Plasmonic Directional Coupler Research Articles

Three-Dimensional Analysis of an Ultrashort Optical Cross-Bar Switch Based on a Graphene Plasmonic Coupler

A high-performance electro-optical cross-bar switch based on a vertical graphene plasmonic directional coupler is proposed. The structure consists of two pattern-free graphene plasmonic waveguides which have major advantages over their previous counterparts. An ultrashort switching length of just 223 nm is obtained at the frequency of 37.5 THz. To the best of our knowledge, this switching length is remarkably shorter than those in the recently reported structures. Our designed structure is simulated using a three-dimensional finite-difference time-domain method. According to the illustrated results, extinction ratios of 10.5 and 9.52 dB are obtained at the cross and bar ports, respectively, and a wide spectral width of more than 1 μ m is presented. The required switching voltage is also calculated as 6 V and a very high 3 dB bandwidth of more than 60 GHz is obtained. An extremely low switching energy of 3.8 fJ/bit is also estimated.

Optimization of Biased PT-Symmetric Plasmonic Directional Couplers

We study a plasmonic directional coupler with a metallic cladding and dielectric core in the presence of balanced gain and loss, and demonstrate that such a photonic structure can possess the parity-time (PT) symmetry even when optical losses in metal are taken into account. We analyze the modal dispersion of the coupler when losses in metal are negligible and then study how substantially increased losses modify the modal dispersion and coupler's properties. For realistic parameters, we demonstrate a novel approach for recovering the property of the PT-symmetry by introducing unbalanced loss and gain in both cores of the coupler.

Plasmonic Directional Couplers Based on Multi-Slit Waveguides

We propose and investigate the performance of the plasmonic directional couplers based on two dimensional multi-slit plasmonic waveguides, employing the finite difference time domain simulation method. The idea behind the directional properties of the directional couplers is the interference of two wave components, in and out of phase, at the coupled and isolated ports, respectively. The coupler is also analyzed by an analytic method. The simulation results comply well with those of the analytic considerations. The effects of variations of the coupler structural parameters, crosstalk between the input and output ports, and the overall structure loss are also investigated.

Broadband efficient directional coupling to short-range plasmons: towards hybrid fiber nanotips.

We present a broadband and efficient short-range plasmonic directional coupler design, for the delivery and collection of deeply sub-wavelength radiation to tapered plasmonic nanowires. We show a proof-of-concept design using a planar geometry operating at wavelengths between 1.2 -2.4 μm, showing that the propagation characteristics predicted by an Eigenmode analysis are in excellent agreement with finite element simulations. This analytical formulation is straightforward to implement and immediately provides the power-exchange properties of hybrid plasmonic waveguides. An investigation of both waveguide delivery and collection performance to and from a plasmonic nano-tip is performed. We show that this design strategy can be straightforwardly adapted to a realistic hybrid fiber geometry, containing wire diameters more than one order of magnitude larger than the planar geometries, with important applications in all-fiber plasmonic superfocussing, and nonlinear plasmonics.

2D 주기적 패턴으로 구성된 격자 구조형 방향성 결합기의 모드 특성

Longitudinal transmission-line modal theory is applied to analyze the guiding mode characteristics along 1D & 2D grating patterns of plasmonic grating-assisted directional couplers (P-GADC) based on silicon waveguide. By defining supermodes amenable to rigorous analytical solutions and interference between even and odd modes, the field distributions of TE modes for each grating patterns are evaluated. The numerical result reveals that the field distribution with maximum coupling efficiency occurs at P-GADC composed by square grating pattern. That is, it reveals at a minium gap condition of grating period $d_{min}

Silicon Optical Modulators Based on Tunable Plasmonic Directional Couplers

For the development of Si-based next generation electronic-photonic integrated circuits, a silicon optical modulator is designed based on tunable plasmonic directional couplers. The characteristics are investigated using numerical simulations with a full-vectorial beam propagation method. The center waveguide in the three-core coupler is a MOS-type hybrid plasmonic waveguide that consists of a Si-ITO-SiO2-Si structure. By electrically tuning the ITO's refractive index, the coupling efficiency of the directional coupler is switchable and, hence, we can obtain modulated optical signals at the outer waveguide, which is configured with a vertical offset. The extinction ratios are 6.7 and 2.3 dB at 1.31- and 1.55-μm wavelength, respectively. The optical modulator can be transformed into a plasmonic absorption modulator based on a two-core directional coupler at the cost of a long coupling length. The proposed silicon optical modulator has the potential to play a key role in complementary metal-oxide-semiconductor-compatible 3-D silicon photonic integrated circuits.

