Two-photon nutation excited in a two-level spin system by microwave and RF fields

Two-photon transient nutation is observed in a two-level spin system (E′₁ centers in crystalline quartz) using a transverse microwave field and a linearly polarized rf field oriented along a static magnetic field in the electron paramagnetic resonance. Nutation is excited when the sum of the energies of a microwave photon and a rf photon is equal to the energy difference between two spin states. The two-photon nature of nutation is confirmed by measuring its frequency as a function of the amplitude and frequency of the rf field as well as the amplitude of the microwave field. The amplitude of the effective field of two-photon transitions is measured. It is shown that the decay rate of two-photon nutation is close to the decay rate for one-photon nutation and is determined by the spin-spin interaction between E′₁ centers.     

Inducing disallowed two-atom transitions with temporally entangled photons

Two uncoupled two-level atoms cannot be jointly excited by classical light under general circumstances, due to destructive interference of excitation pathways in two-photon absorption. However, with temporally entangled light, two-atom excitation is shown possible. Photons arising from three-level cascade decay are intrinsically ordered in time of emission. This field correlation induces a joint resonance in the two-atom excitation probability via suppression of one of the time-ordered excitation pathways. The relative gain in two-photon absorption increases with the time-frequency entanglement.     

Limits for two-photon ande + e − decay widths of positron-electron scattering resonances for√s=1.78 to 1.92 MeV

Motivated by the observation of energy- and momentum-correlatede ⁺ e ^? pairs in heavy ion collisions, a search for resonances ine ⁺ e ^? scattering at the corresponding energies has been made. Thee ⁺ e ^? decay channel is analyzed in a time window from ～ 10^?13 s to ～ 10^?11 s with a set-up optimized for low-background detection of delayede ⁺ e ^? decays. The two-photon decay channel of a hypothetical resonance is investigated by measuring the two-photon annihilation-in-flight excitation function. New upper limits for the partiale ⁺ e ^? decay width Γ_ee of a few meV are derived for total centre-of-mass energies √s between 1.78 and 1.92 MeV, taking into account the dilepton as well as the two-photon decay of a neutral resonance.     

Charmonium spectroscopy and decay at CLEO-c

We report recent results on charmonium spectroscopy and decay from the CLEO-c experiment at the Cornell electron-positron storage ring accelerator,CESR.Most of the results are based on the analysis of 54 pb-1 of luminosity collected at the ψ(2S) resonance,corresponding to 27 M ψ(2S) decays.We concentrate on radiative decays of ψ(2S) and J/ψ,on two-body mesonic decay of χcJ,on hadronic decay of the hc,and on higher multipoles in the two-photon cascade ψ(2S)→γχcJ,χcJ→γJ/ψ.     

Two-photon cooperative decay in a cavity in the presence of a thermalized electromagnetic field

We study the cooperative two-photon spontaneous decay of an excited atomic system in a microcavity whose size is of the order of several wavelengths of atomic radiation. We show that a thermalized electromagnetic field in the microcavity strongly affects the two-photon cooperative spontaneous emission of radiation. The increase in the rate of spontaneous cooperative decay is due to the presence of a small number of thermalized photons in a microcavity mode. At low temperatures, the two-photon absorption probability is found to be a linear function of the two-photon flux, and photon superbunching is observed. Zh. éksp. Teor. Fiz. 115, 1153–1167 (April 1999)     

Stationary inversion of a two level system coupled to an off-resonant cavity with strong dissipation

We present an off-resonant excitation scheme that realizes pronounced stationary inversion in a two level system. The created inversion exploits a cavity-assisted two-photon resonance to enhance the multiphoton regime of nonlinear cavity QED and survives even in a semiconductor environment, where the cavity decay rate is comparable to the cavity-dot coupling rate. Exciton populations of greater than 0.75 are obtained in the presence of realistic decays and pure dephasing. Quantum trajectory simulations and master equation calculations help elucidate the underlying physics and delineate the limitations of a simplified rate equation model. Experimental signatures of inversion and multiphoton cavity QED are predicted in the fluorescence intensity and second-order correlation function measured as a function of drive power.     

Coherent decay of positronium bose condensate

The rate of self-stimulated emission of photon pairs by pseudoscalar particles from Bose condensate is calculated. Growing with density, this rate exceeds the density-independent rate of spontaneous two-photon decay at plausible density values of positronium gas, thus opening, in principle, the way to the annihilation gamma ray laser realization.     

Inhibition of two-photon absorption due to dipole-dipole interaction in nanoparticles

We have investigated the inhibition of two-photon absorption in photonic crystals doped with an ensemble of four-level nanoparticles. The particles are interacting with one another by the dipole-dipole interaction. Dipoles in nanoparticles are induced by a selected transition. Numerical simulations have been performed for an isotropic photonic crystal. Interesting phenomena have been predicted such as the inhibition of the two-photon absorption due to the dipole-dipole interaction. It has also been found that the inhibition effect can be switched on and off by tuning a decay resonance energy within the energy band of the crystal. A theory of dressed states has been used to explain the results.     

Simulations of the dynamics of optical bistability with fluctuations

Numerical integrations of the Langevin-equation for the fluctuating light-field in a nonlinear resonator are presented. As an xample, the nonlinearity of the two-photon resonance of the excitonic molecule in semiconductors is treated. Time-dependent switching processes and the response of the system to nonstationary incident fields are investigated with the technique of molecular dynamics simulations. Using many independent realizations of the stochastic process, the time-dependence of the field dostribution function is determined. Furthermore, it is shown that the decay of metastable states can be described by simple exponential decay rates.     

