Asymmetric Fano Line Shape Research Articles

Plasmonic wavelength demultiplexers based on tunable Fano resonance in coupled-resonator systems

We investigate a plasmonic waveguide system using a 2-dimension finite element method, which consists of a metal-insulator-metal waveguide coupled with a stub resonator and a baffle. Numerical simulations results show that the sharp and asymmetric Fano-line shapes can be created by placing a baffle into the waveguide, and the main dependent factors of Fano-like spectra response are discussed. Based on the above results, we design three kinds of plasmonic waveguides with possible applications in wavelength demultiplexing. An analytic model based on the transfer matrix formalism is utilized to describe and explain this phenomenon. These results could be applied for developing ultra-compact wavelength division multiplexing devices in highly all-optical integration system.

Antenna-coupled microcavities for terahertz emission

We have investigated the capacitive coupling between dipolar antennas and metal-dielectric-metal wire microcavities with strong sub-wavelength confinement in the terahertz region. The coupling appears in reflectivity measurements performed on arrays of antenna-coupled elements, which display asymmetric Fano lineshapes. The experimental data are compared to a temporal coupled-mode theory and finite elements electromagnetic simulations. We show that the Fano interferences correspond to coupling between a subradiant mode (microcavity) and a superradiant mode (antennas). This phenomenon allows one to enhance and control the radiative coupling of the strongly confined mode with the vacuum. These concepts are very useful for terahertz optoelectronic devices based on deep-sub-wavelength active regions.

Tuning the transmission lineshape of a photonic crystal slab guided-resonance mode by polarization control

We demonstrate a system consisting of a two-dimensional photonic crystal slab and two polarizers which has a tunable transmission lineshape. The lineshape can be tuned from a symmetric Lorentzian to a highly asymmetric Fano lineshape by rotating the output polarizer. We use temporal coupled mode theory to explain the measurement results. The theory also predicts tunable phase shift and group delay.

Coupled-Resonator-Induced Fano Resonances for Plasmonic Sensing with Ultra-High Figure of Merits

Fano resonances are numerically predicted in an ultracompact plasmonic structure, comprising a metal-isolator-metal (MIM) waveguide side-coupled with two identical stub resonators. This phenomenon can be well explained by the analytic model and the relative phase analysis based on the scattering matrix theory. In sensing applications, the sensitivity of the proposed structure is about 1.1 × 103 nm/RIU and its figure of merit is as high as 2 × 105 at λ = 980 nm, which is due to the sharp asymmetric Fano line-shape with an ultra-low transmittance at this wavelength. This plasmonic structure with such high figure of merits and footprints of only about 0.2 μm2 may find important applications in the on-chip nano-sensors.

Lorentz Meets Fano in Spectral Line Shapes: A Universal Phase and Its Laser Control

Symmetric Lorentzian and asymmetric Fano line shapes are fundamental spectroscopic signatures that quantify the structural and dynamical properties of nuclei, atoms, molecules, and solids. This study introduces a universal temporal-phase formalism, mapping the Fano asymmetry parameter q to a phase φ of the time-dependent dipole response function. The formalism is confirmed experimentally by laser-transforming Fano absorption lines of autoionizing helium into Lorentzian lines after attosecond-pulsed excitation. We also demonstrate the inverse, the transformation of a naturally Lorentzian line into a Fano profile. A further application of this formalism uses quantum-phase control to amplify extreme-ultraviolet light resonantly interacting with He atoms. The quantum phase of excited states and its response to interactions can thus be extracted from line-shape analysis, with applications in many branches of spectroscopy.

Reduced linewidth multipolar plasmon resonances in metal nanorods and related applications

Using dark-field scattering spectroscopy, we study the multipolar plasmon resonances in single crystallized silver nanorods. The lineshapes and homogenous linewidths of the surface plasmon resonances (SPRs) of different orders are analyzed and compared. The high-order resonances are found to sustain asymmetric Fano lineshapes and their linewidths are narrower than the dipolar resonance. A quantitative comparison using the finite element method reveals more than a three times reduction in the linewidth for the third order resonance, as compared with the dipolar one. These narrow linewidths result from the smaller radiative damping of the multipolar SPRs. Benefiting from the reduced damping, multipolar SPRs in nanorods are better candidates for many plasmonic applications, including increased-sensitivity single particle SPR sensors and reduced-threshold nanolasers.

