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  • Plasmonic nanoantennas: enhancing light-matter interactionsat the nanoscale

    Shobhit K. Patel and Christos Argyropoulos*

    Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, 68588 Lincoln, NE, USA

    Received 3 September 2015 / Accepted 5 November 2015

    Abstract The research area of plasmonics promises devices with ultrasmall footprint operating at ultrafast speedsand with lower energy consumption compared to conventional electronics. These devices will operate with light andbridge the gap between microscale dielectric photonic systems and nanoscale electronics. Recent research advance-ments in nanotechnology and optics have led to the creation of a plethora of new plasmonic designs. Among the mostpromising are nanoscale antennas operating at optical frequencies, called nanoantennas. Plasmonic nanoantennas canprovide enhanced and controllable light-matter interactions and strong coupling between far-field radiation and local-ized sources at the nanoscale. After a brief introduction of several plasmonic nanoantenna designs and their well-established radio-frequency antenna counterparts, we review several linear and nonlinear applications of differentnanoantenna configurations. In particular, the possibility to tune the scattering response of linear nanoantennas andcreate robust optical wireless links is presented. In addition, the nonlinear and photodynamic responses of differentlinear and nonlinear nanoantenna systems are reported. Several future optical devices are envisioned based on theseplasmonic nanoantenna configurations, such as low-power nanoswitches, compact ultrafast light sources, nanosensorsand efficient energy harvesting systems.

    Key words: Plasmonics, Nanoantennas, Metamaterials, Nonlinear optics.

    1 Introduction

    Antennas are usually referred to transducers which canconvert electrical power into propagating electromagneticwaves and vice versa [1, 2]. Owing to the fundamental impor-tance of this functionality, their use at microwave and radio fre-quencies (RF) is applied in several devices around us. Theirdesign has seen major developments starting from the inven-tion of wireless telegraphy by Marconi around the turn of19th century. During the past decades, the rapid developmentof electronics and wireless communications led to greaterdemand for wireless devices that can operate at different fre-quency bands, such as mobile telecommunications systems,Bluetooth devices, wireless local area networks, and satellitecommunications. Additionally, several communication devices,e.g. cell phones, demand the antenna apparatus to be very com-pact and confined in an ultrasmall dimensional footprint[311]. These two requirements have triggered extensiveresearch on the design of compact and complex antennadesigns for RF and microwave frequency applications.

    On a different context, recent developments in nanoscaleoptical systems have generated a major interest in the opticalcounterpart of the well-established RF/microwave antennas,the so-called plasmonic nanoantennas. The design of thesemetallic nanoantennas is different from the well-establishedRF/microwave antennas in two important aspects. First, theassumption of metals behaving as perfect electrical conductors(PEC), which is usually followed in microwave frequencies, isno longer valid at optical frequencies. The electric fields canpenetrate inside metals at this high frequency regime and theOhmic losses are substantially higher compared to microwavefrequencies. Second, metallic and dielectric interfaces can sus-tain surface plasmon polaritons (SPPs) waves in the visibleconfined at nanoscale regions [12, 13]. Therefore, the responseof the aforementioned optical plasmonic nanoantenna struc-tures is drastically different compared to their microwave coun-terparts. The most notable example among these differences isthe strong field confinement obtained at the subwavelengthnanogap region of metallic nanoantennas. This nanogap regioncan be used as feeding or receiving point of the nanoantennasystem [14]. Additionally, new physical phenomena arise atnanoscale light-matter interaction regions. Their theoretical*e-mail: [email protected]

    EPJ Appl. Metamat. 2015, 2, 4 S.K. Patel & C. Argyropoulos, Published by EDP Sciences, 2015DOI: 10.1051/epjam/2015006

    Available online at:http://epjam.edp-open.org

    This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0),which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

    OPEN ACCESSREVIEW

    http://www.edpsciences.org/http://dx.doi.org/10.1051/epjam/2015006http://epjam.edp-open.orghttp://epjam.edp-open.orghttp://creativecommons.org/licenses/by/4.0/

  • study requires the development of new analytical and modelingtools to account for deviations from the classical RF antennatheory to the optical and quantum response of plasmonic nano-antennas. As a result, serious efforts have been recentlydevoted to extend the well-established and thoroughly studiedconcepts of radio frequency antennas to their optical counter-parts [1522].

