Asymmetric Line Shape Research Articles

Origin of strong and narrow localized surface plasmon resonance of copper nanocubes

Inexpensive copper nanoparticles are generally thought to possess weak and broad localized surface plasmon resonance (LSPR). The present experimental and theoretical studies show that tailoring the Cu nanoparticle to a cubic shape results in a single intense, narrow, and asymmetric LSPR line shape, which is even superior to round-shaped gold nanoparticles. In this study, the dielectric function of copper is decomposed into an interband transition component and a free-electron component. This allows interband transition-induced plasmon damping to be visualized both spectrally and by surface polarization charges. The results reveal that the LSPR of Cu nanocubes originates from the corner mode as it is spectrally separated from the interband transitions. In addition, the interband transitions lead to severe damping of the local electromagnetic field but the cubic corner LSPR mode survives. Cu nanocubes display an extinction coefficient comparable to the dipole mode of a gold nanosphere with the same volume and show a larger local electromagnetic field enhancement. These results will guide development of inexpensive plasmonic copper-based nanomaterials.

Nanotrimer enhanced optical fiber tips implemented by electron beam lithography

Here we present a novel fabrication approach that allows for the implementation of sophisticated planar nanostructures with deep subwavelength dimensions on fiber end faces by electron beam lithography. Specifically, we planarize the end faces of fiber bundles such that they are compatible with planar nanostructuring technology, with the result that fibers can be treated in the same way as typical wafers, opening up the entire field of nanotechnology for fiber optics. To demonstrate our approach, we have implemented densely-packed arrays of gold nanotrimers on the end face of 50 cm long standard single mode fibers, showing asymmetrical resonance lineshapes that arise due to the interplay of diffractive coupling of the individual timer response at infrared wavelengths that overlap with the single mode regime of typical telecommunication fibers. Refractive index sensing experiments suggest sensitivities of about 390 nm/RIU, representing the state-of-the-art for such a device type. Due to its unique capability of making optical fibers compatible with planar nanostructuring technology, we anticipate our approach to be applied in numerous fields including bioanalytics, telecommunications, nonlinear photonics, optical trapping and beam shaping.

Analysis of resonance asymmetry phenomenon in resonator integrated optic gyro**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA02005003) and the National Natural Science Foundation of China (Grant Nos. 61274066, 61504138, and 61474115).

Resonator integrated optic gyro (RIOG) is a high-accuracy gyroscope based on the Sagnac effect. The waveguide-type ring resonator is a key rotation sensing element in the RIOG. An asymmetric resonance line shape is found in the optic resonator. These asymmetries will induce offset errors when the phase modulation spectroscopy technique (PMST) is applied to the RIOG. The polarization errors and the difference among normal mode losses are found to be the two main sources of resonance asymmetry in an experiment. These sources are fully investigated and their contributions to the offset errors are compared. The analysis shows that proper modulation frequencies in clockwise (CW) and counterclockwise (CCW) directions can reduce an RIOG bias error. A transmissive resonator is recommended to obtain a better resonance line shape.

Dual Fano resonance control and refractive index sensors based on a plasmonic waveguide-coupled resonator system

A compact plasmonic waveguide-coupled resonator system, which consists of a metal–insulator–metal (MIM) waveguide with a double stub side-coupled to a ring cavity, is designed and numerically investigated using finite element method (FEM). Simulation results show that dual Fano resonances with sharp asymmetric line shape are observed in the transmission spectrum. One of the Fano resonances originates from the coupled-resonator effect of the two stub cavities, whereas the other is induced by the ring resonator. The two resonance peaks exhibit different dependencies on the parameters of the structure because of varying mechanisms and can be easily tuned. Moreover, the maximum refractive index sensitivity and Q-factor of the proposed structure are 1000 nm/refractive index unit (RIU) and 41, respectively. The maximum figure-of-merit (FOM∗) is approximately 4200 at 0.71 μm, which is due to the sharp asymmetric Fano line shape with an ultra-low transmittance at this wavelength. The proposed structure is expected to provide guidelines for the design of nano-sensors.

