Asymmetric Fano Resonance Research Articles

Microwave-assisted coherent and nonlinear control in cavity piezo-optomechanical systems

We present a cavity piezo-optomechanical system where microwave and optical degrees of freedom are coupled through an ultra-high frequency mechanical resonator. By utilizing the coherence among the three interacting modes, we demonstrate optical amplification, coherent absorption and a more general asymmetric Fano resonance. The strong piezoelectric drive further allows access to the large-amplitude-induced optomechanical nonlinearity, with which optical transparency at higher harmonics through multi-phonon scattering is demonstrated.

Fano Interference Between Higher Localized and Propagating Surface Plasmon Modes in Nanovoid Arrays

Surface plasmon resonance-based optical properties for gold nanovoid arrays were analyzed using three-dimensional finite-difference time-domain method, revealing that two asymmetric Fano resonances, with extremely low reflectivity and narrow line width, are caused by the strong interference between localized quadrupolar and hexadecapolar modes and propagating surface plasmon mode. The two Fano resonances produce significant electromagnetic field enhancement, resulting in 7 orders of magnitude surface-enhanced Raman scattering (SERS) enhancement, with about 100 times more than that produced by single propagating surface plasmon polaritons (SPPs) or localized surface plasmon resonance (LSPR) mode, including dipolar mode. Their narrow line width and giant field strength also allow highly sensitive sensing of refractive index; the sensitivities and figure of merits for such Fano resonance-based sensor are predicted to be 789 and 642 nm/RIU, 39 and 31, respectively. These plasmonic characteristics, thus, would enable the nanovoid arrays for using in wide practical applications including surface-enhanced spectroscopy and surface plasmon sensor.

Magnetic-based Fano resonance of hybrid silicon-gold nanocavities in the near-infrared region.

Direct interference between the orthogonal electric and magnetic modes in a hybrid silicon-gold nanocavity is demonstrated to induce a pronounced asymmetric magnetic-based Fano resonance in the total scattering spectrum at near-infrared frequencies. Differing from the previously reported magnetic-based Fano resonances in metal nanoparticle clusters, the narrow discrete mode provided by the silicon magnetic dipole resonance can be directly excited by external illumination, and greatly enhanced electric and magnetic fields are simultaneously obtained at the Fano dip.

Optical properties of nitrogen-doped graphene thin films probed by spectroscopic ellipsometry

Nitrogen-doped graphene thin films were prepared by either chemical vapor deposition (CVD) or electrochemical exfoliation (ECE). Their optical properties were determined in the spectral region of 0.73–6.42eV and at temperatures between 200 and 350K by spectroscopic ellipsometry. The parameters of the dispersive structures were derived by numerical fitting of the experimental data to the stacked layer model. The optical absorption spectrum of the CVD-grown thin films is characterized by an asymmetric Fano resonance in the ultraviolet frequency region. In contrast, the line shape of the ECE-grown thin films displays less asymmetric. The excitonic resonance of the nitrogen-doped thin films is overall blue shifted by ~0.2–0.3eV compared with that of undoped analog. We interpret these results in terms of the exothermic nature of triazine molecule adsorption due to binding to graphene's surface via electron rich nitrogen.

Shot noise in the hybrid triple-quantum-dot interferometer coupled to superconductor and normal terminals

Abstract The shot noise of a hybrid triple-quantum-dot (TQD) interferometer has been investigated by employing the nonequilibrium Green's function method, and the general shot noise formula has been derived. The oscillation behaviors of transmission coefficients and shot noise versus the Aharonov–Bohm phase ϕ exhibit asymmetric Fano resonance structure and blockade effect. Sub-Poissonian and super-Poissonian behaviors of shot noise appear in different regimes of terminal bias e V γ contributed by the Andreev reflection, and correlation of Andreev tunneling with the normal electron transport. The inverse resonance and resonance structures emerge in the shot noise and Fano factor with respect to one of the gate voltages in different regimes of e V γ . The asymmetric structure can be enhanced by modifying the energy levels and gate biases of the TQD. The self-correlation and cross-correlation of current components contribute to the enhancement and suppression of shot noise.

