30th NATIONAL RADIO SCIENCE CONFERENCE (NRSC 2013) April 16-18, 2013, National Telecommunication Institute, Egypt B5. Perforated Nanoantenna Relectarray S.H Zainud-Deen1, .A. Malhal*, .M Gabe?, and .. Awadalla1. 1Faculty of Electonic Eng., Menouia Universiy, Eypt. *echonidal@yahoo.com 2Egyptian Russian University, Egypt. ABSTACT This paper presents a design of perforated nanoantenna relectarray. The use of metallic nanostructures made of Silver and/or Gold at appropriate wavelength cause fascinating unusual electromagnetic effects. Relectarray consists of an array of unit cell made rom Silver is investigated. The effect of the number of perforated holes in the unit cell conigurations is investigated for proper relection coeficient phase compensation. A linearly polarized pyramidal nano-hon is used to feed the perforated nanoantenna relectarray. The radiation characteristics of 9 x 9 perforated nanoantenna relectarray are illustrated. A high gain of 20.5 dB is obtained at the designed requency of 735 THz. A comparison between solid Silver sheet with no perforation holes and the proposed perforated relectarray is explained. Kewords: Nanoantenna, Relectarray, Tera-Hertz. I. INTRODUCTION Relectarray antennas have received considerable attention over the years and are quickly inding many applications. Relectarray antennas combine the same features of parabolic relectors and phased arrays providing a directive beam in a desired scanned angle. The most important advantages of relectarrays over phased arrays are the elimination of complexity and losses of the feeding network and the higher eiciency [1-4]. The relectarray consists of an array of unit-cells illuminated by a primary feed. Each unit-cell is designed in such a way to correct the phase of the incident wave as is done in the traditional parabolic relector. In relectarrays, the phase of the relected ield rom each element is adjusted so that the main beam can be directed to a desired direction. Several relectarray approaches are proposed to control the relection phase in the literature [5]. For example, one is to use identical microstrip patches with different-length phase-delay lines attached so that they can compensate for the phase delays over the different paths rom the illuminating feed. The other is to use variable-size patches, dipoles, or rings so that elements can have different scattering impedances and, thus, diferent phases to compensate for the different feed-path delays. To overcome the shortcoming of narrow bandwidth of the relectarray, dual-band multi­ layer relectarrays using variable patch size, anular rings, and crossed dipoles are being developed [6-8]. Generally, conventional antennas act as a source and transformer of electromagnetic (EM) radiation at radio requencies (RF) and microwaves, resulting in their sizes being comparable with the operational wavelength. Recent success in the fabrication of nanoscale elements allows bringing the concept of the RF antennas to optics, leading to the development of optical nanoantennas [9]. Nanoantennas are metal nanostructures used to enhance, conine, receive, and transmit optical ields [10]. Nanoantenna is one of the most developing in plasmonics due to their ability to overcome the size and impedance mismatch between subwavelength emitters and ree space radiation [11]. The ability to redirect propagating radiation and transfer it into localized subwavelength modes at the nanoscale makes the optical nanoantennas highly desirable for many applications. Several applications of nanoantennas have been used such as spectroscopy and high resolution near-ield microscopy [9], subwavelength light coninement and enhancement [12], photovoltaic [13], sensing [14], molecular response enhancement [15], non-classical light emission [16], and communication [17]. Different nanoantennas have been investigated in the literature. By far, research in this nanoantennas ield has utilized the coinage metals, Gold, Copper, and Silver, yet many potential commercial applications would be optimally realized by inexpensive plasmonic materials compatible with either high-technology or high-troughput manufacturing methods. Recently, a method of realizing nanoantennas relectarray system at optical requencies by using nanoparticles of concentric structures with cores made of ordinary dielectrics and shell of plasmonic materials has been suggested in [18]. The scattering resonance of these concentric structures can be tailored at different wavelength range by adjusting the core and shell radii or the material properties. Later, the perforated technique was used in the dielectric Fresnel lens design to obtain proper phase compensation and gain enhancement [19]. The technique of perforating a dielectric sheet eliminates the need to position and bond individual elements in an