Fano Dips Research Articles

Deterministic relation between thermal-phonon dressings and a non-Hermitian multi-Fano interferences router in ion-doped microcrystals

We report the multi-Fano interference obtained through the simultaneous acquisition of bright and dark states in different phase transitions of Eu3+: BiPO4 (7:1, 6:1, 1:1, and 0.5:1) and Eu3+: NaYF4 (1:1/4) crystals. We employ multi-dressed spontaneous four-wave mixing and multi-dressed fluorescence (multi-order) to optimize the strong photon-phonon nested dressing effect resulting in more obvious multi-Fano interference. Firstly, the multi-Fano is produced through interference in continuous and multi-bound states. Secondly, five multi-Fano dips originate from nested five dressings (one photon and four phonons) under symmetrical splitting of 7F1 energy level. We depict that the pure H-phase (0.5:1) sample exhibits the strongest photon-phonon dressed effect (five Fano dips). Further, we investigate high-order non-Hermitian exceptional points in multi-Fano interference by adjusting the ratio of Rabi-frequency to de-phase rate through nested photon and phonon dressing. Our experimental results are validated by theoretical simulations, which may be applied to designing optoelectronic devices such as non-Hermitian multi-Fano interferences (multi-channel) router.

Spectral line-shape controlled by nonlinear Fano interference in Eu3+-doped microcrystals

We investigate nonlinear Fano interference effect in time-resolved excitation spectra of Eu3+-doped microcrystals. Multiple parameters are used to control Fano interference. The transition between bright and dark states depends on the initial phase, cross-Kerr and self-Kerr nonlinear phase. Spectral Line-shape is continuously tuned by delay time, laser power, time gate width, bandwidth of laser, detector position and temperature. Two Fano dips are observed in excitation spectra. The photon–phonon dressing Fano interference model is set up to explain this phenomenon. The tunable nonlinear Fano interference line-shape may be used for quantum devices such as high-contrast all-optical switch and multi-channel optical filter.

Turning a hot spot into a cold spot: polarization-controlled Fano-shaped local-field responses probed by a quantum dot

Optical nanoantennas can convert propagating light to local fields. The local-field responses can be engineered to exhibit nontrivial features in spatial, spectral and temporal domains, where local-field interferences play a key role. Here, we design nearly fully controllable local-field interferences in the nanogap of a nanoantenna, and experimentally demonstrate that in the nanogap, the spectral dispersion of the local-field response can exhibit tuneable Fano lineshapes with nearly vanishing Fano dips. A single quantum dot is precisely positioned in the nanogap to probe the spectral dispersions of the local-field responses. By controlling the excitation polarization, the asymmetry parameter q of the probed Fano lineshapes can be tuned from negative to positive values, and correspondingly, the Fano dips can be tuned across a broad spectral range. Notably, at the Fano dips, the local-field intensity is strongly suppressed by up to ~50-fold, implying that the hot spot in the nanogap can be turned into a cold spot. The results may inspire diverse designs of local-field responses with novel spatial distributions, spectral dispersions and temporal dynamics, and expand the available toolbox for nanoscopy, spectroscopy, nano-optical quantum control and nanolithography.

Plasmonic refractive index sensor with multi-channel Fano resonances based on MIM waveguides

A multi-channel Fano resonant structure is proposed and analyzed based on subwavelength metal–insulator–metal (MIM) waveguides. First, two MIM output ports associated with specific side-coupled cavities are designed to locate at the center and quarter positions of an end-coupled cavity, respectively. Since the interference between the dark and bright modes, dual-channel Fano resonances with asymmetrical lines shapes are obtained at both ports, respectively. High sensitivity and figure of merits are investigated. Besides, phase shifts are also investigated leading to positive and negative group delays available at the Fano peaks and dips, respectively. Likewise, two extra output ports with identical resonant cavities are placed on the other side of the end-coupled cavity. In this case, four-channel Fano resonances with considerable performances are obtained. The proposed structure is analyzed by the coupled mode theory and the finite difference time domain method. It is believed this device can be used as a chip-scale refractive index sensor and optical filter.

