Kerr nonlinear characteristics of plasmonic waveguide devices

Subwavelength conductor-gap-silicon plasmonic waveguides along with compact S-bends and Y-splitters were theoretically investigated and experimentally demonstrated on a silicon-on-insulator platform. A thin SiO2 gap between the conductor layer and silicon core provides subwavelength confinement of light while a long propagation length of 40 microm was achieved. Coupling of light between the plasmonic and conventional silicon photonic waveguides was also demonstrated with a high efficiency of 80%. The compact sizes, low loss operation, efficient input/output coupling, combined with a CMOS-compatible fabrication process, make these conductor-gap-silicon plasmonic devices a promising platform for realizing densely-integrated plasmonic circuits.

The study of quantum cascade lasers (QCL) in the terahertz frequency region by the use of THz time-domain spectroscopy has gained a lot of interest within the last few years [1]. Good physical insight was obtained for THz QCLs employing a single plasmon waveguide (surface plasmon waveguide) [2,3]. In comparison to that the double plasmon or metal-metal waveguide QCL uses a metal/semiconductor/metal structure as a waveguide. THz QCLs with this type of waveguide are showing the highest operating temperatures so far [4]. The metal-metal THz QCLs confine the whole mode in the active region of subwavelength size. In order to investigate metal-metal THz QCLs by THz pulses an efficient method of THz pulse launching needs to be employed. Several methods for coupling of free space THz pulses into the subwavelength waveguide, such as horn antenna, gratings or lenses can be used. A further method is a waveguide emitter [5,6] which benefits from the near infrared excitation beam focus which is much smaller than the corresponding THz focus.

The unique optical and electronic properties of graphene make possible the fabrication of novel optoelectronic devices. One of the most exciting graphene characteristics is the tunability by gating which allows one to realize active optical devices. While several types of graphene-based photonic modulators have already been demonstrated, the potential of combining the versatility of graphene with subwavelength field confinement of plasmonic waveguides remains largely unexplored. Here we report fabrication and study of hybrid graphene–plasmonic waveguide modulators. We consider several types of modulators and identify the most promising one for telecom applications. The modulator working at the telecom range is demonstrated, showing a modulation depth of >0.03 dB μm−1 at low gating voltages for an active device area of just 10 μm2, characteristics which are already comparable to those of silicon-based waveguide modulators while retaining the benefit of further device miniaturization. Our proof-of-concept results pave the way towards on-chip realization of efficient graphene-based active plasmonic waveguide devices for optical communications.

Multimode interference effect in plasmonic subwavelength waveguides and an ultra-compact power splitter

The characteristics of plasmonic nano-slot waveguides, in terms of different metallic materials and geometries, are presented in this paper. We numerically investigage how efficiently light can be guided in straight and bend types of metallic waveguide at nanoscle. For numerical simulation the Drude model incorporated finite difference time domain (FDTD) method is used to handle the dispersive nature of the metals such as gold, silver, copper, aluminum at visible and near infra-red (IR) wavelengths. It is demonstrated that the plasmonic waveguide with nano-slot is an efficient candidate for optical interconnects at the visible and near-IR range of electromagnetic spectrum. The performance of stragiht, 90° and Z-bend nano-slot waveguides is discussed by means of transmission and near-field characteristics.

Integrated photonics has been pursued on a large variety of materials and platforms over more than half a century. In recent decades, silicon has emerged as the preferred optical and substrate material. However, since many optical functionalities cannot be, inherently or conveniently, implemented on silicon-based devices, heterogeneous integration of other materials has been an inevitable constituent of silicon photonics from its inception. This paper reviews recent progress in the heterogeneous integration of dielectrics and compound semiconductors as the core optical material for ultracompact nonlinear integrated photonics, preferably on silicon substrates. Occasionally, important devices on native substrates of the core nonlinear material are also reviewed. The focus is on ultracompact photonic devices and circuits formed on submicron films of lithium niobate (LiNbO3), and the aluminium-gallium-arsenide (AlGaAs) and aluminium-gallium-nitride (AlGaN) families for second-order nonlinear processes, such as second-harmonic generation, and techniques to address phase-matching in them. Integration of second- and third-order nonlinear devices on the same chip appears to be a future trend, thus the related preliminary results are reviewed. A broader introduction to heterogeneously integrated photonics, shortcomings of conventional silicon photonics for application in nonlinear optics, as well as an overview of modeling second-order nonlinear integrated-waveguide devices are also provided for completeness.

Silicon-based integrated photonics became a very attractive technology for guiding and manipulating of light in highly integrated structures due to the large index contrast between silicon and cladding materials allowing for very high mode confinement. Moreover, these structures can be realized by conventional planar CMOS techniques. Different passive devices based on Si nanowire waveguides have been realized using SOI technology or amorphous silicon deposition with applications in highly integrated communication systems for optical networking, bio and sensing, as well as for computer interconnects in data centers. Nevertheless for future use, especially for inter-core and intra-core computer communication, structures allowing for subwavelength light confinement based on surface plasmon waveguiding are an attractive solution. Different methods to decrease or compensate the intrinsic high losses of these structures have been proposed. Here we report our work on design and experimental realization of hybrid plasmonic waveguides and devices that allow for considerable decrease of losses, still keeping sub-wavelength light confinement.

