Influence of the sum frequency on optically pumped parametric amplifiers

Parametric traveling-wave interactions are calculated with the aid of a plane wave approximation, considering the 4 frequencies ω _s , ω _p , ω _I =ω _p ?ω _s and ω _Σ =ω _p +ω _s . Special attention is paid to the case where ω _p ?ω _s . Competition between parametric amplification and upconversion is studied as a function of phase matching and the results are illustrated by means of numerical examples. It is shown that parametric gain disappears if the linear combination of wave vectors, 2k _p-k_I-k_Σ, vanishes. In this situation upconversion with power gain up to about (ω _Σ /ω _s )² is possible. It is concluded that fork _p?k_s the sum frequency ω _Σ can significantly influence parametric forward amplification but does not affect backward wave amplification.     

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.     

Double resonance interference in third-order light mixing

The first observation of interference effects between two sharp resonant contributions to the third-order nonlinear susceptibility χ₍₃₎ (ω₃=ω₀+ω₁?ω₂) is reported. Using two tunable near ir frequencies, ω₀=ω₁ and ω₂, the was seen in crystalline CuCl when 2ω₁ and ω₁?ω₂ were close to the sharp Z₃ exciton at 3.21 eV and the 210 cm^-1 Raman line, respectively. Three different tunable visible frequencies were used to observe similar interference effects in a benzene-cyclohexane mixture when ω₀?ω₂ and ω₁?ω₂ were close to the 992 cm^-1 Raman line of benzene and the 801 cm^-1 Raman line of cyclohexane, respectively.     

Four-wave mixing and hyper-Raman scattering in CuCl

Non-degenerate four-wave mixing using two non-collinear laser beams with frequencies (wavevectors) ω_p, ω_t (k_p, k_t) respectively is studied in CuCl. Two emission lines at frequencies ω⁽¹⁾=2ω_t-ω_p, and ω⁽²⁾=2ω_p-ω_t are observed. Their excitation spectrum is sharply peaked if the phase-match condition k⁽¹⁾=2k_t-k_p is fulfilled. This is the case, if ω_p coincides with the hyper-Raman lines (R⁺_T, R^-_T) of the laser labelled (t) in a well-defined geometrical configuration.     

Singular Resonance in Fluctuation-Induced Electromagnetic Phenomena at the Rotation of a Nanoparticle near the Surface of a Condensed Medium

It has been shown that the rotation of a spherical nanoparticle with the radius R near the surface of a semi-infinite homogeneous medium can result in singular resonance in fluctuation-induced electromagnetic phenomena (Casimir force, Casimir friction, and radiative heat generation). Fluctuation electromagnetic effects increase strongly near this resonance even in the presence of dissipation in the system. The resonance occurs at distances of the particle from the surface d < d₀R(3/4ε″₁(ω₁)ε″₂ (ω₂))^1/3 (where ε″_i(ω_i) is the imaginary part of the dielectric function of the particle or the medium at the frequency of a surface phonon or plasmon polariton ω_i), when the rotation frequency coincides with poles in the photon generation rate at Ω ≈ ω₁ + ω₂. These poles are due to the multiple scattering of electromagnetic waves between the particle and surface under conditions of the anomalous Doppler effect. These poles exist even in the presence of dissipation. At d < d₀, depending on the particle rotation frequency, the Casimir force can change sign; i.e., the attraction of the particle to the surface changes to repulsion. The results can be important for the development of experimental methods for the detection of quantum friction.     

Theory of transient two-photon resonant up-conversion with non-synchronized excitation

The theory of the two-photon resonant up-conversion process (ω_a + ω_a) + ω_c ? ω_d is developed for short pulse excitation. Prominence is given to the case where the pulse at frequency ω_c is not time-synchronized with the pulse ω_a but travels immediately behind it. Not only is the conversion efficiency thereby maximized, but the associated theoretical simplification enables several analytical formulae to be derived which provide new insight into the mechanism of resonant non- linear interactions. In particular, an expression is derived for the maximum useful energy per unit area of the pulse at ω_c. Exceeding this value is of little or no advantage in the non-synchronized case and is positively detrimental when the a- and c-pulses are coincident. The results are supported by computer 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.     

Two-dimensional wigner crystal under magnetic field

Dispersion relations of phonons are obtained in the harmonic approximation for the two-dimensional Wigner crystal under the magnetic field. For small wave vector, the eigen-energies of two modes are ω₊ = ω_c = eH/mc and ω_? → ω_lω_t/ω_c, where ω_l and ω_t are frequencies of longitudinal and transverse phonons in the absence of the field.     

