Ultra-high light confinement and ultra-long propagation distance design for integratable optical chips based on plasmonic technology

A high figure of merit of phonon-polariton waveguide modes with hbn/SiO2/graphene /hBN ribs waveguide in mid-infrared range

Natural hyperbolic materials can confine electromagnetic waves at the nanoscale. In this study, we propose a waveguide design that combines a high quality factor (FOM) with low loss, utilizing hexagonal boron nitride and graphene and gold substrate. The waveguide consists of a dielectric rib with a graphene layer sandwiched between two hBN ribs. Numerical simulations demonstrate the existence of two guided modes in the proposed waveguide within the second reststrahlen band (1360.0 cm-1<ω < 1609.8 cm-1) of hBN. These modes are formed by coupling the hyperbolic phonon polariton (HPhP) of two hBN rib in the middle dielectric rib and are subsequently modulated by a graphene layer. Interestingly, we observe variations in four transmission parameters, namely effective length, figure of merit, device length, and propagation loss of the guided modes, with respect to the operation frequency and gate voltage. By optimizing the waveguide's geometry parameters and dielectric permittivity, the modal properties were analyzed. Simulation results indicate that optimizing the waveguide size parameters enables us to achieve a high FOM of 4.0 × 107. The proposed waveguide design offers a promising approach for designing tunable mid infrared range waveguides on photonic chips, and this concept can be extended to other 2D materials and hyperbolic materials.

Dual Coupled Long-Range Hybrid Surface Plasmon Polariton Waveguide for Sub-Wavelength Confinement

In this paper, a long-range hybrid waveguide for subwavelength confinement based on double SPP coupling is proposed. The hybrid waveguide consists of a metal-based cylindrical hybrid waveguide and a silver nanowire. There are two coupling regions in the waveguide structure that enhance mode coupling. Strong mode coupling enables the waveguide to exhibit both a small effective mode area (0.01) and an extremely long transmission length (700 μm). The figure of merit (FOM) of the waveguide can be as high as 4000. In addition, the cross-sectional area of the waveguide is only 500 nm × 500 nm, allowing optical operation in the subwavelength range, which helps enhance the miniaturization of optoelectronic devices. The excellent characteristics of the hybrid waveguide make it have potential applications in photoelectric integrated systems.

Structure, stability, and electronic and optical properties of TMDC-coinage metal composites: vertical atomically thin self-assembly of Au clusters

Composites of metal clusters supported on transition metal dichalcogenides (TMDCs) often provide promising opportunities for applications in nanoelectronics, catalysis, sensing, etc. In the present investigation, a systematic attempt has been made to unveil the structure and stability of coinage M₆ clusters supported on TMDC (MoS₂ and WS₂) monolayers. The more prominent objective is to explore potential candidates that stabilize the two-dimensional (2D) M₆ clusters on their surface. Periodic energy decomposition analysis (pEDA) was carried out to probe the various interaction energy (IE) components that govern the stability of the M₆ clusters in the composites. Attention has also been devoted to unravelling the electronic and optical properties of these TMDCs/M₆ composites. Moreover, ab initio molecular dynamics (AIMD) simulations were performed to scrutinize the dynamic behaviour of Au cluster on WS₂ monolayer. The results reveal that the coinage M₆ clusters form energetically more stable composites on MoS₂ than WS₂ monolayer. It is worth mentioning that WS₂ promotes the stability of 2D M₆ clusters. Inclusion of dispersion correction marginally altered the geometries of the TMDCs/M₆ composites but its impact on the IE values was significant. AIMD simulation explicitly emphasizes that the WS₂ surface preferentially facilitates the vertical 2D self-assembling of Au atoms and, interestingly, the planarity is mostly retained during the course of simulations. The adsorption of coinage M₆ clusters substantially influences the electronic and optical properties of the TMDCs. HSE06 calculation confirms that the decrease in energy gap is more pronounced in MoS₂/M₆ composites. The outcomes of this study render fundamental insights into the various TMDCs/M₆ composites that would certainly be worthwhile probing for diverse practical applications.

