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Terahertz emission properties from YBCO thin film log-periodic antennas
Physica C 362 (2001) 319±323
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Terahertz emission properties from YBCO thin ®lm log-periodic antennas Hitoshi Saijo a, Masahiko Morimoto a, Toshihiko Kiwa a,b, Masayoshi Tonouchi a,b,* a
Research Center for Superconductor Photonics, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan b CREST, Japan Science and Technology Corporation (JST), 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Received 23 August 2000
Abstract We study terahertz (THz) radiation properties from high-Tc superconductor thin ®lm log-periodic antennas. Three sets of wire grid polarizers are employed to characterize the frequency-dependent polarization of the THz beam. The polarized radiation has clear resonance at frequencies corresponding to the antenna structures. The polarization patterns are thus well explained by the antenna structures. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 07.57.)c; 74.76.Bz; 74.90.n; 85.25.)j Keywords: Terahertz radiation; Femtosecond laser; YBa2 Cu3 O7 d ; Log-periodic antenna; Polarization
1. Introduction Terahertz (THz) radiation from high-Tc superconductors excited with femtosecond optical pulses has opened a new research ®eld in microwave photonics [1,2]. Previously, we reported the radiation from YBa2 Cu3 O7 d (YBCO) thin ®lm dipole and bow-tie antennas. The radiation power from bow-tie antenna is about ten times larger than that from dipole antenna. The results indicate that the
* Corresponding author. Address: Research Center for Superconductor Photonics, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan. Tel.: +81-6-6879-7981; fax: +816-6879-7984. E-mail address: [email protected] (M. Tonouchi).
waveform and the power of the THz radiation strongly depend on the antenna structure. Up to the present, we have detected sub-lW radiation from the YBCO bow-tie antennas [3,4]. In the present work, we examine a log-periodic antenna structure as the THz beam emitter to increase the radiation power. Log-periodic antennas are a family of frequency independent antennas. Their large bandwidth is due to the repetition of a pattern [5]. However, there still exist unknown properties in the log-periodic antenna eect. Thus, we observe the THz radiation waveform from YBCO log-periodic antennas by time domain measurement using wire grid polarizers and characterize the radiation properties through the spectra obtained from the waveforms. We investigate the relationship between the antenna structure and the antenna polarization pattern.
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 6 9 6 - 7
320
H. Saijo et al. / Physica C 362 (2001) 319±323
2. Experimental Fig. 1 shows a self-complimentary log-periodic toothed planar antenna structure. The antenna is based on the bow-tie antenna with some teeth spread on both sides. The central angles of each tooth are de®ned as 45°. The nth tooth is characterized by an outer radius Rn and inner radius rn where rn =Rn 0:7, Rn 1 =Rn 0:49, the maximum and minimum outer radius being 700 and 82 lm, respectively. The antennas are coupled to a 30-lmwidth strip line at the center. A 100-nm-thick YBCO thin ®lm on an MgO substrate is patterned into the antenna using conventional photolithography and ion milling process. We employ three sets of wire grid polarizers to characterize the polarization of the THz radiation, which are located between the emitter and detector in a conventional THz beam measurement system. The ®rst wire grid is used to pick out the polarized radiation by changing the grid angle. We de®ne the antenna axis illustrated in Fig. 1 as the origin of rotation and measured the polarization from 0° to 180° with an interval of 5°. The 45° component of the radiation passed through the ®rst grid is picked out by the second wire grid. The third one is set to detect the radiation polarized parallel to the antenna direction of the detector. Since the LT-GaAs
dipole detector detects only those polarized in the antenna direction, the second grid is inserted to avoid divergence when the polarized emission is calculated from the observed amplitude at ®rst grid angles near 90°. The detected values are recalculated to the original amplitude by taking the second grid eect into account. The sample is cooled down to 17.6 K in a cryostat. The optical pulses are focused to modulate supercurrent at the center bridge of the antenna. The devices are biased with a current of 80 mA and THz radiation is excited by femtosecond laser pulses. The details of the experimental setup have been reported previously [2,6].
