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Analysis of Scattering by Plasmonic Gratings of Circular Nanorods Using Lattice Sums Technique
摘要： A self-contained formulation for analyzing electromagnetic scattering by a significant class of planar gratings composed of plasmonic nanorods, which were infinite length along their axes, is presented. The procedure for the lattice sums technique was implemented in a cylindrical harmonic expansion method based on the generalized reflection matrix approach for full-wave scattering analysis of plasmonic gratings. The method provided a high computational efficiency and can be considered as one of the best-suited numerical tools for the optimization of plasmonic sensors and plasmonic guiding devices both having a planar geometry. Although the proposed formalism can be applied to analyze a wide class of plasmonic gratings, three configurations were studied in the manuscript. Firstly, a multilayered grating of silver nanocylinders formed analogously to photonic crystals was considered. In the region far from the resonances of a single plasmonic nanocylinder, the structure showed similar properties compared to conventional photonic crystals. When one or a few nanorods were periodically removed from the original crystal, thus forming a crystal with defects, a new band was formed in the spectral responses because of the resonant tunneling through the defect layers. The rigorous formulation of plasmonic gratings with defects was proposed for the first time. Finally, a plasmonic planar grating of metal-coated dielectric nanorods coupled to the dielectric slab was investigated from the viewpoint of design of a refractive index sensor. Dual-absorption bands attributable to the excitation of the localized surface plasmons were studied, and the near field distributions were given in both absorption bands associated with the resonances on the upper and inner surfaces of a single metal-coated nanocylinder. Resonance in the second absorption band was sensitive to the refractive index of the background medium and could be useful for the design of refractive index sensors. Also analyzed was a phase-matching condition between the evanescent space-harmonics of the plasmonic grating and the guided modes inside the slab, leading to a strong coupling.
关键词： plasmonics,refractive index sensors,scattering,bandgap structures,periodic structures
更新于2025-09-11 14:15:04
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Stress-induced refractive index changes in laser fibers and preforms
摘要： Refractive index profile of the core is a key design parameter in fiber lasers and amplifiers. During manufacturing, the initial information of the index profile is obtained from the preform, while ultimately the performance is defined by the index profile of the fiber. Depending on stresses and diffusion, the two profiles may be different. It is possible to predict more accurately the laser fiber refractive index when we apply a stress-induced index change model to the measured preform index profile data. The improved capability to predict the fiber index from preform increases the confidence in achieving the designed index profile in fiber, which enables faster process feedback and higher fiber yields.
关键词： laser fiber,tension,optical fiber,modelling,thermal stress,preform,Refractive index change,DND,stress
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Determining the Waveguide Profile using the Overlap Integral
摘要： The determination of the index pro?le in guiding structures is a central problem in applied photonics, ranging from optical ?bers to femtosecond-written waveguides. A non-destructive and relatively easy method consists in the measurement of the index pro?le by measuring the transmitted ?eld. From the transmitted ?eld, the refractive index pro?le is computed by direct inversion of the Helmholtz equation. This technique is called near-?eld method. Here we present a new near-?eld method based upon the inversion of the overlap integral. From the waveguide theory, the power coupled to the m-th mode with pro?le ψm(x) from an input Ein(x) is am = (cid:2) Ein(x)ψ ? (x)dx. If the input Ein(x) is shifted by an amount x0, the overlap am(x0) is the convolution between the pro?le of the input beam Ein and the mode pro?le ψm. The convolution operator implicitly ?lters out the noise, but without creating distorsions or artifacts. An experimental measurement of the transmitted ?eld Eexp will be Nη, where η is a white Gaussian noise and N the noise amplitude. In the previous formulae P is the overall power given by |am(x0)|2. If both amplitude and phase of the transmitted ?eld are simultaneously measured, the overlap am can be inverted for any guided mode m. If only the intensity is measured, the overlap can be inverted only if the waveguide is monomodal. An example of the reconstructed mode in the case of intensity-only measurements is plotted in Fig. 1(a) for ψ0 = cosh(x/w) with w/λ = 4 and N/P = 1 × 10?4. The retrieved ?eld is very close to the exact one, with appreciable differences only for |x/λ | > 10. Important to stress, this range is larger than what it is achievable with a single direct measurement of the transmitted ?eld. The next step is to invert the Helmholtz equation, that is, to compute the second derivative of the retrieved mode. Direct application of the inversion protocol strongly enhances the noise, above all on the tails of the mode. The net result is the appearance of several fake oscillations, even where the retrieval of the mode is good. The problem can be overcome by ?tting the mode tails with a decaying exponential, in accordance with the waveguide theory. The reconstructed guide pro?les shown in Fig. 1(b) do not present arti?cial oscillations on the tails, showing a 10% error with respect to the original waveguide, the error depending slightly on the original signal-to-noise ratio N/P.
