Two-atom two-photon resonance fluorescence

The excitation spectrum arising from the interaction between two identical two-level atoms, one of which is excited in the presence of a strong electromagnetic field (pump field), is investigated when the atomic transition frequency is nearly twice the frequency of the pump field. The excitation spectrum consists of those describing the symmetric and antisymmetric modes, respectively. The spectral functions for the symmetric and antisymmetric modes are derived and discussed in detail. The possibility to measure directly the magnitude of the dipole-dipole interaction energy is discussed.     

One- and two-photon resonance fluorescence of a three-level atom at high photon densities

We consider the possible physical processes that may arise in a three-level atom when only two of its levels interact with a strong electromagnetic field and when the atomic transition frequency is nearly equal to once and twice the frequency of the laser field, respectively. There have been found pronounced cooperative effects in the spectrum of the two-level system, which is in resonance with the laser field, arising from the presence of the third level. The excitation spectra describing the transitions from the first excited state into the second excited state and from that to the ground state consist, apart from the two central peaks, of two pairs of sidebands which are induced by the laser field of the neighbouring system. Detailed expressions of the spectral functions for the physical processes of one- and two-photon resonance fluorescence have been derived and discussed in the limit of high photon densities. The excitation spectrum of the low frequency modes has been considered and discussed in detail. It is found that quantum beats in spontaneous emission may appear in the spectra of the one- and two-photon resonance fluorescence arising from the interference between the two atomic transition frequencies and the frequency of the laser field. The importance of the low frequency modes that occur in the processes in question has been pointed out.     

[Russian Text Ignored]

A nonlinear theory of the interaction of an external pump field with ion-acoustic and h.f. surface waves in a plasma layer with inhomogeneous boundary regions is presented for the case, when the pump field frequency is less than the electron plasma frequency. The evolution of the parametrically excited turbulence and its stationary state are both analytically treated and numerically demonstrated. Dependent on the pump field strength, two mechanisms are to be distinguished: (i) nonlinear generation of damped ion-acoustic harmonics, (ii) cascade excitation of secondary eigenmodes by growing surface oscillations. At only slight overthreshold values, mechanism (i) is responsible for the occurence of the steady state, whereas, for higher pump field strength, the wave amplitudes as well as their saturated levels are mainly determined by mechanism (ii).     

Quantum beats in two-photon resonance fluorescence

The two-photon resonance fluorescence spectrum of a three-level atom is shown to consist of the low frequency modes in addition to the high frequency ones in the limit of high photon densities. The spectral function for the low frequency modes consists of two lorentzian lines describing: the peak occuring at the renormalized beat frequency Δ₊ and that of the zero-photon excitation at the frequency Δ_-, where Δ_±=Δ-3Ω²/2ω_a±Ω²u/2ω_a, u²=1+(2Δω_a/Ω ²)². Here, 2Δ is the energy splitting between the two excited states, ω_a is the photon energy of the pump field and Ω is the Rabi frequency. The peak at the renormalized beat frequency Δ₊ occurs provided that the condition (2Δω_a/Ω²)² > 1 is satisfied. The two-photon laser spectroscopy is expected to be a useful tool for the observation of the low frequency modes in question.     

Three-wave and four-wave forward mixing in photorefractive materials

An amplification mechanism relying mainly on the real part of the arising space charge field in the presence of two strong pump beams is described. The dependence of amplification on frequency detuning, applied electric field and pump beam ratio are investigated in the undepleted beam approximation.     

Resonance fluorescence spectrum of a three-level atom. II. Numerical results

Numerical computations are presented for the excitation spectra arising from the interaction between a three-level atom in the cascade configuration and a strong electromagnetic field whose frequency mode is initially populated. The excitation spectra are considered when the laser field is at resonance with the equally spaced levels of the atom as well as a function of the detunings. The physical process of optical amplification occurs without population inversion and it is more pronounced when the laser field is detuned than when it is at resonance. The shapes of the spectral lines for a number of side-bands are of the absorption-amplification type rather than that of the absorption one. In the presence of detunings as well as in the cooperative two-photon cascade process, the resulting spectra are far more complicated than those occuring at resonance. Results of numerical calculations for a wide range of Rabi frequencies and detunings are presented graphically.     

