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Development of high power THz-TDS system based on S-band compact electron linac
ARTICLE IN PRESS Radiation Physics and Chemistry 77 (2008) 1131– 1135
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Development of high power THz-TDS system based on S-band compact electron linac R. Kuroda a,, N. Sei a, T. Oka b, M. Yasumoto a, H. Toyokawa a, H. Ogawa a, M. Koike a, K. Yamada a, F. Sakai c a
Research Institute of Instrumentation Frontier, National Institute of Advanced Industrial Science and Technology (AIST), Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan c Sumitomo Heavy Industries, Ltd. (SHI), 2-1-1 Yatocho, Nishitokyo, Tokyo 188-8585, Japan b
a r t i c l e in fo
Keywords: Terahertz Time domain spectroscopy THz-TDS Linac Electron beam Laser
abstract The high power terahertz (THz)-time domain spectroscopy (TDS) system has been designed based on Sband compact electron linac at Advanced Industrial Science and Technology (AIST). The THz pulse is expected to have the peak power of about 25 kW with frequency range 0.1–2 THz using the 40 MeV electron beam which has about 1 nC bunch charge with 300 fs bunch length (rms). The aptitude discussion of the EO sampling method with ZnTe crystal was accomplished to apply to our THz-TDS system. The preliminary experiment of the absorption measurements of P-PPV on the Si wafer has been successfully demonstrated using the 0.1 THz coherent synchrotron radiation (CSR) pulse and W-band rf detector. It is confirmed that the intense of the THz pulse is enough to perform the THz-TDS analysis of the sample on the Si wafer. In near future, the investigation of the un-researched materials will be started in the frequency range 0.1–2 THz with our high power THz-TDS system. & 2008 Elsevier Ltd. All rights reserved.
1. Introduction The terahertz (THz) radiation is a useful tool for progressing on biomedical and material studies (Mittleman et al., 1996). Especially, THz-time domain spectroscopy (THz-TDS) has recently emerged as a powerful probe of charge transport in materials, owing to the fact that it provides a probe of the complex conductivity in a wide frequency range with sub-picosecond time resolution (Kawayama et al., 2002). However, even if the conventional laser-driven THz source has a high repetition rate about 100 MHz, its power is quite low about 1 mW which corresponds to pulse energy of about 1017 J/pulse (10 aJ/pulse) with pulse length of about 1 ps. Typical peak power of THz pulse is about 10 mW, so that the investigation of the sample that has large absorption in the THz region is not practical. The high peak power THz source is required instead of the laser-based THz source. Especially, it is difficult to investigate liquid materials using the low power THz-TDS because water is opaque to THz. Consequently, the high power THz-TDS is necessary to measure the complex refractive index of the un-researched electronic materials such as conductive polymers in liquid state and to investigate the spectral fingerprints of the biological samples and so on. While optical rectification has long provided an accessible means to generate terahertz pulses (Chang et al., 2006; Reimann Corresponding author. Tel.: +81 29 861 5104; fax: +81 29 861 5683.
E-mail address: [email protected] (R. Kuroda). 0969-806X/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2008.05.009
et al., 2003), their energies have been well under 100 nJ. The free electron laser sources have been able to generate high power terahertz pulses that have at least 1 mJ of energy (Knippels et al., 1999). The generation of terahertz pulses with 1.5 mJ of singlecycle terahertz pulses using about 50 mJ/pulse Ti:sapphire laser (Blanchard et al., 2007), but the total system is quite large. The generation of single-cycle terahertz pulses via four-wave mixing of the fundamental and the second harmonic of 25 fs pulses from a Ti:sapphire amplifier in air plasma were recently reported (Bartel et al., 2005). Until recently, such high power THz source is not applied to the TDS. On the other hand, coherent synchrotron radiation (CSR)-based THz source, which has high peak power of kW-order has been designed for the high power THz-TDS system using the S-band compact electron linac at National Institute of Advanced Industrial Science and Technology (AIST) in Japan. The CSR THz pulse can be generated in a wavelength between 150 mm and 3 mm (0.1–2 THz) using ultra short electron bunch with a bunch length of 300 fs (rms) and energy of 40 MeV.
