Plasmonic nanoloop array antennaAkram Ahmadi and Hossein Mosallaei*

Applied EM and Optics Laboratory, ECE Department, Northeastern University,360 Huntington Avenue, Boston, Massachusetts 02115, USA

*Corresponding author: hosseinm@ece.neu.edu

Received July 8, 2010; revised September 15, 2010; accepted October 5, 2010;posted October 11, 2010 (Doc. ID 131159); published October 29, 2010

In this Letter, we create an optical nanoantenna array composed of parasitic plasmonic loops that can enhanceradiation characteristics and direct the optical energy successfully. Three metallic loops inspired by the conceptof the Yagi-Uda antenna are optimized around the region where they feature high scattering performance to controlthe radiation beam. The loop geometry, when compared to the dipole configuration, has the benefit of using theavailable aperture in an effective way to provide higher directivity. The angular emission of the nanoloop arrayantenna is highly directive, and a directivity of 8:2 dB for upward radiation is established. © 2010 Optical Societyof AmericaOCIS codes: 240.6680, 260.3910, 350.4600, 350.5610.

The antenna is a key element in the microwave spectrumfor enabling wireless data communication. The extensionof this concept into optics has many applications, andthere has been a growing body of research in recentyears [1–11]. Among the technological applications foroptical antennas, one can find high-resolution micro-scopy and spectroscopy, optical sensors, lasing, solarcells, and efficient solid-state light sources, and they havealso become important in biotechnology and medicine.Noble metals in optics offer a negative permittivity pa-

rameter where a high scattering performance in subwave-length sizes can be achieved. Arraying subwavelengthplasmonic elements in unique configurations can success-fully engineer the optical emission. Recently, opticalnanodipoles made of plasmonic materials and their ar-rangements in the Yagi-Uda definition have been theoreti-cally and experimentally characterized to modify opticalemission by various groups [1–4,6–8]. To achieve a higherdirectivity,wepropose in this Letter touse loop elements inthe Yagi-Uda array. Resonant loop antennas are attractivebecause of their symmetric radiation patterns and thepotential for offering higher directivity.A microwave metallic loop antenna goes to resonance

where βb ¼ 1; 2; 3;… (b is the mean radius of a circularloop) [12]. This means that the required minimum loopdiameter to achieve a high scattering is around 0:3λ. Adifferent scenario is expected in optics as the incidentwave penetrates into the metal and gives rise to oscilla-tion of the free-electron gas generating surface plasmon(SP) modes [3]. The optical antenna is scaled down bythe effective wavelength of the plasmonic material,which depends on the material property and the shapeof the structure [3,10]. For instance, for a rectangularloop antenna (constructed from four cubical arms),the higher the aspect ratio of the metallic arms, the high-er the operating wavelength and, thus, the smaller thesize of the resonator.Here, the physics of the plasmonic loop is integrated

into the design of the optical nanoloop array antenna.First we consider a single plasmonic loop antenna (rec-tangular) of dielectric material εðωÞ, side length l, andthickness t. Figure 1 shows the parameters used in ourmodel. The frequency-dependent dielectric function ofthe metal is described by the Drude model:

εðωÞ ¼ ε0�1 −

ω2p

ωðωþ iγpÞ�; ð1Þ

where ωp is the plasma frequency of the material and thedamping factor γp represents the losses present. Here,the loop antenna is made of silver (ωp ¼ 2π × 2175 THzand γp ¼ 2π × 4:35 THz [13]).

An incident plane wave with wavelength λ illuminatesthe structure and polarizes the material. The materialpolarization provides resonance radiation at a negativematerial constitutive parameter.

The finite-difference time-domain (FDTD) technique[14] is applied to simulate the structure and determinefield characteristics. In the FDTD model, the computa-tional domain is configured with cubical Yee cells withΔ ¼ 5 nm, and the structure is illuminated by a planewave having an x-polarized electric field and travelingin the −z direction. The loop is placed at the center ofthe computational domain (0,0,0), and the scattered fieldis stored at (0,0,30Δ) in front of the loop. The scatteredfield is plotted in Fig. 2. A high scattering performance atthe resonant wavelength of λ ¼ 1:34 μm is observed. Atresonance, the plasmonic loop has a subwavelength sizewith a total length of 4l ¼ λ=3:9. The x and y componentsof the polarized current distribution are plotted at the

Fig. 1. (Color online) Single plasmonic loop antenna illumi-nated by an x-polarized electric field plane wave: l ¼ 85 nm,t ¼ 15 nm.

