Localized Waveguide Research Articles

Efficient Second-Harmonic Generation in Adapted-Width Waveguides Based on Periodically Poled Thin-Film Lithium Niobate.

Frequency conversion process based on periodically poled thin-film lithium niobate (PPTFLN) has been widely recognized as an important component for quantum information and photonic signal processing. Benefiting from the tight confinement of optical modes, the normalized conversion efficiency (NCE) of nanophotonic waveguides is improved by orders of magnitude compared to their bulk counterparts. However, the power conversion efficiency of these devices is limited by inherent nanoscale inhomogeneity of thin-film lithium niobate (TFLN), leading to undesirable phase errors. In this paper, we theoretically present a novel approach to solve this problem. Based on dispersion engineering, we aim at adjusting the waveguide structure, making local waveguide width adjustment at positions of different thicknesses, thus eliminating the phase errors. The adapted waveguide width design is applied for etched and loaded waveguides based on PPTFLN, achieving the ultrahigh power conversion efficiency of second harmonic generation (SHG) up to 2.1 × 104%W-1 and 6936%W-1, respectively, which surpasses the power conversion efficiency of other related works. Our approach just needs standard periodic poling with a single period, significantly reducing the complexity of electrode fabrication and the difficulty of poling, and allows for the placing of multiple waveguides, without individual poling designs for each waveguide. With the advantages of simplicity, high production, and meeting current micro-nano fabrication technology, our work may open a new way for achieving highly efficient second-order nonlinear optical processes based on PPTFLN.

Manipulation of plasmon dephasing time via the coupling between localized surface plasmon resonance and waveguide modes

Manipulation of plasmon dephasing time of the hybrid modes has gained much attention in recent years, which plays critical roles in many important applications of plasmonics. Here, we manipulate the dephasing time of plasmonic field resulting from coupling of the localized surface plasmon (LSP) of the nanosphere array and the waveguide mode supported by the indium tin oxide film with quasi-normal mode-assisted harmonic oscillator model method. It results in the formation of two distinct modes, transverse electric (TE) and transverse magnetic (TM) modes. Such coupling can significantly modify the dynamics of the original plasmonic modes. The dephasing time of the hybrid plasmon modes, encompassing both TE and TM modes, can be extended from 1.25fs to 4.95fs, which is about 2–7 times compared to that of a single localized surface plasmon resonant mode. More importantly, by changing the structural parameters, we can flexibly manipulate either the dephasing time with a fixed wavelength or the resonant wavelength with a fixed dephasing time.

Excitation of hybrid modes in plasmonic nanoantennas coupled with GeSiSn/Si multiple quantum wells for the photoresponse enhancement in the short-wave infrared range

Hybrid structures represented by a plasmonic nanoantenna array located on the surface of GeSiSn/Si multiple quantum wells (MQWs) grown on a silicon-on-insulator (SOI) substrate by molecular-beam epitaxy were developed for applications in the short-wave infrared range. Using numerical modeling methods, the electromagnetic characteristics of metasurfaces based on aluminum nanoantennas located on the surface of GeSiSn/Si nanoheterostructures were studied. It is shown that the spectral and spatial overlap of localized plasmons in nanoantennas and waveguide modes in the semiconductor film leads to the formation of hybrid modes with a Fano-type resonance profile. The possibility of significant photoluminescence (PL) enhancement of GeSiSn/Si MQWs through the use of hybrid plasmonic structures is demonstrated. The PL enhancement reached to 250 % at the wavelength of 1.9 μm, coinciding with the maximum of the reflectance spectrum for the hybrid structure. Based on the hybrid system, represented by the plasmonic nanoantenna array on the top part of GeSiSn/Si MQWs, p-i-n photodiodes were fabricated. The photocurrent enhancement was approximately 7 times at wavelengths of 1.73 µm and 1.8 µm. The excitation of separately localized plasmons and waveguide modes is demonstrated, and their hybridization is also studied.

