On the three-pulse excitation of spin echo signals

Three variants of three-pulse excitation of the quadrupole spin echo are considered. The specific features are determined for each variant of the excitation.     

Dynamic holograms and phase conjugation in multilevel resonant media

The generation of dynamic holograms and four-wave phase conjugation in resonant media has been investigated under conditions of interaction between radiation and excited singlet and triplet states. Two mechanisms of optical control of resonant media non-linearities using independent pump-up to increase the diffraction efficiency of dynamic holograms have been considered. The peculiarities of non-linear recording of holograms, and a variant of quadratic recording, have been investigated. The dependence of the efficiency of diffraction by dynamic holograms on the intensity of interacting waves and the spectroscopic characteristics of the medium has been analysed.     

Resonant excitation of single-pulse nuclear spin echo in magnetically ordered media

An analytical expression for a single-pulse nuclear echo signal in magnetically ordered materials has been obtained taking into account inhomogeneous broadening of the spectroscopic transition and non-uniform distribution of the enhancement factor. The signal has been shown to result from superimposing the nuclear magnetic moment oscillations of the same amplitude and phase that arise upon termination of a radio-frequency pulse. The cause for the effective suppression of oscillations of nuclear magnetic moments in the initial phase of the free precession signal has been determined analytically. The single-pulse echo signal amplitude has been found as a function of the external variable magnetic field strength, the exciting pulse duration, the inhomogeneous NMR line broadening, and the distribution width of the enhancement factor. The results have been compared with experimental data observed in a Co₂MnSi ferromagnetic polycrystalline sample. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 3, pp. 387–394, May–June, 2009.     

Ultrafine particle formation by resonant laser excitation of potassium molecules

In a heat pipe oven containing potassium and a noble gas we have observed a large production of metallic potassium particles upon resonant laser excitation of the K₂ molecules. This is a new method for particle production, based on the strong local heating of the vapor due to the high absorption rate of the laser light. As a consequence of the disequilibrium caused at the gas/vapour boundary by this heating, a supersaturated region is created where the vapor condenses and produces ultrafine particles.     

Investigation of three-pulse photon echo in thick crystal using finite-difference time-domain method

This paper investigates the phenomenon of three-pulse photon echo in thick rare-earth ions doped crystal whose thickness is far larger than 0.002 cm which is adopted in previous works.The influence of thickness on the three-pulse photon echo's amplitude and efficiency is analyzed with the Maxwell-Bloch equations solved by finite-difference timedomain method.We demonstrate that the amplitude of three-pulse echo will increase with the increasing of thickness and the optimum thickness to generate three-pulse photon echo is 0.3 cm for Tm~(3+):YAG when the attenuation of the input pulse is taken into account.Meanwhile,we find the expression 0.09 exp(α'L),which is previously employed to describe the relationship between echo's efficiency and thickness,should be modified as 1.3 · 0.09 exp(2.4 ·α'L) with the propagation of echo considered.     

Stimulated spin echo upon excitation with pseudorandom pulses

Different regimes of excitation of a stimulated spin echo by pseudorandom pulses and short coherent delta-shaped pulses are considered. Radio pulses phase-shifted by a 127-element M sequence are used as pseudorandom signals. The shape of the complex envelope of the stimulated echo is simulated in linear and nonlinear regimes with respect to the phase-shifted pulses. It is demonstrated that the excitation pulses can be described by correlation functions. Appropriate conditions are determined under which the amplitude of the stimulated echo can be greater than the amplitude corresponding to the classical algorithm used for exciting a stimulated echo by three delta-shaped pulses. The results obtained can be used for analyzing the formation of a stimulated photon echo.     

Modified stimulated photon echo with hyperfine structure of resonant levels

The intensity and polarization of a modified stimulated photon echo generated on resonant levels with hyperfine structure are obtained. It is proposed that the polarization properties of this kind of a photon echo can be used for the identification of the structure of the resonant transitions on which the echo is formed as well as for the separation of the modified stimulated photon echo pulse from the third exciting pulse.     

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.     

Current-driven resonant excitation of magnetic vortices

A magnetic vortex core in a ferromagnetic circular nanodot has a resonance frequency originating from the confinement of the vortex core. By the micromagnetic simulation including the spin-transfer torque, we show that the vortex core can be resonantly excited by an ac (spin-polarized) current through the dot and that the resonance frequency can be tuned by the dot shape. The resistance measurement under the ac current successfully detects the resonance at the frequency consistent with the simulation.     

