Theory of cavity-enhanced spontaneous parametric downconversion

In this paper we study photon pairs generated in a nonlinear cavity composed of a nonlinear crystal surrounded by mirrors, by the process of spotaneous parametric downconversion. We analyze two different regimes: singly-resonant cavities where the signal and idler modes are resonant and doubly-resonant cavities where the pump mode is also resonant. We present analytic expressions for the joint spectral amplitude in these two cases and study the reduction in emission bandwidth as the cavity coefficient finesse is increased. We also study the enhancement of the source brightness from the presence of the cavity.     

Band-pass plasmonic filter based on periodic cascade resonant cavities

A band-pass plasmonic filter based on periodic cascade resonant cavities is theoretically designed and analyzed. The transmission modes of surface plasmon polariton are well studied by the finite-difference time-domain method. The structure indicates the band-pass selection capability with several types of modes in the transmission spectra. The results show that the number of the transmission modes depends on the number of the cascade cavities period. These modes exhibit a shift with varying incident intensity involving Kerr medium in cascade cavities. The structure provides flexibility in design for pass-bands filter.     

Non resonant transmission modelling with statistical modal energy distribution analysis

Statistical modal Energy distribution Analysis (SmEdA) can be used as an alternative to Statistical Energy Analysis for describing subsystems with low modal overlap. In its original form, SmEdA predicts the power flow exchanged between the resonant modes of different subsystems. In the case of sound transmission through a thin structure, it is well-known that the non resonant response of the structure plays a significant role in transmission below the critical frequency. In this paper, we present an extension of SmEdA that takes into account the contributions of the non resonant modes of a thin structure. The dual modal formulation (DMF) is used to describe the behaviour of two acoustic cavities separated by a thin structure, with prior knowledge of the modal basis of each subsystem. Condensation in the DMF equations is achieved on the amplitudes of the non resonant modes and a new coupling scheme between the resonant modes of the three subsystems is obtained after several simplifications. We show that the contribution of the non resonant panel mode results in coupling the cavity modes of stiffness type, characterised by the mode shapes of both the cavities and the structure. Comparisons with reference results demonstrate that the present approach can take into account the non resonant contributions of the structure in the evaluation of the transmission loss.     

Acoustical cavity excitation.

A tutorial exposition is given of cavity excitations and it is shown that relatively simple analytical solutions for the excitation of acoustic fields in cavities with lossy walls can be obtained for a broad class of such cavities. The standard analytic procedure whereby one expresses the acoustic field as a sum over the eigenmodes of the cavity is reviewed and its limitations are pointed out. An alternate procedure is to seek to make use of Green's functions depending on a single coordinate and to not express such Green's functions as sums over eigenfunctions. Instead, one expresses the Green's function as a closed form function that has a discontinuous slope at the value of the coordinate corresponding to the source location. Although the class of cavities for which this approach is possible is limited, it is sufficiently broad to be of intrinsic interest and the results may be helpful to those seeking benchmark solutions and analytic insight. Displayed examples are for cavities whose shapes are spheres and cylinders, with interior monopole, dipole, line, and distributed sources. The formulation yields simple expressions for the total power absorbed by the cavity walls.     

四极子量子级联微腔激光器模式理论

介绍了一种计算四极子量子级联微腔激光器模式的方法.推导了本征方程和光场分布公式,对不同变形因子四极子微腔进行了数值计算和模式分析,结果表明四极子量子级联微腔激光器模式具有 “蝴蝶结”形状特征,在=45°、135°、225°、315°的4个方向光输出最强.该计算方法可适用于其它任意形状截面介质上电磁波传输或散射的计算问题.     

含色散介质的一维光子晶体微腔的光学特性和模式调节

采用时域有限差分(FDTD)法计算了含色散介质一维光子晶体微腔的透射谱,研究了缺陷模的频移特性。通过与无色散光子晶体微腔透射谱相比较,发现了介质色散导致的缺陷模频移现象,并详细地分析了中心频率、色散强度和衰减系数等色散介质参量和缺陷模频移的依赖关系。模拟结果显示,缺陷模的频移决定于中心频率、色散强度和衰减系数等色散介质参量的大小,通过合理的调节这些参量,可以有效地调节缺陷模的频率。     

Multipass resonators for annular gain lasers

The need to obtain a coherent output beam may impose severe constraints on the design of lasers with annular amplifying regions. One solution to the mode control problem involves resonant cavities in which the optical fields make more than one pass through the amplifying medium before returning to their starting point on the cavity mirror. Such multipass cavities and the resulting phase-compensated beam modes are considered in detail.     