Toward integrated electrically controllable directional coupling based on dielectric loaded graphene plasmonic waveguide.

We propose and numerically analyze a mid-infrared electrically controllable plasmonic waveguide directional coupler that is composed of two parallel identical straight dielectric loaded graphene plasmonic waveguide and S-shaped waveguide bends. By varying the Fermi energy level of the graphene sheet, the maximum power coupled from the input waveguide to the cross-waveguide and the corresponding coupling length could be effectively tuned. Under different Fermi energy level, this directional coupler could serve as an electrically controlled optical switch or a 3-dB splitter around the wavelength of 10.5μm. Moreover, the size of the entire device is really in sub-wavelength scale making it very facilitative for high density integration.

Compact and Wide-band Hybrid Metal-Insulator Plasmonic Directional Coupler

A compact and wideband plasmonic directional coupler is proposed based on the hybrid metal-insulator slab waveguide (HMISW). Transfer matrix method (TMM) is adopted to analyze the multilayer structure. The hybrid waveguide is composed of a metallic layer of silver and two dielectric layers of silica (SiO2) and silicon (Si) with high index contrast. Coupling length and maximum transferred power are optimized according to dielectric layer thicknesses. It is shown that sub-micron coupling length is achieved with wide bandwidth and high efficiency, being advantageous to conventional directional couplers Full Text: PDF References P. Berini, "Plasmon-polariton waves guided by thin lossy metal films of finite width: Bound modes of symmetric structures", Phys. Rev. B, 61, 10484-10503 (2000). CrossRef R. Zia, M.D. Selker, and M.L. Brongersma, "Leaky and bound modes of surface plasmon waveguides", Phys. Rev. B, 71, 165431 (2005). CrossRef R. Zia, M.D. Selker, P.B. Catrysse, and M.I. Brongersma, "Geometries and materials for subwavelength surface plasmon modes", J. Opt. Soc. Am. A, 21, 2442-2446 (2004). CrossRef G. Veronis, and S. Fan, "Guided subwavelength plasmonic mode supported by a slot in a thin metal film", Opt. Lett., 30, 3359?3361 (2005). CrossRef A. V. Krasavin and A. V. Zayats, "Passive photonic elements based on dielectric-loaded surface plasmon polariton waveguides", Appl. Phys. Lett., 90, 211101 (2007). CrossRef R. F. Oulton, V. J. Sorger, D. A. Genov, D. F. P. Pile, and X. Zhang, "A hybrid plasmonic waveguide for subwavelength confinement and long-range propagation", Nature Photonics, 2, 496-500 (2008). CrossRef M. Z. F. Alam, J. S. Aitchison and M. Mojahedi, "Theoretical Analysis of Hybrid Plasmonic Waveguide", IEEE J. of Selec. Topics in Quant. Elec., 19, 4602008 (2013). CrossRef M. T. Noghani and M. H. V. Samiei, "Analysis and Optimum Design of Hybrid Plasmonic Slab Waveguides", Springer Plasmonics, 8, 1155-1168 (2013). CrossRef A. Boltasseva, T. Nikolajsen, K. Leosson, K. Kjaer, M. S. Larsen, and S. I. Bozhevolnyi, "Integrated optical components utilizing long-range surface plasmon polaritons", IEEE J. of Lightwave Tech., 23, 413?422 (2005). CrossRef Y. Wang, R. Islam, and G. V. Eleftheriades, "An ultra-short contra-directional coupler utilizing surface plasmon-polaritons at optical frequencies", Opt. Exp., 14, 413?422 (2006). CrossRef H. Zhao, X. G. Guang, and J. Huang, "Novel optical directional coupler based on surface plasmon polaritons", Physica E: Low-dim. Systems and Nanostruct., 40, 3025?3029 (2008). CrossRef P. Dastmalchi, N. Granpayeh, and M. Rasouli Disfani, "Investigation of coupling length in a semi-cylindrical surface plasmonic coupler", Springer Appl. Phys. A, 103, 741?744 (2011). CrossRef M. Z. Alam, J. S. Aitchison, and M. Mojahedi, "Polarization-independent hybrid plasmonic coupler for a silicon on insulator platform", Opt. Lett., 37, 3417?3419 (2012). CrossRef P.B. Johnson and R.W. Christy, "Optical Constants of the Noble Metals", Physical Review B, 6, 4370-4379 (1972). CrossRef

