Breaking the loss-confinement barrier in plasmonic devices (Conference Presentation)

Slot-based plasmonic waveguides have attracted significant attention owing to their unique ability to confine light within nanometer-scale. In this context, enhanced localized light-matter interaction and control have been exploited to demonstrate novel concepts in data communication and sensing applications revealing the immense potential of plasmonic slot waveguides. However, inherent light absorption in the metallic parts included is such structures hampers the scaling of plasmonic devices and limits their application diversity. A promising solution of such issues is the use of hybrid plasmo-photonic configurations. Hybrid slot waveguides have been introduced as the means to reduce such propagation losses while maintaining their functional advantages. In addition, their co-integration with low-loss photonic waveguides can enable the development of more complex structures with acceptable overall losses. In such scenario, light needs to be efficiently transferred from the photonic to the plasmonic components and/or backwards. Based on this rationale, in this work a hybrid slot-based structure is adopted to achieve highly efficient light transfer between photonic and plasmonic slot waveguides in the near-infrared spectrum region (λ=1550 nm). This transition is realized with the aid of a directional coupling scheme. For this purpose, a Si3N4 bus waveguide (photonic branch) is located below an Aubased metallic slot (plasmonic branch) forming a hybrid waveguide element. The combined configuration, as it is shown with the aid of numerical simulations , is capable of supporting two hybrid guided modes with quasi-even and odd symmetry allowing the development of a power exchange mechanism between the two branches. In this context, a new directional coupling structure has been designed which can achieve power transmission per transition over 68% within a coupling length of the order of just several microns.

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.

Total internal reflection (TIR) is a ubiquitous phenomenon used in photonic devices ranging from waveguides and resonators to lasers and optical sensors. Controlling this phenomenon and light confinement are keys to the future integration of nanoelectronics and nanophotonics on the same silicon platform. We introduced the concept of relaxed total internal reflection in 2014 to control evanescent waves generated during TIR. These unchecked evanescent waves are the fundamental reason photonic devices are inevitably diffraction-limited and cannot be miniaturized. Our key design concept is the engineered anisotropy of the medium into which the evanescent wave extends thus allowing for skin depth engineering without any metallic components. In this article, we give an overview of our approach and compare it to key classes of photonic devices such as plasmonic waveguides, photonic crystal waveguides and slot waveguides. We show how our work can overcome a long standing issue in photonics nanoscale light confinement with fully transparent dielectric media.

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.

Plasmonic waveguides are promising in nonlinear optical applications because of their subwavelength field confinement, which can strongly enhance light-matter interactions and greatly reduce the device footprint. We investigate the Kerr nonlinear characteristics of two typical plasmonic waveguides and compare their relative merits to a conventional all-dielectric silicon slot waveguide. This work provides useful guidance on designing ultra-compact nonlinear optical devices.

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.

In this paper, two novel waveguides with ELC like-shaped and Z-shaped grooves have been proposed to achieve highly efficient and strongly confined spoof surface plasmon polaritons (SSPPs) propagation. Low-dispersion bands can be realized by such structures with tight field confinement of SSPPs, resulting in size miniaturization of the proposed waveguides. Specifically, our method yields deep physical insight into the effect that the geometrically induced modifications of the supporting structure has on the dispersion properties and field confinement capabilities of SSPPs. In comparison, SSPP waveguide results presented in Aziz (2021 Results Opt. 5 100116) are given which is regarded to have stronger SSPPs field confinement as compared to previously reported different grooves shapes based SSPP waveguides. It is found that the dispersion and waveguide propagation characteristics can be directly manipulated by varying the geometrical parameters of the horizontal and slanted slits of Z-shaped and ELC-like shaped plasmonic waveguides without increasing the lateral dimension of the waveguides. Based on this waveguiding scheme, the proposed waveguides exhibit much lower asymptotic frequency of the dispersion relation and even tighter SSPPs field confinement than I-shaped plasmonic waveguide. Then, broadband transitions with a tapered metallic strip and an array of graded height ELC-like and Z-shaped units with good impedance matching and high mode conversion efficiency are designed. Fully controlled field enhancement functionality has been performed, by using such metamaterial particles in the form of grooves, decorated in SSPP waveguide. Simulated results have demonstrated that the proposed SSPP waveguides have much stronger field confinement than the highly efficient I-shaped grooves based SSPP waveguide. The proposed waveguides can be a significant contribution towards the advancement of plasmonic functional devices and integrated circuits in microwave frequencies.