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resonance frequency in Fig. 3. A successful currentcirculation is observed.To study the radiation performance of the resonant

loop, we excite the loop by an Ex point source placedat ð0; l=2; 0Þ. The 3D power pattern is presented in Fig. 4.The drawing is a series of patterns on planes of constantangles ϕ. Eϕ is almost zero in the vertical plane ϕ ¼ 0, π,while Eθ is small in the vertical plane ϕ ¼ π=2, 3π=2. Tostudy the angular dependence of the emission, the angu-lar directivity Dðθ;ϕÞ is studied. The DðθÞ in two majorplanes, ϕ ¼ 0 and π=2, are plotted in Fig. 5. The maximumdirectivity of D ¼ 2 dB is obtained.The next step will be to enhance the directivity of

the antenna. To achieve this, first the backradiation issuppressed by depositing the loop antenna on a silversubstrate. Then we use two more loop antennas abovethe structure to increase the gain. Figure 6 shows the na-noloop array antenna configuration, inspired by the Yagi-Uda concept. The antenna consists of one exciter andtwo directors, which are printed on the low dielectricsubstrate MgF2 with εd ¼ 1:5 [15]. The silver slab in theback can be envisioned as the reflector. The size andspacing of the directors are adjusted to successfullyengineer the phase of the scattered fields and enhancethe beam radiation in the upward direction. The designparameters are shown in Fig. 6.

Figure 7 shows DðθÞ of the array antenna in the planesϕ ¼ 0 and π=2. The 3D power pattern is also plotted inFig. 8. The maximum directivity is about 8:2 dB, whichis a 4.2 times improvement in the power radiation of asingle dipole performance. It is clearly observed thatwhen compared to the single-loop antenna (Fig. 5), thebeam is more collimated and the half-power beam widthsof 71° and 82° are obtained in the ϕ ¼ 0 and ϕ ¼ π=2planes, respectively. Increasing the number of array ele-ments can result in further enhancement in the radiationperformance. This can be of great advantage for enhanc-ing the optical emission with potential integration inmany emerging optical applications, such as wirelessoptical communications, sensing, and molecular andquantum-dot boosted emissions.

To provide a physical insight of the plasmonic loopsnanoantenna, the near-field performance in the antennaarray is depicted in Fig. 9. In this figure, the magnitude ofthe electric field in the x–y plane is plotted. The Exsource sets up a strong electric field in the exciter loop,which can induce the SP modes on the director loops,and as a result the whole system radiates efficientlyand directs the beam successfully.

In summary, we have investigated a highly directive na-noantenna array whose elements are plasmonic nano-loops. The beam is collimated, and high directivity of8:2 dB is achieved by arraying three loops over a silversubstrate where the total size in the z direction does notexceed 1:125 μm≃ 0:8λ. The size in the transverse direc-tion is 500 nm. The resonant size of the plasmonic loop isscaled down by a factor of about 3.9 compared to the size

Fig. 2. (Color online) Resonance performance of a single plas-monic loop. High scattering occurs at λ ¼ 1:34 μm.

Fig. 4. (Color online) Far-zone power pattern for single plas-monic loop at the operating wavelength.

Fig. 5. (Color online) Directivity (in decibels) for resonantplasmonic loop antenna in planes (a) ϕ ¼ 0 and (b) ϕ ¼ π=2.Maximum directivity is 2 dB.

Fig. 3. (Color online) Polarized current on plasmonic loop atresonant wavelength λ ¼ 1:34 μm: (a) normalized jJxj (dB) and(b) normalized jJyj (dB). The current distribution is similar towhat one observes in microwave for a rectangular loop antennawith 4l≃ λ (the size becomes subwavelength in optics).

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of a microwave loop antenna. The directivity can even befurther enhanced by increasing the number of directors.The loop nanoantenna has the advantage of offering high-er directivity as compared to the traditional dipole anten-na design. The nanoloop array concept proposed in thisLetter can provide significant benefits for optical boostedemissions and relevant nanoscale applications.

This work was supported in part by the United StatesAir Force Office of Scientific Research (AFOSR) and theNational Science Foundation (NSF).

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Fig. 7. (Color online) Directivity (in decibels) for parasiticplasmonic loop array antenna in planes: (a) ϕ ¼ 0 and (b)ϕ ¼ π=2. The emission of the coupled system is highly directedupward. Maximum directivity of 8:2 dB is established.

Fig. 8. (Color online) Far-zone power pattern for the array an-tenna. The power is highly directed toward the upper hemi-sphere, and the backradiation is suppressed. Successfulcollimation as compared to Fig. 4 is illustrated.

Fig. 6. (Color online) Schematic view of nanoloop antenna ar-ray. At the operating wavelength of λ ¼ 1:34 μm, the emitter ele-ment has the resonant size of 4l1 ¼ 340 nm ¼ λ=3:9, and thedirector lengths are 4l2 ¼ 4l3 ¼ 260 nm. The reflector spacingis t1 ¼ 125 nm, and the director spacings are t2 ¼ t3 ¼375 nm. The emitter and the directors are printed on low dielec-tric substrates with εd ¼ 1:5. The silver slab has a thickness ofts ¼ 205 nm. A finite-size structure of ls ¼ 500 nm in thetransverse plane is considered. The yellow arrow shows theexcitation.

Fig. 9. (Color online) Electric field distribution induced on thenanoloops antenna array at the operating wavelength of λ ¼1:34 μm (normalized and plotted in decibels).

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