array. Perforations create different effective dielectric permittivity and make the fabrication of the arrays feasible. The array is made rom one piece of material; and 978-1-4673-6222-11131$31.00 ©2013 IEEE 48 30th NATIONAL RADIO SCIENCE CONFERENCE (NRSC 2013) April 16-18, 2013, National Telecommunication Institute, Egypt inserting air holes with deined diameters and spacing in the required position. The perforations result in lowering the effective dielectric constant for the substrate at the region of air holes. The inite-integration technique, FIT [20] is used to ca1culate the characteristics of the relectarray nanoantenna. In this paper, the radiation characteristics of perforated nanoantennas relectarray are calculated. The paper is organized as folIows. Section 11 describes basic material properties used in designing nanoantennas. Section III offers the complete description of the design of nanoantenna relectarray unit cell and its designing parameters. Finally, in Section IV conclusions are drawn. 11. THEORY Generally, the relectarray transforms the spherical wave emanating rom the feed into a plane wave at the output, like conventional parabolic relector antenna. The array is located in the x-y plane and is illuminated by a feed nano-horn. A required phase distribution, Ji(Xi,Yi), at each element of the array is needed to collimate a beam in the (80, Jo) direction and is determined by pi(Xi'yJ di = = ko(di -sinBo(xi cospo + Yi sinpJ) �(Xi -X [ ) 2 + (Yi - Y[ ) 2 + ( z[ ) (1) 2 (2) where ko is the propagation constant in vacuum, diis the distance rom the feed nano-horn x. y. z} to the element j of the array and xi,) are the coordinates of the cell element . The phase shit of each cell element in the relectarray should between 0 and 360°.The use of metallic nanostructures made of Silver and/or Gold may become promising. The light illumination of such metals at appropriate wavelength may cause fascinating unusual electromagnetic effects [21] leading to promising applications for optical telecommunication, integrated optics and optical sensors. Most of these effects appears due to an excitation of Surface Plasmon Polariton (SPP) resonances. Due to high losses at optical requencies, the assumption of perfect electrical conductor is no longer valid. The dielectric unction of material substrate, used in nanoantennas, have been derived by itting a Drude model given by [18] E = Eo [1 - _ �_ ( - jvp) -:: w w W... 7 ] (3) (4) where o is the angular resonance requency of the antenna, vp is the angular scattering requency, and op is the electron plasma angular requency for bulk material at the operating wavelength A. It can be seen that as the metal conductivity decreases, the losses will increase. In this paper, the desired phase shit compensation required at each unit cell element is obtained by changing the effective permitivity of the Gold substrate trough the variation of the diameters of the holes according to the theory of perforated dielectric material [22]. 111. NUMEICAL RESULTS a. The Unit-Cell The detailed construction of the proposed perforated nanoantenna relectarray unit cell element is shown in Fig.l. The antenna was designed to operate at 735 THz. The proposed square unit cell has length L 600 nm, height = 70 nm, with four identical circular holes of equal radii, and perforation depth d= 35 nm. The unit cell is constructed rom Silver substrate with its material properties are introduced using the Drude model. The Silver angular scattering requency vp is (4.35x 1012 rad/sec) and the electron plasma angular requency op is (1.3665x 1016 rad/sec) at the operating wavelength of A = 408 nm. To ca1culate the required relection coeficient phase compensation in each unit cell, the unit cell is placed in a waveguide simulator [5]. The perfect electric and magnetic wall boundary conditions are applied to the sides of the surrounding waveguide, and result in image planes on all sides of the unit cell to represent an ininite array approximation. A linearly polarized plane wave was used as the excitation of the unit cells inside the waveguide simulator and only normal incidence angle was considered. There are several limitations to the ininite array approach. Fist, all elements of the relectarray are identical; this is clearly not the case in the real relectarray in which the diameters of the holes in each cell element must vary according to the required phase compensation. Secondly, the relectarray itself is not ininite in extent. = 49 30th NATIONAL RADIO SCIENCE CONFERENCE (NRSC 2013) April 16-18, 2013, National Telecommunication Institute, Egypt x x z Fig. 1. a. 3-D view b. Side view The detailed construction of the perforated relectarray nanoantenna unit cell. -.