Asymmetric surface plasmon resonances revisited as Fano resonances

Attenuated total reflection spectra of planar metal-dielectric structures obtained in the Otto and Kretschmann configurations exhibit resonance dips due to excitation of surface-plasmon polaritons. Although the dips have asymmetric line shapes, the asymmetry has not received much attention and the Lorentzian symmetric line shape has commonly been used in the spectral analyses. We analyze the reflectivity spectra of two- and three-layer plasmonic systems based on the electromagnetic and coupled-mode theories, and demonstrate that the asymmetric line shapes can be regarded as the Fano line shapes. In the resonance region, the spectra calculated as Fano line shapes using our approximate expressions are in excellent agreement with those obtained by rigorous electromagnetic calculations. The present formulations of the reflectivity allow us to gain insight into physical origins of characteristic behavior of the plasmonic resonances.

Fano Interference between Bulk and Surface States of a Dirac Semimetal Cd_{3}As_{2} Nanowire.

Dirac semimetals possess Fermi-arc surface states, which will be a set of discrete surface subbands in a nanowire due to the quantum confinement effect. Here, we report a tunable Fano effect induced by the interference between the discrete surface states and continuous bulk states of a Dirac semimetal Cd_{3}As_{2} nanowire. The discrete surface bands lead to a zero bias peak in conductance as the Femi level is tuned to across the surface subbands. The Fano resonance results in an asymmetric line shape in the differential conductance dI/dV spectrum. Furthermore, the Fano interference would introduce an additional phase into the Weyl orbits and lead to a modification of the oscillation frequency. The results are valuable for further understanding the exotic quantum transport properties of topological semimetals.

Optical study of Dirac fermions and related phonon anomalies in the antiferromagnetic compound CaFeAsF

We performed optical studies on CaFeAsF single crystals, a parent compound of the 1111-type iron-based superconductors that undergoes a structural phase transition from tetragonal to orthorhombic at $T_s$ = 121 K and a magnetic one to a spin density wave (SDW) state at $T_N$ = 110 K. In the low temperature optical conductivity spectrum, after the subtraction of a narrow Drude peak, we observe a pronounced singularity around 300 cm$^{-1}$ that separates two regions of quasilinear conductivity. We outline that these characteristic absorption features are signatures of Dirac fermions, similar to what was previously reported for the BaFe$_2$As$_2$ system [1]. In support of this interpretation, we show that for the latter system this singular feature disappears rapidly upon electron and hole doping, as expected if it arises from a van Hove singularity in-between two Dirac cones. Finally, we show that one of the infrared-active phonon modes (the Fe-As mode at 250 cm$^{-1}$) develops a strongly asymmetric line shape in the SDW state and note that this behaviour can be explained in terms of a strong coupling with the Dirac fermions.

Plasmonic Multichannel Refractive Index Sensor Based on Subwavelength Tangent-Ring Metal⁻Insulator⁻Metal Waveguide.

In this paper, a multichannel refractive index sensor based on a subwavelength metal–insulator–metal (MIM) waveguide coupled with tangent-ring resonators is proposed. When two tangent-ring resonators were placed above the MIM waveguide, Fano resonance with asymmetrical line shape appeared in the transmission spectrum due to the interference between the light–dark resonant modes. The sensitivity and figure of merit were as high as 880 nm/RIU and 964, respectively. Through adding more tangent-ring resonators, multiple Fano resonances with ultrasharp peaks/dips were achieved in the wavelength range of 800–2000 nm. Besides, negative group delays were also observed in the Fano resonant dips. Two-dimensional finite-difference time-domain (FDTD) method was used to simulate and analyze the performances of the proposed structures. These kinds of multiring structures can find important applications in the on-chip optical sensing and optical communication areas.

Coherent Dirac plasmons in topological insulators

We explore the ultrafast reflectivity response from photo-generated coupled phonon-surface Dirac plasmons in Sb$_2$Te$_3$ topological insulators several quintuple layers thick. The transient coherent phonon spectra obtained at different time frames exhibit a Fano-like asymmetric line shape of the $A^2_{1g}$ mode, which is attributed to quantum interference between continuum-like coherent Dirac-plasmons and phonons. By analyzing the time-dependent asymmetric line shape using the two-temperature model (TTM), it was determined that a Fano-like resonance persisted up to $\approx$1 ps after photo-excitation with a relaxation profile dominated by Gaussian decay at $\leq$200 fs. The asymmetry parameter could be well described by the TTM for $\geq$200 fs, therefore suggesting the coherence time of the Dirac plasmon is $\approx$200 fs.