Effective field and the Bloch-Siegert shift at bichromatic excitation of multiphoton EPR

The dynamics of multiphoton transitions in a two-level spin system excited by transverse microwave and longitudinal RF fields with the frequencies ω_mw and ω_rf, respectively, is analyzed. The effective time-independent Hamiltonian describing the “dressed” spin states of the “spin + bichromatic field” system is obtained by using the Krylov-Bogoliubov-Mitropolsky averaging method. The direct detection of the time behavior of the spin system by the method of nonstationary nutations makes it possible to identify the multiphoton transitions for resonances ω₀ = ω_mw + rω_rf (ω₀ is the central frequency of the EPR line, r = 1, 2), to measure the amplitudes of the effective fields of these transitions, and to determine the features generated by the inhomogeneous broadening of the EPR line. It is shown that the Bloch-Siegert shifts for multiphoton resonances at the inhomogeneous broadening of spectral lines reduce only the nutation amplitude but do not change their frequencies.     

The turbulence of capillary waves on the surface of liquid hydrogen

It is experimentally demonstrated that the surface excitation of liquid hydrogen at a low frequency results in the turbulent mode in a system of capillary waves. The experimental results are in good agreement with the theory of weak wave turbulence. The pair correlation function of the surface deviations is described by the exponential function ω^m. The exponent m decreases in magnitude from m=?3.7±0.3 to ?2.8±0.2 when the pumping at a single resonant frequency changes to broadband noise excitation. Measurements are made of the dependence of the boundary frequency ω_b of the upper edge of the inertial range in which the Kolmogorov spectrum is formed on the wave amplitude η_p at the pumping frequency. It is demonstrated that the obtained data are well described by a function of the form ω_b∝η _p ^4/3 ω _p ^23/9 .     

Time-resolved nonlinear surface plasmon optics

A new surface-sensitive method of time-resolved optical studies is proposed. The method consists in the independent excitation of several surface electromagnetic waves (SEW) by two laser femtosecond pulse beams with varied time delay Δτ and distance Δr between corresponding excitation regions on the surface. To fulfill the phase-matching condition for plasmon-photon coupling, metal grating is used. Due to nonlinear plasmon interaction, the optical radiation with ω₁ + ω₂ and 2ω₁ ? ω₂ (where ω₁, ω₂ are corresponding laser beam frequencies) is generated. The intensity of this nonlinear response versus Δτ and Δr are studied. The direct measurements of the SEW temporal properties are presented. Experiments of this type are important for the development of femtosecond surface plasmon optics.     

On the theory of Boson peak in glasses

A universal mechanism of the Boson peak formation in glasses is proposed. The mechanism is based on the concept of interacting quasilocal oscillators. Even in the case of weak interaction, the low-frequency spectrum becomes unstable. Due to anharmonicity, the system undergoes a transition into a new stable configuration. As a result, below some characteristic frequency ω_c, proportional to the typical strength of interaction, the renormalized density of states becomes a universal function of ω with a Boson peak feature; i.e., the reduced density of states g(ω)/ω² has a maximum at a frequency ω_b?ω_c. We derive an analytic form of this function.     

固态核磁共振半整数四极体系中心跃迁对梳状脉冲的响应

半整数四极核的中心跃迁对梳状脉冲序列的响应随四极常数ω_q而变化。在梳伏脉冲的平均射频汤强度ω_e(以弧度·秒^-1为单位)满足ω_q/ω_e<1/5时,测量中心线所得的谱,位于频率ω_e/2π处;而当ω_q/ω_e>2时,所得的谱线在2ω_e/2π处,理论计算与实验结果是一致的。本文得到的结论表明,梳状脉冲序列将可能成为研究这 关键词：     

What is measured in a time-resolved stimulated Raman experiment?

We have performed four-beam time-resolved stimulated Raman measurements on liquid CS₂. To interpret our results we have applied the response formalism for the third-order polarization to such an experiment. It turns out that the measured quantity is proportional to (a convolution of) Abs[χ⁽³⁾(-ω_s, ω_s, ω_l, -ω_l)]. As a consequence in this four-beam e xperiment one picks up electronic contributions in contrast to a two-beam experiment where Im[χ⁽³⁾] is observed.     

Enhancement of low energy phonon structure by high energy spectral weight

We have investigated the effect on the tunneling characteristics of adding weight to the electron phonon spectral density α²((ω)F(ω) at an energy ω_E well above its cut off ω_D. This not only adds structure in the current (I) as a function of voltage (V) at ω_E but more importantly enhances the structure below ω_D although no addition to the strength of the spectral density is made in that region.     