Efficient second harmonic generation at quasi-bound states in the continuum in hybrid nanostructures of 2D plasmonic array and waveguide

We numerically investigate the second harmonic generation (SHG) in hybrid plasmonic–photonic structures consisting of an array of periodic gold nanodisks covered by a nonlinear dielectric layer. In the designed hybrid systems, symmetry-protected bound states in the continuum (BICs) at normal incidence, and Friedrich–Wintgen BICs due to the coupling between localized surface plasmon resonances (LSPRs) and waveguide modes at oblique incidence, are observed under both transverse-electric (TE) and transverse-magnetic (TM) polarized illuminations. SHG near the BICs is studied, and the dramatically enhanced nonlinear optical response at quasi-BICs is obtained. Such enhancement is ascribed to the enhanced local field due to hybrid modes of LSPRs and photonic modes. The results indicate that hybrid plasmonic-dielectric systems supporting BICs at both TE- and TM-polarized incidences are promising platforms for linear and nonlinear photonic devices.

High-efficiency broadband perfect absorber based on a multilayered pyramid structure

In this paper, an ultra-broadband perfect absorber based on a four-layer pyramid metamaterial and a metal insulator metal (MIM) structure has been demonstrated and investigated. Finite-difference time-domain (FDTD) results indicate that the absorption originates from localized surface plasmons and waveguide mode resonance effect. An average absorption of 96.18 % with bandwidth ranging from 308 nm to 2387 nm was achieved, and the average absorption in the entire wave band (200–2800 nm) was approximately 94.64%. The results of the impedance matching theory are consistent with the FDTD simulation. Polarization insensitivity was investigated by studying the absorption characteristic of TE and TM polarizations. The effect of top and bottom materials was investigated and compared in detail. Moreover, by adjusting the geometrical size, the absorption performance can be fine- tuned. In addition, the proposed broadband absorber exhibited excellent absorption stability and environmental refractive index tolerance. Without noble metal materials, the proposed structure can reduce the fabrication costs. We believe that the proposed broadband absorber has potential applications in solar cells, infrared imaging, and thermal emitters.

Experimental and theoretical investigation of waveguided plasmonic surface lattice resonances.

Plasmonic nanostructures are good candidates for refractive index sensing applications through the surface plasmon resonance due to their strong dependence on the surrounding dielectric media. However, typically low quality-factor limits their application in sensing devices. To improve the quality-factor, we have experimentally and theoretically investigated two-dimensional gold nanoparticle gratings situated on top of a waveguide. The coupling between the localized surface plasmon and waveguide modes results in Fano-type resonances, with high quality-factors, very similar to plasmonic surface lattice resonances. By combining plasmonic surface lattice resonance and waveguide theory, we present a theoretical framework describing the structures. By immersing the fabricated samples in three different media we find a sensitivity of ∼50 nm/RIU and figure of merit of 8.9, and demonstrate good agreement with the theory presented. Further analysis show that the sensitivity is very dependent on the waveguide parameters, grating constant and the dielectric environment, and by tuning these parameters we obtain a theoretical sensitivity of 887 nm/RIU.

Numerical demonstration of surface lattice resonance excitation in integrated localized surface plasmon waveguides.

We numerically show that surface lattice resonances (SLR) in periodic localized surface plasmon (LSP) waveguides integrated on a dielectric waveguide can be excited via in-phase evanescent coupling, by incident propagation vector outside the light cone and without any constraint on the structural symmetry. FDTD simulations show that the coupling between wideband LSP resonances and narrowband SLR results in a Fano-like resonance, showing few nanometers large sharp spectral features that may be exploited for achieving new functions for integrated optics and sensing.

A Review: Surface Plasmon Resonance-Based Biosensor for Early Screening of SARS-CoV2 Infection

Rapid viral diagnosis is essential to contain the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV2) in the community and ensure effective patient management. However, the current gold standard for diagnosis of viral disease depends on the time-consuming molecular technique. Surface plasmon resonance (SPR) phenomenon has been widely applied in the analysis of biomolecular interactions and analytes detection, which is currently being investigated as a quick diagnostic method of numerous viral infections. In this regard, researchers have performed experimental studies dedicated to enhancing the sensitivity and specificity of the SPR sensor performance for utilization in the medical diagnostic field. In this review, we summarized various SPR application including the recent advancements and advantages of different types of SPR-based biosensor such prism-based, localized surface plasmon resonance (LSPR), fiber optic, optical grating, and optical waveguide for the detection of SARS-CoV2. We proposed various technical recommendations to improve the sensitivity, accuracy, time response, and cost-saving means of the SPR sensor. A special focus was set for sensitivity enhancement aspects in terms of the type of prism, nanomaterials, support layer with type and shape of optical fibers for early, swift, and ultra-sensitive detection of viral infection. With the highlighted impacts of SPR application, this review frames the current potential and challenges of SPR in comparison to the current gold standard of viral infection diagnostic strategy.