关键词： refractive index,overlap integral,Helmholtz equation,waveguide profile,near-field method
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Boson Band Vibrations Aid Refractive Index Mapping of Waveguides in High Index Chalcogenide Glass
摘要： The boson band mapping of waveguides fabricated by femtosecond laser inscription was used for the first time to identify and understand the material densification profile in chalcogenide glasses. The refractive index mapping of waveguides in high refractive index dielectrics is problematic as there are no known techniques. Refracted near field technique cannot be carried out in glasses with refractive index higher than 1.6 due to the lack of availability or toxic nature of the index matching oil. Though other techniques based on holographic and phase microscopies are available, sample preparation steps compatible with reliable measurement are tedious and in some cases, like diamond, is almost impossible. In this work, using the information obtained from the Bosonic band vibrations, we explain waveguide formation in Gallium Lanthanum Sulphide (GLS) glass. The boson peak shows up in any amorphous material regardless of its constituents or stoichiometry and is observed in the low frequency region between 0 – 100 cm-1. Their origin is a well debated and controversial topic. Here we make use of its response to the physical properties of the material.
关键词： refractive index mapping,waveguides,boson band vibrations,chalcogenide glass,femtosecond laser inscription
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Threshold Effects and Metastability in Solitary Refractive Index Wells
摘要： Threshold effects and trapping subsequent to the collision of a projectile wave packet and a target are ubiquitous in quantum mechanics. In nonlinear optics, the interaction between a soliton (S) and a dispersive wave (DW) exhibits similarities to wave-packet scattering and offers means for all-optical control [1]. However, threshold effects akin to those in quantum mechanical potential scattering when a bound-state eigenvalue is close to the continuum, and generation of metastable states due to scattering resonances, are not directly allowed. Such phenomena are of high interest in the context of optical technologies concerning storage and manipulation of optical data. Here, we exploit an elaborate yet fabricable nonlinear waveguide [2] to realize an all-optical analogue of a quantum mechanical potential well in terms of the refractive index change induced by a solitary wave. Besides threshold effects this allows for an unexpected trapping mechanism with the ability to engineer metastable states with controllable lifetimes. Observation of the above effects requires S and DW to copropagate at nearly identical group velocities. Our setup features an alternating sequence of regions of normal dispersion (N1?N3) and anomalous dispersion (A1, A2), exhibiting group delay and group velocity dispersion pro?les shown in Figs. 1(a,b). We use numerical simulations of an unidirectional propagation equation for the analytic signal [3]. Firstly, we demonstrate threshold effects in the collision of a “projectile” DW in A2 and a “target” soliton in A1, manifested by peaks in the fraction of transmitted DW energy (Fig. 1(c)) and culminating in its re?ectionless transmission at integer potential strengths νeff de?ned by the DW center frequency (Fig. 1(d)). Off-threshold, interference with re?ected DW components causes a polychotomous wave train (Fig. 1(e)). As evident from the spectrograms in Figs. 1(h,i), inside S, DW exhibits a time-domain structure resembling the quasi bound state closest in νeff. Secondly, we show that a projectile DW launched in N3, colliding with a target soliton in A1, experiences a partial down-shift to a frequency range in A2, group-velocity matched to the soliton. The down-shifted frequency components perceive the soliton as time-domain potential-well to which they are con?ned. Nonzero overlap of the trapped state with the adjacent region of normal dispersion causes its decay, resulting in metastability. Trapping and decay of a metastable state (Figs. 1(f,j)) might also lead to a huge time-delay (Figs. 1(g,k)). We further observe re?ectionless transmission, total (bounce-off) re?ection and trapping in the collision of unequal solitons residing in different regions of anomalous dispersion. Besides the fundamental aspect of providing a quantum analog system, this might pave the way for new applications in optical technologies such as all-optical manipulation of data.