Graphene-based terahertz tunable plasmonic directional coupler

We propose and numerically analyze a terahertz tunable plasmonic directional coupler which is composed of a thin metal film with a nanoscale slit, dielectric grating, a graphene sheet, and a dielectric substrate. The slit is employed to generate surface plasmon polaritons (SPPs), and the metal-dielectric grating-graphene-dielectric constructs a Bragg reflector, whose bandgap can be tuned over a wide frequency range by a small change in the Fermi energy level of graphene. As a graphene-based Bragg reflector is formed on one side of the slit, the structure enables SPP waves to be unidirectionally excited on the other side of the slit due to SPP interference, and the SPP waves in the Bragg reflector can be efficiently switched on and off by tuning the graphene's Fermi energy level. By introducing two optimized graphene-based Bragg reflectors into opposite sides of the slit, SPP waves can be guided to different Bragg reflectors at different Fermi energy levels, thus achieving a tunable bidirectional coupler.

Measurement of a compact colorless 3 dB hybrid plasmonic directional coupler.

We fabricated and measured a compact 3dB hybrid plasmonic directional coupler for silicon photonics integrated circuits with a length of 21.2μm. The coupler has a 50∶50 coupling ratio over a spectral bandwidth of more than 100nm around a wavelength of 1.55μm and has an insertion loss of less than 1dB.

SPM and XPM nonlinear effects in plasmonic directional couplers, considering the ponderomotive metal nonlinearity.

In this paper, a two-dimensional nonlinear plasmonic directional coupler (2D-NPDC), with 90° waveguide bends, has been numerically analyzed by the finite-difference time-domain (FDTD) method, considering the nonlinear response of metal due to the ponderomotive force. It has been shown that the required switching power of the 2D-NPDC is 0.05% of that when only the dielectric is nonlinear and the nonlinearity of metal is neglected. Also, the cross-phase modulation (XPM) nonlinear effect has been investigated, which the power for switching is decreased significantly compared to the one with the self-phase modulation (SPM) effect.

Low Loss, High Extinction Ration and Ultra-Compact Plasmonic Polarization Beam Splitter

In this letter, an ultra-compact plasmonic polarization beam splitter (PBS) is proposed and investigated by numerical simulations using the finite element method. The PBS is based on a three-core plasmonic directional coupler, which uses a long range surface plasmon polaritons waveguide as the middle waveguide to achieve polarization selective coupling. The calculations show that with proper structural parameters, the PBS with low insertion losses of 0.17 and 0.25 dB for TE and TM polarizations, respectively, and high extinction ratios of 20.17 and 19.83 dB for TE and TM polarizations, respectively, can be realized at a telecommunication wavelength of 1550 nm. Furthermore, an insertion loss and an extinction ratio > 14 dB can be realized across the entire C-band for both TE and TM polarizations.

Dispersion properties of coupled waveguides with loss and gain: a full-vectorial analysis

Waveguide structures with a balance of loss and gain have been frequently discussed in the photonic community not only as photonic analogues of quantum-mechanical systems with parity-time (PT) symmetry (breaking), but also as components with potentially interesting technical applications. In this contribution, properties of mutually coupled photonic waveguides with loss and gain are numerically analyzed using our proprietary full-vectorial Fourier modal method. Beside the classical case of a directional coupler with a balance of loss and gain, more general waveguide coupler structures exhibiting the behaviour analogous to PT-symmetry breaking, potentially applicable for photonic switching, are analyzed, too. For applications, simultaneous mutually balanced change of both loss and gain is evidently impractical. For a directional coupler structure with loss in one channel and gain in the other channel it is shown by rigorous numerical modelling that the behaviour analogous to PT-symmetry breaking can be realized by varying gain (or loss) only. Finally, it is confirmed by rigorous numerical modelling that the exceptional point does exist also in a very high-contrast asymmetric structure of a plasmonic directional coupler with strongly localized gain in one channel.

