A hybrid plasmonic waveguide for subwavelength confinement and long-range propagation

Buttcoupling is the most efficient way to excite surface plasmon polariton (SPP) waves at dielectric/metal interfaces in order to realize applications in broadband and ultra-compact integrated circuits (IOCs). We propose a reasonable waveguide structure to efficiently excite and collect the SPP waves supported in a plasmonic trench waveguide in the long-haul telecommunication wavelength range. Our simulation results show that the coupling efficiency between the dielectric strip waveguides and a plasmonic trench waveguide can be optimized, which is dominated by the zigzag propagation path length in the dielectric strip loaded on the metal substrate. It is noted that nearly a 100% coupling efficiency can be achieved when the distance between the excitation source and the plasmonic waveguide is about 6.76 μm.

Numerical investigation of finite thickness metal-insulator-metal structure for waveguide-based surface plasmon resonance biosensing

Surface Plasmon Polaritons (SPPs) are coherent electron-plasma oscillations at the interface between a conductor and a dielectric and are promising candidates to overcome the diffraction limit of light in plasmon-waveguides. Noble metals are widely used for SPP modes in the visible range. However, these metal-based optical devices show a weak electromagnetic field confinement and strong transmission losses in the mid-infrared region, making it insufficient for SPP excitation in this wavelength region. In this paper, we studied long-range surface plasmon propagation on graphene/SiO2 and graphene/Si3N4 waveguides on a biaxial, garnet substrate. This plasmonic waveguide supports long-range SPP modes with effective mode confinement and lower transmission losses. The simulation results show that the dispersion of SPPs can be effectively tuned by changing the fermi energy and electron mobility of graphene layer, temperature and geometrical and material parameters of plasmon waveguides.

The excitation of surface plasmon-polariton (SPP) waveguide modes in subwavelength dielectric ridges deposited on a thin gold film has been characterized and optimized at telecommunication wavelengths. The experimental data on the electromagnetic mode structure obtained using scanning near-field optical microscopy have been directly compared to full vectorial three-dimensional finite element method simulations. Two excitation geometries have been investigated where SPPs are excited outside or inside the dielectric tapered region adjoint to the waveguide. The dependence of the efficiency of the SPP guided mode excitation on the taper opening angle has been measured and modeled. Single-mode guiding and strong lateral mode confinement of dielectric-loaded SPP waveguide modes have been characterized with the near-field measurements and compared to the effective-index method model.

Surface plasmon polaritons (SPPs) manipulation on metal surfaces is important for constructing ultracompact integrated micro/nano-optical devices and systems. We employ the methodology of surface electromagnetic wave holography (SWH) to design holographic groove patterns for controlling SPPs with complicated wavefronts traveling on metal surface. SPPs are scattered by these deli groove patterns and interfere with each other to form desired SPP wavefronts. Several devices are demonstrated to control the intensities and phases of SPPs, such as focusing a plane SPP or diverging SPPs to two points with different phases, and focusing SPPs with complicated beam profile to a point. The finite-difference time-domain simulations show that in all cases, the predesignated functionalities are fully achieved by the designed plasmonic holographic structures. The results strongly support the power of SWH for shaping the complicated wavefront of in-plane transporting SPPs.

Experimental analysis of waveguide-coupled surface-plasmon-polariton cone properties

Vertical directional coupling between a metal-insulator-metal (MIM) plasmonic waveguide and a conventional dielectric waveguide is investigated. The coupling length, extinction ratio, insertion loss and coupling efficiency of the hybrid coupler are analyzed. As an example, when the separation between the two waveguides is 250 nm, a maximum coupling efficiency of 73%, an insertion loss of -1.4 dB and an extinction ratio of 16 dB can be achieved at a coupling length of 4.5 microm at 1.55 microm wavelength. A particular feature of this hybrid coupler is that it is highly tolerant to the structural parameters of the plasmonic waveguide and the misalignment between the two waveguides. The performance of this hybrid coupler as a TM polarizer is also analyzed and a maximum extinction ratio of 44 dB and an insertion loss of -0.18 dB can be obtained. The application of this hybrid coupler includes the signal routing between plasmonic waveguides and dielectric waveguides in photonic integrated circuits and the polarization control between TE and TM modes. In addition, it provides an approach for efficiently exciting MIM plasmonic modes with conventional dielectric modes.

Nanofocusing of Surface Plasmons at the Apex of Metallic Tips and at the Sharp Metallic Wedges. Importance of Electric Field Singularity

Renewed and growing interest in the field of surface plasmon polaritons (SPPs) comes from a rapid advance of nanostructuring technologies. The desired nanostructures are usually fabricated by electron- or ion-beam lithography. An alternative approach is the application of two-photon polymerization (2PP) or nonlinear lithography. Both these technologies are based on nonlinear absorption of near-infrared femtosecond laser pulses. With 2PP, the fabrication of three-dimensional micro-objects and photonic crystals with a resolution down to 100 nm is possible. In this contribution, we study applications of advanced femtosecond laser technologies for the fabrication of SPP structures. We demonstrate that resulting structures can be used for excitation, guiding, and manipulation of SPPs on a subwavelength scale. Characterization of these structures is performed by detection of the plasmon leakage radiation (LR). 2PP allows the fabrication of dielectric waveguides, splitters, and couplers directly on metal surfaces. The fabricated dielectric structures are also very efficient for the excitation of SPPs. Using these structures, excitation and focusing of the resulting plasmon field can be achieved.