关键词： soliton,solitary refractive index wells,dispersive wave,metastability,nonlinear optics,threshold effects
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Brightness Enhancement in Non-Hermitian VCSELs
摘要： Vertical Cavity Surface Emitting lasers (VCSELs) are compact and efficient light sources useful for a variety of applications. However, due to lack of a transverse mode control mechanism, such lasers suffer from poor spatial beam quality, intrinsic spatiotemporal instabilities and nonlinear destabilizing effects such as filamentation and spatial hole burning [1]. Therefore, there is a need for new strategies to manipulate the light wave dynamics to enhance the stability of VCSELs. Recently, non-Hermitian media have become a flexible platform for new functionalities such as asymmetric coupling, unidirectional invisibility, single mode lasing [2-3]. In this presentation, we propose a novel stabilization mechanism for VCSLEs to obtain bright and narrow beams. The mechanism relies on non-Hermitian configuration of the laser potential, achieved by simultaneous spatial modulation of the refractive index and gain-loss profiles. In particular, we consider axisymmetric non-Hermitian potentials expressed as: n(r)=nR cos(qr)-inI cos(qr-?) where nR and nI are the amplitude of the refractive index and gain-loss modulations, and ? is the relative phase shift between them. Such potentials may confine the emitted light around the central part of VCSELs, through unidirectional-inward radial coupling among the transverse modes [4]. The interplay of the relative strength and relative phase of the index and gain-loss modulations manipulate the wave dynamics of such lasers to emit powerful and narrow beams of high brightness. We use the mean-field paraxial model to study the spatiotemporal dynamics of such VCSELs with non-Hermitian potentials. The output emission of conventional VCSEL and modified VCSEL with concentric non-Hermitian configuration is shown in Fig. 1(a,b), illustrating irregular and stable localized pattern, respectively. We assess the performance through the central intensity enhancement [see Fig. 1(c)] and field concertation [see Fig. 1(d)]. The spatial dynamics for a representative point is provided in Fig. 1(e). The stationary intensity profile and its corresponding cross-section show that intensity is strongly concentrated at r=0 due to the favoured radial coupling of inward propagating waves, as depicted in the transverse field flow on the inset [see Fig. 1(e)].
关键词： VCSELs,refractive index,non-Hermitian media,gain-loss profiles,spatial modulation,brightness enhancement
更新于2025-09-11 14:15:04
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Fano Resonance in a MIM Waveguide with Two Triangle Stubs Coupled with a Split-Ring Nanocavity for Sensing Application
摘要： Herein, a compact refractive index nanosensor comprising a metal-insulator-metal (MIM) waveguide with symmetric two triangle stubs coupled with a circular split-ring resonance cavity (CSRRC) is theoretically presented. An analysis of the propagation characteristics of the designed structure is discussed employing the finite element method (FEM). The calculation results revealed that a Fano resonance outline emerged, which results from an interaction between the continuous broadband state of the waveguide with two symmetric triangle stubs and the discrete narrowband state of the CSRRC. The influence of geometric parameters on sensing properties was studied in detail. The maximum sensitivity reached 1500 nm/RIU with a high figure of merit of 65.2. The presented structure has great applications for on-chip plasmonic nanosensors.
