Effect of spatially nonlocal versus local optical response of a gold nanorod on modification of the spontaneous emission

The spontaneous emission rate of a two-level quantum emitter(QE)near a gold nanorod is numerically investigated.Three different optical response models for the free-electron gas are adopted,including the classical Drude local response approximation,the nonlocal hydrodynamic model,and the generalized nonlocal optical response model.Nonlocal optical response leads to a blueshift and a reduction in the enhancement of the spontaneous emission rate.Within all the three models,the resonance frequency is largely determined by the aspect ratio(the ratio of the nanorod length to the radius)and increases sharply with decreasing aspect ratio.For nanorod with a fixed length,it is found that the larger the radius is,the higher the resonance frequency is,and the smaller the enhancement is.However,if the length of the nanorod increases,the peak frequency falls sharply,while the spontaneous emission enhancement grows rapidly.For nanorod with a fixed aspect ratio,the peak frequency decreases slowly with increasing nanorod size.Larger nanorod shows smaller nonlocal effect.At a certain frequency,there is an optimal size to maximize the enhancement of the spontaneous emission rate.Higher order modes are more affected by the nonlocal smearing of the induced charges,leading to larger blueshift and greater reduction in the enhancement.These results should be significant for investigating the spontaneous emission rate of a QE around a gold nanorod.     

Modification of the spontaneous emission of quantum dots near the surface of a three-dimensional colloidal photonic crystal

This paper demonstrates experimentally and numerically that a significant modification of spontaneous emission rate can be achieved near the surface of a three-dimensional photonic crystal.In experiments,semiconductor core-shell quantum dots are intentionally confined in a thin polymer film on which a three-dimensional colloidal photonic crystal is fabricated.The spontaneous emission rate of quantum dots is characterised by conventional and time-resolved photoluminescence (PL) measurements.The modification of the spontaneous emission rate,which is reflected in the change of spectral shape and PL lifetime,is clearly observed.While an obvious increase in the PL lifetime is found at most wavelengths in the band gap,a significant reduction in the PL lifetime by one order of magnitude is observed at the short-wavelength band edge.Numerical simulation reveals a periodic modulation of spontaneous emission rate with decreasing modulation strength when an emitter is moved away from the surface of the photonic crystal.It is supported by the fact that the modification of spontaneous emission rate is not pronounced for quantum dots distributed in a thick polymer film where both enhancement and suppression are present simultaneously.This finding provides a simple and effective way for improving the performance of light emitting devices.     

Objective estimates of cochlear tuning by otoacoustic emission analysis

A new method is presented for estimating cochlear tuning starting from measurements of either the transient evoked otoacoustic emission latency or the spontaneous otoacoustic emission minimal spacing. This method could be useful in obtaining indirect information about the tuning curve, particularly for subjects that, like neonates, cannot be studied with psycho-acoustical techniques. Theoretical models of the acoustic transmission along the cochlea based on the transmission line formalism predict a relation between the otoacoustic emission latency and the frequency. This relation depends on the tuning curve, i.e., the frequency dependence of the quality factor of the cochlear resonances. On the other hand, models for the generation of spontaneous emissions based on the concept of coherent scattering from cochlear random inhomogeneities imply an independent relation between the tuning curve and the minimal frequency spacing between spontaneous emissions. In this study, experimental measurements of the otoacoustic emission latency and of the minimal spacing between spontaneous emissions are presented. Theoretical relations are derived, which connect these two measured quantities and the tuning curve. The typically longer latency of neonates implies a higher degree of tuning at high levels of stimulation.     

Calculations of spontaneous emission rates of a single-impurity molecule in uniaxial host crystals

In a uniaxial host crystal the spontaneous emission rate of the dipole transition of the zero-phonon line of a single-impurity molecule depends on the angle between the transition dipole moment and the optical axis of a crystal and as well as on the ordinary and extraordinary refractive indices. The relative spontaneous emission rate (the spontaneous emission rate divided by the spontaneous emission rate in the case when the transition dipole moment is parallel to the optical axis) is determined through a simple formula by one coefficient. Here this coefficient is calculated as a function of the extraordinary refractive index for 40 values of the ordinary refractive index on the interval from 1.25 to 3.20. For comparison, the effects caused by the nearness to the plane interface between cinnabar crystal (HgS) and air, as an example, are calculated.     

原子在两个平行镜面间两层电介质板中的自发辐射率

利用光子的闭合轨道理论,我们研究了原子在两个平行镜面间两层电介质板（折射率分别为n1,n2）中的自发辐射率. 自发辐射率呈现出多周期的振荡结构。自发辐射率的傅立叶变换中的每一个峰和光子从原子出发到返回原子的一条闭合轨道相对应。结果表明自发辐射率和两层电介质的宽度和折射率有关。和只有一层电介质的辐射率比较,当两层电介质的折射率n1 和 n2 差别很小时, 两层电介质之间分界面的反射效应可以忽略;但是当二者的差别很大时,发射效应变得非常重要且自发辐射率中的振荡减弱。本文的结果为原子在不同电介质间的自发辐射率的研究提供了新的理解。     

Spontaneous emission suppression via quantum path interference in coupled microcavities

We examine theoretically the spontaneous emission rate in optical microstructures with cavity resonances that overlap in both position and frequency. Using projection techniques, we show that the spontaneous emission in such structures can be accurately described by the direct emission and quantum path interference of emission into a few discrete resonant modes, even though the exact infinite-dimensional problem involves a coupling to the continuum of radiation states. Moreover, we obtain an efficient numerical time-domain method for determining the spontaneous emission rate that incorporates these effects, including the suppression of spontaneous emission into some modes.     

