Electron transport through a quantum cavity

Electron transport through a quantum cavity coupled with two one-dimensional waveguides is studied using a generalized scattering matrix method. In a symmetric N-channel cavity model, we are able to obtain an exact solution that predicts the electron energies at which the transmission of electron waves become zero. We found that the zero of transmission is closely related to the longitudinal resonance through inter-channel scattering, in particular, to the resonance of the highest propagating mode inside the cavity. This model provides a simple way to calculate the electron transmission through a cavity which could be useful in quantum waveguide engineering.     

Motion of dissipative solitons in a laser cavity with a smooth transverse inhomogeneity

The effect of the inhomogeneity gradient in a large-aperture class A laser with a saturable absorber on the transverse motion of dissipative solitons and their clusters in the laser cavity is analytically and numerically studied. A soliton with a nonzero topological charge is found to move rectilinearly in the steady-state regime but in a direction that differs from the direction of the cavity length gradient, which is connected with a nontrivial internal structure of the soliton. The steady-state motion of soliton clusters can be curvilinear, including translational and rotational motions of their center.     

Improving both transverse mode discrimination and diffraction losses in a plano-concave cavity

The paper considers the interest in using two apertures rather than one in a plano-concave cavity for improving the discrimination between TEM₀₀ and TEM₁₀ modes while keeping the diffraction losses unchanged or even decreasing them. Such an investigation involves stochastic methods like simulated annealing that can be used if and only if the resonant field computation time is low. Fortunately, a quad-double arithmetic recurrence formula allows to compute round-trip operators faster than any adaptative numerical integrator with the same accuracy.     

Design of a higher harmonic cavity for the SSRF storage ring

Higher harmonic cavity used in the third generation synchrotron light source increases the Touschek lifetime. The higher harmonic cavity of Shanghai Synchrotron Radiation Facility (SSRF) is a 1.5GHz passive superconducting cavity. Its higher order modes (HOM) are extracted by a ferrite HOM damper out of the cryostat. Multi-cell cavity is chosen concerning the voltage. The harmonic cavity dynamics, beam dynamics with passive harmonic cavity and the design of single cell cavity are included in this paper.     

Design of a higher harmonic cavity for the SSRF storage ring

Higher harmonic cavity used in the third generation synchrotron light source increases the Touschek lifetime.The higher harmonic cavity of Shanghai Synchrotron Radiation Facility(SSRF)is a 1.5 GHz passive superconducting cavity.Its higher order modes(HOM)are extracted by a ferrite HOM damper out of the cryostat.Multi-cell cavity is chosen concerning the voltage.The harmonic cavity dynamics,beam dynamics with passive harmonic cavity and the design of single cell cavity are included in this paper.     

Electron bunch train excited higher-order modes in a superconducting RF cavity

Higher-order mode(HOM) based intra-cavity beam diagnostics has been proved effective and convenient in superconducting radio-frequency(SRF) accelerators.Our recent research shows that the beam harmonics in the bunch train excited HOM spectrum,which have much higher signal-to-noise ratio than the intrinsic HOM peaks,may also be useful for beam diagnostics.In this paper,we will present our study on bunch train excited HOMs,including a theoretical model and recent experiments carried out based on the DC-SRF photoinjector and SRF linac at Peking University.     

Calculations for shortening the bunch length in storage rings using a harmonic cavity

Using the Hefei Light Source phase Ⅱ project （HLS- Ⅱ） as an example, a theoretical analysis of shortening the bunch lengths using a higher harmonic cavity （HHC） is given. The threshold voltage of an active HHC and the threshold tuning angle of a passive HHC are first analysed. The optimum tuning angle for the constant detuning scenario and the optimum harmonic voltage for the constant voltage scenario are presented. The calculated results show that the reduced bunch length is about half that of the nominal bunch. The bunch lengths vary from 11 mm at 0.1 A to 7 mm at 0.4 A for the constant detuning scenario, while the bunch lengths are around 7 mm over the beam current range for the constant voltage scenario. In addition, the synchrotron frequency spread is increased. It indicates that HHC may be used to reduce the bunch length and increase the Landau damping of synchrotron oscillations in a storage ring.     

Calculations for shortening the bunch length in storage rings using a harmonic cavity

Using the Hefei Light Source phaseⅡproject(HLS-Ⅱ)as an example,a theoretical analysis of shortening the bunch lengths using a higher harmonic cavity(HHC)is given.The threshold voltage of an active HHC and the threshold tuning angle of a passive HHC are first analysed.The optimum tuning angle for the constant detuning scenario and the optimum harmonic voltage for the constant voltage scenario are presented.The calculated results show that the reduced bunch length is about half that of the nominal bunch.The bunch lengths vary from 11 mm at 0.1 A to 7 mm at 0.4 A for the constant detuning scenario,while the bunch lengths are around 7 mm over the beam current range for the constant voltage scenario.In addition,the synchrotron frequency spread is increased.It indicates that HHC may be used to reduce the bunch length and increase the Landau damping of synchrotron oscillations in a storage ring.     