一类约瑟夫森结混沌系统的谐和共振控制

引入外激和参激两种不同形式的谐和共振激励,探讨了一类约瑟夫森结(Josephson junction)系统的混沌控制问题.利用Melnikov方法研究了异宿混沌的生成和抑制,得到了在一定的控制激励振幅范围内,能确保异宿混沌被控制住,而且推导出控制激励与系统的激励两者之间的相位差和两者频率之间的共振阶数应满足的关系式.从定性的角度说明相位差在异宿混沌的控制中确实有着至关重要的影响,而且,数值方法的研究表明可通过调节相位来控制非自治系统中的稳态混沌.通过分析、比较外激和参激两种不同的共振激励对约瑟夫森结系统的异宿混沌的控制效果,得到对于较小的共振频率,宜采用参激激励,而对于较大的共振频率,宜采用外激激励. 关键词： 混沌控制 谐和共振激励 相位控制 Melnikov方法     

Electron–hole liquid formation by exciton and biexciton resonant excitation in CuCl

The dynamics of a cold photoexcited electron–hole system created by resonant excitation with picosecond optical pulses are studied by time-resolved emission and pump–probe measurements. A rapid build-up of strong metallic reflection in the mid-infrared region was observed, which indicates the creation of electron–hole plasma via the exciton or biexciton Mott transition. Transient reflection spectra clearly show the transformation of high-density electron–hole plasma into a metallic colloid-like state within 10 ps. In the meantime, broad plasma emission turns to a narrow emission band, in which the spectral profile does not change within several tens of picoseconds. These features in the emission spectra disappear when switched to the band-to-band excitation. An analysis of the reflection and emission spectra reveals that the density of metastable electron–hole liquid is as high as 10²⁰ cm⁻³. This indicates that the plasma formation via the Mott transition is crucial to eliminate excess heating for reaching the low-temperature states of an electron–hole ensemble.     

Polarization properties of thick anisotropic diffraction holograms recorded with infrared-polymerizable media

The polarization properties of thick anisotropic holographic gratings are studied theoretically and experimentally. The dependences of the diffraction efficiency, ellipticity, and polarization orientation of the diffraction beam on the azimuth angle of polarization of an incident Bragg beam are derived. The experimental data are shown to agree well with the analytical calculations. It is found that the diffraction characteristics can be controlled in a wide range of incident polarization.     

Stochastic excitation of a stimulated spin echo

The excitation of a stimulated spin echo by two coherent pulses of Gaussian white noise and a δ-function-like radio pulse is studied. The mean value of the complex envelope of the echo is obtained by solving the Bloch equations. The behavior of the mean value of the envelope of the stimulated echo is investigated for a Lorentzian shape of the inhomogeneously broadened absorption line. It is shown that in an excitation regime which is linear for noise pulses the envelope corresponds to the Fourier transform of the line-shape function. Nonlinear distortions of the mean value of the envelope of a stimulated echo can arise outside the linear excitation regime. Zh. Tekh. Fiz. 67, 100–104 (October 1997)     

Parametric excitation of two internally resonant oscillators

The response of two-degree-of-freedom systems with quadratic non-linearities to a principal parametric resonance in the presence of two-to-one internal resonances is investigated. The method of multiple scales is used to construct a first-order uniform expansion yielding four first-order non-linear ordinary differential (averaged) equations governing the modulation of the amplitudes and the phases of the two modes. These equations are used to determine steady state responses and their stability. When the higher mode is excited by a principal parametric resonance, the non-trivial steady state value of its amplitude is a constant that is independent of the excitation amplitude, whereas the amplitude of the lower mode, which is indirectly excited through the internal resonance, increases with the amplitude of the excitation. However, in addition to Poincaré-type bifurcations, this response exhibits a Hopf bifurcation leading to amplitude- and phase-modulated motions. When the lower mode is excited by a principal parametric resonance, the averaged equations exhibit both Poincaré and Hopf bifurcations. In some intervals of the parameters, the periodic solutions of the averaged equations, in the latter case, experience period-doubling bifurcations, leading to chaos.     

Coherent population control and three-pulse photon echoes: Their dependence on phase of excitation pulse

We discuss analytically here coherent population control and photon echoes of two-level atoms excited by two excitation pulses of different characteristics: (i) the excitation pulses with the same carrier-envelope phases and (ii) the excitation pulses, which are obtained by intensity-modulation of a continuous-wave laser beam and are free from incidental phase- and frequency-modulation. A perturbative calculation, which includes the effect of relaxations during pulse excitations shows that both the coherently controlled population and the echo field show considerably different behaviors depending on the characteristics of the excitation pulses. We also find that the peak position of the echo field can be controlled by adjusting the pulse widths of the excitation pulses.