Fano resonances in prism-coupled multimode square micropillar resonators

We report Fano resonances in a multimode square glass micropillar resonator; the resonances were obtained by using angle-resolved prism coupling. Our experiments reveal characteristically asymmetric line shapes of high-Q resonances and of detuned low-Q resonances in multimode reflection spectra. The asymmetric resonance line shapes evolve for an approximately pi phase within a 0.5 degree range of reflection angles. We model our observed asymmetric multimode resonances by the far-field interference between a light wave that is evanescently coupled with a high-Q mode orbit and a coherent light wave that is refractively coupled with a detuned low-Q mode orbit.     

Complex-number asymmetry parameters of the optical Fano effect in ring resonators

We studied the sharp asymmetric Fano line shape in fiber ring resonator systems and provided an explicit expression for asymmetry parameters using the physical parameters of the system. The fiber ring system was controllable and reconfigurable, allowing us to produce a variety of Fano line shapes in different configurations. Experimentally observed asymmetric spectral structures were fully reproduced using the complex-number asymmetry parameters, validating the approximations used to reduce the analytical expression for the line shape to the phenomenological Fano formula. The results may be useful in the design of on-chip application systems.     

Shape of Fano resonance line spectra calculated for silver nanorods

Using theoretical tools, we numerically demonstrated Fano line shapes in the scattering spectra of silver rods resulting from different mechanisms. One of the Fano line shapes is due to the coupling of an in-plane quadrupole and a dipole mode in a single rod. Two nodes were observed at the resonance wavelength, each of which is located at a quarter of the rod length from the two ends. The Fano resonance is strengthened when the silver rod is cut at the two nodal positions. The second mechanism that gives rise to a new Fano resonance peak occurs when the symmetry of the rod is broken and is a result of the asymmetric coupling between the two excited dipoles.     

ESR spectroscopy at 75 GHz by using a whispering gallery mode dielectric resonator and an electromagnet: Part I

Disc-shaped Whispering Gallery Mode (WGM) dielectric resonators were used to obtain ESR spectra of powder samples at 75 GHz. A mechanically and electronically controlled Gunn diode was used while the static magnetic fieldB ₀ was obtained by modifying a Bruker electromagnet. WGM resonators having a diameter of 30 mm were prevalently used. The samples were contained in polyethylene holders suitably designed so that the resonator could retain its original circular symmetry and the sample could stay as close as possible to the curved outer rim of the resonator.Q-factors of the order of 10⁴ were obtained at room temperature when single crystal quartz was used for the disc resonator. Anyway, the value ofQ could be easily controlled by using different materials for the WGM disc. The gained experience allows one to foresee that WGM resonators can be used with similar results also at frequencies up to 150 and 300 GHz. ESR spectra of many organic and inorganic samples will be reported in Part II of this paper. Here, an evaluation is made of the intensity of the electromagnetic field on a selected sample. Suitable holders for liquid samples are under study.     

A X-band ESR apparatus using a Whispering Gallery Mode dielectric resonator

The behaviour of a disc-shaped dielectric resonator (DR) working in the Whispering-Gallery Modes (WGM) when used for X-band ESR experiments has been tested. It has been verified that the WGM in the resonator can be excited by using the microwave bridge of a Bruker spectrometer. The microwave field is well confined in the DR also in the absence of metal shieldings as demonstrated by the high Q values of the detected resonances. A large number of such resonances has been detected from eight up to twelve gigahertz making the DR a very broadband device. ESR spectra of various powder and aqueous samples have been obtained and compared with those of the same samples recorded by using a metal cavity. The relative amplitude of the spectra demonstrated that for aqueous samples the sensitivity of DR is similar to that of the metallic cavity. Further investigations are in progress to enhance the overall performance.     

外力驱动下耦合弹簧振子的法诺共振

法诺共振是物理体系中普遍存在的一种非对称共振现象,它最早起源于量子物理,其微观图像是原子谱线中窄的分离态与宽的连续态之间的相干干涉.本文利用经典力学体系中两个弹簧振子的耦合,使其中一个弹簧振子受到周期性外力的驱动,成功类比了量子力学中的法诺共振现象.通过分析每个弹簧振子的动力学方程,严格求解它们的振动公式,从而得到每个...     

Some characteristics of resonant electromagnetic modes in a dielectric sphere

A set of formulae have been developed for calculations of the optical characteristics of droplets under conditions of whispering gallery mode (WGM) excitation. The WGM excitation is shown to cause the ripple structure in the Mie extinction cross section. The Chýlek equation for the distance between two neighboring morphology-dependent resonances (MDRs) in a spectrum has been revised. The equation is presented for the spherical-resonator finesse allowing for light scattering on inhomogeneities of the resonator surface, etc. A comparison of the theory and data in the literature is made and some peculiarities of spectra are explained.     