IrOx/Nafion Catalyst for Oxygen Evolution: Effect of Surface Oxide on Activity and Stability

High catalyst loading is a major barrier for large-scale production of proton exchange membrane water electrolysis (PEMWE) despite its advantages over other electrolysis technologies. [1]. [2,3] Herein, a low iridium loading composite catalyst layer of iridium oxide/Nafion (IrOx/Nafion) is developed using reactive spray deposition technology (RSDT). In this process, the catalyst synthesis and catalyst-coated membrane (CCM) fabrication are combined into one step. The activity and stability for oxygen evolution reaction (OER) are tested through rotating disk electrode (RDE) measurements and membrane electrode assembly (MEA), respectively. The iridium particles has an average size of ~2 nm resulting in very high surface area and superior activity over planar and electrodeposited IrO2 thin films. In this study, two flame conditions (F-1 and F-2) are used for synthesizing the iridium particles and a correlation between the surface oxide layer and the electrochemical performance is observed. Based on preliminary results of X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM), we hypothesize that the iridium particles exhibit core-shell structure with a metallic iridium core and mixed iridium (IV and III) oxides shell. Higher flame temperature and longer residence time in the flame (F-2 condition) increases the amount of surface oxide and forms a thicker oxide layer. The oxide composition of the particle can be determined from XPS results (Figure 1 a and b) employing the fitting model proposed in reference [4]. The oxide composition for sample F-1 and F-2 are 54% and 90%, respectively. On the other hand, electron diffraction pattern from TEM analysis shows a metallic phase, which suggests the oxides may reside only on the surface since XPS is surface sensitive. Electrochemical measurements have shown that the iridium particles with higher oxide content improves the OER mass activity by ~3 fold over particles with lower oxide content. Furthermore, the catalyst stability in electrolysis test is improved from 250 hours to 4500 hours with the higher oxide content. Detailed surface analysis is carried out using XPS and high-resolution TEM to obtain the chemical state or change of iridium in particles synthesized under different flame conditions and to investigate the role of surface oxide in the enhancement of activity and stability. Figure 1. XPS of iridium particles synthesized in F-1 (a) and F-2 condition (b) fitted with asymmetric Doniach-Sunjic line shape.

Field induced transient current in one-dimensional nanostructure

Field-induced transient current in one-dimensional nanostructures has been studied by a model of an electron confined in a 1D attractive Gaussian potential subjected both to electrodes at the terminals and to an ultrashort pulsed oscillatory electric field with the central frequency ω and the FWHM pulse width Γ. The time-propagation of the electron wave packet has been simulated by integrating the time-dependent Schrödinger equation directly relying on the second-order symplectic integrator method. The transient current has been calculated as the flux of the probability density of the escaping wave packet emitted from the downstream side of the confining potential. When a static bias-field E0 is suddenly applied, the resultant transient current shows an oscillatory decay behavior with time followed by a minimum structure before converging to a nearly constant value. The ω-dependence of the integrated transient current induced by the pulsed electric field has shown an asymmetric resonance line-shape for large Γ while it shows a fringe pattern on the spectral line profile for small Γ. These observations have been rationalized on the basis of the energy-level structure and lifetime of the quasibound states in the bias-field modified confining potential obtained by the complex-scaling Fourier grid Hamiltonian method.

Fano line shapes of leaky surface acoustic waves extending from supersonic surface wave points

This paper highlights the close connection between two issues that are attracting considerable attention in diverse areas of physics including photonics, acoustics, fluid dynamics and the dynamical properties of periodic structures, namely bound states in a continuum (BIC), and asymmetric Fano resonance line shapes. The discussion is framed in the context of solid state acoustics, with supersonic surface acoustic waves (supersonic-SAW) taken as representative examples of BIC. One of these pertains to the surface of an anisotropic solid and the other to an isotropic layer on an isotropic substrate. These are surface modes lying within a continuum of bulk radiating states, which at isolated values of the system parameters become uncoupled from these states. We demonstrate how for generic non-symmetry-protected supersonic-SAW, varying these parameters leads to the unfolding of this mode into a leaky pseudo-SAW, with the resonance line shape conforming accurately to the Fano expression. With passage through the SSAW point in parameter space, the Fano q parameter diverges and changes sign. It is anticipated that this is a fairly general feature of the unfolding of BIC with variation of relevant control parameters.