Terahertz hybrid plasmonic waveguides with ultra-long propagation lengths based on multilayer graphene-dielectric stacks

To develop on-chip photonic devices capable of transmitting terahertz signals beyond the propagation distance of millimeter while keeping deep subwavelength field confinement has been a challenging task. Herein, we propose a novel multilayer graphene-based hybrid plasmonic waveguide (MLGHPW) consisting of a cylindrical dielectric waveguide and hyperbolic metamaterials. The device is based on alternating graphene and dielectric layers on a rib substrate, operating in the terahertz range (f = 3 THz). We couple the fundamental dielectric waveguide mode with the fundamental volume plasmon polarition modes originated from the coupling of plasmon polaritons at individual graphene sheets. The resulting hybrid mode shows ultra-low loss compared with the conventional GHPW modes at the comparable mode sizes. The present MLGHPW demonstrated a few millimeters of propagation length while keeping the mode area of 10^-3A₀, where A₀ is the diffraction-limited area, thus possessing a thirty times larger figure of merit (FoM) compared to other GHPWs. The additional degree of freedom (the number of graphene layers) makes the proposed MLGHPW more flexible to control the mode properties. We investigated the geometry and physical parameters of the device and identified optimal FoM. Moreover, we analyzed the crosstalk between waveguides and confirmed the potential to construct compact on-chip terahertz devices. The present design might have the possible extensibility to other graphene-like materials, like silicene, germanen, stanene etc.

Influence of Spatial Dispersion on Propagation Properties of Waveguides Based on Hyperbolic Metamaterial

In this work, we study the effect of spatial dispersion on propagation properties of planar waveguides with the core layer formed by hyperbolic metamaterial (HMM). In our case, the influence of spatial dispersion was controlled by changing the unit cell's dimensions. Our analysis revealed a number of new effects arising in the considered waveguides, which cannot be predicted with the help of local approximation, including mode degeneration (existence of additional branch of TE and TM high-β modes), power flow inversion, propagation gap, and plasmonic-like modes characterized with long distance propagation. Additionally, for the first time we reported unusual characteristic points appearing for the high-β TM mode of each order corresponding to a single waveguide width for which power flow tends to zero and mode stopping occurs.

Nanostructured hybrid plasmonic waveguide in a slot structure for high-performance light transmission

Squeezing light to nanoscale is the most vital capacity of nanophotonic circuits processing on-chip optical signals that allows to significantly enhance light-matter interaction by stimulating various nonlinear optical effects. It is well known that plasmon can offer an unrivaled concentration of optical energy beyond the optical diffraction limit. However, the progress of plasmonic technology is mainly hindered by its ohmic losses, thus leading to the difficulty in building large-area photonic integrated circuits. To significantly increase the propagation distance of light, we develop a new waveguide structure operating at the telecommunication wavelength of 1,550 nm. It consists of a nanostructured hybrid plasmonic waveguide embedded in a high-index-contrast slot waveguide. We capitalize on the strong mode confinement of the slot waveguide and reduce mode areas with the nanostructured hybrid plasmonic configuration while maintaining extremely low ohmic losses using a nanoscale metal strip. The proposed design achieves a record propagation distance of 1,115 µm while comparing with that of other designs at a mode area of the order of 10^-5A₀ (A₀ is the diffraction-limited area). The mode characterization considering fabrication imperfections and spectral responses show the robustness and broadband operation range of the proposed waveguide. Moreover, we also investigated the crosstalk to assess the density of integration. The proposed design paves the way for building nanophotonic circuits and optoelectronic devices that require strong light-matter interaction.

Highly confined dielectric guiding mode in nanoridges embedded in a conventional slot waveguide

Plasmonic waveguides can offer a promising solution beyond the optical diffraction limit. However, the cost of shrinking mode sizes reflects in metallic ohmic losses that lead to a short propagation distance of light, hindering the practical applications of plasmonic waveguides. Herein, we tackled the practicality of a novel CMOS-compatible all-dielectric waveguide structure that exploits electromagnetic boundary conditions of both the continuous normal component of the electric displacement field and the tangential component of the electric field at a high-index-contrast interface, which allows the attainment of mode areas comparable with those of plasmonic waveguides and theoretical lossless. The proposed waveguide comprises two oppositely contacted nanoridges with semicircular tops embedded in a conventional slot waveguide. By stepping on the strong electric field in the low-index slot region of the slot waveguides, the nanoridges squeeze the mode areas further with a guiding mechanism identical to that of a surrounding slot waveguide. Through the design of the geometry parameters, the calculated mode area of the reported structure achieved an unprecedented order of 4.21 × 10^-5A₀, where A₀ is the diffraction-limited area. The mode area dependence on fabrication imperfections and spectral response showed the robustness and broadband operation. Moreover, on the basis of extremely tight mode confinements, the present waveguide even outperformed the hybrid plasmonic waveguides in lower crosstalk. The proposed idea makes the realization of practically feasible nanoscale photonic integrated circuits without any obstructions by the limited propagation distance of light for plasmonic waveguides, thereby expanding its applications in various nanophotonic and optoelectronics devices requiring strong light-matter interaction within nanoscale regions.