Probing mid-infrared surface interface states based on thermal emission.

Probing mid-infrared surface wave radiation remains a big challenge for a long time. The lack of convenient and quick mid-infrared surface wave radiation probing methods limits the development of the integrated mid-infrared materials and devices. In this work, we propose a scheme to construct and probe the mid-infrared surface wave radiation of interface state in the waveguide through thermal emission. A superlattice composed of alternately placed periodic meta-crystals is designed to construct an array of interfaces to realize the interface states through the transverse electrical waveguide modes with a tolerance in structural parameters. By heating the structure, we employ angular resolved thermal emission spectroscopy to directly and quickly verify the dispersion of mid-infrared interface states, which have specific frequencies, angles, and polarizations. Moreover, we establish a thermal imaging microscopy to probe the local waveguide interface state directly for the first time. This proposed infrared probing method based on thermal emission can be generalized to probe the mid-infrared surface wave in other systems, such as surface plasmon waves in graphene or surface phonon waves in two-dimensional materials in the mid-infrared range.

Low-threshold and narrow-linewidth perovskite microlasers pumped by a localized waveguide source

Abstract For the widely used vertically pumped (VP) method with a free-space beam, very little pump power is absorbed by the gain materials in microlasers because of the large spatial mismatch of areas between laser modes and free-space pump beams together with small thicknesses of gain materials, resulting in a high pump power threshold. Here, an in-plane-waveguide-pump (IPWP) method with a localized waveguide source is proposed to reduce pump power threshold of perovskite microlasers. Owing to reduced spatial mismatch of areas between laser modes and localized waveguide sources as well as increased absorption distances, the pump power threshold of the IPWP method is decreased to approximately 6% that of the VP method. Moreover, under the same multiple of the pump power threshold, the laser linewidth in the IPWP method is narrowed to approximately 70% that in the VP method. By using the IPWP method, selective pumping two adjacent (separation 2 or 3 μm) parallel-located perovskite microlasers is experimentally demonstrated, and no crosstalk is observed. This IPWP method may have applications in low-energy and high-density microlasers and photonic integrated circuits.

Surface Temperature Mapping of a Metal Plate Using Ultrasound-Guided Wave Technique

Abstract Surface temperature mapping is crucial for the monitoring and control of an object of interest, such as furnace, reactor pipes carrying hot fluids, or a component under a temperature-dependent process. While the use of waveguides for temperature measurement is well documented in literature, the attachment of the waveguide to a metallic component poses challenges. These include the relationship between the local waveguide temperature and that of the metal component, and wave leakage into the component. In this paper, the authors study the propagation of shear horizontal (SH) guided wave in a strip waveguide and its interaction with the notch embodiments in the waveguide. The effects of the type of notch and its depth on the SH mode characteristics are investigated through simulation studies. The mode of attachment of the waveguide to the metal component is by means a slot made in the component. The area of contact between the waveguide and metal component is optimized such that there is minimum wave leakage into the bulk material. Based on the simulation results, a waveguide strip is fabricated and used to monitor the local surface temperature of a test metal component. The waveguide is calibrated by correlating the time of flight (ToF) shift in the waveforms against reference temperature values. Thereafter, the instantaneous temperature of the metal component is determined from the calibration equations. A set of experimental trials are performed to check for repeatability. The experiments are conducted in near steady-state conditions for better accuracy in the measurements.

Highly efficient second harmonic generation of thin film lithium niobate nanograting near bound states in the continuum

Bound states in the continuum (BICs) are ubiquitous physical phenomena where such states occur due to strong coupling between leaky modes in side lossy systems. BICs in meta-optics and nanophotonics enable optical mode confinement to strengthen local field enhancement in nonlinear optics. In this study, we numerically investigate second-harmonic generation (SHG) in the vicinity of BICs with a photonic structure comprising one-dimensional nanogratings and a slab waveguide made of lithium niobate (LiNbO3, LN). By breaking the symmetry of LN nanogratings, BICs transition to quasi-BICs, which enable strong local field confinement inside LN slab waveguide to be supported, thereby resulting in improving SHG conversion with lower pump power of fundamental frequency (FW). With a peak intensity of 1.33 GW cm−2 at the FW, our structure features a second-harmonic conversion efficiency up to 8.13 × 10−5 at quasi-BICs. We believe that our results will facilitate the application of LN in integrated nonlinear nanophotonic.