Fano resonances in prism-coupled square micropillars

We report Fano resonances in the frustrated total internal reflection (TIR) spectra of a prism-coupled square micropillar. Our angle-selective frustrated TIR technique reveals characteristically asymmetric resonance line shapes, which evolve spectrally over approximately a 2pi phase in the far field within a subdegree range of reflection angles. We theoretically model the asymmetric line shapes by the interference between a high-Q resonance that is evanescently coupled and partially confined by TIR, and a coherent background that is total internally reflected at the prism surface without coupling to the micropillar.     

低压下飞秒激光诱导Cu等离子体的诊断及空间分布实验研究

本文分别利用Gauss、Lorentz和Voigt函数对10Pa背景气压下飞秒激光诱导Cu等离子体光谱进行了拟合分析,结果表明,光谱在150ns内与Lorentz线型符合较好,200ns后与Gauss线型符合较好,Voigt线型与实验谱线一直符合较好。本文还对等离子体的电子密度和温度进行了诊断,并给出了它们随时间的演化;由发射光谱强度的时空演化,推知了等离子体的空间分布;利用时间飞行谱(TOF)得出了等离子体初期沿垂直靶面的膨胀速度。     

A wide-band Whispering Gallery Mode dielectric resonator having multiple high-Q cavities for high-field ESR spectroscopy

The behaviour of a disc-shaped dielectric resonator made of sintered alumina and working in the Whispering Gallery Mode (WGM) has been studied in the frequency interval from about 8 up to about 26 GHz. Many equidistant resonances have been detected in this frequency interval each of which being characterized by a satisfactoryQ-value due to the very good self-confinement of the microwave field which is typical of WGM propagation. Particular care has been devoted to the optimization of the coupling between the resonating disc and the microwave source. Examples of ESR spectra obtained on each of the detected resonances are reported which show the variation of the lineshape with the increase of the frequency. The obtained results allow one to foresee that the same dielectric disc can be used for ESR spectroscopy also at frequencies much higher than those up to now exploited.     

红外激光载波包络相位对氦原子的极紫外光(XUV)吸收谱的量子调控研究

本文通过数值求解双电子含时薛定谔方程, 研究了利用红外(IR)超短超强激光的载波包络相位(CEP)对氦(He)原子的极紫外光(Extreme Ultra-Violet, XUV)吸收谱进行量子调控的可能性. 当XUV作用到He原子上时, 原子存在两个电离通道: 无明显电子关联的直接电离和带强烈电子关联的间接电离(即通过双激发态自发电离). 两个通道相互干涉可在XUV吸收谱中形成人们所熟知的Fano共振线型, 并且谱线的形状由两个通道间的比例决定. 通过引入另外一束IR激光, 我们发现, 原子的XUV吸收谱将发生明显改变, 即伴随着超短脉冲CEP的改变而 呈现出从Fano线型到Lorentz线型的周期性连续变化. 上述结果表明, 通过合理地控制超短脉冲的CEP可以有效地调控两个电离通道之间的量子干涉, 进而为探测和操控原子中的极端超快电子关联提供可能.     

Whispering gallery mode profiles in a coated microsphere

The properties of whispering gallery modes (WGM) of a microsphere can be tailored by coating the resonator with a thin film. If the coating has a higher refractive index than the microsphere core, the resulting structure will have a configuration which has similarities both with a classical WGM resonator and with a planar waveguide. By choosing the film thickness and refractive index we can tailor the degree of confinement of the WGMs inside the sphere core, coating and surrounding medium, which will in turn influence the WGMs contact with its surroundings. We present a study of the effect of the coating on the WGM mode profiles in particular of a silicon coating on a silica sphere.     

Transmission characteristics of a Fabry-Perot etalon-microtoroid resonator coupled system

The transmission spectra of a Fabry-Perot etalon coupled to a microtoroid resonator are studied theoretically and experimentally. The resonance line shapes depend strongly on the resonance wavelength detuning and coupling strength between the two resonators. A wide variety of line shapes, ranging from a single to triple peaks, symmetric to asymmetric Fano-like peaks, and notches were predicted and observed experimentally. The capability to modify the spectral line shapes by tuning the coupling between or losses of two resonators may find applications in optical filtering, switching, sensing, and dispersion engineering.     

