Surface plasmon polaritons (SPPs) are electromagnetic (EM) modes propagating along metal‐dielectric interfaces, in which surface collective excitations of free electrons in the metal are coupled to evanescent EM fields in the dielectric. Various SPP modes can be supported by flat and curved, single and multiple surfaces, exhibiting remarkable properties, including the possibility of concentrating EM fields beyond the diffraction limit, i.e. on the nanoscale, while enhancing local field strengths by several orders of magnitude. This unique feature of SPP modes, along with the ever-increasing demands for miniaturization of photonic components and circuits, generates an exponentially growing interest in SPP-mediated radiation guiding and SPP-based waveguide components. Here we review the current status of this rapidly developing field, starting with a brief presentation of the main planar SPP modes along with the techniques employed for their excitation and manipulation by sets of nanoparticles. We then describe in detail various SPP-based waveguide configurations that ensure two-dimensional mode confinement in the plane perpendicular to the propagation direction and compare their characteristics. Excitation of SPP waveguide modes and recent progress in the development of SPP-based waveguide components are also discussed, concluding with our outlook on challenges and possible future developments in this field. (Some figures may appear in colour only in the online journal) This article was invited by Horst-Guenter Rubahn.

http://iopscience.iop.org/article/10.1088/0034-4885/76/1/016402/pdf

Radiation guiding with surface plasmon polaritons

In contrast to designing nanophotonic devices by tuning a handful of device parameters, we have developed a computational method which utilizes the full parameter space to design linear nanophotonic devices. We show that our method may indeed be capable of designing any linear nanophotonic device by demonstrating designed structures which are fully three-dimensional and multi-modal, exhibit novel functionality, have very compact footprints, exhibit high efficiency, and are manufacturable. In addition, we also demonstrate the ability to produce structures which are strongly robust to wavelength and temperature shift, as well as fabrication error. Critically, we show that our method does not require the user to be a nanophotonic expert or to perform any manual tuning. Instead, we are able to design devices solely based on the user's desired performance specification for the device.

Nanophotonic computational design

We present an improved analytical model describing transmittance of a metal-dielectric-metal (MDM) waveguide coupled to an arbitrary number of stubs. The model is built on the well-known analogy between MDM waveguides and microwave transmission lines. This analogy allows one to establish equivalent networks for different MDM-waveguide geometries and to calculate their optical transmission spectra using standard analytical tools of transmission-line theory. A substantial advantage of our model compared to earlier works is that it precisely incorporates the dissipation of surface plasmon polaritons resulting from ohmic losses inside any metal at optical frequencies. We derive analytical expressions for transmittance of MDM waveguides coupled to single and double stubs as well as to N identical stubs with a periodic arrangement. We show that certain phase-matching conditions must be satisfied to provide optimal filtering characteristics for such waveguides. To check the accuracy of our model, its results are compared with numerical data obtained from the full-blown finite-difference time-domain simulations. Close agreement between the two suggests that our analytical model is suitable for rapid design optimization of MDM-waveguide-based compact photonic devices.

Improved transmission model for metal-dielectric-metal plasmonic waveguides with stub structure

The realization of practical on-chip plasmonic devices will require efficient coupling of light into and out of surface plasmon waveguides over short length scales. In this letter, we report on low insertion loss for polymer-on-gold dielectric-loaded plasmonic waveguides end-coupled to silicon-on-insulator waveguides with a coupling efficiency of 79 ± 2% per transition at telecommunication wavelengths. Propagation loss is determined independently of insertion loss by measuring the transmission through plasmonic waveguides of varying length, and we find a characteristic surface-plasmon propagation length of 51 ± 4 μm at a free-space wavelength of λ = 1550 nm. We also demonstrate efficient coupling to whispering-gallery modes in plasmonic ring resonators with an average bending-loss-limited quality factor of 180 ± 8.

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Efficient Coupling between Dielectric-Loaded Plasmonic and Silicon Photonic Waveguides

We develop a highly efficient approach for the modulation of photonic signals at the nanoscale, combining an ultrasubwavelength plasmonic guiding scheme with a robust electroabsorption effect in degenerate semiconductors. We numerically demonstrate an active electro-optical field-effect nanoplasmonic modulator with a revolutionary size of just 25 × 30 × 100 nm(3), providing signal extinction ratios as high as 2 at switching voltages of only 1 V. The design is compatible with complementary metal-oxide-semiconductor (CMOS) technology and allows low-loss insertion in standard plasmonic and Si-photonic circuitry.

/pdf/photonic-signal-processing-on-electronic-scales-electro-52ox0w2y0d.pdf

Photonic signal processing on electronic scales: electro-optical field-effect nanoplasmonic modulator.