左手介质对谐振腔谐振频率的影响

在谐振腔设计过程中, 谐振腔的品质因数以及谐振频率都是需要考虑的关键因素. 传统的方法是通过减小谐振腔的尺寸或者利用高次模来提高谐振腔的谐振频率, 但是由于两种方法都有其局限性, 导致设计结果并不理想. 通过理论计算与模拟仿真相结合的方法, 对影响谐振腔谐振频率的因素进行分析, 得出了填充介质的材料属性与谐振腔谐振频率的关系. 理论计算显示: 当用“左手介质”作为谐振腔的填充物质时, 可以在不改变谐振腔尺寸的基础上提高谐振频率. 高频结构仿真器(high frequency structure simulator)的仿真数据也证明了以上结果, 从而得出谐振腔的谐振频率可以不受谐振腔尺寸的限制. 相较于传统理论而言, 研究结论有进一步的发展, 为探索和设计新颖的谐振腔提供了理论依据.     

2D Analysis of multimode photonic crystal resonant cavities with the finite volume time domain method

In this paper an accurate analysis of two-dimensional (2D) Photonic Crystal (PhC) based multimode resonant cavities is carried out. The analysis is performed with a robust and accurate Finite Volume Time Domain (FVTD) technique. The analysis proves the ability of the FVTD method to extract different resonant modes from a multimode PhC resonant cavity with the use of appropriate source profiles. A detailed explanation on how the source is engineered and used to excite different modes is given. Furthermore, parameters such as resonant frequency and quality factor for each resonant mode are accurately calculated.     

半导体超常媒质矩形谐振腔(英文)

设计并研究了两个有不同各向异性参数的半导体超常媒质矩形谐振腔,其中填充了部分右手半导体超常媒质和部分左手半导体超常媒质。通过作图法研究了充满非色散的右手半导体超常媒质和色散的左手半导体超常媒质的谐振腔中谐振模式的解。结果显示,谐振腔的谐振模式取决于半导体超常媒质的色散关系。讨论了每种谐振腔中包含的六种情况,分别表示两个区域的传播常数为实数和虚数的不同的组合,并且揭示了构建具有无相差特性的谐振腔的几何参数要求。     

实现多共振模的一维异质结构光子晶体

用两种正折射率材料设计了一维光子晶体异质结构模型,并利用传输矩阵法对该光子晶体的透射谱进行了理论研究。结果发现:光通过光子晶体时,在较宽的禁带范围内出现透射率为100%的多条共振模,且共振模的数目与光子晶体结构周期数m相对应;当入射角一定时,共振模的位置可以通过介质的折射率、厚度来调制;当入射光入射角度增大时,多共振模出现整体蓝移,同时共振模宽度变窄。这些特性有望应用于多通道窄带滤波器的设计。     

Local resonant characteristics of a layered cylinder embedded in the elastic medium

Three kinds of resonant modes of a single layered circular elastic cylinder embedded in the elastic medium are analysed by considering the oscillation of the scatter＇s core, based on the fact that the core moves as a rigid body when the shell material is very compliant. The resonant frequencies of the single resonator acquired by our method are in good agreement with those calculated by the local interaction simulation approach （LISA） for the local resonant phononic crystal. Therefore, the local resonant characteristics of a single layered circular elastic cylinder can be used to evaluate the resonant frequencies of the phononic crystal. The effects of the geometrical and physical parameters of the shell and the core are also studied in details. This work is significant for designing the locally resonant phononic crystal based on the local resonant characteristics of the single resonator, and the resonant frequencies can be tuned by selecting the geometrical sizes and the materials.     

Elastic resonances of a periodic array of fluid-filled cylindrical cavities embedded in an elastic matrix

The resonant scattering by a periodic infinite array of fluid-filled cylindrical cavities in an elastic matrix is studied. The exact reflection and transmission coefficients of the array are calculated by means of a multiple scattering formalism taking into account all the interactions between the cavities. Numerical results are next given for low frequencies for which only the longitudinal and transverse zero modes propagate. A first study based on the analysis of the transmission coefficients clearly shows that the resonances of the array can be classified into two sets: those close to the resonances of a single cavity and those due to a resonant coupling between a cavity and its nearer neighbors. The resonant coupling is due to the interaction between the whispering-gallery surface waves propagating around each cavity. In the case of cavities with very close spacing, it is observed that the dispersion curves of the waves propagating along the array can also be classified into two sets: those with a positive group velocity have cut-off frequencies that correspond to the resonances of a single cavity, those with a negative group velocity have cut-off frequencies that correspond to the resonances resulting from the strong coupling. A new method for the analysis of the resonances is presented. It is based on the properties of the scattering matrix and consists in studying the resonant eigenvalues of the scattering matrix of the array once the background is removed. For the detection of very fine resonances, as well as in the separation of several resonances very close to each other, this method proves to be more efficient than one based on the analysis of the reflection and transmission coefficients.     