Fano resonances in a multimode waveguide coupled to a high-Q silicon nitride ring resonator

Silicon nitride (Si3N4) optical ring resonators provide exceptional opportunities for low-loss integrated optics. Here we study the transmission through a multimode waveguide coupled to a Si3N4 ring resonator. By coupling single-mode fibers to both input and output ports of the waveguide we selectively excite and probe combinations of modes in the waveguide. Strong asymmetric Fano resonances are observed and the degree of asymmetry can be tuned through the positions of the input and output fibers. The Fano resonance results from the interference between modes of the waveguide and light that couples resonantly to the ring resonator. We develop a theoretical model based on the coupled mode theory to describe the experimental results. The large extension of the optical modes out of the Si3N4 core makes this system promising for sensing applications.

Highly Sensitive Plasmonic Sensor Based on Fano Resonance from Silver Nanoparticle Heterodimer Array on a Thin Silver Film

We investigate the optical spectrum of a multilayer metallic slab using multiple-scattering formalism. A thin silver film is attached to a periodic array of heterodimers consisting of two vertically spaced silver nanoparticles of different radii. Depending on the radius of nanoparticles, heterodimer array presents a simple nanoscale geometry which gives rise to remarkable plasmonic properties of multipolar resonances. Due to the coherent interference of the localized nanoparticle plasmons (discrete mode) and surface plasmon polaritons of metallic film (continuous mode), the reflection spectrum represents a sharp asymmetric Fano resonance dip, which is strongly sensitive to the refractive index of the surrounding embedded dielectric host. The physical features contribute to a highly efficient plasmonic sensor for refractive index sensing with sensitivity of ∼1.5 × 10−3 RIU/nm.

Optical properties of a two-dimensional nanodisk array with super-lattice defects

Surface plasmon resonance modes in a two-dimensional gold nanodisk array incorporated with super-lattice defects are investigated in this paper. Both transmission and absorption spectra of the super-lattice metal nanodisk array are calculated. The transmission spectrum exhibits an asymmetric Fano resonance lineshape and a narrow linewidth peak between two transmission dips. The electric field and polarization charge distributions at the transmission peak and dip wavelengths are also calculated. It is explained that the surface plasmon resonance modes associated with the large period nanodisks and small period nanodisks give rise to the Fano resonance lineshape and the anomalous transmission peak. We also retrieve the effective optical constants of the metal nanodisk array film. The effective optical constants exhibit strong chromatic dispersion and a small attenuation at the peak transmission wavelength. The strong dispersion and small attenuation can potentially be used for slow light.

Resonant Transparency and Non-Trivial Non-Radiating Excitations in Toroidal Metamaterials

Engaging strongly resonant interactions allows dramatic enhancement of functionalities of many electromagnetic devices. However, resonances can be dampened by Joule and radiation losses. While in many cases Joule losses may be minimized by the choice of constituting materials, controlling radiation losses is often a bigger problem. Recent solutions include the use of coupled radiant and sub-radiant modes yielding narrow asymmetric Fano resonances in a wide range of systems, from defect states in photonic crystals and optical waveguides with mesoscopic ring resonators to nanoscale plasmonic and metamaterial systems exhibiting interference effects akin to electromagnetically-induced transparency. Here we demonstrate theoretically and confirm experimentally a new mechanism of resonant electromagnetic transparency, which yields very narrow isolated symmetric Lorentzian transmission lines in toroidal metamaterials. It exploits the long sought non-trivial non-radiating charge-current excitation based on interfering electric and toroidal dipoles that was first proposed by Afanasiev and Stepanovsky in [J. Phys. A Math. Gen. 28, 4565 (1995)].

Fano resonances in ultracompact waveguide Fabry-Perot resonator side-coupled lossy nanobeam cavities

We propose and demonstrate experimentally an ultracompact resonant system exhibiting sharp asymmetric Fano-resonance, in which a tiny photonic crystal nanobeam cavity is laterally coupled to a waveguide Fabry-Perot resonator. Under the presence of cavity intrinsic loss, we find that the system produces sharper and higher extinction ratio Fano resonance spectra only if appropriate phase is selected. The whole device (∼6 μm2) is implemented on silicon on insulator platform, and experimentally we observe steep-slope Fano resonance with extinction ratio of 7.3 dB, slope above 5 dB/nm, and average insertion loss less than 1.8 dB.