Bifunctional resonance effects of classical electromagnetically induced transparency and Fano response using a terahertz metamaterial resonator.

A terahertz metamaterial resonator consisting of two rectangular strips embedded in a closed-ring resonator is designed and investigated in this paper that can simultaneously generate the classical electromagnetically induced transparency effect and two Fano resonance modes. The formation of the transparent window and the Fano dips can be attributed to the coupling of the localized resonance modes between the closed-ring resonator and the two embedded rectangular strips. The effect of embedded rectangular strip sizes on the transmitted performance is discussed, and it is found that the resonant performance of the transparent window and Fano effect depends mainly on the length of the rectangular strips in the longitudinal direction. Based on this, two different device applications related to the embedded rectangular strips are discussed. The metamaterial proposed here could open up new avenues toward the control of terahertz waves in many technology-related areas.

A low lasing threshold and widely tunable spaser based on two dark surface plasmons

We theoretically demonstrate a low threshold and widely tunable spaser based on a plasmonic nanostructure consisting of two sets of disk-rings (TSDR). The TSDR nanostructure supports two dark surface plasmons (SPs), which are excited simultaneously by two bright SPs at Fano dips. The two dark SPs support lower effective mode volume, higher quality factor and higher Purcell factors. When the dark SPs serve as the pumping and lasing mode of a spaser, the spaser has a lower lasing threshold, a higher pump absorption efficiency and a lower threshold absorbed pump power than the spaser based on a bright SP. In addition, the lasing and pumping wavelengths of the spaser proposed in this article can each be tuned over a very wide wavelength range. Our results should be significant for the development of spasers.

Interaction between graphene-coated nanowires revisited with transformation optics.

The interaction between graphene-coated nanostructures provides interesting optical properties not found in isolated graphene plasmonic structures. However, full-analytical solutions, which can provide deep physical insights underlying the hybrid graphene plasmonic systems, are difficult to achieve. In this Letter, we deploy the theory of transformation optics to study the plasmonic interactions between two dielectric-core-graphene-shell nanowires. The scattering and absorption spectra as well as the field distributions are derived analytically. We find that the interaction between two graphene-coated nanowires results in polarization-independent multi-frequency Fano dips, which show a broadband red shift of bonding modes and a blue shift of anti-bonding modes when the nanowires approach each other. The analytical tool presented here offers a rigorous study of graphene plasmonic compound and can be extended to treat more complicated cases.

Fano resonance in Al nano-dolmen plasmonic structure for enhanced biosensing

Aluminum nanoparticles or nanolayers are low cost and high quality materials for plasmonic sensing. Here we systematically studied the Fano characteristics of Al nano-dolmen and achieved the continuously turning the Fano dips in visible ranges. The Fano dips were significantly changed through varying the arm length of the designed dolmen structure. To further optimize and characterize the Fano resonance of Al nano-dolmen, an intuitive coupling function is used to fit the Fano line shape. More importantly, we showed that our designed nanostructures exhibit a remarkable spectral plasmonic sensitivity to the tiny refractive index change of the local dielectric environment. The refractive index changes that we measured from 1.0 to 1.4 covered most of the ranges that induced by the general molecular bindings like single-stranded DNAs and antibody-antigens.

Asymmetrically engineered metallic nanodisk clusters for plasmonic Fano resonance generation

In this paper, we numerically introduce a planar metamolecule that generates plasmonic Fano resonance. The engineered molecule consists of closely packed asymmetric gold nanodisks deposited on a glass substrate operating at visible and near-infrared wavelengths. The asymmetric arrangement of nanodisks plays a key role in Fano resonance generation. The induced extinction cross-section spectroscopy has a Fano-like shape owing to interference between bright and dark plasmonic modes sustained by the asymmetric nanodisk clusters. The Fano dips are shown to be highly sensitive to the interdisk gaps as well as to the surrounding environment. As a result, we introduce a potential refractive index nanosensor having a sensitivity of 660 nm/RIU and a figure of merit of 4.75. The proposed metamolecule holds potential for various applications, such as Fano-induced enhancement of solar energy harvesting, molecular fluorescence, and photo upconversion.