3. Results and discussion Fig. 2(a) shows typical waveforms radiated from the YBCO log-periodic antenna at various grid angles. The electromagnetic wave radiation is observable for a time period of more than 70 ps after arrival of the optical pulse, and their frequency spectra are extremely complicated because they contain the radiation due to the wave propagation along the antenna edges and its re¯ection at the corners. There exists a frequency sweep except the case at a grid angle of 0°; the THz radiation bursts at ®rst with high frequencies and is followed by the waves with low frequencies. This eect has been reported on the THz radiation from the semiconductor photoconductive switches with a spiral antenna and a log-periodic antenna [7,8]. Corresponding amplitude spectra are given in Fig. 2(b). We see a number of resonant peaks, which would originate in the resonance of the wave running along the antenna edges. The frequencies are generally explained by the resonance between the wavelength km and the structural length. It is well known that the log-periodic antenna is resonant when arc length ln is equal to km =2. They are given by ln
Fig. 1. A micrograph of the prepared YBCO thin ®lm logperiodic antenna.
p Rn r n 2 2
and
k0 km p em
with mean dielectric constant em 1 er =2, where er is the dielectric constant of the substrate,
H. Saijo et al. / Physica C 362 (2001) 319±323
321
Fig. 2. (a) Detected electrical pulses at various grid angles and (b) corresponding amplitude spectra.
and k0 is the free-space wavelength [7,9]. It has also been reported that the resonance exists when ln is given by pR=2 or 3pR=4 where R is Rn or rn [10]. Calculated frequencies agree with the resonance explained by the above model as indicated by the open triangles. Although some of the peaks can be explained as higher order of the resonance, there exist unexplainable peaks such as the one at 0.31 THz. Examples of the polarization patterns of the THz beam radiated from the log-periodic antennas are shown in Fig. 3 for the frequencies corresponding to the resonance with the arc lengths. The radius is the amplitude and the periphery is the grid angle. It is observed that the THz beam is strongly polarized perpendicular to the axis, which indicates that the resonance corresponding to the matching between the arc length and the wavelength is major component. Slight tilt in the polarization might be attributed to the coexistence of the wave resonating along the direction from the center to the far edges of the each tooth. It seems quite natural to assume that there exists a resonance between the center of the antenna and the tooth. Some of the frequencies as indicated by the closed triangles in Fig. 2(b) correspond to the resonance from the center to the far
Fig. 3. Polarization patterns at the resonant peaks corresponding to the wavelength equal to the arc length in the antenna structure. Types A±C correspond to the arc angles of 135°, 45°, and 90°, respectively.
edges of the each tooth. The resonant frequencies are calculated without modi®cation of the dielectric constant. This suggests that the resonance would originate in the impedance matching rather than the running waves. To con®rm this eect, we attached MgO hemispherical lens on the backside of the MgO
322
H. Saijo et al. / Physica C 362 (2001) 319±323
Fig. 4. (a) THz waveforms measured with the MgO lens at various grid angles and (b) corresponding amplitude spectra.
substrate, which enhances the radiation from the central area of the antenna with a diameter of 200 lm. Fig. 4(a) shows THz waveforms measured under the same conditions except the MgO lens. The waveforms are more simpli®ed as compared to those measured without the lens although the similar frequency sweep is still observable. Fig. 4(b) shows the corresponding frequency spectra. We see similar resonance at frequencies below 0.4 THz, which can be explained by a quarter of the wavelength equal to the distance between the
center and the far edge of the each tooth as indicated by the drop lines. Polarizations are summarized in Fig. 5(a) and (b), which notably diers from the ones measured without the MgO lens. The resonance agrees with the one coupled to the each tooth except a length of 118 lm. This is attributed to the fact that the radiation measured from the sample with the MgO lens contains the frequency component emitted from the bow-tie antenna with an antenna length of 82 lm and a bow angle of 90° at frequencies
Fig. 5. Polarization patterns at the resonant frequencies corresponding to the quarter wavelength equal to the length between the center and the far edge of the each tooth.
H. Saijo et al. / Physica C 362 (2001) 319±323
above 0.4 THz. The result indicates that there exists a resonance coupled along the diameter of the tooth of the log-periodic antennas. 4. Conclusions We observed THz radiation from YBCO thin ®lm log-periodic antennas, and studied its polarization using three sets of the wire grid polarizers. The waveform of the THz radiation from the antenna contains a distinctive frequency sweep. The spectra of the radiation have some geometric resonance at frequencies when the average arc length of the near and far edge of each tooth, or the arc length of either edge is equal to a quarter of the wavelength, corresponding to the wavelength equal to four times the outer radius of the teeth. It is also con®rmed that the beam is strongly polarized perpendicular to the axis, which indicates that the resonance corresponding to the matching between the arc length and the wavelength is major component in the THz beam. The complicated waveform and frequency spectra also suggest that there exists the resonance originating in the impedance matching rather than the running waves. Acknowledgements We are grateful to Prof. Masanori Hangyo for his persistent encouragement and helpful discus-
323
sions. M. Tonouchi acknowledges the partial support by a Grant-in-Aid for Scienti®c Research on Priority Areas (A) under grant no. 10142101 and a Grant-in-Aid for Scienti®c Research (B) under grant no. 12450146, from the Ministry of Education, Science, Sports, and Culture, Japan.
