that the focal length decreases with a temperature decrease, which can be explained by the increase of Re(b) with the tem- perature decrease in Figure 1(a). The THz transmission is 45.1%, 41.3%, 36.3%, and 29.6% at 325, 295, 270, and 255 K, respectively. These results are due to loss increases from tem- perature decreases as shown in Figure 1(b). Figure 4 shows the phase distributions according to the slit location and width at four different temperatures. The different temperature conditions build up different phase fronts, which lead to the different focal lengths. 5. CONCLUSIONS In summary, we have theoretically demonstrated the focusing effect of plasmonic lenses based on InSb microslits. We also observed the extraordinary transmission of THz radiation through an InSb subwavelength slit array. THz transmission of more than 70%, using InSb plasmonic lenses, may be possible. Furthermore, we have presented a method for tuning the focal length of the InSb plasmonic lenses. The results of this article offer a possible way to realize a tunable THz plasmonic lens with high transmission, which can be used in many applications that require tunable focal lengths, such as in 3D terahertz imag- ing and sensing. ACKNOWLEDGMENT Partial financial support of this work by the Basic Research Pro- gram of the Office of Naval Research is greatly appreciated. REFERENCES 1. J.G. Rivas, C. Janke, P.H. Bolivar, and H. Kurz, Transmission of THz radiation through InSb gratings of subwavelength apertures, Opt Express 13 (2005), 847–859. 2. H. Shi, C. Wang, C. Du, X. Luo, X. Dong, and H. Gao, Beam manipulating by metallic nano-slits with variant widths, Opt Express 13 (2005), 6815–6820. 3. L. Verslegers, P.B. Catrysse, Z. Yu, J.S. White, E.S. Barnard, M.L. Brongersma, and S. Fan, Planar lenses based on nanoscale slit arrays in a metallic film, Nano Lett 9 (2009), 235–238. 4. S. Kim, Y. Lim, H. Kim, J. Park, and B. Lee, Optical beam focus- ing by a single subwavelength metal slit surrounded by chirped dielectric surface gratings, Appl Phys Lett 92 (2008), 013103. 5. G.-G. Zheng and X.-Y. Li, Optical beam manipulation through two metal subwavelength slits surrounded by dielectric surface gratings, J Opt A: Pure Appl Opt 11 (2009), 075002. 6. C.A. Baron and A.Y. Elezzabi, Active plasmonic devices via elec- tron spin, Opt Express 17 (2009), 7117–7129. 7. J.A. Sanchez-Gil and J.G. Rivas, Thermal switching of the scatter- ing coefficients of terahertz surface plasmon polaritons impinging on a finite array of subwavelength grooves on semiconductor surfa- ces, Phys Rev B73 (2006), 205410. 8. X.-Y. He, Comparison of the waveguide properties of gap surface plasmon in the terahertz region and visible spectra, J Opt A: Pure Appl Opt 11 (2009), 045708. 9. P. Ruffieux, Y. Scharf, H.P. Herzig, R. Vo ¨lkel, and K.J. Weible, On the chromatic aberration of microlenses, Opt Express 14 (2006), 4687–4694. 10. T. Thio, H.F. Ghaemi, H.J. Lezec, P.A. Wolff, and T.W. Ebbesen, Surface-plasmon-enhanced transmission through hole arrays in Cr films, J Opt Soc Am B 16 (1999), 1743–1748. V C 2010 Wiley Periodicals, Inc. ANALYZING IMPEDANCE CHARACTERISTICS OF MONOPOLE ANTENNAS WITH DIFFERENT CHASSIS WIDTHS FOR DUAL RESONANCE-BASED BROADBAND OPERATION Jinwoo Jung, 1 Hyeonjin Lee, 2 and Yeongseog Lim 1 1 Department of Electronics Engineering, Chonnam National University, Gwang-ju, Korea; Corresponding author: crazytis@hanmail.net 2 Department of Electrical and Electronics, Dongkang College, Gwang-ju, Korea Received 2 July 2009 ABSTRACT: Variation of the impedance characteristics between two monopole antennas, according to the width of chassis is analyzed. Dual resonance-based broadband antennas are proposed on the basis of the analyzed results. The experimental antennas are an inverted L-shaped monopole antenna and a printed meander line monopole antenna. The dimensions of the derived antennas with a chassis are appropriate for a USB dongle application. V C 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 981–984, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25082 Key words: broadband; chassis; dual resonance; monopole; antenna 1. INTRODUCTION Recently, there has been increasing demand for various types of multimedia services of transceiver units in wireless communica- tion systems. This trend has brought about considerable interest in the development of broadband antennas [1]. The representative method for broadband operation of antenna is to use dual resonance [2–4]. For a dual resonance- based broadband antenna, the impedance characteristics of an antenna are very important. These characteristics are strongly influenced by the geometry of the radiating elements. Further- more, when the antenna is mounted on a chassis (ground plane or circuit board) of the wireless device, the impedance charac- teristics of the antenna depend strongly on the size of the chas- sis and the position of the antenna on it. As the combined behavior of the antenna element and device chassis determines the performance, the chassis dimensions are an essential compo- nent of mobile device antenna design [4–7]. In this article, addressing broadband operation with dual res- onance of mobile device antennas, we analyzed the variation of the impedance characteristics between two monopole antennas, according to the width of the chassis. To observe the effect of Figure 4 The phase distributions according to the slit location and width with four different temperatures. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com] DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 4, April 2010 981