Controlling spontaneous emission and threshold-less laser oscillation with optical microcavities

We describe the alteration of spontaneous emission of materials in optical microcavities having dimensions on the order of the emitted wavelength. Particular attention is paid to one-dimensional optical confinement structures with pairs of planar reflectors (planar microcavities). The presence of the cavity causes great modifications in the emission spectrum and spatial emission intensity distribution accompanied by changes in the spontaneous emission lifetime. Experimental results are shown for planar microcavities containing GaAs quantum wells or organic dye-embedded Langmuir-Brodgett films as light emitting layers. Also discussed are the laser oscillation properties of microcavities. A remarkable increase in the spontaneous emission coupling into the laser oscillation mode is expected in microcavity lasers. A rate equation analysis shows that increasing the coupling of spontaneous emission into the cavity mode causes the disappearance of the lasing threshold in the input-output curve. Experimentally verification is presented using planar optical microcavities confining an organic dye solution. The coupling ratio of spontaneous emission into a laser mode increases to be as large as 0.2 for a cavity having a half wavelength distance between a pair of mirrors. At this point, the threshold becomes quite fuzzy. Differences between the spontaneous emission dominant regime and the stimulated emission dominant regime are examined with emission spectra and emission lifetime analyses.     

Amplitude noise enhancement caused by nonlinear interaction of spontaneous emission field in laser diodes

The amplitude noise spectrum of the total laser output of a nearly single-mode laser diode is enhanced at low frequencies due to the nonlinear interaction between the spontaneous photons and the coherent laser field. This enhancement strongly depends on the spontaneous emission rate, resulting in a dependence of the amplitude noise spectral shape on the spontaneous emission rate. A linearized analytical model which predicts that the amplitude noise spectral shape is independent of the spontaneous emission rate is inadequate. The experimental data are consistent with a numerical simulation of the full nonlinear model.     

Phase control of spontaneous emission from a double-band photonic crystals

Through picture of dressed states, this paper investigates the spontaneous emission spectrum from a microwave-driven three-level atom embedded in a double-band photonic crystals. The physical dynamics of the phase dependent phenomenon is analysed by comparing two models `upper level coupling' and `lower level coupling'. When the phase is changed from 0 to $\pi$, the variety of spontaneous emission spectra from either of the two models are inverse to each other, in which the relative height and width of peaks are determined by the density of states in photonic crystals.     

光子闭合轨道理论对单镜面附近原子自发辐射率的解释

利用腔量子电动力学计算了原子在全反射镜面附近的自发辐射率。结果表明,所得的震荡谱和原子到镜面的距离有关。通过傅立叶变换,得到与这些振荡谱相对应的频率谱。为了解释这种现象,我们首次将闭合轨道理论用于研究辐射原子在单镜面附近的自发辐射率,给出了对原子自发辐射现象新的理论解释。     

Enhancement or quenching effect of metallic nanodimer on spontaneous emission

The plasmonic effects of a metallic (Au or Ag) nanodimer on the excitation and emission of a single emitter placed within the gap of the nanodimer are studied to identify its overall performance (enhancement or quenching) for the spontaneous emission. The process of a spontaneous emission is divided into two stages for analysis: the excitation and the subsequent emission stages. For the excitation stage, the amplification of the local electric field around the gap region is studied to show the converging-lens effect of the nanodimer for focusing an incident light. For the emission stage, the apparent quantum yield of an electric dipole (the excited emitter) in the presence of the nanodimer is studied in terms of its radiative and nonradiative decay rates. Both models are simulated by the multiple multi-pole methods for solving Maxwell's equations. The results indicate that the overall enhancement factor of a metallic nanodimer on the spontaneous emission depends not only on its dimension (radius and gap) but also on the absorption and emission spectra of the emitter. Moreover, there is an optimal dimension (radius and gap) of a nanodimer for obtaining the maximum enhancement to a specific spontaneous emission. In addition, the observed emission spectrum of the emitter can be modified by the nearby nanodimer (a low-pass filter), and its lifetime can be reduced by two or three orders of magnitude due to the energy transfer between them.     

耗散环境下二比特体系的量子关联性质研究

利用Quantum Discord(QD)判据,研究了与热库相互作用的二比特体系的量子关联性质.讨论了在不同初态下体系量子关联随时间的变化,以及热库的平均光子数m,n和原子的自发辐射率γ对体系量子关联性质的影响.结果表明,在不同的初态下,体系可以得到不同性质的量子关联;而且当原子自发辐射率γ取固定值时,QD的衰减会随m,n取值的减小而减慢,热库平均光子数都为零的情况下能够得到最大范围的量子关联;此外,当热库平均光子数m,n取确定值时,随着γ取值的减小,QD的衰减也会随之减慢,此时同样会得到较大范围的量子关联.说明较小的热库平均光子数以及原子自发辐射率γ能够减弱QD的衰减,从而获得生命力较强的体系量子关联.     

