Guided resonance in negative index photonic crystals: a new approach

Abstract

The behavior of a negative refraction photonic crystal slab irradiated with out-of-plane incident beam is an unexplored subject. In such an experimental configuration, guided mode resonance appears in the reflection spectrum. We show that, in this case, the light coupled inside the photonic crystal is backpropagating. A relationship with the negative index properties is established using a new approach in which the guided resonance is recovered by modeling the photonic crystal layer with a simple Lorentz resonator using the Fresnel reflection formula. Light: Science & Applications (2014) 3, e120; doi:10.1038/lsa.2014.1; published online 3 January 2014Keywords: diffraction gratings; guided mode resonance; negative index; photonic crystals INTRODUCTIONIn 1902, Wood observed the presence of narrow bright and dark bands in the reflectivity spectrum of an optical grating. These bands were dependent on the polarization of the incident light, and because they could not be explained by grating theory, they were classified as anomalies. 1 This effect was theoretically explained for the first time by Rayleigh 2 and then by Hessel and Oliner 3 in 1965, who demonstrated that these anomalies in the reflections from gratings were related to the excitation of surface waves on metallic grating structures. Similar resonant anomalies have been observed in various materials with a periodic patterning applied to a surface that can support excitations, such as plasmon polariton resonance or sharp spectral features in shallow grating waveguide structures. 4 In particular, the reflection and transmission of an incident wave on a photonic crystal (PhC) slab can produce sharp resonance in the spectrum when the radiation is coupled with the modes of the structure. [5][6][7] Guided mode resonance has been well studied in the photonic crystal literature. Due to the extremely narrow shape of the resonance when superposed on the background reflection, guided mode resonances can be used to design optical bandpass filters with elevated symmetrical responses and low side-bands 8,9 or distributed feedback lasers with a high Q factor. 10 Moreover, the confinement of the optical field within the slab can be used to trap 11 single particles or to enhance signals from fluorescent elements, thus enabling high-sensitivity sensors. 12 In this paper, we reveal novel insight into the reflectivity properties of a negative refraction photonic crystal slab. In particular, by studying a hexagonal airhole lattice in silicon, we highlight that the out-of-plane incoming radiation is negatively refracted in the structure. While the in-plane properties of negative refraction in a photonic crystal slab have been studied over the last years, [13][14][15][16][17][18][19][20] this is the first experimental demonstration of negative refraction detected out-of-plane. In particular, we provide imaging of the radiation coupled into a photonic crystal slab

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