folded half-wavelength dipole antennas have higher antenna resistances than the full-wavelength dipole antenna and are able to achieve higher output powers.
TERAHERTZ FOLDED HALF-WAVELENGTH DIPOLE ANTENNA FOR HIGH OUTPUT POWER Kyungsik Moon, Haewook Han, and Ikmo Park† Department of Electrical and Computer Engineering Pohang University of Science and Technology San 31 Hoja-dong, Nam-gu, Pohang, 790-783, Korea †
Department of Electrical and Computer Engineering Ajou University 5 Wonchon-dong, Youngtong-gu, Suwon, 443-749, Korea Abstract — Photoconductive folded half-wavelength dipole antennas have been introduced and demonstrated successfully for high power THz applications. The pulsed terahertz signals are measured in the time-domain terahertz setup. Using the simple model, the measured THz signals were analyzed. The calculated signals are in good agreements with the measured signals. It is shown that the folded half-wavelength dipole antennas have higher antenna resistances than the full-wavelength dipole antenna and are able to achieve higher output powers. Index Terms — THz antenna, folded half-wavelength dipole antenna, photoconductive antenna, .
I. INTRODUCTION Recent advances in terahertz (THz) technology have made possible a new field of applications related to the spectroscopy, sensing, and imaging [1]. Despite the increasing use of THz systems in many research areas, the virtual absence of practical, compact THz sources has limited widespread use of THz technologies. Many efforts have been made to make practical sources in THz frequencies, but no THz sources concurrently satisfy size, output power level, and operating temperature requirements. A photoconductive antenna is an attractive THz source because of its compactness and wide tunability at room temperature [2-5]. However, the photoconductive antenna has the significant disadvantage of low output power. This is mainly due to the high impedance inherent to the photomixer. When an antenna with moderate input impedance is connected to a photomixer, the power transfer from the photomixer to the antenna is poor due to the severe impedance mismatching. Impedance matching between the photomixer and the antenna can be improved by increasing the antenna resistance, which has the result of increasing the power radiated from the antenna. Resonant types of antennas, such as full-wavelength dipole and slot antennas, have moderate antenna resistance and they have been used for low power CW applications where wide band operations are not needed [6, 7].
In this paper, we report the design and measurement of folded λ/2 dipole antennas for high output power in the THz region. It is shown that the folded λ/2 dipole antennas have much higher antenna resistance than the conventional full-wavelength dipole antenna, and therefore they are able to achieve higher output power. II. PHOTOCONDUCTIVE ANTENNA DESIGN There are several types of folded dipole antennas, all of which are characterized by high antenna resistance. Since photoconductive antennas need DC biases, antennas with closed loops cannot be used. Given this restriction, the folded λ/2 dipole antennas shown in Fig. 1 are suitable for photoconductive antenna applications. The currents, as indicated by arrows in Fig. 1, are equal in individual wires and in phase. Therefore, the current distribution of the folded λ/2 dipole antenna is similar to that of a simple λ/2 dipole antenna, as is the radiation pattern [8].
λ/2
(a)
λ/2
(b) Fig. 1. Schematics of (a) three-wire folded λ/2 dipole antenna and (b) five-wire folded λ/2 dipole antenna.
Fig. 2 shows the input resistances of the fullwavelength dipole antenna and the folded λ/2 dipole antennas. They are calculated using MWS, which is a commercially available electromagnetic (EM) simulator based on the method of finite integral time domain. Initially, all antennas were designed to have a resonant frequency at around 1.0 THz. At resonance, the input resistances of the full-wavelength dipole antenna, the three-wire folded λ/2 dipole antenna, and the five-wire folded λ/2 dipole antenna are 256 Ω, 517 Ω, and 640 Ω, respectively. The input resistance of the folded λ/2 dipole antennas are larger than that of the full-wavelength dipole antenna, and, as a result, the output power from the folded λ/2 dipole antennas are expected to be larger than that from the full-wavelength dipole antenna.