Induced high-order resonance linewidth shrinking with multiple coupled resonators in silicon-organic hybrid slotted two-dimensional photonic crystals for reduced optical switching power in bistable devices

Shrinking the linewidth of resonances induced by multiple coupled resonators is comprehensively analyzed using the coupled-mode theory (CMT) in time. Two types of coupled resonators under investigation are coupled resonator optical waveguides (CROWs) and side-coupled resonators with waveguide (SCREW). We examine the main parameters influencing on the spectral response such as the number of resonators (n) and the phase shift (φ) between two adjacent resonators. For the CROWs geometry consisting of n coupled resonators, we observe the quality (Q) factor of the right- and left-most resonant lineshapes increases n times larger than that of a single resonator. For the SCREW geometry, relying on the phase shift, sharp, and asymmetric resonant lineshape of the high Q factor a narrow linewidth of the spectral response could be achieved. We employ the finite-difference time-domain (FDTD) method to design and simulate two proposed resonators for practical applications. The proposed coupled resonators in silicon-on-insulator (SOI) slotted two-dimensional (2-D) photonic crystals (PhCs) filled and covered with a low refractive index organic material. Slotted PhC waveguides and cavities are designed to enhance the electromagnetic intensity and to confine the light into small cross-sectional area with low refractive index so that efficient optical devices could be achieved. A good agreement between the theoretical CMT analysis and the FDTD simulation is shown as an evidence for our accurate investigation. All-optical switches based on the CROWs in the SOI slotted 2-D PhC waveguide that are filled and covered by a nonlinear organic cladding to overcome the limitations of its well-known intrinsic properties are also presented. From the calculations, we introduce a dependency of the normalized linewidth of the right-most resonance and its switching power of the all-optical switches on number of resonator, n. This result might provide a guideline for all-optical signal processing on a silicon PhC chip design.

Ultralow-power on-chip all-optical Fano diode based on uncoupled nonlinear photonic-crystal nanocavities

We achieved an ultralow-power Fano-like diode in integrated photonic circuits through two cascaded and uncoupled photonic-crystal microcavities, coated with a nonlinear layer constructed from polycrystalline indium-tin oxide doped with gold nanoparticles. The multicomponent nanocomposite layer helps to obtain an extremely large nonlinearity enhancement on account of the quantum confinement effect, hot-electron injection, and local-field enhancement effect. Thus, the strong nonlinearity enhancement, asymmetric Fano-like spectrum lineshape, and asymmetric light confinement for the forward and backward incidence cases guarantee the nonreciprocal transmission of signal light. A compact size of 7 μm is maintained for the photonic-crystal microcavities and the transmission contrast reached 15 dB. Besides, we achieved 1.5 kW cm−2 operating threshold intensity (cut down by a thousand times). This work assists chip-integrated optical computing on the basis of nonlinear photonic-crystal microcavities.

A Plasmonic Sensor Array with Ultrahigh Figures of Merit and Resonance Linewidths down to 3 nm

Surface plasmon polaritons (SPPs) are extremely sensitive to the surrounding refractive index and have found important applications in ultrasensitive label-free sensing. Reducing the linewidth of an SPP mode is an effective way to improve the figure of merit (FOM) and hence the sensitivity of the plasmonic mode. Many efforts have been devoted to achieving a narrow linewidth by mode coupling, which inevitably results in an asymmetrical lineshape compromising the performance. Instead, the SPP modes are directly narrowed by elaborately engineering periodic plasmonic structures with minimized feature sizes to effectively reduce the radiative losses. A narrow linewidth smaller than 8 nm is achieved over a wide wavelength ranging from 600 to 960 nm and a minimum full width at half maximum of 3 nm at 960 nm. Benefiting from the almost perfect Lorentzian lineshape and the extremely narrow linewidth, a record FOM value of 730 is obtained. The sensor is capable of detecting bovine serum albumin with an ultralow concentration of 10-10 m. The sensor has great potential for practical application for its ultrahigh FOM, broad working wavelength, and ease of high-throughput fabrication.

Strain Control of Exciton-Phonon Coupling in Atomically Thin Semiconductors.

Semiconducting transition metal dichalcogenide (TMDC) monolayers have exceptional physical properties. They show bright photoluminescence due to their unique band structure and absorb more than 10% of the light at their excitonic resonances despite their atomic thickness. At room temperature, the width of the exciton transitions is governed by the exciton-phonon interaction leading to strongly asymmetric line shapes. TMDC monolayers are also extremely flexible, sustaining mechanical strain of about 10% without breaking. The excitonic properties strongly depend on strain. For example, exciton energies of TMDC monolayers significantly redshift under uniaxial tensile strain. Here, we demonstrate that the width and the asymmetric line shape of excitonic resonances in TMDC monolayers can be controlled with applied strain. We measure photoluminescence and absorption spectra of the A exciton in monolayer MoSe2, WSe2, WS2, and MoS2 under uniaxial tensile strain. We find that the A exciton substantially narrows and becomes more symmetric for the selenium-based monolayer materials, while no change is observed for atomically thin WS2. For MoS2 monolayers, the line width increases. These effects are due to a modified exciton-phonon coupling at increasing strain levels because of changes in the electronic band structure of the respective monolayer materials. This interpretation based on steady-state experiments is corroborated by time-resolved photoluminescence measurements. Our results demonstrate that moderate strain values on the order of only 1% are already sufficient to globally tune the exciton-phonon interaction in TMDC monolayers and hold the promise for controlling the coupling on the nanoscale.