[Russian Text Ignored]

The stability of the electromagnetic plasma confinement by powerful external s-polarized pump waves is considered. The parametric excitation of standing electromagnetic waves along the plasma boundary with frequencies close to the frequency of the pump wave leads to a periodic density modulation of the plasma boundary. The density disturbances along the direction of the external wave field are connected to the excitation of transverse p-polarized surface waves while the modulation in the direction perpendicular to the pump field are created by the parametric interaction between the external wave and s-polarized trapped leaking oscillations. Only when the leaking waves are excited the scale length of the modulation is larger than half the free space wave length of the incident radiation.     

Balance-equation approach to terahertz-field-driven magnetotransport in semiconductors

We investigate the magnetotransport in semiconductors under the influence of a dc or slowly-varying electric field, an intense polarized radiation field of terahertz frequency, and a uniform magnetic field, being in arbitrary directions and having arbitrary strengths. Effective force- and energy-balance equations are derived by using a gauge that the magnetic field and the high-frequency radiation field are described by a vector potential and the dc or slowly-varying field by a scalar potential, and by distinguishing the slowly-varying velocity from the rapidly-oscillating velocity related to the high-frequency field. These equations, which include the elastic photon process and all orders of multiphoton absorption and emission processes, are applied to the examination of the effect of a terahertz radiation on the magnetophonon resonance of the longitudinal resistivity in the transverse configuration in nonpolar and polar semiconductors. We find that the previous zero-photon resonance peaks are suppressed by the irradiation of the terahertz field, while many new peaks, which may be related to multiphoton absorption and emission processes, emerge and can become quite distinct, at moderately strong radiation field. Received 17 May 1999     

Choice of soft pulse shapes for signal excitation in chemical shift selective imaging

The choice of soft pulse shapes for chemical shift selective excitation in chemical shift imaging is discussed. In the presence of inhomogeneities in the static magnetic field resulting from susceptibility anomalies, it is important to optimise pulse bandshape and frequency offset as well as bandwidth, in order to minimize artefacts arising from excitation of unwanted resonances. A comparison of the use of Gaussian and sinc shaped excitation pulses in the chemical shift micro-imaging of grapes serves to illustrate some of the effects that may be observed.     

Effect of a periodic magnetic field on waves in a cold plasma

The effect of a spatially-uniform periodic magnetic field on waves propagating perpendicular to it in a cold plasma is considered. Assuming that the pump field is weak, the method of multiple time scales is used to treat the problem. The results show that, unlike the pump electric field, the pump magnetic field does not produce any parametric excitation of the transverse waves, no matter what the value of the frequency mismatch is.     

Spin-flip Raman laser

The properties of the spin-flip Raman laser (SFR laser) which depend on stimulated Raman scattering from mobile conduction electrons in InSb under an external magnetic field are presented. The essential parameters are derived from a macroscopic treatment of the stimulated Raman effect and the microscopic theory of the scattering cross-section, and are compared with experimental results. Output pulse powers as large as 1 kW have been measured for 10.6 and 5.3 μm excitation radiation and continuous powers of 1 W for continuous excitation with a 5.3 μm pump source. The SFR laser offers some interesting applications in physics and chemistry, since its frequency is proportional to the applied magnetic field and its linewidth can be made smaller than 1 kHz.     

Decay instability of lower hybrid wave in a cold homogeneous plasma

We have pointed out the possibility of parametric excitation of a low frequency wave, propagating almost perpendicular to the magnetic field in a cold homogeneous plasma, by a lower hybrid pump wave. The growth rate and threshold power are calculated.     

Terahertz radiation of Bloch oscillators excited by an electromagnetic field in lateral semiconductor superlattices

The action of a strong high-frequency electromagnetic field on a lateral semiconductor superlattice is considered based on the quasi-classical electron transport theory in the self-consistent wave formulation. The theory predicts that a lateral superlattice can emit terahertz radiation wave trains, which are associated with periodic excitation of Bloch oscillations in the superlattice arising because of the development of transient processes in it in a variable self-consistent electric field. The conditions necessary for observing Bloch oscillator radiation were found. The spectral composition of radiation transmitted through the superlattice and the energy efficiency of frequency multiplication related to Bloch oscillator excitation were calculated.     

On a Destabilizing Effect of Plasma Inhomogeneity on the Two-Plasmon Decay Instability

It has been shown that plasma inhomogeneity reduces the threshold of the parametric instability of two-plasmon decay and, in particular, allows the excitation of plasmons with the same frequency by a spatially uniform pump field.     

