Multiphoton transitions in a spin system driven by strong bichromatic field

EPR transient nutation spectroscopy is used to measure the effective field (Rabi frequency) for multiphoton transitions in a two-level spin system bichromatically driven by a transverse microwave (MW) field and a longitudinal radio-frequency (RF) field. The behavior of the effective field amplitude is examined in the case of a relatively strong MW field, when the derivation of the effective Hamiltonian cannot be reduced to first-order perturbation theory in ω₁/ω_rf(ω₁ is the microwave Rabi frequency, ω_rf is the RF frequency). Experimental results are consistently interpreted by taking into account the contributions of second and third order in ω₁/ω_rf evaluated by Krylov-Bogolyubov-Mitropolsky averaging. In the case of inhomogeneously broadened EPR line, the third-order correction modifies the nutation frequency, while the second-order correction gives rise to a change in the nutation amplitude due to a Bloch-Siegert shift.     

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.     

Suppression of inhomogeneous line broadening of Mössbauer resonance by a spinning magnetic field

A spinning magnetic field, satisfying the so-called “magic-angle” condition, is widely used for the suppression of inhomogeneous line broadening of spin transitions in nuclear magnetic resonance (NMR) spectroscopy. We show that the redefined “magic-angle” condition can also be used for the narrowing of γ-ray nuclear transitions in the Mössbauer spectroscopy.     

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.     

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.     

On the low-temperature thermal conductivity of Ni-doped Al2O3

An attempt is made to explain the lower resonance anomaly in the thermal conductivity of Ni-doped Al₂O₃ by using τ_r ～ ω^-4s(ω² ? ω²_r)². An agreement between theory and experiment needs s = 0.71, and this is interpreted to mean that the frequency dependence of impurity-phonon coupling is approximately described by ω^0.71 for phonons having frequencies in close neighbourhood of ω_r.     

Stark effect on donor transitions in high-purity GaAs

A quantitative study of line narrowing of the 1s?2p_- transitions in hydrogenic GaAs donors with increasing magnetic field shows that the observed widths are due to Stark-effect inhomogeneous broadening. From the parameters involved we conclude that internal electric fields should very often play a dominant role in determining lineshapes of shallow impurity optical transitions in high purity partly compensated semiconductors at low temperatures.     

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：     

Magnetic properties of the tetranitrosyl-iron complex Fe<Subscript>2</Subscript>(SC<Subscript>3</Subscript>H<Subscript>5</Subscript>N<Subscript>2</Subscript>)<Subscript>2</Subscript>(NO)<Subscript>4</Subscript>

The magnetic properties of the binuclear nitrosyl-iron complexes Fe₂(SC₃H₅N₂)₂(NO)₄ are investigated. It is demonstrated that several types of particles, such as dimers with a pair of spins 1/2, dimers with a pair of spins 5/2, and paramagnetic particles with spin 3/2, make a contribution to the magnetic properties of the complexes. A decrease in the temperature below 25 K leads to a change in the shape of the EPR spectra corresponding to these dimers, so that Lorentzian lines (homogeneous broadening) transform into Gaussian lines (inhomogeneous broadening). This is accompanied by a stepwise change in the EPR line width and g factors. The change in the line shape indicates that complexes become asymmetric at low temperatures, possibly, due to the decrease in the spin exchange frequency below the frequency of the microwave field of the spectrometer.     

A multifrequency EPR study of poly(PADPA) synthesized with Trametes versicolor laccase from the aniline dimer p-aminodiphenylamine (PADPA) in the presence of anionic vesicles

Poly(PADPA) synthesized with the enzyme Trametes versicolor laccase (TvL) at pH = 3.5 in aqueous solution from the aniline dimer p-aminodiphenylamine (PADPA) was studied by EPR spectrometry at X-band and W-band frequencies. For the synthesis of poly(PADPA), vesicles formed from AOT (sodium bis-(2-ethylhexyl)sulfosuccinate) were used as structure-directing templates, and TvL/O₂ as catalyst and oxidant. The isolated product is abbreviated as “poly(PADPA)-TvL-AOT” to distinguish it from poly(PADPA) obtained by other means. The EPR spectrum of poly(PADPA)-TvL-AOT recorded at room temperature is complex. It can be decomposed into two separate Dyson–type spectral components. Furthermore, the spectra measured at high frequency exhibit a line broadening behavior in comparison to the spectra recorded at low frequency where there is no line broadening. This behavior was used to estimate the effective inter-chain spin exchange interaction for each monitored spectral component. The obtained effective distances between the polymer (or oligomer) chains in both identified components of poly(PADPA)-TvL-AOT can be explained by considering slightly different interactions between the chains and the AOT molecules present in the sample. Additionally, due to different average g tensor values, g_av, of these components, it seems that each spectral component originates from one of two different molecular subunits (or local structural motifs) within the polymer (or oligomer) chains.     

