基于硅基光子器件的Fano共振研究进展

硅基光子技术的发展为新型微纳光学功能器件和片上系统提供了高可靠、高精度的实现手段.采用硅基光子技术构建的具有连续(准连续)模式微腔与离散模式的微腔耦合产生的Fano共振现象得到了广泛关注.Fano共振光谱在共振波长附近具有不对称且尖锐的谐振峰,传输光的强度在共振波长附近从0突变为1,该机制可显著提高硅基光开关、探测器、传感器,以及光非互易性全光信号处理的性能.本综述分析了Fano共振的一般数学表述,总结了当前硅基光子微腔耦合产生Fano共振的理论模型研究现状,讨论了不同类型硅光器件实现Fano共振的方法,比较各种方案优劣及适用场合,梳理了Fano共振在全光信号处理方面的应用研究情况.最后探讨存在的一些问题及未来可能的相关研究方向.     

Andreev-Fano Effect in a Hybrid Normal-Metal/Superconductor Interferometer

We report on a new type of Fano effect, named as Andreev-Fano effect, in a hybrid normal-metal/supeeconductor (N/S) interferometer embedded with a quantum dot. Compared with the conventional Fano effect, AndreevFano effect has some new features related to the characteristics of Andreev reflection. In the Iinear response regime, the line shape is the square of the conventional Fano shape, while in the nonlinear transport, a sharp resonant structure is superposed on an expanded interference pattern, which is qualitatively different from the conventional Fano effect. The phase dependence of the hybrid N/S interferometer is also distinguished from those of all-N or all-S interferometers.     

Fano resonances in semiconductor superlattices

We use semiconductor superlattices as a model system for the investigation of Fano resonances. In absorption the excitonic transitions of the Wannier–Stark ladder show the typical asymmetric line shape due to coupling to the continuum of lower-lying transitions. The unique feature of these Fano resonances is that they allow to continuously tune the key parameter – the coupling strength Γ between the discrete state and the degenerate continuum – by varying the bias voltage. Using this feature, we directly show that the Fano coupling leads to a fast polarization decay. We also investigate the dependence of the Fano parameters on the structure of the superlattice and compare with an extensive theoretical model of the resonances.     

Andreev-Fano Effect in a Hybrid Normal-Metal/Superconductor Interferometer

We report on a new type of Fano effect, named as Andreev-Fano effect, in a hybrid normal-metal/superconductor (N/S) interferometer embedded with a quantum dot. Compared with the conventional Fano effect, Andreev-Fano effect has some new features related to the characteristics of Andreev reflection. In the linear response regime, the line shape is the square of the conventional Fano shape, while in the nonlinear transport, a sharp resonant structure is superposed on an expanded interference pattern, which is qualitatively different from the conventional Fano effect. The phase dependence of the hybrid N/S interferometer is also distinguished from those of all-N or all-S interferometers.     

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.     

Multiple Fano resonances in nanorod and nanoring hybrid nanostructures

Multiple Fano resonances of plasmonic nanostructures have attracted much attention due to their potential applications in multicomponent biosensing. In this paper, we propose a series of hybridized nanostructures consisting of a single nanoring and multiple nanorods to generate multiple Fano resonances. One to three Fano resonances are achieved through tuning the number of nanorods. The interaction coupling process between different components of the nanostructures is recognized as the mechanism of multiple Fano resonances. We also theoretically investigate the applications of the produced multiple Fano resonances in refractive index sensing. The specific properties of multiple Fano resonances will make our proposed nanostructures beneficial to high-sensitivity biosensors.     

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.     

The phase of Fano resonance in Aharonov–Bohm interferometer with a quantum dot embedded

We study the Fano effect of a closed Aharonov–Bohm mesoscopic interferometer, with a quantum dot embedded in one of the paths, in the presence of dephasing which is introduced by the Büttiker model. An exact analytic formula of conductance including dephasing factor is derived and can be written as an extended Fano form with a complex Fano parameter q. While the total conductance is unitary, the coherent part has broken unitarity due to dephasing, and thus has continuous phase shift. Our results agree with recent experimental measurements.     

Shot noise of the spin inelastic tunneling through a quantum dot with single molecule-magnet

We have studied the quantum fluctuations of inelastic spin-electron scattering in quantum dot with an embedded biaxial single molecule-magnet and particularly investigated the zero-frequency shot noise and Fano factor in different magnetic fields. It is found that the shot noise and Fano factor exhibit a stepwise behaviour as bias increases in the presence of interaction between the electron and molecule-magnet for a weak magnetic field. As magnetic field becomes strong, a dip is displayed in the shot-noise-bias curve due to the suppression of inelastic shot noise caused by the quantum tunneling of magnetisation. Because of the spontaneous inelastic tunneling at zero bias, a small shot noise occurs, which results in the case of Fano factor F >> 1. Moreover, our results show that the sweeping speed can also influence the shot noise and Fano factor obviously.     

Bragg scattering induces Fano resonance in photonic crystals

We present a study of a Fano resonance between a narrow Bragg band and disorder-induced continuum in photonic crystals where the continuum is either of the broad band Fabry-Pérot scattering in an imperfect one-dimensional photonic crystal or Mie scattering in an imperfect three-dimensional photonic crystal. Our experimental studies of synthetic opals have demonstrated how the Fano resonance may lead to a transmission spectrum exhibiting a Bragg dip with an asymmetric profile or a Bragg rise.     