铷蒸气Λ-型系统中的受激Raman谱与光泵效应

从理论和实验上研究了⁸⁵Rb 原子蒸气D1线系统中的受激Raman现象. 在Λ-型能级系统中, 抽运光对弱探测光的影响有两部分: 受激Raman激发和光泵效应. 在一定的抽运光强度和失谐量条件下, 受激Raman谱表现为增益谱或损耗谱; 以光泵效应导致的吸收谱为背景时, 便分别形成了窄线宽透明窗口或线宽差异很大的吸收双线. 理论模拟和实验结果基本符合. 基于这些研究, 从新的角度阐述了在Λ-型系统中的电磁感应透明和Autler-Townes分裂这两个重要现象与受激Raman谱之间的关系. 关键词： 受激Raman谱 光泵效应 电磁感应透明 吸收双线     

All-optical modulation at 1.3 μm wavelength in the inGaAsP/InP system

All-optical, normal-to-surface modulation in InGaAsP epitaxial layers, lattice matched to InP, is investigated. Close to the gap wavelength of 1.3 m a transmission increase under optical excitation is observed. A modulation depth of 34% is achieved for 0.8 mW pump power at 790 nm wavelength. The modulation frequency is limited by excess carrier lifetime. A 3 dB frequency of 80 MHz is achieved with a 10 dB decrease at 400 MHz. A lateral electric field enlarges the bandwidth but decreases the modulation depth. For weak excitation the experiments are well described in terms of direct electronic transitions between parabolic bands or in terms of simple band filling. The devices are well suited for parallel optical data processing.     

Comparison of SRS thresholds under pumping by Gaussian and Bessel beams

In the paraxial approximation, it has been shown that the quasi-stationary SRS amplification coefficient in the near-threshold region is determined by the product of the pump intensity in the beam center and the overlap integral of the transverse structure of the pump and the Stokes component. Expressions for the overlap integral and the scattering powers have been obtained and analyzed for the following cases: the pump and the Stokes field are Gaussian beams; the pump and the Stokes field are Bessel beams; the pump is a Bessel beam and the Stokes field is an axial Gaussian beam. Relations between the SRS excitation thresholds in different cases have been found.Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 71, No. 6, pp. 778–787, November–December, 2004.This revised version was published online in April 2005 with a corrected cover date.     

Cooperative two-photon resonance fluorescence of a three-level atom

We have calculated the Green function describing the physical process where a three level atom interacts with a strong pump field as well as with a weak perturbing signal field in the limit of high photon densities. The theory is then used to describe the cooperative two-photon resonance fluorescence which occurs when the sum of the two atomic transition frequencies is nearly twice the frequency of the pump field. Our atomic system is equivalent to a two-level atom when the degeneracy of the ground state is removed by a Stark field. The excitation spectrum has been found to consist of new bands and sidebands which exist only in the presence of the Stark field. The resonance process which occurs when the Stark splitting is in the neighbourhood of the energy shift induced by the laser field has been discussed in detail.     

Transient saturation absorption spectroscopy excited near the band gap at high excitation carrier density in GaAs

Transient saturation absorption spectroscopy in GaAs thin films was investigated using femtosecond pump and supercontinuum probe technique at excitation densities higher than 1×10^{19}cm^{-3}. The Coulomb enhancement factor of the electron－hole plasma results in a spectrum hole at the pump wavelength. Two distinct transmission peaks at two sides of the pump wavelength are observed, arising from the bleaching of transitions from the heavy- and light-hole bands to the conduction band. The dynamic process of the transient saturation absorption is fitted using a bi-exponential function. The fast decay process is dominated by the carrier-phonon scattering and the slow process may be attributed to the electron－hole recombination.     

Displacement power spectrum measurement by CPMG in constant gradient

The modulation of spin phase produced by Carr-Purcell-Meiboom-Gill (CPMG) sequence in combination with constant magnetic field gradient is appropriate to probe the displacement power spectrum (DPS). The spin-echo attenuation is directly proportional to the DPS value at the applied modulation frequency. Relaxation and selective excitation effects can be factored out while probing the DPS. The modulation frequency is adjusted by varying the pulse separation time while the gradient strength and the time of acquisition are kept constant. In designing the experiment gradient strength limitations, imposed by off-resonance effects, as well as limitations arising from using Gaussian phase approximation must be considered. An effective experimental strategy is presented, supported by experimental results for free and restricted diffusion.