- •• one hole .......... two holes L ----- three holes - lour holes Two holes L L 75 80 85 90 95 1 0 Hole radius (nm) Three holes FOf holes a. The unit cell with different number of holes c 0 ) � t :: b. The relection coeicient phase variation -3 -.- .• one hole ..•••.••.• 4 - -5 70 two holes ----- three holes 75 80 90 85 Hole radius (mm) our holes 95 100 c. The relection coeficient magnitude variation Fig. 2. The variation of the relection coeficient phase variation of the unit cell with different number of perforated holes. The separation between the holes and the number of holes are optimized to maximize the relection rom the structfe as shown in Fig.2a. The variation of the relection coeicient phase versus the hole radius for one, two, three and fOf holes at 735 THz is determined using the FIT technique and is shown in Fig.2b. The unit cell having only one hole produce a phase shit ranging rom 0° to about 200°, by increasing the number of the holes the relection coeicient phase is increased. The unit cell having two holes achieves phase of 250°, unit cell with three holes achieves 300°, and inally the unit cell having fOUf holes achieves 360° relection coeficient phase variation. The noticed variation of the relection coeficient phase is due to the variation of the effective dielectric constant of the Silver material. Fig.2c shows the variation of the magnitude of the relection coeicient in dBs for unit cell with 50 30th NATIONAL RADIO SCIENCE CONFERENCE (NRSC 2013) April 16-18, 2013, National Telecommunication Institute, Egypt different number of perforated holes. The magnitude of the relection coeicient of the unit cell having only one­ hole changes rom -1.9 dB to 0 dB. By increasing the number of holes in the unit cell, the magnitude of the relection coeicient is increased. The unit cell having two holes achieves relection coeicient magnitude variation rom 1.3 dB to 0 dB, unit cell with three holes achieves relection coeficient magnitude variation rom -1 dB to 0 dB, and inally the unit cell having four holes achieves relection coeficient magnitude variation rom -0.2 dB to 0 B. b. Nano-Horn Antenna Different nano-horn antennas made rom nano-materials operating at THz range are investigated in [23, 24]. A linearly polarized pyramidal nano-hon antenna is used to feed the perforated nanoantenna relectarray. The nano­ hon was positioned such that the array was prime-focus fed. The nano-horn antenna is constructed rom Gold substrate with its material properties introduced using the Drude model. The Gold angular scattering requency vp is (1.2566xlQ13 rad/sec) and the electron plasma angular requency Op is (1.2566xlQ1 6 rad/sec) at the operating wavelength of A= 408 m. The feed nano-horn was Lh=1080 nm long, with an aperture size bxa of 1700 m x 850 nm as shown in Fig. 3a. The 3-D radiation patten rom the nano-horn antenna at 735 THz is shown in Fig. 3b. The nano-horn antenna has a gain of 11.7dB at 735 THz. The radiation pattens for the pyramidal nano-hon antenna in E-plane and H-plane are shown in FigA at 735 THz. The irst sidelobe levels ( SLL) in the E- and H-planes are approximately -18.95dB and -18.86 dB below the main peaks respectively. dU 11 .7 9.55 7.43 5.31 3.18 a 1.06 -2.57 -7.72 -12.9 -18 -23.2 - a. 3-D view of horn antenna 28 . 3 b. 3-D gain pattern Fig.3. The detailed construction ofthe feeding hon and the 3-D gain radiation patten at =735 THz. 15 --�- 15 -�- " D ii s :: "j l s :: "j l -25 ---_�__�_�__�_�__l -60 -180 120 -120 180 60 o Eleation (degrees) -120 a. E-plane -60 60 o Eleation (degrees) 120 180 b. H-plane FigA. The normalized gain radiation patens of the feeding hon antenna in different planes at 735 THz. c. 9x9 Nanoantenna Relectarray A schematic of the perforated nanoantenna relectarray showing the feed is shown in Fig.5. 9x 9 unit cell elements are used to construct the array. The element spacing between the unit cells is 600 m. The relectarray has total dimensions of 5400 x 5400 m2 located in the x-y plane. The perforated nanoantenna relectarray is symmetrical about the x-axis and y-axis. The feeding horn is located at a distance F=5400 m (FID=l) in the normal direction of the array plane. Table 1 describes the phase shit and the corresponding hole radii relevant to the unit 51 30th NATIONAL RADIO SCIENCE CONFERENCE (NRSC 2013) April 16-18, 2013, National Telecommunication Institute, Egypt cells of the irst quadrant of the array. The E and H-plane radiation patterns at 735 THz of the perforated nanoantenna relectarray are shown in Fig. 6. The irst SLL is 10.34 dB and 8.9 dB in E-plane and H-plane respectively. The HPBW of the relectarray is 4 degrees in E-plane and 5.7 degrees in H-plane. The 3-D radiation pattern of the relectarray at f=735 THz is shown in Fig.7a. The perforated nanoantenna relectarray has a gain of 20.5 dB at 735 THz. Most of the radiated ield is relected through the perforated nanoantenna relectarray and the back