Symmetric Graphene Dielectric Nanowaveguides as Ultra-Compact Photonic Structures

A symmetric graphene plasmon waveguide (SGPWG) is proposed here to achieve excellent subwavelength waveguiding performance of mid-infrared waves. The modal properties of the fundamental graphene plasmon mode are investigated by use of the finite element method. Due to the naturally rounded tips, the plasmon mode in SGPWG could achieve a normalized mode field area of ~10⁻⁵ (or less) and a figure of merit over 400 by tuning the key geometric structure parameters and the chemical potential of graphene. In addition, results show that the modal performance of SGPWG seems to improve over its circular counterparts. Besides the modal properties, crosstalk analysis indicates that the proposed waveguide exhibits extremely low crosstalk, even at a separation distance of 64 nm. Due to these excellent characteristics, the proposed waveguide has promising applications in ultra-compact integrated photonic components and other intriguing nanoscale devices.

Antimony Nanopolyhedrons with Tunable Localized Surface Plasmon Resonances for Highly Effective Photoacoustic-Imaging-Guided Synergistic Photothermal/Immunotherapy

Antimony (Sb), a typical group VA semimetal, has rarely been studied both experimentally and theoretically in plasmonic photothermal therapy, possibly due to the lack of effective morphology-controllable methods for the preparation of high-quality Sb nanocrystals. In this study, an effective ligand-guided growth strategy to controllably synthesize Sb nanopolyhedrons (Sb NPHs) with ultrahigh photothermal conversion efficiency (PTCE), good photothermal stability, as well as biocompatibility is presented. Furthermore, the modulation effect of different morphologies on localized surface plasmon resonance (LSPR) of Sb NPHs in experimentation is successfully observed. When the resonance frequency of the Sb NPHs is matched well with the excitation wavelength (808 nm), the PTCE of the Sb NPHs is as high as 62.1%, which is noticeably higher compared to most of the reported photothermal agents. The Sb NPHs also exhibit good photothermal stability. In addition, Sb-NPHs-based multifunctional nanomedicines are further constructed via loading 1-methyl-d-tryptophan on PEGylated Sb NPHs for a highly efficient photoacoustic-imaging-guided synergistic photothermal/immune-therapy of tumors in vivo. This work can stimulate further theoretical and experimental investigations of Sb NPHs and other semimetal nanomaterials regarding their LSPR properties and inspire various potential applications of semimetals in biomedicine and sensors.

Mid-infrared electro-optic modulator based on a graphene-embedded plasmonic rib waveguide with ultrahigh electro-optic wavelength tuning

A graphene-embedded plasmonic rib waveguide (GEPRW) is designed for the mid-infrared electro-optic modulator. The mode characteristics and electro-optic (EO) modulation performances are simulated and optimized by using the finite element method. The results show that propagation length of 10³mm and figure of merit of 10⁶ are obtained by adjusting the bias voltage applied to the GEPRW. The EO wavelength tunings are -66.69 and -78.87nm/V for peak L and peak R in the loss spectra when w=3µm and h₁=2µm. For a 100 µm long GEPRW, the modulation depths of ∼96.4,∼97.1,∼93.7, and ∼94.9%, and FWHMs of ∼30,∼74,∼34, and ∼59nm can be achieved when λ=1.55, 1.87. 1.89, and 2.23 µm. The EO modulator based on the GEPRW has a wide wavelength tuning range from 1.05 to 2.23 µm. It has high modulation depth, low insertion loss, and broad bandwidth, which can be used as EO tunable devices such as optical interconnects and optical switches.