Magnetic excitations and amplitude fluctuations in insulating cuprates

We present results from light scattering experiments on three insulating antiferromagnetic cuprates, YBa$_2$Cu$_3$O$_{6.05}$, Bi$_2$Sr$_2$YCu$_2$O$_{8+\delta}$, and La$_2$CuO$_4$ as a function of polarization and excitation energy {using samples of the latest generation. From the raw data we derive symmetry-resolved spectra.} The spectral shape in $B_{1g}$ symmetry is found to be nearly universal and independent of the excitation energy. The spectra agree quantitatively with predictions by field theory [\onlinecite{Weidinger:2015}] facilitating the precise extraction of the Heisenberg coupling $J$. {In addition, the asymmetric line shape on the high-energy side is found to be related to amplitude fluctuations of the magnetization. In La$_2$CuO$_4$ alone minor contributions from resonance effects may be identified.} The spectra in the other symmetries are not universal. The variations may be traced back to weak resonance effects and extrinsic contributions. For all three compounds we find support for the existence of chiral excitations appearing as a continuum in $A_{2g}$ symmetry having an onset slightly below $3J$. In La$_2$CuO$_4$ an additional isolated excitation appears on top of the $A_{2g}$ continuum.

An asymmetric fitting function for condensed-phase Raman spectroscopy.

Asymmetric lineshapes are experimentally observed in the Raman spectra of different classes of condensed matter. Determination of the peak parameters, typically done with symmetric pseudo-Voigt functions, in such situations yields unreliable results. While a wide choice of asymmetric fitting functions is possible, for the function to be practically useful, it should satisfy several criteria: simple analytic form, minimum parameters, description of the symmetric shape as "zero case", estimation of the desired peak parameters in a straightforward way and, above all, adequate description of the experimental data. In this work we formulate the asymmetric pseudo-Voigt function by damped perturbation of the original symmetric shapes with one asymmetry-related parameter. The damped character of the perturbation ensures by construction consistent behavior of the line tails. We test the asymmetric function by fitting the experimental Raman spectra. The results show that the function is able to describe a wide range of experimentally observed asymmetries for different natures of asymmetric broadening, including 3D and 2D crystals, nanoparticles, polymers, and molecular solids and liquids.

High-Sensitivity Refractive Index Sensing Based on Fano resonances in a Photonic Crystal Cavity-Coupled Microring Resonator

Aiming for high-sensitivity refractive index biosensing, we investigate Fano resonance in a one-dimensional photonic crystal cavity-coupled microring resonator (PCMRR) on a silicon chip. Calculated results show that utilizing the asymmetric line shape of the Fano resonance, the detection limit can be reduced by a factor of 5 and the intensity detection sensitivity of PCMRR can be increased by a factor of 5 compared with a regular silicon-on-insulator, reaching the values of ~7.2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> RIU and ~1.76 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> dB/RIU, respectively. Owing to its simplicity, compact footprint, and excellent performances, our device would be a promising candidate for optical label-free sensing applications.

Generalisation of phonon confinement model for interpretation of Raman line-shape from nano-silicon

A comparative analysis of two Raman line-shape functions has been carried out to validate the true representation of experimentally observed Raman scattering data for semiconducting nanomaterials. A modified form of already existing phonon confinement model (PCM) incorporates two basic considerations, phonon momentum conservation and shift in zone centre phonon frequency. After incorporation of the above mentioned two factors, a rather symmetric Raman line-shape is generated which is in contrary to the usual asymmetric Raman line-shapes obtained from nanostructured semiconductor. By fitting an experimentally observed Raman scattering data from silicon nanostructures, prepared by metal induced etching, it can be established that the Raman line-shape obtained within the framework of PCM is a true representative Raman line-shape of sufficiently low dimensions semiconductors.