Spin resonant optical mixing in InSb — CW-measurements and transient effects

Optical four-wave mixing due to the resonant spin nonlinearity in n-InSb was observed in magnetic fields up to 1.4 T. The mixing of two CO-laser frequencies ω₁ and ω₂ = ω₁ - Δω yields radiation with frequencies ω₁ + 2Δω, ω₁ + Δω, ω₁ - 2Δω, and ω₁ - 3Δω. We found the resonant contribution to the third order nonlinear susceptibility arising from the spin-flip interaction to increase by more than two orders of magnitude when ω₁ is varied from 1760 cm^-1 to 1896 cm^-1. With the lasers working simultaneously Q-switched we observed radiation at ω₁ + Δω and ω₁ - 2Δω due to the mixing caused by the nonparabolicity of the conduction electrons even without an external magnetic field. At the resonant magnetic field the intensity of the mixed radiation is increased by more than one order of magnitude and is also observed when the laser pulses do not hit the sample simultaneously as long as the time difference between the two pulses does not exceed 450 ns. This points to a spin dephasing time T₂ of about 100 ns.     

激光拍频波加速器的参数选择

本文研究了激光拍频波加速器中的三波相互作用过程。结果表明:适当地选择参数σ=ω_p/(ω₁-ω₂)以及β=ω₂/ω₁(ω_p,ω₁,ω₂分别为等离子体频率及两束激光频率),即可用较弱的激光造成很强的拍频等离子体波,并使注入等离子体的电子获得显著的能量增益。 关键词：     

Feldgleichungen der TREDERschen Gravitationstheorie,die aus einem Variationsprinzip ableitbar sind. I

In the frame work of TREDER 's gravitational theory we consider two classes of field equations which are derivable from two classes of LAGRANGE ian densities Ω⁽¹⁾(ω₁, ω₂), Ω⁽²⁾(s?₁, s?₂). ω₁, ω₂; s?₁, s?₂ are parameters. Ω⁽²⁾(ω₁, ω₂) gives us field equations which are up to the post-NEWTON ian approximation in the sense of NORDTVEDT , THORNE and WILL equivalent to the field equations given by BRANS and DICKE . For ω₂ = ?1 ?2ω₁ field equations follow from Ω⁽¹⁾(ω₁, ?1 ?2ω₁) which are in the above mentioned sense of post-NEWTON ian approximation equivalent to EINSTEIN 's equations. The field equations following from Ω⁽¹⁾(ω₁, ω₂) have a cosmological model with the well known cosmological singularities for T → ± ∞ in case that ω₁/(1 +3ω₁ +ω₂) ? γ > 0. For ω₁/(1 +3ω₁ +ω₂) ≤ 0 cosmological models with no cosmological singularities exist. From Ω⁽²⁾(s?₁, s?₂) we obtain field equations which at the best give us perihelion rotation 7% above EINSTEIN 's value and light deflection 7% below the corresponding EINSTEIN 's value. But in that case we are able to show the existence of a cosmological model without any cosmological singularity.     

Effect of the Bloch-Siegert shift on the frequency responses of Rabi oscillations in the case of nutation resonance

The dynamics of a two-level spin system dressed by bichromatic radiation is studied under the conditions of double resonance when the frequency of one (microwave) field is equal to the Larmor frequency of the spin system and the frequency of the other (radio-frequency) field ω_rf is close to the Rabi frequency ω₁ in a microwave field. It is shown theoretically that Rabi oscillations between dressed-spin states with the frequency ? are accompanied by higher-frequency oscillations at frequencies nω_rf and nω_rf ± ?, where n = 1, 2,.... The most intense among these are the signals corresponding to n = 1. The counter-rotating (antiresonance) components of the RF field give rise to a shift of the dressed-state energy, i.e., to a frequency shift similar to the Bloch-Siegert shift. In particular, this shift is manifested as the dependence of the Rabi-oscillation frequency ? on the sign of the detuning ω₁ ? ω_rf from resonance. In the case of double resonance, the oscillation amplitude is asymmetric; i.e., the amplitude at the sum frequency ω_rf + ? increases, while the amplitude at the difference frequency ω_rf ? ? decreases. The predicted effects are confirmed by observations of the nutation signals of the electron paramagnetic resonance (EPR) of E′₁ centers in quartz and should be taken into account to realize qubits with a low Rabi frequency in solids.     

Double resonance in the system of coupled Josephson junctions

The effect of LC shunting on the phase dynamics of coupled Josephson junctions has been examined. It has been shown that additional (rc) branches appear in the current-voltage characteristics of the junctions when the Josephson frequency ω_J is equal to the natural frequency of the formed resonance circuit ω_rc. The effect of the parameters of the system on its characteristics has been studied. Double resonance has been revealed in the system at ω_J = ω_rc = 2ω_LPW, where ω_LPW is the frequency of a longitudinal plasma wave appearing under the parametric-resonance conditions. In this case, electric charge appears in superconducting layers in the interval of the bias current corresponding to the rc branch. The charge magnitude is determined by the accuracy with which the double resonance condition is satisfied. The possibility of the experimental implementation of the effects under study has been estimated.