Pico-second transient luminescence of resonantly excited excitonic molecules in CuCl

Time-resolved luminescence spectra of excitonic molecules in CuCl created by the two-photon resonance excitation are studied at 4.2 K with a time resolution of ～ 60 psec. Two sharp emission lines observed are ascribed to the radiative decay of “cold” excitonic molecules with k ～ 2K₀. Stimulated emission process gives a considerable influence on luminescence properties.     

Simple cases of spontaneous emission from an atom-photon cluster localized in a microcavity

The spontaneous emission from an atomic ensemble localized in a microcavity with the participation of microcavity photons and an external broadband quantized electromagnetic field at the Raman resonance of photons with an optically forbidden (two-photon) atomic transition has been studied. The average spontaneous decay intensity has been calculated for simple cases. It is shown that the dynamics of spontaneous emission from this atomic ensemble differs generally from the conventional superradiance (spontaneous emission of an atomic ensemble at a one-photon optically allowed transition from excited to the ground state. When the atomic ensemble is strongly excited, the delay times and the emission pulse shape differ significantly. The parameter ranges where the spontaneous emission from the atomic ensemble under consideration at a two-photon Raman transition can be described as conventional superradiance with renormalized parameters are found. In the case of single excitation the photon emission probability depends on the number of photons and atoms in the microcavity.     

原子内壳层双光子衰变的相对论效应和屏蔽效应

本文系统地计算了H原子,类H-Xe离子及Xe原子内壳层的双光子衰变速率。阐明其相对论效应和电子屏蔽效应。并与最新的实验结果和他人的理论计算结果进行了比较和分析讨论。 关键词：     

A scheme for two-photon lasing with two coupled flux qubits in circuit quantum electrodynamics

We theoretically study the system of a superconducting transmission line resonator coupled to two interacting superconducting flux qubits.It is shown that under certain conditions the resonator mode can be tuned to two-photon resonance between the ground state and the highest excited state while the middle excited states are far-off resonance.Furthermore,we study the steady-state properties of the flux qubits and resonator,such as the photon statistics,the spectrum and squeezing of the resonator,and demonstrate that two-photon laser can be implemented with current experimental technology.     

Control of four-level quantum coherence via discrete spectral shaping of an optical frequency comb

We present experiments demonstrating high-resolution and wide-bandwidth coherent control of a four-level atomic system in a diamond configuration. A femtosecond frequency comb is used to excite a specific pair of two-photon transitions in cold 87Rb. The optical-phase-sensitive response of the closed-loop diamond system is studied by controlling the phase of the comb modes with a pulse shaper. Finally, the pulse shape is optimized resulting in a 256% increase in the two-photon transition rate by forcing constructive interference between the mode pairs detuned from an intermediate resonance.     

4<Emphasis Type="Italic">s</Emphasis>-1<Emphasis Type="Italic">s</Emphasis> two-photon decay in hydrogen atom with allowance for cascades

The 4s-1s two-quantum decay in the hydrogen atom is calculated in the framework of quantum electrodynamics with allowance for cascades. It is shown that the pure two-photon contribution cannot be separated from the contribution of cascade processes. Interference between the transitions of these two types is comparable in magnitude with the pure two-photon contribution. The separation of pure two-photon contributions is discussed recently in connection with the role of two-photon decays in the separation of radiation from matter in the early Universe.     

Photoemission from multiply excited surface plasmons in Ag nanoparticles

Two-photon photoemission spectroscopy using femtosecond laser pulses is used to investigate the excitation and decay mechanisms of the surface plasmon resonance in Ag nanoparticles grown on graphite. The resonant excitation of this collective excitation leads to a two-orders-of-magnitude-enhanced two-photon photoemission yield from a graphite surface with Ag nanoparticles compared to the yield from pure graphite. From the shape of the photoemission spectra, the polarization dependence of the photoemission yield and the excitation probabilities for different excitation pathways we conclude that excitation with 400-nm femtosecond laser pulses leads to the coherent multiple excitation of the surface plasmon in the Ag nanoparticles. This multiply excited plasmon mode can decay via the coupling to a single-particle excitation leading to the emission of an electron if its final state is located in the continuum. The surface plasmon in metallic nanoparticles is a model system to investigate collective excitations in multiphoton processes. Received: 26 June 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

双光子共振荧光分布

本文研究了一二能级原子系的双光子共振荧光现象。通过对系统Hamiltonian进行一规范么正变换,并求解荧光场算符的运动方程,从而得出双光子共振荧光的频谱及强度分布。 关键词：     

双光子过程耗散耦合腔阵列中的量子相变

基于准玻色方法, 利用平均场理论解析求解了环境作用下双光子过程耦合腔阵列体系的哈密顿量, 得到了体系序参量的解析表达式, 并讨论了耗散对体系超流-Mott绝缘相变的影响. 研究结果表明: 双光子共振情况下系统重铸相干的腔间耦合率临界值为(ZJ/β)= (ZJ/β)_c'≈ 0.34;双光子相互作用过程比单光子过程具有更大的耗散率, 系统维持长程相干状态的时间更短, 而实现重铸相干的腔间耦合率的临界值更大.     

Scattering of electrons in image-potential states by steps

Image-potential states on several stepped copper surfaces were studied by time, energy, and angle-resolved two-photon photoelectron spectroscopy. For different step densities binding energies and effective masses were determined in the energy-resolved and decay rates in the time-resolved mode of two-photon photoemission. The asymmetry with respect to the electron motion up or down the steps of the total decay rates is enhanced for less-ordered step arrangements. Elastic interband scattering strongly depends on details of the backfolding of the bands by the periodic step structure. Elastic intraband scattering is of similar importance as inelastic decay may contribute significantly to the asymmetry of the decay rates. PACS 73.20.At; 79.60.Bm; 72.10.Fk