2. Feasibility study 2.1. System design of THz-TDS based on S-band compact linac In our concept design for the high power THz-TDS, total system should be compact and installed in a room of middle size about
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10 m 10 m including all components. The 40 MeV compact S-band linac has a electron injector, an achromatic arc section for the bunch compressor, a 901 bending magnet and laser systems. The electron injector consists of a laser photocathode rf gun which has the BNL-type S-band 1.6 cell cavity with Cs2Te photocathode load-lock system and a solenoid magnet for the emittance compensation. It can generate the low-emittance electron beam with more than 1 nC. The electron beam can be accelerated up to about 40 MeV using S-band linac and the rf source of a 20 MW klystron. The electron beam is compressed down to 300 fs using the magnetic bunch compressor. The CSR of THz region is generated from the ultra short and high charge electron bunch at the 901 bending magnet located after Q-triplet downstream from the bunch compressor. The THz CSR pulse is extracted from the z-cut quartz window for the THz-TDS. Fig. 1 shows a top view of the THz-TDS system based on S-band compact electron linac. 2.2. Theoretical and experimental THz CSR generation Synchrotron radiation less than critical frequency oc is coherently emitted from a ultra short electron bunch (sz). Its frequency is expressed by oc ¼ pc=sz .
Fig. 2. Enhancement factor of CSR as a function of frequency by changing electron rms bunch length (500, 300, 100 fs, incoherent radiation).
(1)
The total photons (Itot) with both of incoherent and coherent radiation are derived from equations Itot ¼ Iinc ð1 þ ðN 1Þf ðoÞÞ
(2)
and 2
f ðoÞ ¼ eðosz Þ
=2
.
(3)
Here, Iinc is the photons of incoherent radiation, N is the number of electrons in the bunch and f(o) is the Fourier transform of the longitudinal electron density for Gaussian bunches with bunch length sz (Blum et al., 1991). In Fig. 2, the enhancement factor as a function of frequency, Itot/Iinc, was calculated by changing the electron bunch length from 100 to 500 fs with 1 nC and 40 MeV against the in CSR yield of about 0.1 THz which is normalized to 1.
Fig. 3. Detected signal of W-band detector.
In Fig. 3, about 0.1 THz CSR radiation generated from 40 MeV (g ¼ 78) electron bunch with less than 500 fs and 1 nC was estimated to be about 2.5 pJ/mm2/pulse at 60 cm down stream from a radiation point by a W-band rf detector (WiseWave FAS-10SF-01) which has a sensitive area of 1 mm 2 mm, the sensitive range 0.075–0.11 THz and its signal of 500 mV corresponds to 1 mW. As a result of Fig. 2, the total energy of extracted THz CSR with a range 0.1–2 THz was estimated to be about 5 nJ/pulse within area of about 200 mm2 at 60 cm because the synchrotron radiation has divergence of 1/g. In case of 300 fs electron bunch, we can obtain the total energy of about 65 nJ with range 0.1–2 THz and its peak power is estimated about 25 kW.
2.3. THz-TDS system with EO sampling method
Fig. 1. THz-TDS system based on S-band linac.
In our design of the THz-TDS system (Fig. 4), generated THz pulse is extracted from the z-cut quartz window and focused to a sample using off-axis parabolic mirrors. Transmitted THz pulse is
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Fig. 4. THz-TDS scheme.