Cross-polarized excitons in double-wall carbon nanotubes

Optical absorption in double-wall carbon nanotubes for light polarized perpendicular to the tube axis is studied by taking into account exciton effects and depolarization effects within an effective-mass theory. The Coulomb interaction is suppressed by not only intrawall screening effects but also interwall screening, leading to the reduction of exciton binding energies and band gaps. When two tubes are both semiconducting, a clear exciton peak still survives even under depolarization effects for the outer tube, but the exciton peak of the inner tube has an asymmetric Fano line shape due to the coupling with continuum states of the outer tube. When a double-wall nanotube contains a metallic tube, either inner or outer, the exciton of the semiconducting tube loses its peak structure under depolarization effects.

A double-ring Mach–Zehnder interferometer sensor with high sensitivity

We theoretically proposed a cost-effective refractometer sensor, which includes an unbalanced two-ring Mach–Zehnder interferometer and can exhibit sharp asymmetric Fano lineshape. Our calculated results show this device has high sensitivity and can measure refractive index change in one of the resonators down to 10−9 refractive index units, which is two orders of magnitude higher than previous Fano lineshape based sensors.

Magneto-transport in a distorted Aharonov–Bohm ring

We investigate the two-terminal electronic transport of a model mesoscopic ring threaded by a magnetic flux and embedded in between two semi-infinite metallic electrodes. The ring is locally distorted to bring a pair of atomic sites residing in the two opposite arms into close proximity. The proximity of the two atoms is modeled by an inter-arm tunnel hopping matrix element, and is shown to lead to nontrivial features in the transmission and the density of states of the system. The asymmetric Fano lineshape and its reversal, the Aharonov–Bohm oscillations in the conductance and the device aspect of the system are critically examined.

Full frequency-domain approach to reciprocal microlasers and nanolasers–perspective from Lorentz reciprocity

We develop a frequency-domain formulation in the form of generalized eigenvalue problems for reciprocal microlasers and nanolasers. While the goal is to explore the resonance properties of dispersive cavities, the starting point of our approach is the mode expansion of arbitrary current sources inside the active regions of lasers. Due to the Lorentz reciprocity, a mode orthogonality relation is present and serves as the basis to distinguish various cavity modes. This scheme can also incorporate the asymmetric Fano lineshape into the emission spectra of cavities. We show how to obtain the important parameters of laser cavities based on this formulation. The proposed approach could be an alternative to other computation schemes such as the finite-difference-time-domain method for reciprocal cavities.

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.

Compact and high-resolution plasmonic wavelength demultiplexers based on Fano interference

Using strong couplings of different Fabry-Perot (FP) resonators in metal-insulator-metal waveguides, a compact plasmonic wavelength demultiplexer is numerically demonstrated with high wavelength resolution. In the demultiplexer, it is found that new right-angle resonators emerge with bandwidth narrower than that of the isolated FP resonators. These narrowband right-angle resonators interfere with the broadband FP resonators, resulting in Fano-line shapes in the transmission spectra. Consequently, these sharp and asymmetric Fano-line shapes considerably increase the resolution of wavelength demultiplexing, which is significantly narrower than the full width of the isolated FP resonator.

FANO-TYPE SPIN-FLIP MESOSCOPIC TRANSPORT THROUGH AN AHARONOV–BOHM INTERFEROMETER RESPONDED BY AC MAGNETIC FIELDS

We have investigated the mesoscopic transport properties of an Aharonov–Bohm (AB) interferometer composed of a direct tunneling path and a resonant tunneling path incorporating a quantum dot (QD). The QD is perturbed by a rotating magnetic field (RMF) and an oscillating magnetic field (OMF). The nonresonant tunneling (NRT) path takes significant role in the spin and charge currents, and the Fano profiles display in the currents due to the modification of NRT strength. The spin current exhibits completely different behaviors as the source–drain bias eV = 0 and eV≠0. The spin current is symmetric versus gate voltage when the source–drain bias eV = 0; however, it becomes asymmetric Fano-line shape resulting from the interference effect when eV≠0. The OMF induced photon-assisted tunneling is quite different from the ones induced by the oscillating electric field and RMF. The tunnel currents vary dramatically with the evolution of the AB magnetic flux, and the Fano-type spin and charge currents are adjusted by OMF, RMF, AB flux, gate voltage, the source–drain bias, as well as the NRT strength to generate novel Fano-type photon-assisted spin and charge currents.

