Hybrid waveguide-surface plasmon polariton modes in a guided-mode resonance grating Chunlei Tan n , Janne Simonen, Tapio Niemi Q1 Optoelectronics Research Q2 Centre, Tampere University of Technology P.O. Box 692, 33101 Tampere, Finland article info Article history: Received 22 February 2012 Received in revised form 3 July 2012 Accepted 4 July 2012 Keywords: Surface plasmon polariton Dielectric waveguide Coupling Hybrid mode Guided mode resonance abstract We analyze the coupling between surface plasmon polaritons in a metal grating and the guided modes of a dielectric waveguide. Our model structure is a gold wire grating on a slab waveguide made of silicon nitride on silica wafer. The excitation of guided-mode resonances, surface plasmon polariton modes and hybrid waveguide-plasmon modes are observed in numerical simulations. Our experiments verify the existence of the predicted modes. These hybrid modes add significant degrees of freedom in designing structures for plasmonic applications. & 2012 Published by Elsevier B.V. 1. Introduction Surface plasmon polaritons (SPPs) are collective oscillations of free electrons coupled with light on the interface of conductor and dielectric [1]. SPPs are characterized by their considerable enhancement and tight spatial confinement of the electromag- netic energy. Surface plasmons have been intensively investigated and they have found various applications in e.g., Raman spectro- scopy and biomolecular sensing, which are the most established application fields for plasmonics [2,3]. Recent research on SPPs is expanding our knowledge on both isolated and periodic metallic nanostructures which exhibit localized particle plasmons, diffrac- tive coupling between the metal nanostructures and local field enhancement in the small gaps between particles. These phenomena enable exciting applications in various fields, such as subwavelength light guiding, superlensing, transmission enhancement through opaque metal films and new nanoscopic light sources [4–7]. Guided-mode resonance (GMR) is also a well-known phenom- enon related to subwavelength diffraction gratings or photonic crystals coupled with waveguides [8–10]. The basic mechanism of GMR is diffraction which couples incident light to waveguide modes. Depending on the design of the grating, the angle of incidence and the parameters of the waveguide, the GMRs can be observed as narrow or broadband resonances in the reflection or transmission spectrum. Contrary to propagating SPP modes introduced above, the electromagnetic field of a localized plasmon can be quasistatically approximated for a metal particle whose dimensions are signifi- cantly smaller than the wavelength of light. The localized plas- mon resonances are much wider than SPP resonances and they typically take place at wavelengths shorter than 1 mm, depending on the shape of the particle, its dielectric environment and the plasma resonance frequency of the metal. Strong coupling between waveguide modes and localized particle plasmons has been previously observed [11] and found to produce ultranarrow absorption and transparency peaks [12,13]. The coupling between propagating SPPs and waveguide modes has also been recently studied [14] but not in the case of gratings. As is well known, introducing a periodic structure facilitates band-folding at the edge of the Brillouin zone, therefore enabling finer control over the hybridization process. The coupling of SPPs to the waveguide modes has been analyzed theoretically in a structure consisting of a corrugated metal film and a waveguide with a thick buffer layer in between [15]. In this case, the co- propagating SPPs and the waveguide modes were shown to couple due to phase matching by a long-period grating engraved in the metal film. In this paper we report on a numerical and experimental study of a metal–dielectric grating-waveguide structure. The structure is designed to work at telecommunication wavelengths around 1500 nm. Different from those in Ref. [14]and Ref. [15], the waveguide and SPP modes are excited by light perpendicular to 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99 Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/optcom Optics Communications 0030-4018/$ - see front matter & 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.optcom.2012.07.027 n Q3 Corresponding author. E-mail addresses: Chunlei.Tan@tut.fi, tclwhu@gmail.com (C. Tan). Please cite this article as: C. Tan, et al., Hybrid waveguide-surface plasmon polariton modes in a guided-mode resonance grating, Optics Communications (2012), http://dx.doi.org/10.1016/j.optcom.2012.07.027 Optics Communications ] (]]]]) ]]]–]]]