detected by EO sampling method using the femtosecond Ti:Sa laser and EO crystal, and the THz pulse temporal waveform can be measured by the pump–probe method. The spectrum and the phase information of the sample is obtained by the Fourier transform of the waveform. The EO crystal should have highspeed time response against such ultra short THz pulse. Nahata et al. had already measured about 270 fs (FWHM) THz pulse by EO sampling method with a [11 0]-oriented ZnTe crystal in 1996 so that its time response was enough high speed and the ZnTe crystal could be applied for our design. A fs-Ti:sapphire laser is used as a probe, which is chirped by a grating stretcher, amplified by a regenerative amplifier and compressed to about 50 fs by a grating compressor. This probe pulse passes through an optical delay stage and Gran Laser Prism (GLP) for high linearly polarization. The linearly polarized probe pulse and the THz pulse are co-propagated through a [11 0]oriented ZnTe crystal. The transmitted probe pulse passes through a quarter wave plate and a polarizer analyzer. The ZnTe crystal as EO transceiver for THz wave was established by Chen et al. (2001). The probe transmission of the polarizer is T ¼ (1+sin G/2), where G is magnitude of induced phase retardation expressed by G¼
2pL 3 pETHz L n g41 ETHz ¼ . l V l=2
(4)
Here, l is the probe wavelength, L is the crystal length, n is the probe refractive index, g41 is the EO coefficient involved in the Pockels effect, ETHz is the THz electric field, and Vl/2 is the halfwave voltage of about 3 kV at 800 nm. In this equation, the phase retardation is increased with the crystal length L, but the length is limited by the phase matching and the coherence length between the THz pulse and the probe pulse (Nahata et al., 1996). The phase matching condition for the optical rectification process in the EO crystal is given by Dk ¼ kðo þ oTHz Þ kðoÞ kðoTHz Þ ¼ 0,
(5)
where o and oTHz are the probe and THz frequencies, respectively, and o and (o+oTHz) lie within the spectrum of the optical pulse. An equivalent equation can be written for EO sampling method and the coherence length lc ( ¼ p/Dk) is expressed with the dispersion in the optical spectral range by lc ¼
pc pc ¼ oTHz neff nTHz . dn oTHz n l nTHz dl
Fig. 5. Coherence length of ZnTe crystal as a function of THz frequency w/ and w/o probe dispersion.
where f (oTHz/2p) is THz frequency in THz unit. The coherence length as a function of THz frequency for ZnTe crystal with and without the probe dispersion is shown in Fig. 5. The crystal length should be less than about 2.7 mm for the 0.1–2 THz detection. In our system, the maximum phase retardation of probe pulse is estimated about p/2 with THz pulse (0.1–2 THz) of about 25 kW peak power corresponding to about 100 kW/cm2 peak power density which is obtained by focusing the probe pulse to about 3 mm radius on the ZnTe crystal of about 2.7 mm length. The limited detected spectrum is related with the time resolution in the time domain and less than 500 fs time resolution should be required for the 0.1–2 THz frequency region, but it is easily achieved by our system. 2.4. Synchronization between THz pulse and probe pulse The temporal synchronization and quite low jitter between the THz pulse and the probe pulse are strictly required for the accelerator-based THz-TDS because the source deference between THz and probe laser comparing with the laser-based THz source. The synchronization has been accomplished by the synchronization between the driving laser for the rf gun and the probe laser. The driving laser for the electron beam generation and the probe laser are the same mode-lock frequency of 79.3 MHz, which is 1/36 of 2856 MHz for the electron accelerator. Two lasers were synchronized to the master with 36th harmonics signal of their repetition frequencies (79.3 MHz) by a phase-locked loop (PLL) feedback control against the 2856 MHz accelerator frequency. Consequently, the relative timing jitter have been obtained less than 10 fs for the 150 fs laser Compton X-ray generation (Sakai et al., 2003) and it is also enough for the THz-TDS. 2.5. Numerical calculation sample model of THz-TDS
(6)
Here, c is the speed of light, nTHz is THz refractive index, and neff is the effective refractive index of probe pulse, which is estimated to be about 3.22. The THz refractive index is obtained from qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 nTHz ¼ ð289:27 6f Þ=ð29:16 f Þ, (7)
To estimate the dynamic range and the resolution of the EO sampling method, we assumed the sample, which has the absorption in around 0.5 THz and the reference THz spectrum generated from the electron bunch with 40 MeV, 1 nC, 300 fs in Fig. 2. Fig. 6 shows the reference and transmission THz spectra in a range 0.01–2 THz. Fig. 7 shows the temporal waveform of the reference and transmission THz pulses calculated by the Fourier
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Fig. 8. Setup of the absorption measurement with 0.1 THz CSR pulse using quarts window, parabolic antenna, W-band wave guide and detector.
Fig. 6. Reference and sample transmission THz pulse spectra assumed for calculation.