References [1] M. Hangyo, S. Tomozawa, Y. Murakami, M. Tonouchi, M. Tani, Z. Wang, K. Sakai, S. Nakashima, Appl. Phys. Lett. 69 (1996) 1948. [2] M. Tonouchi, M. Tani, Z. Wang, K. Sakai, S. Tomozawa, M. Hangyo, Y. Murakami, S. Nakashima, Jpn. J. Appl. Phys. 35 (1996) 2624. [3] M. Tani, M. Tonouchi, M. Hangyo, Z. Wang, N. Onodera, K. Sakai, Jpn. J. Appl. Phys. 36 (1997) 1984. [4] M. Tonouchi, M. Tani, Z. Wang, K. Sakai, M. Hangyo, N. Wada, Y. Murakami, IEEE Trans. Appl. Supercond. 7 (1997) 2913. [5] W.L. Stutzman, G.A. Thiele, Antenna Theory and Design, Wiley, 1981. [6] M. Morimoto, H. Saijo, M. Yamashita, M. Tonouchi, M. Hangyo, in: Y. Tanabe (Ed.), Advances in Superconductivity XII, Springer, Tokyo, 2000, p.1111. [7] M.M. Gitin, F.W. Wise, G. Arjavalingam, Y. Pastol, R.C. Compton, IEEE Trans. on Antennas and Propagation 42 (1994) 335. [8] Y. Pastol, G. Arjavalingam, J.M. Halbout, Electron. Lett. 26 (1990) 133. [9] H. Shimakage, Y. Uzawa, M. Tonouchi, Z. Wang, IEEE Trans. Appl. Supercond. 7 (1997) 2595. [10] J. Chen, E. Kobayashi, K. Nakajima, T. Yamashita, J. Zmuidzinas, Ext. Abs. of ISEC'99, 21±25 June, Berkeley, USA, 1999, p. 222.
www.elsevier.com/locate/physc
Terahertz emission properties from YBCO thin ®lm log-periodic antennas Hitoshi Saijo a, Masahiko Morimoto a, Toshihiko Kiwa a,b, Masayoshi Tonouchi a,b,* a
Research Center for Superconductor Photonics, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan b CREST, Japan Science and Technology Corporation (JST), 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Received 23 August 2000
Abstract We study terahertz (THz) radiation properties from high-Tc superconductor thin ®lm log-periodic antennas. Three sets of wire grid polarizers are employed to characterize the frequency-dependent polarization of the THz beam. The polarized radiation has clear resonance at frequencies corresponding to the antenna structures. The polarization patterns are thus well explained by the antenna structures. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 07.57.)c; 74.76.Bz; 74.90.n; 85.25.)j Keywords: Terahertz radiation; Femtosecond laser; YBa2 Cu3 O7 d ; Log-periodic antenna; Polarization
1. Introduction Terahertz (THz) radiation from high-Tc superconductors excited with femtosecond optical pulses has opened a new research ®eld in microwave photonics [1,2]. Previously, we reported the radiation from YBa2 Cu3 O7 d (YBCO) thin ®lm dipole and bow-tie antennas. The radiation power from bow-tie antenna is about ten times larger than that from dipole antenna. The results indicate that the
* Corresponding author. Address: Research Center for Superconductor Photonics, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan. Tel.: +81-6-6879-7981; fax: +816-6879-7984. E-mail address: [email protected] (M. Tonouchi).
waveform and the power of the THz radiation strongly depend on the antenna structure. Up to the present, we have detected sub-lW radiation from the YBCO bow-tie antennas [3,4]. In the present work, we examine a log-periodic antenna structure as the THz beam emitter to increase the radiation power. Log-periodic antennas are a family of frequency independent antennas. Their large bandwidth is due to the repetition of a pattern [5]. However, there still exist unknown properties in the log-periodic antenna eect. Thus, we observe the THz radiation waveform from YBCO log-periodic antennas by time domain measurement using wire grid polarizers and characterize the radiation properties through the spectra obtained from the waveforms. We investigate the relationship between the antenna structure and the antenna polarization pattern.