The study of the electron spin exchange efficiency in liquid solutions of radicals at various EPR frequencies

The electron spin exchange efficiency in toluene solutions of stable nitroxide radicals at different EPR frequencies has been studied. The experimental results are in good agreement with the theory. The spin exchange constantsK _e measured at X-band and 2-mm band coincide. TheK _e values measured from the exchange broadening of the EPR lines do not depend on the value of the nitrogen nuclear spinI _N     

Spin forbidden transitions in the Ka = 0 subband of NO2 in the λ = 592.5 nm region

In a high resolution laser excitation spectrum of NO₂, lines were recorded which do not follow the selection rule ΔN = ΔJ = ΔF of “spin allowed” transitions. Line positions and intensities of these “spin forbidden” lines were investigated for all rotational lines up to N″ = 12 of the K_a = 0 subband around λ = 592.5 nm. While the observed line intensities of “spin allowed” transitions can be well described by the J-coupling scheme, neither the J- nor the G-coupling scheme sufficiently describes the “spin forbidden” transitions. The observations can be fitted satisfactorily by perturbation theory, in which the Fermi interaction in ²A₁ is treated as the perturber. This looks similar to a superposition of J and G scheme in the ²A₁ ground state.     

Determination of coupling constants from superconvergence sum rules in vector meson — Vector meson scattering

Dynamical relations between masses and coupling constants have been studied using the superconvergent sum rule technique in vector meson — vector meson scattering. Unessential complications due to the spin have been removed by defining a set of 25 KSF invariant amplitudes. Commonly accepted analyticity properties and asymptotics estimated arguing along the line of unitarity then lead to superconvergent sum rules for three amplitudes. Their saturation by one-intermediate-particle contributions in the processesωρ→ ωρ, ωB→ωB andωA ₁→ωA ₁ results in a system of nine coupled equations which have been approximately solved for coupling constants and aρ-ω-B- A ₁ mass sum rule.     

The oscillator frequency in resonating-group-method calculations

For the example α-¹⁶O we present simple RGM calculations with equal (ω = ω′) and different (ω ≠ ω′) oscillator frequencies for the two nuclei. In the case ω ≠ ω′ and ω → ω′ the one-channel RGM space for angular momentum zero contains all ²⁰Ne states with four particles in the s-d shell and total spatial symmetry ($\text{[?] = [4]}$). These states form a basis for low-lying excited 0⁺ states of ²⁰Ne. The ω ≠ ω′ calculation (as well for ω → ω′ as for realistic ω, ω') exhibits, therefore, resonances corresponding to these states. We present also the calculation for ω = ω′ where these physically important resonances are missing.     

Nonlinear and multiquantum effects in dual-frequency EPR experiment: the intensities of sideband resonances

Two microwave frequencies ω₁ and ω₂ simultaneously exciting a paramagnetic spin probe near the resonance condition ω₁ = γB ₀ effect a pattern of resonances occurring at multiples of the difference frequency δ = ω₂ ? ω₁. Their intensities, measured in the absorption or dispersion mode in the magnetic field-sweep experiment, decrease rapidly as the distance from the centerband increases. Numerically solved Bloch equations are used to discuss the intensities of the transverse components of magnetization up to seven, harmonics of the frequency difference δ. In conclusion, it is suggested that the experimental investigation of these patterns can be used for the purpose of a continuous-wave monitoring the relaxation rates of spin probes.     

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.     

Nonlinear resonances in the absorption spectrum of the three-level atomic V system in strong polyharmonic and weak biharmonic fields

A three-level atomic system with a strong three-mode field and a probing biharmonic field at transitions 1→2 and 1?3 (1 is a common lower level), respectively, is studied theoretically by numerical simulation and an analysis of the mathematical expressions derived for the particular case of a symmetric arrangement of the strong field components relative to the transition frequency ω₂₁. The absorption spectrum of the probing field components contains parametric supernarrow resonances against the background of resonances of the nonlinear interference effect. The ratios of the differences in the frequencies of the strong and probing fields, at which the supernarrow resonances appear, are found analytically. The results coincide with those obtained from numerical simulation.     

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.