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

Plasmonic hybrids are introduced to replace photonic optical hybrids for coherent reception. These plasmonic circuits offer a compact footprint and a fabrication compatible with many electronic platforms as one only relies on metallic layer stacks. To mitigate the plasmonic losses, we propose new designs with shortest propagation paths. Simulations predict plasmonic hybrids with compact footprints of less than 1 µm2 and excess losses below 1 dB. In the experiment we fabricated plasmonic hybrids with 1 µm2, excess losses of 3.3 dB, and excellent broadband performance for signals over more than 100 nm. These results need to be compared against reference optical hybrids with typical excess losses of 0.5 dB in the C-band. Our demonstrations show a path towards a fabrication platform with increasingly complex and compact plasmonic devices that can be cointegrated with electronics.

A silicon-based plasmonic waveguide was designed and fabricated for use at telecommunications wavelengths. This waveguide is interfaced to the silicon photonics platform by use of a tapered silicon-on-insulator waveguide. Simulations indicate that this scheme excites the transverse magnetic plasmonic mode and that the electric fields are confined to the silicon-gold interface. Transmitted power is measured for several device lengths and the propagation distance and coupling efficiency are found to be 2.00 μm and 38.0%, respectively. These results demonstrate the potential for integration between silicon photonics and silicon plasmonic devices and demonstrate the ability to incorporate silicon-based plasmonic devices into complimentary metal-oxide-semiconductor electronic and photonic circuitry.

To achieve subwavelength mode confinement, we propose a hybrid plasmonic waveguide consisting of a metal nanoridge embedded in a silica-covered hollow silicon ridge waveguide on a metal substrate. The mode confinement, propagation length, and figure of merit are optimized by controlling the geometry of the waveguide. At the optimal figure of merit, the normalized mode area of 3.0 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> (λ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /4) is achieved while retaining a propagation length of 163.0 μm at the telecom wavelength of 1550 nm. The coupling strength of a directional coupler composed of two such waveguides is analyzed to examine the degree of integration of photonic integrated circuits. A strong dependence on the coupling length makes the proposed structure very favorable for use in various plasmonic devices. Furthermore, transmission through a 90° bend (studied to examine the compactness of the proposed waveguide), is observed to be 80% at the small radius of curvature of 0.7 μm.

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.

We report a ternary-coupled plasmonic system consisting of excitons of J-aggregated dye, localized surface plasmon polaritons of Ag nanoparticles, and propagating surface plasmon polaritons of continuous Ag film. J-aggregate dyes are uniformly self-assembled on colloidally synthesized Ag nanoprisms forming plexcitonic nanoparticles, which are placed at a distance nanometers away from the Ag thin film. The reflection measurements, corroborated by theoretical predictions, reveal that the strong coupling of plasmon polaritons and plexcitons results in a newly formed plasmon-exciton-plasmon hybridized state that we call here, reportedly for the first time, a plexcimon state. The hybrid plasmonic system shows dispersion characteristics similar to a coupled resonator optical waveguide. The group velocity of the plexcimon state approaches zero at the band edges. The ultrahybrid plasmonic system presented here is promising for a variety of light-matter interaction studies, including polariton lasers, plasmonic devices, plasmonic waveguiding, and spectroscopy.

Using plasmonic waveguide for interconnects application is very promising direction to achieve high density integration a good size compatibility with electronic devices. Thus, proposing compact and efficient functional plasmonic devices is or prime essential to achieve the required system functionalities. Power splitters are widely used as one of the important component of the optical interconnects and integrated photonic and plasmonics devices. We propose a simple, ultra compact and wideband balanced power divider. The advantage of this device is compactness and ability to split the power over wideband with negligible imbalance. The device is based on plasmonic slot waveguide configuration and has submicron total foot print. To achieve the proposed optimized design, a simple and novel analytical model is utilized for modelling the behavior or any plasmonic structure using circuit model.