446 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 16, NO. 2, MARCH/APRIL 2010 Optical Nanoantennas Coupled to Photonic Crystal Cavities and Waveguides for Near-Field Sensing Francisco J. Gonz´ alez, Member, IEEE, and Javier Alda Abstract—The performance of dipole nanoantennas coupled to two different waveguides, a cylindrical waveguide and a photonic crystal waveguide, illuminated with a polarized electromagnetic wave incident from the air and from the substrate is analyzed by numerical simulations. When illuminated from the air, the pho- tonic crystal waveguide antenna showed two resonances spectrally far apart from each other: one of them corresponds to the main antenna resonance and the other one to the modes excited in the photonic crystal cavity. The cylindrical waveguide antenna shows only the main antenna resonance. No significant antenna response was observed at a polarization perpendicular to the main axis of the antenna. Illumination from the substrate did not increase the response of the waveguide-coupled antenna. These results show that antenna-coupled waveguides could be used as detectors for near-field applications where polarization sensitivity and dual band operation are desired. Index Terms—Near-field sensing, numerical simulations, optical nanoantennas, photonic crystal waveguides. I. INTRODUCTION N EAR-FIELD measurements can be a useful tool in the design and analysis of optical and near-infrared devices by providing complete field patterns and polarization character- istics [1]. Some techniques, such as scanning near-field optical microscopy, have been developed for the actual measurement of the electromagnetic field in the near-field regime [2], [3]. When combining optical antennas with atomic force microscopy (AFM), it is possible to expand the sensing capabilities to the near field where exciting new phenomena appear [4]. Most of the techniques used in near-field measurements require deli- cate equipment and complex postprocessing algorithms [5]. A useful approach for near-field measurements is to transform an isolated optical antenna element into a working near-field detector. This transformation requires the coupling of a trans- duction mechanism along with the optical antenna itself. This has been done successfully in the past giving rise to a new kind of optical detectors also known as antenna-coupled detectors. Manuscript received June 2, 2009; revised June 17, 2009. First published September 22, 2009; current version published April 7, 2010. This work was supported in part by the University Complutense de Madrid and in part by the Ministry of Science of Spain (TEC2006-1882). The work of F. J. Gonz´ alez was supported in part by the Consejo Nacional de Ciencia y Tecnolog´ ıa (CONACyT) under Grant CB-2006-60349 and in part by the Fondos Mixtos- San Luis Potosi (FOMIX-SLP) under Grant FMSLP-C01-87127. F. J. Gonz´ alez is with the Coordinaci´ on Para la Innovaci´ on y la Aplicaci´ on de la Ciencia y la Tecnolog´ ıa, Universidad Aut´ onoma de San Luis Potos´ ı, San Lu´ ıs Potos´ ı SLP 78210, M´ exico (e-mail: javier.gonzalez@uaslp.mx). J. Alda is with the Applied Optics Complutense Group, School of Op- tics, University Complutense of Madrid, Madrid 28037, Spain (e-mail: j.alda@opt.ucm.es). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSTQE.2009.2027444 So far, nanoantennas have been used as optical detectors in the visible [6] and the infrared [7] regions. They can be fabricated using electron-beam lithography and can be tuned to different wavelengths by changing their size and shape [8]. Lithographic nanoantennas can discriminate different polarizations [6], [7] and due to their small dimensions can easily be coupled to waveguides and optical fibers. Waveguides, specially optical fibers, can be used in sensing applications because of their ability to transmit light in a flexible and compact fashion and have potential applications in chem- ical, biological, and environmental detection [9]. On the other hand, photonic crystals, or photonic bandgap materials, are pe- riodically modulated dielectric or metallic structures that give rise to bands where the propagation is prohibited for a certain frequency range [10]. Photonic crystals have been used in an- tenna technology to suppress surface waves, create controllable beams, and design high-gain antennas with a single feed [11]. Photonic crystal waveguides are photonic bangap materials with a linear defect that supports a linearly localized mode without relying on total internal reflection like regular waveguides [12]; similar to these types of devices, photonic crystal fibers have been developed and used as an alternative to conventional opti- cal fibers. Diverse applications can arise when combining photonic crys- tal waveguides and optical antennas. In this paper, we will focus on the use of optical antennas as probes for monitoring the near-field electromagnetic response of photonic crystals and waveguides. This analysis also constitutes a useful example to understand how the optical antenna serves as a coupling element interacting with a complex nanophotonic structure. In order to do this, the electric current across the feed point of a dipole antenna will be evaluated; this approach has been used to pre- dict the experimental response of fabricated devices [13]. At the same time, we will analyze the effect of the antenna as an excitation element able to modify the resonant characteristics of the photonic crystal. The minimal spatial footprint of optical antennas, along with their spectral characteristics, and polariza- tion selectivity make their use possible as optical nanoprobes for near-field measurements. In Section II, we describe the photonic crystal structure an- alyzed. We use a nanocavity constructed with dielectric rods immersed in air. The nanocavity supports eigenmodes within a band in the infrared. This design has been previously ana- lyzed to take into account its tolerance against fabrication de- fects [14], [15]. The incidence has been made from the air and from the substrate to properly compare these two modes of oper- ation. The computational results are analyzed in Section III. Fi- nally, Section IV summarizes the main conclusions of this paper. 1077-260X/$26.00 © 2009 IEEE Authorized licensed use limited to: Univ Complutense de Madrid. Downloaded on April 21,2010 at 08:42:30 UTC from IEEE Xplore. Restrictions apply.