lobe is due to the variation of the conductivity of the relectarray material. The perforated nanoantenna relectarray gain variation as a unction ofrequency is shown in Fig. 7b. z Fig. 5. The detailed 3-D construction of the 9x9 perforated nanoantenna relectarray. Table 1 The phase shit and the corresponding hole radii relevant to the unit cells of the irst quadrant of the array. ll � : "j l 50.4° 88.03nm 79.5 1° 87.lnm 165.79° 79.36 nm 306.27° 72.33 nm 136.55° 80.87 nm 79.5° 87.1 nm 108.44° 82.99 nm 194.21° 78.21 nm 333.9° 70.97nm 163.17° 79.45 nm 165.79° 79.36 nm 194.21° 78.21 m 278.50° 73.86nm 55.88° 87.93nm 242.21° 76.11nm 306.27° 72.33 nm 333.9° 70.97nm 55.88° 87.93nm 189.65° 78.39nm 11.38° 91.14nm 136.55° 80.87 nm 163.17° 79.45 m 242.21° 76.11nm 11.38° 91.14nm 187.2° 78.47nm 25 25 20 20 15 15 10 10 ll 5 � : 0 "j l -5 -10 5 0 -5 -10 -15 -15 -20 -20 -25 -180 -120 -60 0 60 120 180 x -25 -180 -120 -60 0 60 120 180 Elevation (degrees) b. H-plane Elevation (degrees) a. E-plane Fig. 6. The gain pattens plot far a 9 Perforated ----'Horn - 9 center-feed center-beam perforated nanoantenna relectarray at 735 THz. 52 30th NATIONAL RADIO SCIENCE CONFERENCE (NRSC 2013) April 16-18, 2013, National Telecommunication Institute, Egypt 25 dB 20 20.8 17 13.2 9.46 15 m � 5.67 c 1.89 10 � -1.75 -5.24 5 -8.73 -12.2 -15.7 0 725 -19.2 730 735 Frequency liz) 740 745 a. 3-D gain pattern at =735 THz b. The gain variation versus requency Fig .7. The 3-D gain radiation patteren at =735 THz and the gain variation versus requency and for a 9 perforated nanoantenna relectarray. 25 25 ----- 728.5 Iz 20 � c B ----- 741.5 Iz 15 10 -7351Hz 10 5 5 l . 0 0 c ·5 � ·10 ·15 · 5 ·10 · ·20 9 ----- 728.5 Iz 20 15 x 15 ·20 ·120 Eleation (degrees) a. E-plane Elewtion (degrees) b. H-plane Fig. 8. The gain radiation patterns plot for a 9 x 9 center-feed center-beam perforated nanoantenna relectarray at different requencies. x z Fig. 9. The detailed 3-D construction of the solid Silver sheet. The gain is varied rom 14.5 dB to 20.5 dB over the requency range with 1 dB gain variations bandwidth (6.5 THz). Theoretical radiation pattens are shown in Fig.8 at different requencies to check the bandwidth of the array. At the extreme requencies, the radiation pattens are similar, with some increase in sidelobe levels. A solid Silver sheet with the same dimensions of the proposed 9 x 9 perforated nanoantenna relectarray (5400 x 5400 nm2) but with no holes is shown in Fig. 9. The solid array is fed with the same nano-hon antenna placed at a distance F=D as that in Fig. 5. The radiation patterns rom the solid sheet rom Silver material (with no holes) with the same size of the nanoantenna relectarray are shown in Fig.l0. The gain of the solid Silver sheet is almost a copy of the gain 53 30th NATIONAL RADIO SCIENCE CONFERENCE (NRSC 2013) April 16-18, 2013, National Telecommunication Institute, Egypt of the horn except that the side lobes are a little stronger. This is because the solid Silver sheet has a inite size. It is quite Iikely that the side lobes are Airy features (difraction limit) since the caIculated area of the sheet is only about 12 wavelengths in size. The nano-horn antenna in ront of the solid sheet blocks a part of the relected beam and reduce the main beam. The gain variation versus requency of the solid sheet is illustrated in Fig.ll. Due to the lack of phase compensation in the solid surface, the gain is much less than the ree space hon antenna gain. In the solid surface, there is no transformation of the spherical wave into a plane wave. 25 --, 25 --, 20 - Silver solid sheet 20 15 15 ----'Horn 10 10 5 5 c o 'i ) Silver solid sheet ----'Horn - o -5 -15 -20 -25 ---� 120 -180 -120 180 -60 o 60 Elevation (degrees) a. E-plane -25-��-180 -120 120 -60 o 60 180 Elevation (degrees) b. H-plane Fig. 10. The gain radiation patterns plot for a solid Silver sheet at =735 THz. 25 20 n 15 � c "i ) 10 5 0 725 730 735 Frequency (THz) 740 745 Fig .11. The gain variation versus requency for a solid Silver sheet. IV. CONCLUSION In this paper, a design of perforated nanoantenna relectarray is investigated. A perforated Silver material is used to construct the array unit cell. The number of the perforation holes is optimized in order to obtain relection coeicient phase variation rom 0° to 360° with maximum relection magnitude (�1). The radiation characteristics of linearly polarized pyramidal nano-hon antenna are investigated. The nano-horn antenna has a gain of 11.7 dB at 735 THz. The irst side lobe levels (SLL) in the E- and H-planes are approximately -18.95 dB and -18.86 B below the main peaks respectively. Using the proposed unit cell a 9 x 9 perforated nanoantenna relectarray is designed. The irst SLL is 10.34 B and 8.9 dB in E-plane and H-plane respectively. 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