Influence of Lens Size and Shape on the Performance of ... - IEEE Xplore

The 2014 International Conference on Advanced Technologies for Communications (ATC'14)

Influence of Lens Size and Shape on the Performance of a Terahertz Traveling-wave Stripline Dipole Antenna Truong Khang Nguyen1,2,3, Thanh Tu Nguyen1, Le Khoa Dang1, Fabian Rotermund2, and Ikmo Park3,* 1

Faculty of Electronics and Telecommunications, University of Science HCM City, Vietnam 2 Division of Energy Systems Research, Ajou University, Suwon, Korea 3 Department of Electrical and Computer Engineering, Ajou University, Suwon, Korea * [email protected]

II.

Abstract—In this paper, the influence of a substrate lens made of high-permittivity material on the overall performance of a traveling-wave stripline dipole antenna is presented. Input impedance and radiation characteristics of the antenna are fully investigated over a broad frequency range up to 5.0 THz. The results show that the lens shape represented by the ratio of the extension length to the lens radius primarily determines the best possible antenna gain and radiation spectral bandwidth. The antenna gain response exhibits an increased level of sensitivity to the lens shape as the lens size increases, and this is particularly important when it comes to optimizing large substrate lenses.

The geometries of the stripline dipole and the lens substrate in this study are shown in Fig. 1. The stripline dipole was patterned on a thin GaAs (εr = 12.9) substrate and backed by an extended hemispherical Si (εr = 11.7) lens. The width and the length of the stripline dipole are represented by wd and L, respectively. A rectangular gap, g, was used for antenna excitation and two square pads, p, were used for applying voltage bias at the dipole terminations. The antenna was driven by a wire port with a constant 1V source introduced in this gap. This feeding approach can be employed to characterize the properties of the antenna itself ignoring the response of the photoconductive material to the optical signal. The GaAs substrate had a thickness of TGaAs and a square size of W. The lens had an extension length and radius denoted as TExt and R, respectively. Fixed and dependent design parameters used throughout the study are summarized in Table 1.

Keywords—terahertz antenna, traveling wave, stripline dipole, extended hemispherical lens.

I.

INTRODUCTION

Terahertz time-domain spectroscopy (THz-TDS) systems using a photoconductive antenna (PCA) for the generation and detection of coherent THz radiation propagating through free space have recently started to attract considerable interest [1– 4]. The typical emitter/detector PCA is lithographically patterned on a high-resistivity silicon substrate lens to increase the coupling of THz waves between the device and the free space. Therefore, the geometry of the antenna and lens require a careful design and optimization to increase the overall performance of the PCA [5–12]. Our previous reports have revealed the impact of the bias line length and connection scheme on the characteristics of a THz coplanar stripline dipole antenna [13, 14]. Numerical investigation was carried out using a semi-infinite substrate model in order to characterize solely the antenna properties and eliminate the effect of the substrate. Recently, comparative studies of stripline dipole antenna on semi-infinite and lens substrates at terahertz frequency have been reported [15, 16].

GaAs substrates with different thicknesses (TGaAs) were selected for different lens radii (R), for example, TGaAs = 100 μm for R = 0.5 mm, TGaAs = 200 μm for R = 1.0 mm, and TGaAs = 300 μm for R = 1.5 mm. This allowed for the maintenance of the substrate lens with an extended hemispherical shape having the same initial ratio of T/R = 0.2 and provided the same percentage increase in the overall volume, that of the GaAs substrate and Si lens, for a fair comparison.

(a)

In this paper, we design a stripline dipole antenna on a substrate lens and investigate the influence of lens size and shape on the antenna’s overall characteristics. The substrate lens with a size of a few millimeters in diameter and an extended hemispherical shape is chosen for investigation over a broad frequency range up to 5.0 THz. The investigation is performed using a full-wave simulator, Microwave Studio by CST [17], which allows complete characterization of the performance of such a lens-coupled antenna and monitoring of the radiation properties of the antenna at several frequency points within one simulation run.

978-1-4799-6956-2/14/$31.00 ©2014 IEEE

ANTENNA GEOMETRYAND MODELING

(b) Fig. 1. Geometries of (a) a traveling-wave stripline dipole and (b) a thin GaAs substrate backed by an extended hemispherical Si lens.

417


TABLE I.

FIXED AND DEPENDENT DESIGN PARAMETERS OF THE ANTENNA THROUGHOUT THE STUDY

Parameter

wd

g

p

W

L

T

Value (μm)

10

5

200

2R

W-2p

TGaAs + TExt

III.

ANTENNA CHARACTERISTICS

Figure 2 shows the antenna gain responses with respect to the T/R ratio variation calculated for different lens radii. The T/R ratio, representing the lens shape, was changed simply by varying the extension length behind the hemispherical position, while the GaAs substrate thickness was fixed. In this study, the the small index discontinuity at the interface between the GaAs substrate and the silicon lens was ignored. For a lens diameter of 1.0 mm [Fig. 2(a)], the differences in the gain curves were relatively small with increments of 0.02 in the T/R ratio from 0.34 to 0.38. However, the gain curves in the 2.0 mm lens diameter case changed at a higher rate for each increment of 0.02 in T/R in comparison with the case with a 1 mm lens diameter [Fig. 2(b)]. Particularly, the gain curves changed significantly with the same T/R ratio increment for the case with the 3 mm lens diameter, as can be observed in Fig. 2(c).