Quantum interference effect of resonant transport in nano-scale systems

We propose a new method of analyzing the quantum interference effect of the resonant transport in ballistic open systems. The new method is to obtain the resonant eigenvalues by computing the norm of the retarded and advanced Green's functions. Using the method, we illustrate for a fullerene and an AB-ring the relation between each resonant state and each asymmetric conductance peak, namely the Fano peak. We show that the combination of resonant states determines the symmetry of a conductance peak and that the Fano peak is caused by the asymmetry of the numerator of the conductance around a resonance. The Fano peak appears not only due to the quantum interference effect as often claimed, but more generally due to the resonant transport.     

Optically mediated spin current and Fano resonance in an Aharonov-Bohm ring with a quantum dot

We study the spin-polarized transport and Fano resonance in an Aharonov-Bohm (AB) interferometer with an embedded quantum dot, where the dot is irradiated by continuous circularly polarized light. Compared with the conventional Fano form, the resonance line shape is found to be deformed by the interplay between the external irradiation and the Coulomb repulsion. The Fano resonance peaks are split due to the shift of the effective energy level in the dot by Rabi oscillation of electron-heavy hole pairs. The direction and magnitude of spin current polarization can be modulated by the device parameters. Furthermore, the direct tunneling between two leads can induce a sharp sign reversal of spin polarization, the system thus operates as a rectifier for spin current polarization.     

Splitting of the Fano resonance by Coulomb interactions in a two-dot quantum ring

We study the splitting of the Fano resonance in a Aharonov–Bohm interferometer with a quantum dot in each of its arms. Both intra- and inter-dot Coulomb repulsions are taken into account by employing the Keldysh nonequilibrium Green’s function technique. The single narrow Fano resonance in the noninteracting case is split into two in the presence of either intra- or inter-dot Coulomb interaction. We find that four Fano peaks emerge in the conductance or local density of states spectra when the two kinds of interactions exist simultaneously. Such behavior holds true for the accompanying broad Breit–Wigner type resonance. We also show that the positions of the Fano peaks can be tuned with the aid of the magnetic flux penetrating through the ring, which might have practical applications in device design or quantum computation.     

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.     

ac field-induced Fano resonances in a parallel-coupled double quantum dot system

The effects of an ac electric field on the Fano resonance in a parallel-coupled double quantum dot system are investigated theoretically. The field can induce the photon-assisted Fano resonances for both symmetrical and asymmetrical parallel configurations. The magnitude and position of the photon-assisted Fano peak can be tuned by the ac field strength and frequency, respectively. Furthermore, the Fano resonance can appear with increasing the field frequency for both the symmetrical and asymmetrical configurations. This provides an efficient mechanism to control the Fano resonance. The photon-electron pumping effects for the symmetrical and asymmetrical cases are also studied in the weak- and strong-coupling regime.     

Photoelectron ionization spectra in a system interacting with a neighbor atom

We study long-time photoelectron ionization spectra of a system interacting with a neighbor two-level atom under the influence of a laser field. The system has at least a ground state and a continuum of states that are free with respect to one electron. In a weak light field, a Fano resonance is revealed. Also we consider an atomic autoionization level of the system, which leads to two Fano resonances under the influence of a weak field.     

Collapse of resonance in quasi-one-dimensional quantum channels

We study the resonance structure of the conductance (transmissivity) of a quasi-one-dimensional channel that contains an attractive impurity of finite dimensions and derive an exact expression for the scattering matrix. We show that an impurity of finite dimensions may cause a set of Fano resonances to appear in the transmissivity. We also find that due to the coherent interaction the Fano resonances can collapse and discrete levels may appear in the continuum. Finally, we establish the wave function of the discrete levels and study the channel transmissivity in the critical regime. Zh. éksp. Teor. Fiz. 116, 263–275 (July 1999)     

A new approach to the determination of the Fano factor for semiconductor detectors

Values of the Fano factor are widely scattered in the literature, indicating the difficulties in its determination. We chose to analyze both the escape peaks and parent peaks of Ge detector spectra where the parent peak penetration depth and the escape peak escape depth are much larger than the size of the incomplete charge collection region. Hence, the escape peaks are expected to be free of low‐energy tailing as, although the actually deposited energy is low, it is deposited beyond the incomplete charge collection region. It was found that the product of electron–hole pair creation energy (ε) and the Fano factor (F) has an energy dependence at low energies, as is expected from electron transport theories. Although ε is expected to have its own energy dependence, if a reference value of 2.96 eV is assumed for ε, then the Fano factor values varied between 0.059 and 0.083. The escape peak is less advantageous for Si, hence a different method was used for Si(Li) detectors. Assuming the reference value of 3.8 eV for ε, the Fano factor was found to be 0.063 at 5.9 keV x‐ray energy. We consider the Fano factors reported here as upper limits, rather than the mean values. Copyright © 2004 John Wiley & Sons, Ltd.     

Coulomb-modified Fano resonance in a one-lead quantum dot

We investigate a tunable Fano interferometer consisting of a quantum dot coupled via tunneling to a one-dimensional channel. In addition to Fano resonance, the channel shows strong Coulomb response to the dot, with a single electron modulating channel conductance by factors up to 100. Where these effects coexist, line shapes with up to four extrema are found. A model of Coulomb-modified Fano resonance is developed and gives excellent agreement with experiment.