basis of the model, the radiated powers expected under CW operation are plotted in Fig. 7. The antennas are designed to operate at about 1 THz. It is expected that the radiation power peaks, which are proportional to the antenna resistances, will be observed at resonant frequencies. However, the result for the folded λ/2 dipole antennas are quite different than predicted. This is likely because the photocarriers of the photoconductive folded λ/2 dipole antennas may be generated between the wires due to the large spot size of the input beam. This results in added capacitance between the wires. The additional capacitance, which was not considered in the EM
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III. RESULTS AND DISCUSSIONS The antennas are fabricated on low-temperature-grown gallium arsenide (LT-GaAs) substrate with 1 µm thickness. The metal, composed of 0.05 µm thick titanium for adhesion and 0.3 µm thick gold, is 3 µm wide. The full-wavelength dipole antenna is 88 µm long and the folded λ/2 dipole antennas are 60 µm long. The wire spacing in the folded λ/2 dipole antennas are 2 µm. The photomixer is made up of 3 µm wide metal electrodes separated by a gap of 5 µm. Photographs of the fabricated folded λ/2 dipole antennas are shown in Fig. 3. The antennas were excited by a femtosecond laser as the preliminary stage of the continuous wave (CW) operation. A typical THz time-domain spectroscopy setup was used to measure the THz pulse signals [9, 10], and the measured detected signals are shown in Fig. 4. In order to analyze the measured signals, the simple analytical model is derived based on the equivalent circuit shown in Fig. 5 [6, 11]. Total radiated power from the antenna is given by Prad ( ω ) =
500
FWDA 3WFDA 5WFDA
400 300 200 100 0 0.0
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Frequency (THz) Fig. 2. Simulated input resistances for a full wavelength dipole antenna and three-wire folded dipole antenna.
2 1 i (ω ) 2
(a)
GL (ω ) 2
⎡⎣GL (ω ) + G ( ω ) ⎤⎦ + ⎡⎣ BL (ω ) + ω C ⎤⎦
2
(1)
where i (ω ) is the ideal current source, G(ω) is an output conductance, C is a shunt capacitance, and YL(ω) is a load admittance including the antenna and DC bias lines. GL(ω) and BL(ω) are the real and imaginary parts of YL(ω), respectively. The validity of the equation was proved by the bolometric measured data [6, 12]. Using Eq. (1), the detector currents are calculated in the frequency domain, including the detector responses [10]. Results of these calculations are shown in Fig. 6. The calculated signals are in good agreement with the measured signals, which indicates the validity of our analytical model. On the
(b) Fig. 3. Photographs of fabricated antennas. (a) Three-wire folded λ/2 dipole antenna. (b) Five-wire folded λ/2 dipole antenna.
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simulation, causes the position of the radiation power peak to move to the lower frequency with decreasing peak power value. We expect that this problem can be remedied by changing the physical dimensions of the folded λ/2 dipole antennas and tuning out the capacitances.
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(c) Fig. 6. Comparisons of the THz signals between the measured and the calculated signals. (a) Full-wavelength dipole antenna. (b) Three-wire folded λ/2 dipole antenna. (c) Five-wire folded λ/2 dipole antenna.
Fig. 5. Equivalent circuit for the photomixer coupled to the antenna.
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[5]
S. Matsuura, G. A. Blake, R. A. Wyss, J. C. Pearson, C. Kadow, A. W. Jackson, and A. C. Gossard, “A traveling-wave THz photomixer based on angle tuned phase matching,” Appl. Phys. Lett., vol. 74, pp. 2872-2874, May 1999.
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FWDA 3WFDA 5WFDA 0.0
1.0 0.0
0.5
1.0
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Frequency (THz) Fig. 7. Predicted radiation power from the antenna.
IV. CONCLUSION The photoconductive folded λ/2 dipole antennas have been introduced and its capacity for high power THz generation has been successfully demonstrated. The simple analytical model is derived based on the equivalent circuit, and the measured pulsed THz signals are shown to be in good agreement with the analytical model. The CW output powers from the antennas have been predicted by applying the simple analytical model. We expect from the experiments and analyses that the folded λ/2 dipole antennas are able to achieve higher output power than the full-wavelength dipole antenna.
[10] P. Uhd Jepsen, R. H. Jacobsen, and S. R. Keiding, “Generation and detection of terahertz pulses from biased semiconductor antennas,” J. Opt. Soc. Am. B, vol. 13, 2424-2436, 1996. [11] E. R. Brown, F. W. Smith, and K. A. McIntosh, “Coherent millimeter-wave generation by heterodyne conversion in low-temperature-grown GaAs photoconductors,” J. Appl. Phys., vol. 73, pp. 1480-1484, 1993.
ACKNOWLEDGEMENT This work was supported by the Korea Science and Engineering Foundation through the National Research Laboratory Program.
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S. Verghese, K. A. McIntosh, and E. R. Brown, “Optical and terahertz power limits in the lowtemperature-grown GaAs photomixers,” Appl. Phys. Lett., vol. 71, pp. 2743-2745, Nov. 1997.
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