Tunneling into clean heavy fermion compounds: origin of the Fano line shape

Recently observed tunneling spectra on clean heavy-fermion compounds show a lattice periodic Fano line shape similar to what is observed in the case of tunneling to a Kondo ion adsorbed at the surface. We show that the translation symmetry of a clean surface in the case of weakly correlated metals leads to a tunneling spectrum which shows a hybridization gap but does not have a Fano line shape. By contrast, in a strongly correlated heavy-fermion metal the heavy quasiparticle states will be broadened by interaction effects. The hybridization gap is completely filled in this way, and an ideal Fano line shape of width ～2TK results. In addition, we discuss the possible influence of the tunneling tip on the surface, in (i) leading to additional broadening of the Fano line and (ii) enhancing the hybridization locally, hence adding to the impurity type behavior. The latter effects depend on the tip-surface distance.     

Fano profile in a novel double cavity optomechanical system with harmonically bound mirrors

We investigate a double cavity optomechanical system generating single and double Fano resonance (multi-Fano). By altering a single parameter, the tunnelling rate g of the middle mirror, we are able to switch between single and double Fano line shapes. The first spectral line shape is stronger in the case of multi-Fano than in the case of single Fano. Also the behaviour of the steady state value of the displacement of the middle mirror, with respect to g, heavily influences the behaviour of double Fano lines in our scheme. This tunability along with using a single pump and signal/probe laser has an added advantage in situations where only low power consumption is available.     

Photon trajectories in incoherent atomic radiation trapping as Lévy flights

Photon trajectories in incoherent radiation trapping for Doppler, Lorentz, and Voigt line shapes under complete frequency redistribution are shown to be Lévy flights. The jump length (r) distributions display characteristic long tails. For the Lorentz line shape, the asymptotic form is a strict power law r(-3/2), while for Doppler the asymptotic is r(-2)(ln(r)(-1/2). For the Voigt profile, the asymptotic form always has a Lorentz character, but the trajectory is a self-affine fractal with two characteristic Hausdorff scaling exponents.     

Crystalline MgF2 whispering gallery mode resonators for enhanced bulk index sensitivity

We report on experiments on refractrometric sensing with crystalline Whispering Gallery Mode (WGM) resonators made of magnesium fluoride, which has a refractive index that is only slightly larger than that of water (Δn ≈ 0.05). The resulting evanescent field of a WGM resonator placed in an aqueous environment penetrates therefore deep into the surrounding medium, which makes it a promising candidate for sensing applications. We measured a bulk index sensitivity of 1.09 nm/RIU (refractive index unit) in a resonator with a radius of R = 2.91 mm and intrinsic Q-factors of more than 10⁸ in aqueous environments. Furthermore, we describe the fabrication process of crystalline WGM resonators.     

Existence and collapse of Fano resonances as a function of pinning field in simple mono-mode magnetic circuits

We consider, in the frame of long-wavelength Heisenberg model, the effect of a pinning field on the spin wave band gaps and transmission spectra of a simple magnonic device. This simple device is composed of an infinite one dimensional (1D) monomode waveguide (the backbone) along which N (N’) side resonators are grafted at two sites. Using a Green’s function method, we obtained closed-form expressions for the transmission coefficients for various systems of this kind. The amplitude, the phase, and the phase time of the transmission are discussed as a function of frequency and the strength of the pinning field parameter. In the presence of the pinning field at the ends of the side branches, the transmission probability may exhibit resonances of the Fano type. It is shown that the transition from strong to weak pinning leads to Fano line shapes with gradually smaller asymmetry. This also results in a modification of the position and the width of the resonance but the transmission amplitude remain unaffected. Furthermore, it is shown that the presence of the pinning field causes systematic collapse of certain Fano resonances. These results may provide useful means for the design of narrow-frequency optical or microwave filters.     

Fano resonance and spectral compression in a ring resonator drop filter with feedback

We propose a ring resonator drop filter with feedback loop which can form the other ring cavity to produce an asymmetric Fano-resonance line shape and mode compression. By properly adjusting the feedback loop, the asymmetric line shapes of the transmission spectra can be controlled and selectively mode compression of the circuiting intensity in the feedback loop is achieved. Such characteristics are useful for applications in ring resonator-based photonic devices such as all-optical switches, sensors and intracavity frequency selection single-mode laser.     

Whispering gallery microcavity lasers

Whispering gallery mode (WGM) optical microresonators have attracted intense interests in the past decades. The combination of high quality factors (Q) and small mode volumes of modes in WGM resonators significantly enhances the light‐matter interactions, making them excellent cavities for achieving low threshold and narrow linewidth lasers. In this Review, the progress in WGM microcavity lasers is summarized, and the laser performance considering resonator geometries and materials as well as lasing mechanisms is discussed. Label‐free detection using WGM resonators has emerged as highly sensitive detection schemes. However, the resolution is mainly limited by the cavity Q factor which determines the mode linewidth. Microcavity lasers, due to their narrow laser spectral width, could greatly improve the detection resolution. Some recent developments in sensing using microcavity lasers are discussed.