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 6 9 6 - 7
320
H. Saijo et al. / Physica C 362 (2001) 319±323
2. Experimental Fig. 1 shows a self-complimentary log-periodic toothed planar antenna structure. The antenna is based on the bow-tie antenna with some teeth spread on both sides. The central angles of each tooth are de®ned as 45°. The nth tooth is characterized by an outer radius Rn and inner radius rn where rn =Rn 0:7, Rn 1 =Rn 0:49, the maximum and minimum outer radius being 700 and 82 lm, respectively. The antennas are coupled to a 30-lmwidth strip line at the center. A 100-nm-thick YBCO thin ®lm on an MgO substrate is patterned into the antenna using conventional photolithography and ion milling process. We employ three sets of wire grid polarizers to characterize the polarization of the THz radiation, which are located between the emitter and detector in a conventional THz beam measurement system. The ®rst wire grid is used to pick out the polarized radiation by changing the grid angle. We de®ne the antenna axis illustrated in Fig. 1 as the origin of rotation and measured the polarization from 0° to 180° with an interval of 5°. The 45° component of the radiation passed through the ®rst grid is picked out by the second wire grid. The third one is set to detect the radiation polarized parallel to the antenna direction of the detector. Since the LT-GaAs
dipole detector detects only those polarized in the antenna direction, the second grid is inserted to avoid divergence when the polarized emission is calculated from the observed amplitude at ®rst grid angles near 90°. The detected values are recalculated to the original amplitude by taking the second grid eect into account. The sample is cooled down to 17.6 K in a cryostat. The optical pulses are focused to modulate supercurrent at the center bridge of the antenna. The devices are biased with a current of 80 mA and THz radiation is excited by femtosecond laser pulses. The details of the experimental setup have been reported previously [2,6].
3. Results and discussion Fig. 2(a) shows typical waveforms radiated from the YBCO log-periodic antenna at various grid angles. The electromagnetic wave radiation is observable for a time period of more than 70 ps after arrival of the optical pulse, and their frequency spectra are extremely complicated because they contain the radiation due to the wave propagation along the antenna edges and its re¯ection at the corners. There exists a frequency sweep except the case at a grid angle of 0°; the THz radiation bursts at ®rst with high frequencies and is followed by the waves with low frequencies. This eect has been reported on the THz radiation from the semiconductor photoconductive switches with a spiral antenna and a log-periodic antenna [7,8]. Corresponding amplitude spectra are given in Fig. 2(b). We see a number of resonant peaks, which would originate in the resonance of the wave running along the antenna edges. The frequencies are generally explained by the resonance between the wavelength km and the structural length. It is well known that the log-periodic antenna is resonant when arc length ln is equal to km =2. They are given by ln
Fig. 1. A micrograph of the prepared YBCO thin ®lm logperiodic antenna.
p Rn r n 2 2
and
k0 km p em
with mean dielectric constant em 1 er =2, where er is the dielectric constant of the substrate,
H. Saijo et al. / Physica C 362 (2001) 319±323
321
Fig. 2. (a) Detected electrical pulses at various grid angles and (b) corresponding amplitude spectra.
and k0 is the free-space wavelength [7,9]. It has also been reported that the resonance exists when ln is given by pR=2 or 3pR=4 where R is Rn or rn [10]. Calculated frequencies agree with the resonance explained by the above model as indicated by the open triangles. Although some of the peaks can be explained as higher order of the resonance, there exist unexplainable peaks such as the one at 0.31 THz. Examples of the polarization patterns of the THz beam radiated from the log-periodic antennas are shown in Fig. 3 for the frequencies corresponding to the resonance with the arc lengths. The radius is the amplitude and the periphery is the grid angle. It is observed that the THz beam is strongly polarized perpendicular to the axis, which indicates that the resonance corresponding to the matching between the arc length and the wavelength is major component. Slight tilt in the polarization might be attributed to the coexistence of the wave resonating along the direction from the center to the far edges of the each tooth. It seems quite natural to assume that there exists a resonance between the center of the antenna and the tooth. Some of the frequencies as indicated by the closed triangles in Fig. 2(b) correspond to the resonance from the center to the far
Fig. 3. Polarization patterns at the resonant peaks corresponding to the wavelength equal to the arc length in the antenna structure. Types A±C correspond to the arc angles of 135°, 45°, and 90°, respectively.