Efficient dielectric–plasmonic interconnect is significant in the design of future electronic–photonic integrated circuits that deal with large data transfer. We propose three designs and analysis of small-footprint couplers that are located at the interface between dielectric and plasmonic waveguides. To the best of our knowledge, our proposed couplers outperform all the works reported in the literature in several aspects including size and coupling efficiency (CE). Our results indicate that the optimum dimensions of the proposed couplers are determined based on whether the coupler is located in the metal side or dielectric side, or the coupler extends equally in both types of materials. The proposed couplers work over a broad frequency range achieving a CE above 88% at the optical communications wavelength 1550 nm. In addition, our results indicate that the CE can be further increased to above 93% by increasing the width of the dielectric waveguide before it is connected to the coupler. Moreover, our proposed designs provide a considerable alignment tolerance, which is needed when aligning the dielectric waveguide to the metal–dielectric–metal waveguide. Our proposed couplers have an impact on the design and miniaturization of nanoscale all-optical devices.

https://www.spiedigitallibrary.org/journals/optical-engineering/volume-59/issue-10/107101/Mode-coupling-enhancement-from-dielectric-to-plasmonic-waveguides/10.1117/1.OE.59.10.107101.pdf

Mode coupling enhancement from dielectric to plasmonic waveguides

We propose a novel method to enhance the coupling efficiency between a dielectric waveguide and a planar photonic crystal (PC). The proposed method is based on introducing structural imperfections to change the mode size and shape inside the taper to match that of the PC line-defect waveguide. This method involves changing both the size and position of the inner taper rods. We found that the transmission efficiency is above 88% for the size change method, while it is above 94% for the position change method. The order of which method to apply first is important. Our results show that applying the size change method, followed by the position change method increases the coupling to about 96% without affecting the transmission spectrum of the structure. We also found that the input and output taper geometries are different because light faces different scattering mechanisms when entering or exiting the line defect waveguide. (2009 Wiley Periodicals, Inc). © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1454–1459, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25222

Compact and ultra‐low‐loss planar photonic crystal taper

We present novel designs and fabrication of ultra-compact coupler and 1×2 splitter using plasmonic waveguides. The coupling efficiency is 88% for the former one and 45% for each branch for the latter one.

Ultra-compact nanoplasmonic splitter

Abstract. To achieve ultrahigh-density nanoplasmonic integrated circuits, low-loss conventional dielectric waveguides (CDWs) are used to transfer light into and out of the metal–dielectric–metal (MDM) plasmonic waveguides. We show that sandwiching the MDM plasmonic waveguide between two CDWs forms a Fabry–Perot cavity-like structure, which causes oscillations in the transmission coupling efficiency (TCE) into the output CDW as the length of the MDM waveguide is increased. Three types of compact air-gap couplers (AGCs) were used at the interface between those two types of waveguides to enhance the coupling efficiency between them. Our simulation results indicate that tapering CDW before it is connected to AGC not only enhances TCE into the output CDW by 9% over a wide spectrum range but also reduces the need for high-precision fabrication techniques to align AGC at the interface. Moreover, we achieved a TCE into the output CDW of 77% optimized for the optical communications wavelength of 1550 nm when the length of the MDM plasmonic waveguide is 600 nm. In addition, we showed that our proposed design has large fabrication tolerance by investigating the change in its spectrum response as various parameters of the design dimensions differ from that of the targeted optimum dimensions. We found that increasing the dimensions of AGC reduces the width of the spectrum, whereas increasing the width of CDW with its tapered part shifts the spectrum to the right by 100 nm per 40 nm increase in the width. We also found that increasing the refractive index of the MDM plasmonic waveguide with AGC controls the cutoff wavelength, in which TCE is almost zero at wavelengths shorter than the cutoff wavelength, and consequently provides a unique advantage in using our proposed design in sensing applications.

Evaluation of the coupling characteristics of a sandwiched plasmonic waveguide between two dielectric waveguides using air-gap couplers to achieve high-performance optical interconnects

Using a square lattice to reduce the cross talk between two photonic crystal structures composed of silicon pillars in air and air holes in silicon shows a reduction of -90.60 dB and -30 dB, respectively.

Rami A. Wahsheh

Papers

Mode coupling enhancement from dielectric to plasmonic waveguides

Compact and ultra‐low‐loss planar photonic crystal taper

Ultra-compact nanoplasmonic splitter

Evaluation of the coupling characteristics of a sandwiched plasmonic waveguide between two dielectric waveguides using air-gap couplers to achieve high-performance optical interconnects

Cross Talk Reduction by Photonic Crystal Cavities