利用高阶拉盖尔-高斯横模精确测量法布里-珀罗腔内原子的运动轨迹

研究了冷原子与法布里-珀罗腔内拉盖尔-高斯横模强耦合相互作用体系的透射光谱, 分析了透射光谱与原子在腔中运动轨迹的关系. 结果表明, 与厄米特-高斯横模相比, 拉盖尔-高斯横模的腔场与原子的最大耦合系数几乎不随阶数的增加而变化, 使得探测光谱的对比度受模式阶数的影响较小. 在拉盖尔-高斯横模场分布的圆环边缘附近, 原子运动轨迹的微小偏移会引起透射光谱的很大变化, 因此在这些位置可以实现原子运动轨迹的高精度探测.     

多间隙耦合腔电子电导与模式稳定性

根据空间电荷波小信号基础理论,建立了多间隙耦合腔中单个间隙电子电导的计算模型与模式稳定性分析模型。以3间隙耦合腔为例,推导出了各个间隙电子电导的计算公式。通过理论计算与仿真模拟,研究了3间隙耦合腔中各个模式的电子电导特性,并进行了间隙中注波互作用研究与模式稳定性分析。模型计算发现：各个间隙不同模式的电子电导不同,第3间隙内电子电导受注电压及间隙距离影响最大,对整个间隙内的注波互作用及电路稳定性的影响也最大。该模型还可以用于分布作用速调管注波互作用的计算模拟。     

多间隙耦合腔电子电导与模式稳定性

根据空间电荷波小信号基础理论,建立了多间隙耦合腔中单个间隙电子电导的计算模型与模式稳定性分析模型。以3间隙耦合腔为例,推导出了各个间隙电子电导的计算公式。通过理论计算与仿真模拟,研究了3间隙耦合腔中各个模式的电子电导特性,并进行了间隙中注波互作用研究与模式稳定性分析。模型计算发现:各个间隙不同模式的电子电导不同,第3间隙内电子电导受注电压及间隙距离影响最大,对整个间隙内的注波互作用及电路稳定性的影响也最大。该模型还可以用于分布作用速调管注波互作用的计算模拟。     

In-situ modal inspection based on transverse second harmonic generation in single CdS nanobelt

Based on the transverse second-harmonic generation(TSHG) effect, we demonstrate a method for in-situ modal inspection of nonlinear micro/nanowaveguides. Pumping lights are equally split and coupled into two ends of a single Cd S nanobelt(NB). As pumping light counter-propagates along the NB, transverse second-harmonic(TSH) interference patterns are observed. The influence of multimode interaction on the TSHG effect is discussed in detail. Using fast Fourier transform,TSH interference patterns are analyzed, indicating the existence of at least four modes inside the NB. Experimental beat lengths are found to be in agreement with calculated results.     

Control of transverse mode pattern in a helium-neon laser using the cat’s eye cavity

This paper introduces an innovative method to control and select transverse mode pattern in a He-Ne laser. The cat’s eye cavity is established by using a cat’s eye reflector as the reflecting mirror. Laser transverse mode pattern varies continuously when the distance between the convex lens and the concave mirror is changed, which form the cat’s eye reflector. By Newton-Cotes method, the numerical solution of two-dimensional diffraction integral equation in the cat’s eye cavity is obtained, the laser operating modes under various parameters are analyzed, and the eigenvalue η_mn and diffraction power loss corresponding with every transverse mode are calculated. The parameters assuring fundamental transverse mode output are figured out, which match the experiment results well. Therefore, a new convenient real-time means for the control and selection of the laser transverse mode is realized.     

Electron band structure detection by high-order optical harmonic generation in solids

As the threshold field strength for the breakdown in air significantly exceeds the maximum measured thundercloud strength 3 kV/cm/atm, the problem of lightning initiation remains unclear. According to the popular idea, lightning can be initiated from streamer discharges developed in the enhanced electric field in a vicinity of hydrometeors. To test the idea, we carry out numerical simulations of positive streamer development around charged water drops at air pressure typical at thundercloud altitudes and at different background fields, drop sizes and charges. With real drop sizes and charges, the electric field required for the streamer formation is stronger than the measured fields; therefore, second mechanism is required to amplify the local field.     

On the completeness of the stationary transverse modes in an optical cavity

A general statement of the completeness of the set of steady-state transverse modes supported by a generalized optical cavity is provided. The cavity may be either of the stable or unstable type, and may be either a travelling-wave ring or standing-wave linear resonator. This completeness relation is strictly valid provided there is at most a single diffracting aperture within the cavity.     

Dynamic all-optical tuning of transverse resonant cavity modes in photonic bandgap fibers

Photonic bandgap fibers for transverse illumination containing half-wavelength microcavities have recently been designed and fabricated. We report on the fabrication and characterization of an all-optical tunable microcavity fiber. The fiber is made by incorporating a photorefractive material inside a Fabry-Perot cavity structure with a quality factor Q >200 operating at 1.5 microm. Under short-wavelength transverse external illumination, a 2 nm reversible shift of the cavity resonant mode is achieved. Dynamic all-optical tuning is reported at frequencies up to 400 Hz. Experimental results are compared with simulations based on the amplitude and kinetics of the transient photodarkening effect measured in situ in thin films.