Tunable THz wave absorption by graphene-assisted plasmonic metasurfaces based on metallic split ring resonators

Graphene plasmonics has been introduced as a novel platform to design various nano- and microstructures to function in a wide range of spectrum from optical to THz frequencies. Herein, we propose a tunable plasmonic metamaterial in the THz regime by using metallic (silver) concentric microscale split ring resonator arrays on a multilayer metasurface composed of silica and silicon layers. We obtained an absorption percentage of 47.9% including two strong Fano resonant dips in THz regime for the purely plasmonic metamaterial without graphene layer. Considering the data of an atomic graphene sheet (with the thickness of ~0.35 nm) in both analytical and experimental regimes obtained by prior works, we employed a graphene layer under concentric split ring resonator arrays and above the multilayer metasurface to enhance the absorption ratio in THz bandwidth. Our numerical and analytical results proved that the presence of a thin graphene layer enhances the absorption coefficient of MM to 64.35%, at the highest peak in absorption profile that corresponds to the Fano dip position. We also have shown that changing the intrinsic characteristics of graphene sheet leads to shifts in the position of Fano dips and variations in the absorption efficiency. The maximum percentage of absorption (~67%) was obtained for graphene-based MM with graphene layer with dissipative loss factor of 1477 Ω. Employing the antisymmetric feature of the split ring resonators, the proposed graphene-based metamaterial with strong polarization dependency is highly sensitive to the polarization angle of the incident THz beam.

Fano Resonance Based on End-Coupled Cascaded-Ring MIM Waveguides Structure

Though adding a groove to a plasmonic end-coupled perfect ring (PR) resonator, two additional resonance modes, which can be controlled by the length of the groove, will arise in this proposed ring-groove (RG) joint metal-insulator-metal (MIM) waveguide. By further cascading, the PR resonator and the RG joint resonator, single and dual Fano resonances with asymmetric line shapes are obtained due to the interference effects between the dark modes and the bright modes. High figure of merit and high refractive-index sensitivity are achieved, and thus this structure is suitable for the biochemistry sensing area. Interestingly, normal and abnormal dispersions are also investigated for the Fano peaks and dips, respectively. The performances of the proposed structure are investigated by using the finite-difference time-domain method.

Tailoring the Multiple Fano Resonances in Nanobelt Plasmonic Cluster

We theoretically explore the appearances and characteristics of Fano resonances in novel-designed nanobelt cluster, which shows strong modulation depths, and the Fano dips can be tailored independently. The underlying physical mechanisms contributing to Fano resonances are discussed with the electric field enhancement and charge distributions. The results demonstrate that dark-mode excitation is not a necessary condition for the existence of Fano resonances in the nanobelt cluster. The lineshape of the scattering spectra can be regulated effectively by adjusting the geometrical parameters. The effect of the incident illumination polarization orientation is also numerically investigated, indicating that Fano resonances can be completely switched off when the nanobelt cluster is induced by 90°. Moreover, multiple Fano resonances would occur as the number of the nanobelt is increased, which is explained by a physical mechanism based on temporal coupled-mode theory.

Double Fano resonances excited in a compact structure by introducing a defect

In this letter, the plasmonic system composed of a defective silver nanostrip and a complete silver nanostrip is theoretically investigated to achieve two Fano resonances. A quadrupole mode cannot be directly excited on a complete nanostrip, while it can be fulfilled by introducing the defect. This defective silver nanostrip supports both of the superradiant dipole and quadrupole modes, which produce bright-dark coupling and detuning modes mechanisms and further result in two asymmetric Fano dips in the transmission spectrum. Moreover, the Fano resonance characteristics in the two nanostrips can be tuned by modulating the gap distance between the two nanostrips, the parameters of the defect, and the electric-field polarization angle. Our structure may have potential applications for nanosensor in highly integrated circuits.