透明阴极实现相对论磁控管跳频的仿真北大核心CSCD

基于一个6腔同腔结构相对论磁控管,透明阴极金属条个数与磁控管腔数相同时相对论磁控管易于工作在2π模式,减少为腔体数目一半时易于工作在π模式,提出了旋转扇形透明阴极金属条角向位置实现相对论磁控管中心频率跳变的方案.经仿真优化,设计了外径15mm,6个扇形金属条的透明阴极,每个扇形金属条的角向宽度为20°.运用粒子模拟软件,仿真分析了角向位置金属条与阳极块相对应及金属条与谐振腔相对应两种情况,在工作磁场保持0.75T,调节工作电压在600~800kV 内变化时,模拟结果表明,相对论磁控管可以很稳定地分别工作在2π模式和π模式,即通过旋转透明阴极实现相对论磁控管频率跳变.     

Design of miniaturization resonant cavities using metamaterial

Based on the method of transformation optics, we show the miniaturization of a resonant cavity filled with metamaterials. Because the electromagnetic behavior in the transformation space is invariant in the original space, the quality factor Q and the resonant modes of a miniaturized resonant cavity filled with metamaterial are the same as for those in the original space. Theoretical analysis and numerical simulation confirms that the miniaturization of resonant cavities is effective.     

Multichannel W3 Y-branch filter in a two dimensional triangular-lattice photonic crystal slab

We demonstrate the design, fabrication and characterization of a highly efficient multichannel W3 Y-branch filter in a two dimensional triangular-lattice photonic crystal slab. The coupling properties between high-order waveguide modes and fundamental resonant modes are investigated. By finely adjusting the size of resonant cavities, four higher-order mode channels with different output wavelengths are experimentally realized, which is in agreement with the theoretical simulations. The results show that this kind of filter may be useful in optical integrated circuits with high coupling and transmission efficiency.     

Analytic theory of a tapered gyrotron resonator

An analytic theory has been derived for determining the eigenfrequencies, RF-field distribution and Q of the TE_mpq modes of a gyrotron resonator consisting of a circular cylinder joined to a slowly tapered section. Explicit results are obtained for a linear taper. The cavity modes are found to have an RF-field distribution which is useful for prebunching the electron beam and enhancing efficiency. For high Q cavities, the cavity Q depends on axial mode number q as q^–2, which is important for mode discrimination. Proper selection of taper length is found to reduce the Q of high q modes, also aiding in mode discrimination. The present approach may be applied to other forms of weakly irregular cavities, such as cavities with nonlinear tapers.Work supported by U.S.D.O.E. Contract DE-AC02-78ET-51013Supported by U.S. Department of Energy     

单负材料一维光子晶体的透射谱特性

利用传输矩阵法研究了单负材料一维光子晶体(AB)m(ADBDB)n(AB)mA的透射谱,发现:透射谱中出现2个共振隧穿模,其位置和间距可由周期数m或n,以及介质层厚度d调节控制。改变m,会出现2个恒定间距的共振隧穿模;改变dA,共振隧穿模间距增大,且当dA≥25mm时,间距增大加剧;改变n,共振隧穿模逐渐趋于简并,当n≥6时,两隧穿模合二为一;改变dD,两共振隧穿模亦逐渐趋于简并,当dD≥20mm时,两隧穿模亦合二为一。这些特性可为利用光子晶体设计可调性高品质单通道、双通道滤波器提供参考。     

Energy Levels of Coupled Plasmonic Cavities

We demonstrate the hybridization of the plasmonic modes in directly coupled whispering gallery cavities fabricated on silver films and present the mode patterns and energy levels using cathodoluminescence spectroscopy.Although the energy of the most antisymmetrically coupled modes is higher than that of the corresponding symmetrically coupled ones,the contrary cases happen for small quantum number modes.We attribute the phenomenon to the different surface plasmon polariton paths between the symmetrically and antisymmetrically coupled modes.These results provide an understanding of the resonant properties in coupled plasmonic cavities,which have potential applications in nanophotonic devices.     

Effect of a pulse entrapping on weak localization of waves in a resonant random medium

We consider theoretically a new physical effect in coherent backscattering enhancement (CBE) of electromagnetic or acoustic non-stationary waves from a discrete random medium under condition of Mie resonant scattering. The effect manifests itself as an angle-cone broadening of a short pulsed signal CBE from the resonant random medium, compared with the case of a non-resonant random medium. The cone broadening is associated with a pulse-entrapping effect when the pulse, while propagating within the resonant random medium, spends most of the time being 'entrapped' inside scatterers. A theory for the predicted effect is based on, first, the well known relation between the contributions of the ladder and cyclical diagrams to the time spectral density of the wave electric field coherence function and, second, a recently derived radiative transfer equation with three Lorentzian kernels of delay describing a pulse entrapping in an ensemble of resonant point-like scatterers. Using the generalized Chandrasekhar H-function, we obtain an exact analytic expression for the non-stationary albedo of the semi-infinite resonant random medium, taking into account the phenomena of a pulse CBE and entrapping. A simple analytic asymptotics is found for the albedo of the later part of the scattered pulse. This asymptotics shows quantitatively how the entrapping affects the peak amplitude and peak line shape of the CBE of a short pulse.