The miniaturization of polarization beam splitters (PBSs) is vital for ultradense chip-scale photonic integrated circuits. However, the small PBSs based on complex hybrid plasmonic structures exhibit large fabrication difficulties or high insertion losses. Here, by designing a bending multimode plasmonic waveguide, an ultrabroadband on-chip plasmonic PBS with low insertion losses is numerically and experimentally realized. The multimode plasmonic waveguide, consisting of a metal strip with a V-shaped groove on the metal surface, supports the symmetric and antisymmetric surface plasmon polariton (SPP) waveguide modes in nature. Due to the different field confinements of the two SPP waveguide modes, which result in different bending losses, the two incident SPP waveguide modes of orthogonal polarization states are efficiently split in the bending multimode plasmonic waveguide. The numerical simulations show that the operation bandwidth of the proposed PBS is as large as 430 nm because there is no resonance or interference effect in the splitting process. Compared with the complex hybrid plasmonic structure, the simple bending multimode plasmonic waveguide is much easier to fabricate. In the experiment, a broadband (Δλ≈120 nm) and low-insertion-loss (<3 dB with a minimum insertion loss of 0.7 dB) PBS is demonstrated by using the strongly confined waveguide modes as the incident sources in the bending multimode plasmonic waveguide.

Surface plasmon polaritons (SPPs) carry transverse optical spin within the evanescent field, which has enabled the demonstration of various chiral light–matter interactions in classical and quantum systems. To achieve high spin selectivity in the interactions, the elliptical polarization of the evanescent field should be made circular, but the engineering principle of the degree of circular polarization (DOCP) of SPPs has been lacking. In this study, we theoretically and numerically investigate the scaling behavior of the DOCP of the SPP field with respect to the modal effective refractive index (neff). The DOCP of the SPP field exhibits power-law scalability to the effective refractive index in the one-dimensional (1D) layered system regardless of the material, structural geometry, and excitation wavelength. The power-law scalability is also confirmed in two-dimensional (2D) waveguide structures for in-plane and out-of-plane SPP fields, but the scaling exponents vary depending on the distance from the waveguide boundaries by the reduced symmetry of the given system. Due to Lorentz reciprocity, the power-law scalability can be extended to the coupling directionality of chiral emitters toward the plasmonic waveguide. To this end, we propose a chiral photonic platform for enhanced light–valley interaction, which utilizes simultaneous enhancement of the DOCP and coupling directionality. An incident SPP can excite a chiral emitter with high spin selectivity that unidirectionally couples the emitted light into the plasmonic waveguide, depending on the valley polarization of excitons in 2D materials. Our work provides a ground rule for designing chiral nanophotonic systems and paves the way for the exploration of scale-free phenomena of electromagnetic waves.

A new, rigorous theory is presented for analyzing radiation and transmission of electromagnetic waves in uniformly curved sections of slab waveguides. General fields are expressed, in this theory, in terms of a spectral integral of eigenmodes with continuous eigenvalues. The theory is applied to the investigation of mode transmission characteristics of a circularly bent, finite‐length dielectric slab waveguide, sandwiched between the two straight sections. It is demonstrated that the analysis method is also applicable, though approximately, to the study of radiation and transmission of electromagnetic fields in a uniform bend of a three‐dimensional dielectric waveguide.

Surface plasmon polariton (SPP) Airy beams are very attractive in theory and applications. We propose a new method to design dielectric relief holographical structures for controlling the SPP wave propagations and generating SPP Airy beams on metal surface. In the dielectric relief holography (DRH) method, both phase and amplitude are considered. The thickness of designed dielelctric films is proportional to the intensity of SPPs interference in holography. The films designed by the DRH method are coated on metal surface and can control SPPs propagation effectively. The complicated Airy beams can also be generated through the films designed by the method. The finite difference time domain (FDTD) method is used to test the functionalities of the designed structures and find the optimal parameters. The structures designed by the DRH method can be manufactured with traditional die or corroding methods. The investigation on the method extends SPP device manufacture methods, and therefore may open up the possibility for mass industrial manufacture of plasmonic devices.

Surface plasmon polaritons (SPPs) can be launched with an organic nanowire that serves as both light source and dielectric waveguide in a single organic/metal nanowire heterojunction. Efficient modulation of the output signals from the silver tip can be achieved via the alternation of incident polarizations, which is further used to design and realize prototypical photonic-plasmonic logic devices. These findings are essential for incorporating plasmonic waveguides as practical components into hybrid high-capacity photonic circuits.

Efficient excitation of surface plasmon polaritons (SPPs) remains one of the most challenging issues in areas of plasmonics related to information communication technologies. In particular, combining high SPP excitation efficiency and acceptance of any polarization of incident light appeared to be impossible to attain due to the polarized nature of SPPs. Here we demonstrate plasmonic couplers that represent arrays of gap SPP resonators producing upon reflection two orthogonal phase gradients in respective linear polarizations of incident radiation. These couplers are thereby capable of efficiently converting incident radiation with arbitrary polarization into SPPs that propagate in orthogonal directions dictated by the phase gradients. Fabricated couplers operate at telecom wavelengths and feature the coupling efficiency of ∼25% for either of two linear polarizations of incident radiation and directivity of SPP excitation exceeding 100. We further demonstrate that an individual wavelength-sized unit cell, representing a meta-scatterer, can also be used for efficient and polarization sensitive SPP excitation in compact plasmonics circuits. Devices capable of converting light of any polarization into surface plasmon polaritons could aid the development of on-chip optical circuits. Anders Pors and co-workers from the University of Southern Denmark and the University of Burgundy in France have now fabricated such a ‘coupler’ from gradient metasurfaces — arrays of miniature gold patches on a thin dielectric layer covering a metal-coated glass substrate. Their device is compatible with wavelengths of light in the telecommunications window (around 1,500 nm) and does not require a specific polarization in order to excite plasmons. Furthermore, it allows the direction of plasmon propagation to be controlled by changing the polarization of the incident beam. Calculations indicate that the coupling efficiency could be as high as 40%, with a directivity of around 50:1.