SNS junctions in nanowires with spin-orbit coupling: Role of confinement and helicity on the subgap spectrum

We study normal transport and the subgap spectrum of superconductor-normal-superconductor (SNS) junctions made of semiconducting nanowires with strong Rashba spin-orbit coupling. We focus, in particular, on the role of confinement effects in long ballistic junctions. In the normal regime, scattering at the two contacts gives rise to two distinct features in conductance: Fabry-Perot resonances and Fano dips. The latter arise in the presence of a strong Zeeman field $B$ that removes a spin sector in the leads (helical leads), but not in the central region. Conversely, a helical central region between nonhelical leads exhibits helical gaps of half-quantum conductance, with superimposed helical Fabry-Perot oscillations. These normal features translate into distinct subgap states when the leads become superconducting. In particular, Fabry-Perot resonances within the helical gap become parity-protected zero-energy states (parity crossings), well below the critical field ${B}_{c}$ at which the superconducting leads become topological. As a function of Zeeman field or Fermi energy, these zero modes oscillate around zero energy, forming characteristic loops, which evolve continuously into Majorana bound states as $B$ exceeds ${B}_{c}$. The relation with the physics of parity crossings of Yu-Shiba-Rusinov bound states is discussed.

Multiple Fano interferences in a plasmonic metamolecule consisting of asymmetric metallic nanodimers

We theoretically explore signatures of plasmonic Fano interferences in a subwavelength plasmonic metamolecule consisting of closely packed asymmetric gold nanodimers, which lead to the possibility of generating multiple Fano resonances in the scattering spectrum. This spectral feature is attributed to the interference between bright and dark plasmonic modes sustained by the constituent nanodimers. The excited Fano dips are highly sensitive in both wavelength and amplitude to geometry and background dielectric medium. The tunability of induced Fano resonances associated with enhanced electric fields from the visible to infrared region provides promising applications, particularly in refractive index sensing, light-trapping, and photon up-converting.

Tailoring the negative-refractive-index metamaterials composed of semiconductor-metal-semiconductor gold ring/disk cavity heptamers to support strong Fano resonances in the visible spectrum.

In this study, we investigated numerically the plasmon response of a planar negative-index metamaterial composed of symmetric molecular orientations of Au ring/disk nanocavities in a heptamer cluster. Using the plasmon hybridization theory and considering the optical response of an individual nanocluster, we determined the accurate geometrical sizes for a ring/disk nanocavity heptamer. It is shown that the proposed well-organized nanocluster can be tailored to support strong and sharp Fano resonances in the visible spectrum. Surrounding and filling the heptamer clusters by various metasurfaces with different chemical characteristics, and illuminating the structure with an incident light source, we proved that this configuration reflects low losses and isotropic features, including a pronounced Fano dip in the visible spectrum. Technically, employing numerical methods and tuning the geometrical sizes of the structure, we tuned and induced the Fano dip in the visible range, while the dark and bright plasmon resonance extremes are blueshifted to shorter wavelengths dramatically. Considering the calculated transmission window, we quantified the effective refractive index for the structure, while the substance of the substrate material was varied. Using Si, GaP, and InP semiconductors as substrate materials, we calculated and compared the corresponding figure of merit (FOM) for different regimes. The highest possible FOM was obtained for the GaP-Au-GaP negative-refractive-index metamaterial composed of ring/disk nanocavity heptamers as 62.4 at λ∼690 nm (arounnd the position of the Fano dip). Despite the outstanding symmetric nature of the suggested heptamer array, we provided sharp Fano dips by the appropriate tuning of the geometrical and chemical parameters. This study yields a method to employ ring/disk nanocavity heptamers as a negative-refractive-index metamaterial in designing highly accurate localization of surface plasmon resonance sensing devices and biochemical sensors.

Multiple Fano Resonances in Plasmonic Metamaterials Composed of Al/Al2O3 Nanomatryushka Structures

ABSTRACTMetamaterial structures composed of ordered arrays of metallic nanoparticles (NPs) and nanocavities are able to support strong plasmon and Fano resonances in the optical frequencies, where the appeared Fano dips can be utilized in bio/chemical sensing and spectroscopic purposes with a significant sensitivity. Herein, we utilize two concentric compositional Aluminum (Al) nanoshells (Al/Al2O3) to design nanomatryushka (NM) structures in periodic arrays, where each one of Al NPs is covered by a certain thickness of the oxide layer. Depositing studied Al NM arrays on metasurfaces, we determined the optical response of the metamaterial. It is shown that the proposed structure is able to support multiple strong Fano resonances in the visible spectrum. Evaluating the plasmon response of the metamaterial configuration for the presence of various semiconductor metasurfaces (Silicon and GaP), the quality of Fano dips is analyzed for different regimes. In this method, we measured the accuracy and sensitivity of the metamaterial structure by plotting the linear figure of merit (FoM) and quantifying this parameter.