edges of the each tooth. The resonant frequencies are calculated without modi®cation of the dielectric constant. This suggests that the resonance would originate in the impedance matching rather than the running waves. To con®rm this eect, we attached MgO hemispherical lens on the backside of the MgO
322
H. Saijo et al. / Physica C 362 (2001) 319±323
Fig. 4. (a) THz waveforms measured with the MgO lens at various grid angles and (b) corresponding amplitude spectra.
substrate, which enhances the radiation from the central area of the antenna with a diameter of 200 lm. Fig. 4(a) shows THz waveforms measured under the same conditions except the MgO lens. The waveforms are more simpli®ed as compared to those measured without the lens although the similar frequency sweep is still observable. Fig. 4(b) shows the corresponding frequency spectra. We see similar resonance at frequencies below 0.4 THz, which can be explained by a quarter of the wavelength equal to the distance between the
center and the far edge of the each tooth as indicated by the drop lines. Polarizations are summarized in Fig. 5(a) and (b), which notably diers from the ones measured without the MgO lens. The resonance agrees with the one coupled to the each tooth except a length of 118 lm. This is attributed to the fact that the radiation measured from the sample with the MgO lens contains the frequency component emitted from the bow-tie antenna with an antenna length of 82 lm and a bow angle of 90° at frequencies
Fig. 5. Polarization patterns at the resonant frequencies corresponding to the quarter wavelength equal to the length between the center and the far edge of the each tooth.
H. Saijo et al. / Physica C 362 (2001) 319±323
above 0.4 THz. The result indicates that there exists a resonance coupled along the diameter of the tooth of the log-periodic antennas. 4. Conclusions We observed THz radiation from YBCO thin ®lm log-periodic antennas, and studied its polarization using three sets of the wire grid polarizers. The waveform of the THz radiation from the antenna contains a distinctive frequency sweep. The spectra of the radiation have some geometric resonance at frequencies when the average arc length of the near and far edge of each tooth, or the arc length of either edge is equal to a quarter of the wavelength, corresponding to the wavelength equal to four times the outer radius of the teeth. It is also con®rmed that the beam is strongly polarized perpendicular to the axis, which indicates that the resonance corresponding to the matching between the arc length and the wavelength is major component in the THz beam. The complicated waveform and frequency spectra also suggest that there exists the resonance originating in the impedance matching rather than the running waves. Acknowledgements We are grateful to Prof. Masanori Hangyo for his persistent encouragement and helpful discus-
323
sions. M. Tonouchi acknowledges the partial support by a Grant-in-Aid for Scienti®c Research on Priority Areas (A) under grant no. 10142101 and a Grant-in-Aid for Scienti®c Research (B) under grant no. 12450146, from the Ministry of Education, Science, Sports, and Culture, Japan.
References [1] M. Hangyo, S. Tomozawa, Y. Murakami, M. Tonouchi, M. Tani, Z. Wang, K. Sakai, S. Nakashima, Appl. Phys. Lett. 69 (1996) 1948. [2] M. Tonouchi, M. Tani, Z. Wang, K. Sakai, S. Tomozawa, M. Hangyo, Y. Murakami, S. Nakashima, Jpn. J. Appl. Phys. 35 (1996) 2624. [3] M. Tani, M. Tonouchi, M. Hangyo, Z. Wang, N. Onodera, K. Sakai, Jpn. J. Appl. Phys. 36 (1997) 1984. [4] M. Tonouchi, M. Tani, Z. Wang, K. Sakai, M. Hangyo, N. Wada, Y. Murakami, IEEE Trans. Appl. Supercond. 7 (1997) 2913. [5] W.L. Stutzman, G.A. Thiele, Antenna Theory and Design, Wiley, 1981. [6] M. Morimoto, H. Saijo, M. Yamashita, M. Tonouchi, M. Hangyo, in: Y. Tanabe (Ed.), Advances in Superconductivity XII, Springer, Tokyo, 2000, p.1111. [7] M.M. Gitin, F.W. Wise, G. Arjavalingam, Y. Pastol, R.C. Compton, IEEE Trans. on Antennas and Propagation 42 (1994) 335. [8] Y. Pastol, G. Arjavalingam, J.M. Halbout, Electron. Lett. 26 (1990) 133. [9] H. Shimakage, Y. Uzawa, M. Tonouchi, Z. Wang, IEEE Trans. Appl. Supercond. 7 (1997) 2595. [10] J. Chen, E. Kobayashi, K. Nakajima, T. Yamashita, J. Zmuidzinas, Ext. Abs. of ISEC'99, 21±25 June, Berkeley, USA, 1999, p. 222.