Collectively driven optical nanoantennas

Optical nanoantennas, i.e., elements transforming localized light or waveguide modes into freely propagating fields and vice versa, are vital components for modern nanophotonics. Optical antennas have been demonstrated to cause the Dicke superradiance effect, i.e., collective spontaneous emission of quantum sources. However, the impact of coherent excitation on the antenna performance, such as directivity, efficiency, and Purcell effect, remains mostly unexplored. Herein, using full-wave numerical simulations backed by a quantum model, we unveil that coherent excitation allows controlling antenna multipoles, on-demand excitation of nonradiative states, enhanced directivity, and improving antenna radiation efficiency. This collective excitation corresponds to the states with nonzero dipole moment in the quantum picture, where the quantum phase is well defined. The results of this work bring another degree of freedom----the collective phase of an ensemble of quantum emitters----to control optical nanoantennas and, as such, pave the way to the use of collective excitations for nanophotonic devices with superb performance. To make the discussion independent of the frequency range, we consider the all-dielectric design and use dimensionless units.

Free-Standing Single Ag Nanowires for Multifunctional Optical Probes.

Miniaturized and manipulable optical probes are the foundation for developing in situ characterization devices in confined space. We developed two methods for fabricating free-standing single Ag nanowires (AgNWs) directly at the tip of a glass capillary either by chemical or electrochemical reduction. The electrochemical nature of both methods resulted in a rapid growth rate of AgNWs up to 1.38 μm/s and a controllable length from 5 to 450 μm. The AgNWs with a unique anisotropic structure allow localized surface plasmon resonance and surface plasmon waveguides in the radial direction and axial direction, respectively. We verified the possibility of using single AgNWs as an optical dispersion device and waveguide probe. By controlling the experimental conditions, rough-surface AgNWs with high surface-enhanced Raman scattering (SERS) activity were also fabricated. These SERS-active probes also exhibited advantages in acquiring molecular information from a single living cell.

Reconfigurable locally resonant surface acoustic demultiplexing behavior in ZnO-based phononic crystal

We present the design and numerical investigation of a reconfigurable and miniature locally resonant surface acoustic wave demultiplexer based on a ZnO pillar phononic crystal, for the first time. Hollow cylinder line defects are used as waveguides, due to their good structural controllability over the local resonant waveguiding frequency and bandwidth. Two local resonant surface acoustic waveguides are designed and simulated as the output channels of the demultiplexer, and the shear-horizontal wave transmission spectra are calculated for each channel individually. The designed radio frequency demultiplexing output channels support frequencies of 4.14 GHz and 4.28 GHz, with respective bandwidths of 40 MHz and 60 MHz, while their spatial separation is just about 800 nm. In order to achieve a reconfigurable output characteristic, the effect of acoustoelectric interaction in piezoelectric semiconductors is numerically simulated in this study. The acoustoelectric interaction causes an additional stiffness in ZnO that can be released by adding extra charge carriers, i.e., increasing conductivity, thus changing the effective elasticity of the ZnO structures and the guiding frequencies of the output channels. Two output frequencies show red shifts of about 100 MHz and 150 MHz by extremely increasing the conductivity of ZnO structures from 0.01 S/m to 100 S/m.

Record-high positive refractive index change in bismuth germanate crystals through ultrafast laser enhanced polarizability

Unlike other crystals, the counter intuitive response of bismuth germanate crystals ({text {Bi}}_4{text {Ge}}_3{text {O}}_{12}, BGO) to form localized high refractive index contrast waveguides upon ultrafast laser irradiation is explained for the first time. While the waveguide formation is a result of a stoichiometric reorganization of germanium and oxygen, the origin of positive index stems from the formation of highly polarisable non-bridging oxygen complexes. Micro-reflectivity measurements revealed a record-high positive refractive index contrast of 4.25times 10^{-2}. The currently accepted view that index changes >1times 10^{-2} could be brought about only by engaging heavy metal elements is strongly challenged by this report. The combination of a nearly perfect step-index profile, record-high refractive index contrast, easily tunable waveguide dimensions, and the intrinsic high optical non-linearity, electro-optic activity and optical transparency up to 5.5,upmu {text {m}} of BGO make these waveguides a highly attractive platform for compact 3D integrated optics.