Mid-infrared polarization rotator based on a Si3N4-CaF2 hybrid plasmonic waveguide with asymmetric metal claddings

In this paper, a Si₃N₄-CaF₂ hybrid plasmonic waveguide (HPW) with an asymmetric metal cladding is designed for the mid-infrared polarization rotator (PR). The mode characteristics and polarization rotation performances of the Si₃N₄-CaF₂ HPW-based PR are simulated by using the finite element method. Operating at the wavelength of 3.5 µm, the polarization conversion efficiency between two polarization modes (PM 1 and PM 2) is larger than 99% at a Si₃N₄-CaF₂ HPW length of 104 µm. The Si₃N₄-CaF₂ HPW-based PR maintains good polarization rotation performances within fabrication tolerances from -10 to 10 nm. The polarization rotator based on the Si₃N₄-CaF₂ HPW paves the way to achieve integrated waveplates, driving many important optical functions from free space onto a chip.

LiNbO3 waveguide with embedded Ag nanowire and 3L MoS2 for strong light confinement and ultra-long propagation length in the visible spectral range

A vertical slot LiNbO₃ waveguide with an Ag nanowire and 3L MoS₂ embedded in the low-refractive index slot region is proposed for the purpose of improving light confinement. We find that the proposed waveguide has a novel dielectric based plasmonic mode, where local light field is enhanced by the Ag nanowire. The mode exhibits an extremely large figure of merit (FoM) of 6.5×10⁶, one order of magnitude larger than that the largest FoM of any plasmonic waveguide reported in the literature to date. The waveguide also has an extremely long propagation length of 84 cm in the visible wavelength at 680 nm. Furthermore, the waveguide has a low sub-micro bending loss and can be directly connected to all-dielectric waveguides with an extremely low coupling loss. The proposed vertical slot LiNbO₃ waveguide is a promising candidate for the realization of ultrahigh integration density tunable circuits in the visible spectral range.

Plasmonics for Telecommunications Applications

Plasmonic materials, when properly illuminated with visible or near-infrared wavelengths, exhibit unique and interesting features that can be exploited for tailoring and tuning the light radiation and propagation properties at nanoscale dimensions. A variety of plasmonic heterostructures have been demonstrated for optical-signal filtering, transmission, detection, transportation, and modulation. In this review, state-of-the-art plasmonic structures used for telecommunications applications are summarized. In doing so, we discuss their distinctive roles on multiple approaches including beam steering, guiding, filtering, modulation, switching, and detection, which are all of prime importance for the development of the sixth generation (6G) cellular networks.

Analytic approach to study a hybrid plasmonic waveguide-fed and numerically design a nano-antenna based on the new director

In this paper, a wideband InP-based hybrid plasmonic nano-antenna (HPNA) operating at telecommunication wavelengths has been proposed. Monolithically integrating InP-based lasers with hybrid plasmonic waveguide (HPW) as a feed line of the proposed HPNA on the same InGaAsP/InP wafer can increase the antenna efficiency. A new vertical director has been employed to have a highly directive horizontal radiation pattern. This enhancement is attributed to the efficient coupling between the radiation patterns of arm elements as well as reduced side lobes and back-lobes levels due to the achieved impedance matching. As a result, the directivity has been increased considerably, 3.6 dBi at 193.5 THz (1550 nm) and 1.1 dBi at 229 THz (1310 nm). The HPNA shows the high directivity, total efficiency and quality factor of 11.8, 97.49% and 94.57, respectively. Further, to verify the validity of confining the fundamental TM mode to a thin layer with the lower refractive index, both theoretical and numerical methods have been employed. Therefore, we have derived an analytical formula to investigate the HPW dispersion relation based on the transfer matrix theory and genetic algorithm. Moreover, due to the HPNA ability to receive an optical signal from free space and transmit it to the waveguide based on the reciprocity theorem, the HPNA performance as an optical wireless on-chip nano-link has been investigated analytically and numerically. Additionally, to obtain a high optical power signal and steering the beam angle, the antenna gain and directivity have been calculated with two different types of array structure by controlling the relative phase shift between the array elements and elements number. To validate the array design performance, a three dimensional full-wave numerical simulation and array factor theory have been exploited. The HPNA fabrication is compatible with generic foundry technology.