Theoretical and experimental evidence of Fano-like resonances in simple monomode photonic circuits

A simple photonic device consisting of two dangling side resonators grafted at two sites on a waveguide is designed in order to obtain sharp resonant states inside the transmission gaps without introducing any defects in the structure. This results from an internal resonance of the structure when such a resonance is situated in the vicinity of a zero of transmission or placed between two zeros of transmission, the so-called Fano resonances. A general analytical expression for the transmission coefficient is given for various systems of this kind. The amplitude of the transmission is obtained following the Fano form. The full width at half maximum of the resonances as well as the asymmetric Fano parameter are discussed explicitly as function of the geometrical parameters of the system. In addition to the usual asymmetric Fano resonance, we show that this system may exhibit an electromagnetic induced transparency resonance as well as well as a particular case where such resonances collapse in the transmission coefficient. Also, we give a comparison between the phase of the determinant of the scattering matrix, the so-called Friedel phase, and the phase of the transmission amplitude. The analytical results are obtained by means of the Green's function method, whereas the experiments are carried out using coaxial cables in the radio-frequency regime. These results should have important consequences for designing integrated devices such as narrow-frequency optical or microwave filters and high-speed switches. This system is proposed as a simpler alternative to coupled-micoresonators.

Microsphere Coupled Mach-Zehnder Interferometer for Biochemical Sensor

We present the microsphere coupled Mach–Zehnder interference structure. We theoretically calculate that spectral responses of this structure vary with the glucose solution concentrations. It can produce the sharply asymmetric Fano resonance line shape related to the slope between zero and unity transmission. The variation of the normalized transmission is linearly related to the solution concentration. This structure is the promising highly sensitive biochemical sensor due to high quality factor resonance and steep slope over a very narrow frequency range

Theory of SNAP devices: basic equations and comparison with the experiment

A SNAP (Surface Nanoscale Axial Photonics) device consists of an optical fiber with introduced nanoscale effective radius variation, which is coupled to transverse input/output waveguides. The input waveguides excite whispering gallery modes circulating near the fiber surface and slowly propagating along the fiber axis. In this paper, the theory of SNAP devices is developed and applied to the analysis of transmission amplitudes of simplest SNAP models exhibiting a variety of asymmetric Fano resonances and also to the experimental characterization of a SNAP bottle microresonator and to a chain of 10 coupled microresonators. Excellent agreement between the theory and the experiment is demonstrated.

Plasmonic nanosensor based on Fano resonance in waveguide-coupled resonators

We propose a plasmonic nanosensor based on Fano resonance in the strong-confinement metal-dielectric-metal waveguide side-coupled with a pair of nanoresonators. Due to the coherent interference of the splitting discrete and quasi-continuum modes, the reflection spectrum possesses a sharp asymmetric Fano resonance dip, which is dependent on the cavity-cavity phase and the refractive index change of the dielectric. The physical features contribute to a highly efficient plasmonic sensor for refractive index sensing. The nanosensor yields a sensitivity of ~900 nm/RIU and a figure of merit of ~500, remarkable values compared with those of plasmonic sensors supported by perfect absorbers.

Asymmetric Fano resonance in eye-like microring system

In this paper, the Fano resonance in the “eye-like” microring resonator system (EMRS) is theoretically and experimentally investigated. The asymmetric Fano-resonance line shape of EMRS is generated by adding a microring inside and coupling with the outer ring to produce a nonlinear phase shift. The EMRS was fabricated on silicon-on-insulator wafer by the complementary metal-oxide-semiconductor compatible process. Compared to the conventional single-stage microring add-drop filter structure, the maximum sharpness of the transmission of the drop port is enhanced 3 times, and the drop port extinction ratio is increased by about 20 dB. The experimental results are in good agreement with the theoretical analysis.

