利用分步快速傅里叶变换和四阶龙格-库塔法,对具有一定温升分布的倍频晶体的二次谐波转换过程进行了研究。综合考虑了谐波转换过程中的离散、衍射、二阶、三阶非线性等效应,着重讨论了倍频晶体吸收光能后,晶体内温升分布对晶体内o光和e光的折射率分布的影响,定量分析了温度分布引起的相位失配量、输出光场分布、二次谐波转换效率随倍频晶体温度分布变化的规律。结果表明:在高功率倍频系统中,倍频晶体温升分布引起基频光、倍频光的相位失配,相位失配导致输出光场光强分布的变化以及谐波转换效率的降低。     

Predictions of quantum mechanics and stochastic electrodynamics in travelling wave second harmonic generation

We show that stochastic electrodynamics and quantum mechanics give quantitatively different predictions for the quantum nondemolition (QND) correlations in travelling wave second harmonic generation. Using phase space methods and stochastic integration, we calculate correlations in both the positive-P and truncated Wigner representations, the latter being equivalent to the semi-classical theory of stochastic electrodynamics. We show that the semi-classical results are different in the regions where the system performs best in relation to the QND criteria, and that they significantly overestimate the performance in these regions.     

Magnetization-induced second harmonic generation in a polar ferromagnet

Second harmonic generation (SHG) induced by spontaneous magnetization has been investigated for a polar ferromagnetic crystal of GaFeO3. The Kerr rotation of the second harmonic light becomes gigantic with decreasing temperature below the magnetic transition temperature (approximately =205 K), e.g., as large as 73 degrees at 100 K. The magnetic domains can be visualized by using that large nonlinear Kerr rotation. The spectrum of the magnetization-induced SHG as measured indicates the two-photon resonant electronic process on a Fe3+ ion in the crystal.     

Self pulsing in second harmonic generation

We consider nonlinear optical processes in a cavity maintained far from thermal equilibrium by external pumping. In second harmonic generation we predict a hard mode instability at a critical pump strength indicating the existence of undamped spiking similar to that in strongly pumped lasers. In subharmonic generation above the soft mode transition noted by Graham we predict the appearance of sidebands in the subharmonic spectrum.     

Noncollinear second harmonic generation in KDP

This paper reports experimental and analytical results on optical second harmonic generation when a noncollinear arrangement is employed. The dependence of second harmonic power on crystal length, absorption, beam radius and the angle between the two fundamental waves is obtained. Double refraction of the crystal is taken into account, but the divergence and depletion of the fundamental waves are neglected. The results of the calculations are confirmed by experiments carried out with a KDP crystal in the near field of two intersecting TEM₀₀ ruby laser beams.An extension of the theory to nonlinear interactions other than second harmonic generation - e.g. four-wave interaction - is possible.     

Efficient second harmonic generation in CDA

Efficient second harmonic generation at 0.532 μ has been achieved in a temperature tuned CDA crystal when a narrow beam divergence Nd:YAG laser was used. At a crystal temperature of 40.3°C, a peak second harmonic power of 7.5 MW was obtained with a peak power conversion efficiency of 33%. The temperature variation of the refractive indices in CDA has been determined to be d(n^e_2ω?n⁰_ω)/dT = 7.2 × 10^?5°C^?1.     

On the theory of second harmonic generation

It is shown how the theory of non-linear interaction between the modes of the open resonator, developed previously [1] and used to study resonant second harmonic generation [2], can be applied to second harmonic generation in the absence of a resonator. The conditions under which the second harmonic output, in the small conversion approximation, can be approximated by a Gaussian beam are derived.The optimum value of the focusing parameter I/z₀ (hereI is the length of the crystal, andz ₀ is one-half the confocal parameter of the fundamental beam) for the total output is shown to be 5.68 agreeing precisely with the value given by Boyd and Kleinman [3].     

Enhanced second harmonic generation in a two-dimensional optical micro-cavity

We introduce a two-dimensional Bose–Einstein condensation model consisting of massive photon and photon-pair.Based on the new nonlinear model, the traditional process of second harmonics generation is reinvestigated. In order to describe the process, a new quantum phase, the harmonic phase, is introduced. The order parameter of the new physical phase is also given in this paper.     

Nonlinear acoustic wave generation in a three-phase seabed

Generation of an acoustic wave by two pump sound waves is studied in a three-phase marine sediment, which consists of a solid frame and the pore water with air bubbles in it. To avoid shock-wave formation, the interaction is considered in the frequency range where there is a significant sound velocity dispersion. Nonlinear equations are obtained to describe the interaction of acoustic waves in the presence of air bubbles. An expression for the amplitude of the generated wave is obtained and numerical analysis of its dependence on distance and resonance frequency of bubbles is performed.     

Mode-coupling phenomena in intracavity second harmonic generation

The dependence of intracavity second harmonic generation efficiency on the linear phase detunings in crystal and air is studied both theoretically and experimentally. It is shown that in the presence of feedback at the second harmonic frequency an optimum conversion efficiency can be achieved by a mutually correlated choice of phase mismatch in a nonlinear crystal and the position of the latter inside the cavity.     

锥形声源的二次谐波声场分布

本文推导了锥形声源的非线性二次谐波声场分布。理论表明锥形源二次谐波声场沿径向呈Bessel函数分布,且与传播距离无关。另一个重要结论是这种声场的二次谐波束宽刚好为基波的1/2,而非一般情况下的$1 / \\sqrt{2}$倍,此外,我们指出了这种声束在非线性参量B/A成象或测量中潜在的应用价值.     

二次谐波产生过程中准相位匹配条件的研究

从耦合波基本方程出发,在实空间严格推导出二次谐波产生(SHG)过程中的准相位匹配(QPM)条件.讨论了满足QPM条件时,SHG的转换效率以及相位失配角随波传播距离的变化.着重研究了非线性极化率的周期性调制对提高SHG转换效率的贡献.     

Optical second harmonic generation in ammonium tartrate

Second harmonic generation was observed in ammonium tartrate single crystals by a phase matching technique at the fundamental wavelength 1.153 μm. The nonlinear coefficients were found to be comparable in magnitude to those of KDP. The optical transmission and the refractive indices were also measured.     

Third harmonic generation in the field of a backward pump wave

Third harmonic generation in a nonlinear medium the refractive index of which is negative at the pump frequency and positive at the harmonic frequency with allowance for self-action effects is considered theoretically. An exact solution of equations that describe the parametric interaction of continuous waves has been obtained analytically. The behavior of the solution is analyzed depending on the degree of violation of the phase matching condition. It is shown that, in a definite domain of values of nonlinear susceptibilities of the third order, there exists a critical value of the phase detuning at which the monotonic transform of the pump into the harmonic is changed by a periodical one. This phenomenon does not occur when the third harmonic is generated in ordinary media with a positive refractive index.