    2 Comparison between RF and opticalantennas

    Figures 1a, 1b, 2a, 2b, and 3a, 3b demonstrate a schematiccomparison between radio frequency antennas [7, 23] and theiroptical nanoscale counterparts [24, 25]. In general, RF anten-nas have dimensions on the order of several centimeters. Theyare extremely important devices in modern wireless communi-cations. On the other hand, optical nanoantennas, with typicaldimensions on the order of a few hundred nanometers, have

    several important optical applications due to their ability toachieve extremely high field values localized in subwavelengthhotspots.

    An example of a broadband planar RF bowtie antenna isshown in Figure 1a [23]. Drawing inspiration from its radio-frequency counterpart, the optical bowtie nanoantenna ispresented in Figure 1b. It is made of two gold nanotrianglesseparated by an ultrasmall nanogap (30 nm) [26]. Figure 1cdemonstrates the field intensity enhancement localized in thegap region of this bowtie nanoantenna. The field enhancementwas calculated with full-wave electromagnetic simulations[24]. Very strong field intensity is obtained which is localizedin the nanogap region of the nanoantenna. Metallic bowtienanoantennas can be used to significantly improve the couplingmismatch between external light radiation and nanoscale emit-ters or receivers placed at the nanogap [26]. However, the fab-rication and characterization of these optical nanoantennas ischallenging because of their ultrasmall nanoscale dimensions.Furthermore, the high Ohmic losses of metals at optical fre-quencies pose another severe limitation in their realistic appli-cations. It is interesting that RF bowtie planar antennas havethe advantage of broad bandwidth compared to other well-established RF antennas, such as dipoles [2]. On the otherhand, bowtie nanoantennas are more narrowband comparedto nanodipoles at optical frequencies [24, 26].

    Figure 1. Examples of bowtie (a) RF planar antenna [23] and(b) optical nanoantenna [24], (c) the intensity enhancement of thebowtie nanoantenna shown in (b) [25]. The scale bar in this captionis 100 nm.

    Figure 2. Example of Yagi Uda (a) RF antenna [27] and (b) opticalnanoantenna [25], (c) angular radiation pattern measured (black)and theoretical (red) of the Yagi Uda nanoantenna shown in (b) [25].

    2 S.K. Patel and C. Argyropoulos: EPJ Appl. Metamat. 2015, 2, 4

  • Yagi Uda RF and optical antenna designs are presented inFigure 2 [25, 27]. In general, Yagi Uda RF antennas are builtby multiple antenna elements out of which one feed elementis connected to the transmitter source or receiver circuit [2].They can achieve strong directivity and are ideal candidatesfor long-range communication systems. The phase shift oftheir antenna elements is adjusted so that constructive interfer-ence exists only in the forward direction and not in any otherdirection. They are usually terminated by one reflector ele-ment, which is longer than the emitted radiations wavelengthk, in order to achieve inductive operation at the back side oftheir geometry. The phase of the current traveling along thereflector lags the voltage induced in it and, as a result, itsimpedance is inductive. The reflector is always placed at theback side to reduce the backward and enhance the forwarddirection radiation. The other multiple antenna elements actas radiation directors. They have shorter dimensions than theemitted wavelength k. These directors operate as capacitorsin order to improve the radiation focus in the forward direction.The phase of the current along each director leads the phase ofthe voltage induced in it. This type of phase distribution acrossthe array of directors leads to constructive interference and ulti-mately the phase progression of the radiation. The distancebetween each director element is 0.3 k. In addition, the dis-tance between the feed and reflector is 0.25 k to improve thedirectivity [2, 27]. This complex antenna configuration canprovide substantial increase in directivity compared to a simpledipole.

    To realize a Yagi Uda nanoantenna operating at optical fre-quencies, five gold nanorods are employed as its elements andare placed over a glass substrate (Figure 2b) [25]. Three ofthem are used as directors, one is the feed element and oneis the reflector. The feed element is driven by a quantum dotemitter [25], which ordinary has a pure dipolar radiation

    pattern. The plasmonic reflector is inductively detuned toreduce the backward optical radiation. The directors are capac-itively detuned to direct the optical wave in the forward direc-tion [25]. The positions of these elements are kept in such away that the traveli