关键词： refractive index sensor,metal-insulator-metal waveguide,Fano resonance,finite element method
更新于2025-09-11 14:15:04
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Plasmon induced transparency like transmission properties in compact MIM waveguide side-coupled with U-cavity
摘要： Surface plasmon polaritons (SPPs) can overcome the limitation of diffraction and control light at nanoscale, thus becoming a hotspot in recent years. SPPs based metal-insulator-metal (MIM) plasmonic waveguides using U-cavity and slot cavity are designed. The transmission characteristics are numerically simulated and verified by the coupled mode theory (CMT). Meanwhile, the effects of changing the geometric parameters on the transmission characteristics are also studied. Single and double plasmon induced transparency (PIT) effects are realized through the coupling and the destructive interfering between the transmission paths. Furthermore, characteristics of the refractive index sensing as well as the slow light and fast light effects are also investigated. We hope the designed waveguide structures along with their transmission characteristics have potential application prospects in the area of nanoscale integrated optical devices, such as filters, sensors, switches, slow/fast light devices, and other optoelectronic circuits.
关键词： refractive index sensing,slow light,U-cavity,Surface plasmon polaritons,slot cavity,plasmon induced transparency,MIM plasmonic waveguides,fast light
更新于2025-09-11 14:15:04
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Rectangular and sinusoidal Au-Grating as plasmonic sensor: A comparative study
摘要： Sensitivity of surface plasmon resonance sensor (SPRS) is mainly associated to efficient excitation of surface plasmon polaritons (SPPs) waves. In this work, effect of 50 nm thick Au film designed as rectangular and sinusoidal grating structure on glass substrate has been explored by finite element analysis (FEA) using COMSOL Multiphysics 5.3a. The comparison between the rectangular and sinusoidal gratings for efficient sensing has been presented here. Slit width (SW) for grating device has been optimized by applying far-field and near-field analysis while periodicity of grating structure is kept constant. Far field transmission spectra for both devices have been analyzed by illuminating the grating device through excitation port from the substrate side. Resonance wavelength (RW) dips for plasmonic grating device has been examined in each transmission spectrum for different values of refractive indices (RI). The sensitivity of plasmonic grating device has been calculated by measuring the shift in resonance wavelength by varying refractive index of analyte. The sinusoidal grating supporting fundamental plasmonic mode is found to be more sensitive (717 nm/RIU) as compared to simple rectangular grating (693 nm/RIU). Being highly sensitive to minute change in refractive index, such devices are efficiently applicable in real life.
关键词： Plasmonic sensor,Sinusoidal grating,Finite element analysis,Refractive index sensing,Sensitivity,Rectangular grating
更新于2025-09-11 14:15:04
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Theoretical and experimental study of liquid refractive index measurement method based on the double external-cavity-laser feedback effect
摘要： A liquid refractive index measurement method based on the double external-cavity laser feedback effect is proposed. The method uses a laser beam to simultaneously enter the top and bottom surfaces of the liquid to be tested, with the light reflected from the two surfaces returning to the laser cavity along the original path, forming a double external-cavity system. When the liquid level rises, the self-mixing interference signal, caused by the optical path from the top and bottom liquid surfaces, superimposes on the tuning intensity of the laser, and the refractive index of the liquid can be obtained by demodulating the amplitude-frequency feedback tuning curve. This method offers the following innovative performances. Firstly, the structure of the system is simple and the measuring range is large. Secondly, the measurement results can be traced back to one physical quantity, i.e., the refractive index of air. Thirdly, it has been proved that the laser feedback effect is ubiquitous in all types of lasers, and the change of wavelength does not affect the feedback phenomenon, so that the method is not limited by the type and wavelength of the laser.
关键词： traceable measurement,liquid refractive index,double feedback
更新于2025-09-11 14:15:04