Colloidal quantum dots decorated micro-ring resonators for efficient integrated waveguides excitation

Abstract Micro-ring resonators made of titanium dioxide were decorated with local light sources comprising CdSe/CdS colloidal quantum dot aggregates. The active micro-resonators are operated to achieve efficient evanescent excitation of nearby co-planar integrated waveguides. Coupled-mode analysis and numerical simulations are used to capture the dynamic of the optical interaction between locally activated resonators and integrated waveguides. In this context, we exemplify the key role of resonator intrinsic loss. Next, we show that locally activated or bus-waveguide excited resonators are in optimum waveguide interaction for the same so-called critical coupling condition, although the physical origin of this property is different for each configuration. More importantly, we found that a locally activated resonator is a fabrication imperfection tolerant configuration for the coupling light of local sources into waveguides. This remarkable property originates from the opposite change of the power cycling into the resonator and the waveguide coupling efficiency as a function of the resonator-waveguide separation gap. By operating an 8-μm-radius ring resonator with loaded quality factors around Q = 2100, we experimentally demonstrate a 5.5-dB enhancement of the power coupled into the output waveguide compared to a direct local source waveguide excitation.

Detection of damage in welded joints using high order feature guided ultrasonic waves

Abstract Topographical features such as welds, stiffeners, and bends in plate-like structures have been identified as special local waveguides, which are able to render ultrasonic wave energy preferentially guided along themselves. Such feature guided waves (FGW) at low frequencies have been reported in the literature as an effective screening tool for long-range features, which, however, can only detect relatively large defects. In this study, the existence of high order FGW modes, categorized into multiple wave families, are revealed via the modal analysis of an unbounded welded plate at high frequencies, by using the semi-analytical finite element (SAFE) approach. These FGWs exhibit strong mode confinement in specific regions of the weld, which can potentially offer enhanced interrogation of the localized area in long welds and provide improved detection sensitivity to small defects. A series of high order shear horizontal (SH) type weld-guided modes are investigated in both three-dimensional finite element (3D FE) simulations and experiments, for detecting axial and transverse cracks of sub-wavelength size in the weld bead. A new superposition technique is proposed to coherently enhance the amplitude of the selected mode in a multi-modal waveform, leading to an attractive single-mode inspection capability at high frequencies. Both small cracks are identified numerically and experimentally in the superposed reflection of the SH 1 -like weld-guided wave, and results are in close agreement. The feasibility of using high order weld-guided modes for inspecting small defects inside the weld has been demonstrated.

Angular dependent strong coupling between localized waveguide resonance and surface plasmon resonance in complementary metamaterials

We give direct evidence of both surface plasmon resonance (SPR) and localized waveguide resonance (LWR) contribution to the extraordinary optical transmission in complementary metamaterials. Strong coupling between SPR and LWR are also observed with clear evidence of Rabi splitting and anti-crossing phenomena. The splitting introduces sharp phase shift, which in turn enhances group velocity delay by the incident angle without geometric parameter change. The results not only clarify SPR and LWR effects in the extraordinary optical transmission, but also provide a novel route to control light-metamaterial interaction by angular modulation for on-chip slow light devices.

Low-energy high-speed plasmonic enhanced modulator using graphene.

Graphene, as a type of flexible and electrically adjustable two-dimensional material, has exceptional optical and electrical properties that make it possible to be used in modulators. However, the poor interaction between optical fields and a single atom graphene layer prevents the easy implementation of graphene modulators. Currently available devices often require a larger overlap area of graphene to obtain the desired phase or amplitude modulation, which results in a rather large footprint and high capacitance and consequently increases the energy consumption and reduces the modulation speed. In this paper, a localized plasmonic-enhanced waveguide modulator with high-speed tunability using graphene is proposed for telecommunication applications. Strong modulation of the transmission takes place due to the enhanced interaction between the ultrathin plasmon patches and the graphene, when the plasmons are tuned on- and off-resonance by the gate-tunable graphene. A 400 GHz modulation rate using low gated-voltages with an active device area of 0.2 μm2 and a low consumption of only 0.5 fJ/bit is achieved, which paves the way for ultrafast low-energy optical waveguide modulation and switching.