Novel Wire-Grid Nantenna Array for Optical Communication Systems

Ezzeldin A. Soliman(1), Mai O. Sallam*(1),(2), and Guy A. E. Vandenbosch(2)

(1)The American University in Cairo, Physics Department, AUC Avenue, P. O. Box 74, New Cairo 11835, Egypt

Emails: esoliman@aucegypt.edu, mai.sallam@aucegypt.edu

(2)K. U. Leuven, ESAT/TELEMEIC, Kasteelpark Arenberge 10, B-3001 Leuven, Belgium Email: guy.vandenbosch@esat.kuleuven.ac.be

ABSTRACT: A novel wire-grid array of nanorods is introduced for the first time in this paper. The array consists of five gold radiating rods and four gold perpendicular connecting rods. Via optimizing the lengths of the radiators and connectors, high value of directivity has been obtained. The new array is designed for operation at a wavelength of 1.55 m, which is commonly used by optical communication system. The overall physical and electrical sizes of the array are (0.68 m -by- 1.05 m) and (3g/2 -by- g), respectively, with a maximum directivity of 9.25 dBi. The new array is analyzed using a commercial full-wave finite-difference time-domain package. It provides symmetric radiation pattern with reasonable side-lobe-level and extremely low cross-polarization level. INTRODUCTION Motivated by recent development in nanotechnology, electromagnetic wave transmitters/receivers operating in the optical frequencies, i.e. nano-antennas, are now being fabricated. The nano-antenna, or simply nantenna, is the interface between radiating electromagnetic waves and guided waves, which is the same role of regular antennas in the microwave range of frequencies. Nantennas have a number of inherent advantages over semiconductor transceivers, such as small size in the order of sub-wavelength, fast response, sensitivity to polarization which doubles the information capacity, broadband behavior, and directivity. Unlike microwave antennas, nantennas have very small share in literature. Only a few nantenna configurations are presented so far in literature, such as dipoles [1-3], bow-ties [2,3], fractal bow-tie [4], spiral [5], Yagi-Uda [6], and ring [7]. Although a number of nantenna arrays have been reported [2,5], only one attempt has been made to arrange a number of nantennas in an array configuration for the sake of enhancing the directivity [8]. However, a relatively complicated feeding network made using power splitters has been used. In this paper, a novel high directivity compact array of nantennas is introduced for the first time. The proposed array is in the form of wire-grid [9] that does not require a separate feeding network [10], which makes it simple in design and fabrication. It consists of a number of radiators and perpendicular connectors. The adjustment of the lengths of these wires, results in current with unified direction through the radiators, while the current in the connectors is flowing in opposite directions. Detailed theoretical analysis of the proposed array is presented in the following sections. SINGLE NANOROD

Fig.1 shows a single radiator in the form of rod with width of 30 nm and length of Lrad. The gap width at the center of the nanorod is 30 nm. This rod is made up of gold, which can be treated as a collisional cold plasma medium with permittivity described by a Drude dispersive model with the following parameters: infinite permittivity of 9.069, plasma resonance frequency of 1.3541016 rad/s, and plasma collision frequency of 1.21014 rad/s. The rod is surrounded by free-space everywhere. The excitation is in the form of a localized voltage difference applied at the gap of the nanorod, as shown in Fig.1. The selected operating wavelength is 1.55 m, which is commonly used in optical communication systems. The corresponding frequency is 193.55 THz. The z-component of the magnetic field at 193.55 THz in the xy plane containing the dipole is plotted in Fig. 2, for different values of dipole's length, Lrad. The simulations are performed using the finite-difference time-domain solver of CST Microwave Studio. The domain of calculation is truncated by perfect matched layers (PML). The red and blue colors in the field distributions of Fig. 2 indicate +ve and ve value of Hz, respectively. Hence, one can easily verify that the magnetic field is circulating around the conduction current along the gold nanorod, and around the displacement current through its gap. The objective of displaying the field distribution of Fig. 2 is to determine numerically the value of the guided wavelength of a nano gold rod with 30 nm 30 nm cross-section and surrounded by free-space. Comparing these field

978-1-4673-2831-9/13/$31.00 ©2013 IEEE

2013 International Workshop on Antenna Technology (iWAT)

109

distributions, with the well-known current distributions of electric dipoles with different lengths, it becomes clear that the electrical lengths of the rods in Fig. 2 are (g/4), (g/2), (3g/4), (g), and (5g/4). Hence, the guided wavelength of this structure is approximately 700 nm, which is approximately half of the free-space wavelength of 1.55 m.