Two-photon cooperative emission in the presence of athermal electromagnetic field

The possibility of cooperative spontaneous two-photon emission of an extended radiators system and the influence of the external thermal electromagnetic field on the spontaneous emission rate, in such a system, are investigated. It is concluded that, in an external electromagnetic field, the two-photon cooperative emission rate increases significantly. The importance of this effect on the emission of gamma rays from inverted long-lived isomers triggered by X-ray thermal fields, is emphasized. This revised version was published online in August 2006 with corrections to the Cover Date.     

Spontaneous Emission of a Polarized Atom in a Medium Between Two Parallel Mirrors

Using the photon closed orbit theory, the spontaneous emission rate of a polarized atom in a medium between two parallel mirrors is derived and calculated. It is found that the spontaneous emission rate of a polarized atom between the mirrors is related to the atomic position and the polarization direction. The results show that in the vicinity of the mirror, the variation of the spontaneous emission rate depends crucially on the atomic polarization direction. With the increase of the polarization angle, the oscillation in the spontaneous emission rate becomes decreased. For the polarization direction parallel to the mirror plane, the oscillation is the greatest; while for the perpendicular polarization direction, the oscillation is nearly vanished. The agreement between our result and the quantum electrodynamics result suggests the correctness of our calculation. This study further verifies that the atomic spontaneous emission process can be effectively controlled by changing the polarization orientation of the atom.     

介质腔中自发辐射原子的寿命分布

在本文中,我们研究了半无限大介质板夹层对空腔中激发态原子的自发辐射率的影响,并利用寿命分布函数来描述原子衰变动力学的性质,在对称和非对称的结构中,分别计算了以原子在板中的相对位置为自变量的原子自发辐射率的函数,计算结果表明,原子的自发辐射率与介质腔的宽度以及外层半无限大介质板的折射率有关,介质腔可以增强或抑制激发态原子的激发,当介质腔的宽度足够大时,原子的寿命分布呈现指数衰减的形势,这些理论结果和实验值相符合的非常好.     

Modification of spontaneous emission rate in planar dielectric microcavities – seen as superradiance and subradiance

The modification of the spontaneous emission rate of an atom in a cavity can be seen as superradiance/subradiance induced by virtual image dipoles. This result is used to gain physical insight why the spontaneous emission is enhanced or suppressed in various cavity geometries. A general model for spontaneous emission engineering, which simultaneously handles dispersive and lossy cavities, is derived.     

加深光学速调管自发辐射谱调制的研究

相干谐波储存环自由电子激光的相干增强因子正比于光学速调管自发辐射谱调制率的平方,加深光学速调管自发辐射谱的调制是相干谐波储存环自由电子激光研究的重要一环.介绍在中国科学技术大学800MeV电子储存环上所进行的加深光学速调管自发辐射谱调制的研究工作,调制率已从0.236提高到了0.80,为相干谐波产生实验奠定了基础.     

Spontaneous emission rates of a single-impurity molecule in birefringent host crystals for electric quadrupole transitions

In a birefringent host crystal, the spontaneous emission rate of zero-phonon line of a single-impurity molecule depends on the angles between the transition dipole moment and the principal axes of the dielectric (permittivity) tensor and also on the main dielectric constants of a host crystal. In this paper, the spontaneous emission rate of the electric quadrupole transition of a single-impurity molecule is calculated for uniaxial host crystals with the ordinary reflective indices n_o=1.5, 2, 3 and a variable extraordinary reflective index, and for seven biaxial host crystals (anthracene, chrysene, diphenyl, fluorine, naphthalene, phenanthrene, terphenyl). For the above-mentioned biaxial crystals, differences in the values of spontaneous emission rate of the quadrupole transition in the same host crystal are up to 11%.     

Coherence-controlled stationary entanglement between two atoms embedded in a bad cavity injected with squeezed vacuum

We investigate the entanglement between two atoms in an overdamped cavity injected with squeezed vacuum when these two atoms are initially prepared in coherent states. It is shown that the stationary entanglement exhibits a strong dependence on the initial state of the two atoms when the spontaneous emission rate of each atom is equal to the collective spontaneous emission rate, corresponding to the case where the two atoms are close together. It is found that the stationary entanglement of two atoms increases with decreasing effective atomic cooperativity parameter. The squeezed vacuum can enhance the entanglement of two atoms when the atoms are initially in coherent states. Valuably, this provides us with a feasible way to manipulate and control the entanglement, by changing the relative phases and the amplitudes of the polarized atoms and by varying the effective atomic cooperativity parameter of the system, even though the cavity is a bad one. When the spontaneous emission rate of each atom is not equal to the collective spontaneous emission rate, the steady-state entanglement of two atoms always maintains the same value, as the amplitudes of the polarized atoms varies. Moreover, the larger the degree of two-photon correlation, the stronger the steady-state entanglement between the atoms.