(a)

Figure 3 presents comparison plots at the optimal T/R ratio of 0.36 for three different lens radii R. The larger the lens diameter was, the higher the gain level benefits became due to the larger radiating aperture for the stripline dipole as the lens diameter increased. The average increment of the antenna gain was 5.3 dB as the lens diameter increased from 1 mm to 2 mm, but it was only 3.1 dB from 2 mm to 3 mm [Fig. 3(a)]. This predicts that further increasing of the diameter of the substrate lens could lead to more limited increased antenna gain. The detailed radiation patterns of the antenna will be presented at the conference. The radiation efficiency of the antenna, as shown in Fig. 3(b), was small at low frequencies but gradually increased with increases in frequency and reached saturation at around 2.5 THz. The radiation efficiency of the antenna was increased with increases in the lens diameter, but with smaller improvement levels in comparison with the antenna gain. As illustrated in Fig. 3(c), the fluctuations in the input impedance at low frequencies gradually decreased with an increase in the lens diameter, whereas the antenna impedance in the highfrequency region remained constant regardless of the lens diameter. Larger lens diameters may result in longer stripline dipoles, thereby reducing the current reflected back at the dipole terminations.

(b)

IV.

CONCLUSIONS

The influence of lens size and shape on the impedance and radiation characteristics of a traveling-wave stripline dipole antenna was characterized over a broad THz frequency range up to 5.0 THz. The gain response sensitivity was found to be increased with the increases of the lens diameter, thereby indicating the importance and difficulty of optimizing large substrate lenses. The larger the lens diameter was, the higher the gain level became. This study provides useful guidelines in choosing and designing a proper substrate lens to improve the

(c) Fig. 2. Gain versus frequency of the antenna with different T/R ratios: (a) TGaAs = 100 μm and R = 0.5 mm, (b) TGaAs = 200 μm and R = 1.0 mm, and (c) TGaAs = 300 μm and R = 1.5 mm.

418


output power of a broadband THz system using a photoconductive antenna as emitter/detector. ACKNOWLEDGMENT This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number “103.05-2013.75”. REFERENCES [1]

[2] [3] [4]

(a)

[5]

[6] [7]

[8]

[9]

[10] (b) [11]

[12]

[13]

[14]

[15]

[16]

[17]

(c) Fig. 3. Comparison plots with the optimal T/R ratio of 0.36 for three different lens radii R: (a) gain, (b) radiation efficiency, and (c) input impedance.

419

M. Hangyo, M. Tani, and T. Nagashima, “Terahertz time-domain spectroscopy of solids: A review”, J. Infrared Millim. Terahertz Waves, vol. 26, no. 12, pp. 1661–1690, 2005. S. Dexheimer, Terahertz Spectroscopy: Principles and Applications, New York: CRC Press, 2007. Y. S. Lee, Principles of Terahertz Science and Technology, New York: Springer, 2009. T.-I. Jeon, “THz generation and propagation using femtosecond laser”, In: Park G.S., editor. Convergence of Terahertz Sciences in Biomedical Systems, Abingdon: Springer, 2013, pp. 113–127. D. F. Filipovic, S. S. Gearhart, and G. M. Rebeiz, “Double-slot antennas on extended hemi-spherical and elliptical silicon dielectric lenses”, IEEE Transaction Microw. Theory Technol., vol. 41, no. 10, pp. 1738–1749, 1993. P. Jepsen and S. R. Keiding, “Radiation patterns from lens-coupled terahertz antennas,” Optics Lett., vol. 20, no. 8, pp. 807–809, 1995. J. V. Rudd and D. M. Mittleman, “Influence of substrate-lens design in terahertz time domain spectroscopy”, J. Optics Society Am. B., vol. 19, no. 2, pp. 319–329, 2002. M. T. Reiten and R. A. Cheville, “Effect of spherical aberration and surface waves on propagation of lens-coupled terahertz pulses,” Optics Lett., vol. 30, no. 6, pp. 673–675, 2005. T. K. Nguyen, T. A. Ho, H. Han, and I. Park, “Numerical study of selfcomplementary antenna characteristics on substrate lenses at terahertz frequency”, J. Infrared Millim. Terahertz Waves, vol. 33, no. 11, pp. 1123–1137, 2012. F. Miyamaru, Y. Saito, K. Yamamoto, T. Furuya, S. Nishizawa, and M. Tani, “Dependence of emission of terahertz radiation on geometrical parameters of dipole photoconductive antennas”, Appl. Phys. Lett., vol. 96, pp. 211104(1)–211104(3), 2010. Y. Kamo, S. Kitazawa, S. Ohshima, and Y. Hosoda, “Highly efficient photoconductive antennas using optimum low-temperature-grown GaAs layers and Si substrates”, Jpn. J. Appl. Phys., vol. 53, pp. 032201, 2014. E. Moreno, M. F. Pantoja, F. G. Ruiz, J. B. Roldán, and S. G. García, “On the numerical modeling of terahertz photoconductive antennas,” J. Infrared Millim. Terahertz Waves, vol. 35, no. 5, pp. 432–444, 2014. T. K. Nguyen and I. Park, “Effects of antenna design parameters on the characteristics of a terahertz coplanar stripline dipole antenna”, Prog. Electromagnetics Res. M, vol. 28, pp. 129–143, 2013. T. K. Nguyen, S. Kim, and I. Park, Impact of varying DC bias stripline connection angle on terahertz coplanar stripline dipole antenna characteristics, J. Electromagnetic Waves Application, vol. 27, no. 14, pp. 1725–1734, 2013. T. K. Nguyen and I. Park, “Stripline dipole antenna on a substrate lens at a terahertz frequency,” 7th Global Symp. on Millim. Waves, Seoul, Korea, May 21–23, 2014. T. K. Nguyen and I. Park, “Comparative study of stripline dipole antenna on semi-infinite and lens Substrates at terahertz frequency,” 8th European Conf. on Antennas and Propagat., the Hague, Netherland, April 6–11, 2014. CST Microwave Studio, CST GmbH, 2012, http://www.cst.com.