30 nm30

30 nm Lrad

Fig. 1: Single gold nanorod.

(a) (b) (c) (d) (e) Fig. 2: z-component of the magnetic field in the xy plane at 193.55 THz (log scale): (a) Lrad = 175, (b) Lrad = 350, (c) Lrad = 525, (d) Lrad = 700, and (e) Lrad = 875 nm.

The 3D radiation pattern of the half-wavelength 350 nm nanorod of Fig. 2(b) is shown in Fig. 3, which takes the well-known “donut” shape. The directivity of this rod is 1.8 dBi. Such uniform distribution of the radiated power is inconvenient for point-to-point communication systems, as huge fraction of the total power will be wasted along unwanted directions. For such system, directive radiation pattern is required, which can be achieved via arraying a number of radiating elements. In the following section, a novel arraying technique suitable for nantennas is presented.

Fig. 3: 3D radiation pattern of the 350 nm half-wavelength nanorod at 193.55 THz.

WIRE-GRID ARRAY OF FIVE NANORODS The proposed wire-grid array of five nanorods is shown in Fig. 4. This array consists of five vertical radiators and four horizontal connectors. The lengths of radiators and connectors are Lrad and Lcon, respectively. A directive beam can be achieved by optimizing the values of these two geometrical parameters for maximum directivity. The contour plot in Fig. 5 illustrates how the directivity function is varying with Lrad and Lcon. According to this figure and the parametric study that has been performed, a maximum directivity of 9.25 dBi is obtained when Lrad = 350 nm and Lcon = 325 nm. These values come with no surprise as they are so close to the predicted value of (g/2), which forces the currents on the vertical rods to add constructively. The achieved value of directivity is much higher than that of a single rod with the same Lrad of 350 nm. To describe the theory of operation of the proposed wire-grid nano-array, the distribution of

110

the z-component of the magnetic field in the xy plane calculated using CST at 193.55 THz for the optimum array is shown in Fig. 6. This component of the magnetic field is proportional to electric current flowing through the gold arms of the array. The half-wavelength field distribution terminated by two nulls can be clearly seen along the four outer vertical radiators. The direction of current along these elements is the same, which gives rise to an array factor. On the other hand, the field distribution along the horizontal connecting arms reveal that their currents are flowing in opposite directions, and hence no radiation is expected from these connectors. The field distribution along the central radiator does not obey the conventional half-wavelength standing wave pattern, and consequently its contribution to radiation is less than that of the outer radiators.

Lrad

Lrad

Lrad

Lrad

Lrad

Lcon Lcon

Lcon Lcon

Fig. 4: Structure of the wire-grid array of five nanorods.

6.2925

6.5396

6.78667.0337

7.0337

7.2807

7.2807

7.5278

7.5278

7.5278

7.77497.7749

7.7749

8.0219

8.0219

8.0219

8.0219

8.0219

8.269

8.269

8.269

8.269

8.2698.516

8.516

8.516

8.516

8.516 8.76318.7631

8.76

31

8.76

31

8.7631

8.7631

9.0101

9.0

101

9.0101

250 300 350 400 450250

300

350

400

450

Lcon (nm)

Lra

d(n

m)

Max Directivity

Fig. 5: Directivity of the nano-wire-grid array versus Lrad and Lcon.

Fig. 6: Logarithmic distribution of Hz along the xy plane at 193.55 THz: Lrad = 350 nm, Lcon = 325 nm.