Nanofocusing of light using three-dimensional plasmonic mode conversion

Efficient nanofocusing of light into a gap plasmon waveguide using three-dimensional mode conversion in a strip plasmonic directional coupler is proposed. Unlike conventional nanofocusing using tapering structures, a plasmonic directional coupler converts E(z)-type odd mode energy into E(y)-type gap plasmon mode by controlling phase mismatch and gap spacing. The simulation result shows the maximum electric field intensity increases up to 58.1 times the input intensity, and 17.3% of the light is focused on the nano gap region.

Ultrashort hybrid metal–insulator plasmonic directional coupler

An ultrashort plasmonic directional coupler based on the hybrid metal-insulator slab waveguide is proposed and analyzed at the telecommunication wavelength of 1550 nm. It is first analyzed using the supermode theory based on mode analysis via the transfer matrix method in the interaction region. Then the 2D model of the coupler, including transition arms, is analyzed using a commercial finite-element method simulator. The hybrid slab waveguide is composed of a metallic layer of silver and two dielectric layers of silica (SiO2) and silicon (Si). The coupler is optimized to have a minimum coupling length and to transfer maximum power considering the layer thicknesses as optimization variables. The resulting coupling length in the submicrometer region along with a noticeable power transfer efficiency are advantages of the proposed coupler compared to previously reported plasmonic couplers.

Compact low loss and broadband hybrid plasmonic directional coupler

We propose a novel broadband coupler for silicon photonics using a hybrid plasmonic waveguide section. The hybrid plasmonic waveguide is used to create an asymmetric section in the middle of a silicon nanowire waveguide coupler to introduce a phase delay to allow for a 3-dB power coupling ratio over a 150 nm bandwidth around 1.55 µm. The device is very compact (<8.5 µm) and has a low insertion loss (<0.15 dB).

Nonlinear directional coupling between plasmonic slot waveguides

A numerical investigation of nonlinear switching in plasmonic directional couplers made of two dielectric slab waveguides with metallic claddings is presented. We assume Kerr-nonlinear dielectric and study the influence of geometrical parameters, metallic losses, and metal nonlinearities on coupler characteristics. We observe a general trade-off between losses and nonlinearity levels required for the switching operation. Underlying physical mechanisms that affect the coupler performance are discussed. The obtained results can be useful in design and optimization of the nonlinear plasmonic couplers.

Liquid-Crystal Tunable Long-Range Surface Plasmon Polariton Directional Coupler

A tunable plasmonic directional coupler is proposed based on the electro-optical control of a nematic liquid-crystal layer placed above two side-coupled gold stripe waveguides. It is shown that by proper adjustment of the structural parameters and the control voltage, the coupling length of the structure and the effective modal index of the individual waveguides can be tuned, allowing for the design of electro-optical liquid-crystal plasmonic directional coupler switches.

Long-range plasmonic directional coupler switches controlled by nematic liquid crystals

A liquid-crystal tunable plasmonic optical switch based on a long-range metal stripe directional coupler is proposed and theoretically investigated. Extensive electro-optic tuning of the coupler's characteristics is demonstrated by introducing a nematic liquid crystal layer above two coplanar plasmonic waveguides. The switching properties of the proposed plasmonic structure are investigated through rigorous liquid-crystal studies coupled with a finite-element based analysis of light propagation. A directional coupler optical switch is demonstrated, which combines very low power consumption, low operation voltages, adjustable crosstalk and coupling lengths, along with sufficiently reduced insertion losses.

실리콘 도파로에 기초한 플리즈마 격자 구조형 방향성 결합기의 설계 및 분석

Longitudinal transmission-line modal theory is applied to analyze maximum power transfer in plasmonic grating-assisted directional couplers (P-GADC) based on silicon waveguide. By defining a coupling efficiency amenable to rigorous analytical solutions and interference between even and odd modes, the power exchange of TE modes as a function of propagation distance is evaluated. The numerical result reveals that maximum power transfer occurs at a grating period ${\Lambda}_{eq}