Transmission resonance induced by a δ‐like defect in the Fano–Anderson model with two Fano defects

AbstractWe study theoretically the propagation of waves in a discrete linear chain with two side‐coupled Fano defects and one δ‐like defect, using a modified Fano–Anderson model. Two separate cases, where the Fano defects are coupled to the chain by on‐site coupling or by inter‐site coupling, are considered. We find that the transmission behavior is very sensitive to whether the distance between the Fano defects and the δ‐like defect, l, is even or odd. In the on‐site coupling case, we find that for even values of l, the Fano–Feshbach resonance is sustained in the presence of a δ‐like defect, but the position of the transmission peak is shifted and its shape becomes asymmetric. In the inter‐site coupling case, the width of the asymmetric Fano resonance can be tuned by changing the coupling constants. When l is odd in the inter‐site coupling case, we find that there exist two resonant transmissions in addition to the perfect reflection. In general, the δ‐like defect strongly influences on the transmission resonance but does not alter the perfect reflection.

Enhancing Surface Plasmon Detection Using Template-Stripped Gold Nanoslit Arrays on Plastic Films

Nanostructure-based sensors are capable of sensitive and label-free detection for biomedical applications. However, high-throughput and low-cost fabrication techniques are the main issues which should be addressed. In this study, chip-based nanostructures for intensity-sensitive detection were fabricated and tested using a thermal-annealing-assisted template-stripping method. Large-area uniform nanoslit arrays with a 500 nm period and various slit widths, from 30 to 165 nm, were made on plastic films. A transverse magnetic-polarized wave in these gold nanostructures generated sharp and asymmetric Fano resonances in transmission spectra. The full width at half-maximum bandwidth decreased with the decrease of the slit width. The narrowest bandwidth was smaller than 10 nm. Compared to nanoslit arrays on glass substrates using electron-beam lithography, the proposed chip has a higher intensity sensitivity up to 10367%/RIU (refractive index unit) and reaches a figure of merit up to 55. The higher intensity sensitivity for the template-stripped nanostructure is attributed to a smoother gold surface and larger grain sizes on the plastic film, which reduces the surface plasmon propagation loss.

Experimental controlling of Fano resonance in indirectly coupled whispering-gallery microresonators

We experimentally studied the transmission spectrum of a coupled resonator structure in which a low-Q microdisk and a high-Q microtoroid indirectly interact with each other mediated by a fiber taper. Asymmetric Fano resonances were observed and could be controlled to change periodically by adjusting the distance between the two microresonators. It is revealed that the Fano resonance originates from the coupling of the two modes belonging to the two microresonators. The observed period of distance change is around 8 μm, which shows good agreement with the theoretical prediction by the beat of multiple propagating modes in the fiber taper.

Unconventional ballistic transport through bilayer graphene electrostatic barriers

The asymmetric Fano resonances (FRs) are noted for the first time in the coherent ballistic transmission spectrum of bilayer graphene tunneling structures. This characteristic tunneling transmission that occurs particularly at small glancing incidence mainly arises due to the quadratic dispersion of chiral charge carriers. The Fano line shape is found to be highly sensitive with respect to the direction of incidence of the charge carriers, the applied homogeneous electric field, as well as to the barrier height and width. The absence of the FR at normal incidence is in accordance with the conservation of chirality of the Dirac fermions in bilayer graphene. The sharp antiresonance at the center of the tunneling band arising due to destructive interference between the electron and the holelike states might be responsible for high negative differential conductivity (NDC) in bilayer graphene.

Signature of quantum interference and the Fano resonances in the transmission spectrum of bilayer graphene nanostructure

The well-known asymmetric Fano resonances that results from the quantum interference between the discrete and the continuum states are noted for the first time in the ballistic transmission spectrum of the bilayer graphene tunneling structures. This unconventional tunneling transmission, in stark contrast to the monolayer graphene and to the conventional heterostructures, arises due to the quadratic dispersion of the chiral charge carriers. If the Klein tunneling (the phenomenon for normal incidence) is an unusual characteristic of the massless chiral particles, then the Fano tunneling (the phenomenon for low glancing incidence) would be the specialty for the massive chiral particles. The characteristic features of the Fano line shape are found to be highly sensitive to the direction of incidence of the charge carriers, the applied homogeneous electric field, and to the barrier height. The sharp anti-resonance at the center of the tunneling band arising due to the destructive interference between the electron and the holelike states could probably be responsible for the high negative differential conductance (NDC) in the bilayer graphene. The tunneling conductance in the double barrier structure exhibits a resonant peak with a sharp NDC region for the Fermi energy less than or equal to half of the barrier height. The present findings might have great implications in the preparation of NDC-based devices using bilayer graphene nanostructures.