Asymmetric Fano resonance analysis in indirectly coupled microresonators

We reveal that there exists an indirect interaction between two single-mode microcavities which have no direct coupling. This indirect interaction is mediated by coupling the two microcavities to a common waveguide, and it plays a key role in obtaining the asymmetric Fano line shape of the transport spectrum of the coupled cavity system. Finally, we show that this sharp Fano line shape can contribute to highly sensitive sensing, which is immune to most environmental variations.

Interpretation of Fano lineshape reversal in the reflectivity spectra of photonic crystal slabs

Resonant coupling of light to leaky modes of a photonic crystal slab leads to asymmetric Fano lineshapes in the reflectivity spectra. The generally accepted picture, for a lossless system, is that the sign of the real-valued parameter q controls the asymmetry of a Fano resonance. For the reflectivity of a symmetric slab this situation occurs if the amplitude reflection coefficient of the slab goes through zero. In this article, we show that it is also possible to change the asymmetry of a resonance by angle tuning without reaching a condition of zero amplitude. Moreover, we show that the picture of a real-valued parameter q that controls the asymmetry is incomplete.

Phonon switching and combined Fano-Rice effect in optical spectra of bilayer graphene

Recent infrared measurements of phonon peaks in gated bilayer graphene reveal two striking signatures of electron-phonon interaction: an asymmetric Fano lineshape and a giant variation of the peak intensity as a function of the applied gate voltage. In this Letter we provide a unified theoretical framework which accounts for both these effects and unveils the occurrence of a switching mechanism between the symmetric ($E_g$) and anti-symmetric ($E_u$) phonon mode as dominant channel in the optical response. A complete phase diagram of the optical phonon response is also presented, as a function of both the charge density and the bandgap.

Excitation Profiles and the Continuum in SERS: Identification of Fano Line Shapes

We extend a previous theoretical expression of SERS to include the possibility of coupling of the molecular levels with the continuum of levels in the conduction band in the metal. We modify the expression for excitation profiles to account for interference between nearby conduction band levels, producing an asymmetric Fano line shape. Both charge-transfer and molecular transitions are considered since both can contribute to the SERS enhancement. We test the theory by comparison with the observed profiles in pyridine, p-aminothiopehnol (PATP), and an azo dye (DABS). The experimental profiles are clearly asymmetric, dipping into the continuum on one side and approaching the continuum asymptotically on the other. In each case, an excellent fit of the Fano profile is obtained.

Broad Feshbach Resonance in theLi6−K40Mixture

We study the widths of interspecies Feshbach resonances in a mixture of the fermionic quantum gases 6Li and 40K. We develop a model to calculate the width and position of all available Feshbach resonances for a system. Using the model, we select the optimal resonance to study the {6}Li/{40}K mixture. Experimentally, we obtain the asymmetric Fano line shape of the interspecies elastic cross section by measuring the distillation rate of 6Li atoms from a potassium-rich 6Li/{40}K mixture as a function of magnetic field. This provides us with the first experimental determination of the width of a resonance in this mixture, DeltaB=1.5(5) G. Our results offer good perspectives for the observation of universal crossover physics using this mass-imbalanced fermionic mixture.

Fano-resonance-based Mach-Zehnder optical switch employing dual-bus coupled ring resonator as two-beam interferometer

A kind of Mach-Zehnder optical switch with a dual-bus coupled ring resonator as a two-beam interferometer is proposed and investigated. The analysis based on the transfer matrix method shows that a sharp asymmetric Fano line shape can be generated in the transmission spectra of such a configuration, which can be used to significantly reduce the phase change required for switching. Meanwhile, it can also be found that complete extinctions can be achieved in both switching states if the structural parameters are carefully chosen and the phase bias is properly set. Through tuning the phase difference between the arms of the Mach-Zehnder interferometer, complete extinction can be easily kept within a large range of the ring-bus coupling ratios in the OFF state. By properly modulating the phase change in the ring waveguide, the shift of the resonant frequency and the asymmetry of the transmission spectra can be controlled to finally enable optical switching with a high extinction ratio, even complete extinction, in the ON state. The switching functionality is verified by the finite-difference time-domain simulation.

Compact silicon microring-assisted directional couplers for optical signal processing applications

We propose and demonstrate a compact microring-assisted directional coupler in the silicon-on-insulator material system. An analytical model based on the strongly coupled mode theory shows that the device can be designed to have a wide range of spectral characteristics, including sharp asymmetric Fano line shapes, which can be used for WDM, switching, and sensing applications. The measured transmission spectra of the fabricated device with a 5-mum-radius microring resonator exhibited an asymmetric spectral response characteristic of the Fano resonance as predicted by the analytical model.