Intensifying magnetic dark modes in the antisymmetric plasmonic quadrumer composed of Al/Al2O3 nanodisks with the placement of silicon nanospheres

In this study, a quadrumer cluster composed of Al nanodisks in both symmetric and antisymmetric orientations has been utilized to generate magnetic hot-spots by using coil-type Fano resonances. Determining the accurate geometrical sizes for the examined cluster, we calculated the spectral response of the structure numerically. Utilizing strong plasmon resonance hybridization between Al/Al2O3 nanodisks that are suited in a close proximity to each other, such a finite and simple nanocluster yields intensified hidden magnetic fields |H| as a dark mode and electric |E| as a bright modes. Using and placement of silicon nanospheres in the unoccupied gap distance between proximal Al nanodisks give rise to significant enhancement in the energy and quality of the induced multiple Fano dips. Appearing of multiple Fano resonant modes in a coil-type regime in the UV and visible spectrum helps us to optimize the energy of generated magnetic hot-spots, significantly. Ultimately, we examined the sensitivity of the proposed final quadrumer by considering the behavior of Fano minima. We plotted the linear figure of merit (FoM) based on the Fano resonance energy differences in various conditions over the refractive index. Quantifying the FoM for the studied nanostructure, then we compared the quality of structure with the analogous nanoclusters. This work paves novel methods toward the utilization of Al/Al2O3 nanoparticles as a potential substance to employ in designing nanoclusters that are able to support strong dark resonances as well as bright modes. Wide-range working region, optimized electric and magnetic fields, multiple and high quality Fano dips, high FoM and low-costs are the superior features of the proposed artificial structure in comparison to analogous configurations.

Modulation of Fano resonances in symmetry-broken gold-SiO2-gold nanotube dimers

Fano resonances in the symmetry-broken gold-SiO2-gold (BGSG) nanotubes and the associated dimers have been investigated based on the finite element method. In the BGSG nanotube, the symmetry breaking induced the interactions of the inner gold core and outer gold nanoshell plasmons of all multipolar orders and hence the red-shifts of the plasmon resonance modes and the enhanced quadrupole mode peaks were observed. The interference of the quadrupole mode peak with the subradiant dipole mode caused a Fano-dip in the scattering spectrum. By increasing the core offset-value in the BGSG nanotube, the Fano dip with low energy showed a red-shift and became deeper. Unexpectedly the plasmon coupling between a GSG nanotube and a BGSG nanotube can lead to two strong Fano dips in the scattering spectra of the dimer. It was further noted that the thin side of the BGSG nanotube located at two sides of the dimer gap can lead to the strong near-field coupling between two BGSG nanotubes and hence a deeper and broader Fano dip.

Plasmonic Fano resonance and dip of Au-SiO2-Au nanomatryoshka

This study theoretically investigates Fano resonances and dips of an Au-SiO2-Au nanomatryoshka that is excited by a nearby electric dipole. An analytical solution of dyadic Green's functions is used to analyze the radiative and nonradiative power spectra of a radial dipole in the proximity of a nanomatryoshka. From these spectra, the plasmon modes and Fano resonances that accompany the Fano dips are identified. In addition, the scattering and absorption spectra of a nanomatryoshka that is illuminated by a plane wave are investigated to confirm these modes and Fano dips. Our results reveal that a Fano dip splits each of the dipole and quadrupole modes into bonding and anti-bonding modes. The Fano dip and resonance result from the destructive interference of the plasmon modes of the Au shell and the Au core. The Fano factors that are obtained from the nonradiative power spectra of the Au shell and the Au core of a nanomatryoshka are in accordance with those obtained from the absorption cross section spectra. Moreover, these Fano factors increase as the plasmonic coupling of the Au shell with the core increases for both dipole and quadrupole modes.