Fig. 7: 3D radiation pattern of the wire-grid array at 193.55 THz, Lrad = 350 nm and Lcon = 325 nm.

The 3D radiation pattern of the optimum wire-grid array is shown in Fig. 7, as calculated using CST at 193.55 THz. Now, the beam is narrow and hence suitable for point-to-point communication systems. The calculated directivity is 9.25 dBi. The co-polar components along the E- and H-planes are shown in Fig. 8. The side-lobe-levels in these planes are 22.5 dB and 10.2 dB, respectively. The levels of the cross-polar components are below the co-polar levels by 157 dB and dB in the E- and H-planes, respectively. Such extremely low cross-polarization levels are expected due to the complete symmetry of the proposed structure along the principal planes. This leads to perfect cancellation of the radiation from the connectors' currents. If symmetric mesh is used by the simulation engine, the calculated cross-polarization level should be ideally zero.

111

(a)

(b)

Fig. 8: Co-polar far field components of the optimum wire-grid array at 193.55 THz: (a) E-plane, and (b) H-plane. CONCLUSION A novel wire-grid array is reported in this paper. This array is suitable for point-to-point optical communication systems, where directive beam is required. The proposed array has a number of appealing features, such as: compact size, no need for separate feeding network, symmetric radiation pattern, and extremely low cross-polarization level. The study presented in this paper needs to be extended for wire-grid arrays of more radiators. Also, the substrate effect is not considered in this paper. It is expected that adding a substrate with relatively high dielectric constant, such as silicon, results in uni-directional pattern towards the substrate, which enhances the value of the directivity even more. REFERENCES [1] F. Pelayo Garcia de Arquer, V. Volski, N. Verellen, G. A. E. Vandenbosch, and V. V. Moshchalkov,

“Engineering the input impedance of optical nano dipole antennas: materials, geometry and excitation effect,” IEEE Transactions on Antennas and Propagation, vol. 59, pp. 3144-3153, Sept. 2011.

[2] E. Cubukcu, N. Yu, E. J. Smythe, L. Diehl, K. B. Crozier, F. Capasso, “Plasmonic laser antennas and related devices,” IEEE. J. of Selected Topics in Quantam Elect., vol. 14, 1448-1461, 2008.

[3] H. Fischer and O. J. F. Martin, “Engineering the optical response of plasmonic nanoantennas,” Optics Express, vol. 16, 9144-9154, 2008.

[4] S. Sederberg and A. Y. Elezzabi, “Sierpiński fractal plasmonic antenna: a fractal abstraction of the plasmonic bowtie antenna,” Optics Express, vol. 19, 10456-10461, 2011.

[5] F. Javier González, B. Ilic, J. Alda, G. D. Boreman, “Antenna-coupled infrared detectors for imaging applications,” IEEE J. of Selected Topics in Quantum Elect., vol. 11, 117-120, 2005.

[6] J. Dorfmüller, D. Dregely, M. Esslinger, W. Khunsin, R. Vogelgesang, K. Kern, H. Giessen, “Near-field dynamics of optical Yagi-Uda nanoantennas,” Nano Lett., vol. 11, 2819-2824, 2011.

[7] E. A. Soliman, “Circularly polarized nanoring antenna for uniform overheating applications,” Microwave and Optical Technology Letters, vol. 54, pp. 2209-2214, Sept. 2012.

[8] K. Van Acoleyen, H. Rogier, and R. Baets, “Two-dimensional optical phased array antenna on silicon-on-insulator,” Optics Express, vol. 18, pp. 13655-13660, June 2010.

[9] E. A. Soliman, S. Brebels, G. A. E. Vandenbosch, and E. Beyne, “X-band brick-wall antenna fed by CPW,” Electronics Letters., vol. 34, pp. 836-838, April 1998.

[10] E. A. Soliman, S. Brebels, G. A. E. Vandenbosch, and E. Beyne, “Antenna arrays in MCM-D technology fed by coplanar CPW networks,” IEEE Transactions on Microwave Theory and Techniques, vol. 48, pp. 1065-1068, June 2000.

112