In this paper, we introduce slow-light enhanced nanoscale plasmonic waveguide devices for manipulating light at the nanoscale. In particular, we investigate nanoplasmonic metal-dielectric-metal (MDM) waveguide structures for highsensitivity sensors. Such plasmonic waveguide systems can be engineered to support slow-light modes. We find that, as the slowdown factor increases, the sensitivity of the effective index of the mode to variations of the refractive index of the material filling the structures increases. Such slow-light enhancements of the sensitivity to refractive index variations lead to enhanced performance of active plasmonic devices such as sensors. We consider Mach-Zehnder interferometer (MZI) sensors in which the sensing arm consists of a slow-light waveguide based on a plasmonic analogue of electromagnetically induced transparency (EIT). We show that a MZI sensor using such a waveguide leads to approximately an order of magnitude enhancement in the refractive index sensitivity, and therefore in the minimum detectable refractive index change, compared to a MZI sensor using a conventional MDM waveguide.

In this study, we numerically simulate novel planar plasmonic waveguides and notch filters with excellent guiding and rejection of terahertz (THz) waves with super subwavelength confinement. Our design is based on spoof surface plasmon polaritons-surface plasmon polaritons with a frequency that has been tuned using patterned conductive surfaces. We find that by using patterns of periodically arranged spiral-shaped units, the dispersion characteristics can be engineered at will by tuning the parameters of the spirals. We find that the resulting plasmonic waveguides have much lower asymptotic frequencies and much tighter terahertz field confinement when compared with conventional rectangular-grooved plasmonic waveguides. We show it is possible to design a structure with lateral dimensions that are only 25% the size of the conventional spoof surface plasmon polariton waveguides but with the same asymptotic frequency. Finally, we combined this architecture with broadband couplers to design an ultrawideband low-pass filter with sharp roll-off (cut-off frequency at 1.29 THz) and low insertion loss (<;3 dB). Furthermore, by introducing double ring resonators based on spiral-shaped units, a planar plasmonic notch filter with rejection of more than 17 dB between 0.97 and 0.99 THz is demonstrated. Finally, the proposed waveguides and notch filters may have great potential applications in the promising terahertz integrated plasmonic circuits and systems.

Graphene-based plasmonic waveguide devices for electronic-photonic integrated circuit

Waveguides made of hyperbolic metamaterials have unique properties such as extremely high field enhancement. Here we show that hyperbolic slot waveguides, i.e., nanoscale plasmonic slot waveguides cladded by hyperbolic metamaterials can have greatly enhanced Kerr nonlinear effects at even shorter device lengths and lower pump power compared with the conventional metallic slot waveguides. This result points to novel nanoscale waveguide designs for achieving low-driving-power and compact nonlinear nanophotonic devices that are used in diverse applications at both classical and quantum levels.

Electro-optical modulators which work at the near-infrared range are significant for a variety of applications such as communication and sensing. However, currently available approaches result in rather bulky devices which suffer from low integration and can hardly operate at low power consumption levels. Graphene, an emerging advanced material, has been widely utilized due to its tunability by gating which allows one to realize active optical devices. Plasmonic waveguides, one of the most promising candidates for subwavelength optical confinement, provide a way to manipulate light on scales much smaller than the wavelength. In this paper, we combine the advantages of graphene and plasmonic waveguides and propose a tunable graphene-based hybrid plasmonic modulator (GHPM). Considering several parameters of the GHPM, the modulation depth can reach approximately 0.3 dB·μm−1 at low gating voltages. Moreover, we combine GHPM with metal-insulator-metal (MIM) structure to propose another symmetrical GHPM with a modulation depth of 0.6 dB·μm−1. Our modulators which utilize the light-matter interaction tuned by electro-doped graphene are of great potential for many applications in nanophotonics.

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.

In this paper, we propose a general boundary integral equation (BIE) approach for solving both exterior (e.g., scattering) and interior (e.g., guided wave propagation) problems involving general plasmonic waveguiding structures with arbitrary cross-sectional geometries and with a continuous translational symmetry in the direction of wave propagation. Contrary to the field-based approach which deals with the electric and magnetic fields, we employ a potential-based formalism instead, involving vector and scalar potentials, and match them at the media interfaces. The proposed approach can handle not only conventional plasmonic waveguides on the order of a few hundred nanometers but also those whose critical sizes are a few nanometers and in which the nonlocal hydrodynamic effects need to be accounted for. The BIEs describing the interaction of light with the plasmonic waveguide are solved by the method of moments (MoM) algorithm. Two illustrative examples, the first one of which deals with the scattering problem, while the second computes the dispersion diagram of a plasmonic cylinder, are considered for both local and nonlocal cases. An excellent agreement is observed between the numerical and theoretical results, with the latter being derived from the generalized Mie theory.

The potential of graphene in plasmonic electro-optical waveguide modulators has been investigated in detail by finite-element method modelling of various widely used plasmonic waveguiding configurations. We estimated the maximum possible modulation depth values one can achieve with plasmonic devices operating at telecom wavelengths and exploiting the optical Pauli blocking effect in graphene. Conclusions and guidelines for optimization of modulation/intrinsic loss trade-off have been provided and generalized for any graphene-based plasmonic waveguide modulators, which should help in consideration and design of novel active-plasmonic devices.