Fig. 9. Signals of W-band detector of THz raw signal, Si wafer and Si wafer with P-PPV transmission signals.
Fig. 7. Calculated temporal waveform of reference and transmission THz pulse.
transform of the spectra. As a result, the dynamic range of EO sampling detector required is more than 8 bit and the resolution as a time step of the pomp–probe technique is enough about 500 fs corresponding with about 75 mm of the optical delay stage.
signal of 500 mV corresponded to 1 mW for about 0.1 THz radiation. In this experiment, Si wafer /1 0 0S with 0.5 mm thickness the sample is the precursor polyphenylene vinylene (P-PPV) (poly[p-xylene tetrahydro-thiophenium chloride]) of 2.5 wt% solution in water and spin-coated about less than 100 nm on the Si wafer with 500 rpm, 10 s. The PPV is one of the conductive polymers and the THz-TDS analysis of PPV is expected for measurements of its complex refractive index in near future. 3.2. Results
3. Preliminary experimental 3.1. Setup for the absorption measurement of 0.1 THz To demonstrate the absorption measurements of Si wafer and some layered sample against the THz region wave, the preliminary experiment has been performed using 0.1 THz CSR with the Wband detector. Fig. 8 shows the setup of the absorption measurement using 0.1 THz CSR. The THz pulse was extracted from a z-cut crystal quartz window located at 201 direction in the 901 bending magnet and collected by the parabolic antenna and passed through a W-band wave guide (WR-10), an E-bend and a sample space. The pulse was guided to the W-band rf detector whose
Fig. 9 shows results of the absorption measurements of P-PPV on the Si wafer against the 0.1 THz pulse using the W-band detector. As a result, the absorption of the Si wafer and sample were observed about 50% and 5%, respectively. It is found that the intensity of the THz pulse is strong enough to perform the THz-TDS analysis of the sample on the Si wafer.
4. Summary The design of the high power THz-TDS system based on S-band compact electron linac has been accomplished. The THz pulse has
ARTICLE IN PRESS R. Kuroda et al. / Radiation Physics and Chemistry 77 (2008) 1131–1135
been expected to have the peak power of about 25 kW with frequency range 0.1–2 THz using the 40 MeV electron beam which has about 1 nC bunch charge with 300 fs bunch length (rms). The EO sampling method with ZnTe crystal is suitable for our THz-TDS system and the synchronization between the electron beam and the probe laser of our system is enough for the low timing jitter measurements. The preliminary experiment of the absorption measurements of P-PPV on the Si wafer has been successfully demonstrated using the 0.1 THz CSR pulse and W-band rf detector. It is found that the intense of the THz pulse is enough to perform the THz-TDS analysis of the sample on the Si wafer. In near future, we will complete all components installation for the high power THz-TDS system and start the investigation of the un-researched materials in the frequency range 0.1–2 THz.
Acknowledgments Authors would like to thank KEK-ATF and Waseda University members for their deep help on development of the Cs2Te rf gun system. References Bartel, T., Gaal, P., Reimann, K., Woerner, M., Elsaesser, T., 2005. Generation of single-cycle THz transients with high electric-field amplitudes. Opt. Lett. 30, 2805–2807.
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Blanchard, F., Razzari, L., Bandulet, H.-C., Sharma, G., Morandotti, R., Kieffer, J.-C., Ozaki, T., Reid, M., Tiedje, H.F., Haugen, H.K., Hegmann, F.A., 2007. Generation of 1.5 mJ single-cycle terahertz pulses by optical rectification from a large aperture ZnTe crystal. Opt. Express 15, 13212–13220. Blum, E.B., Happek, U., Sievers, A.J., 1991. Observation of coherent synchrotron radiation at the Cornell linac. Nucl. Instr. Methods A 307, 568–576. Chang, G., Divin, C.J., Liu, C.-H., Williamson, S.L., Galvanauskas, A., Norris, T.B., 2006. Power scalable compact THz system based on an ultrafast Yb-doped fiber amplifier. Opt. Express 14, 7909–7913. Chen, Q., Tani, M., Jiang, Z., Zhang, X.C., 2001. Electro-optic transceivers for terahertz-wave applications. Opt. Soc. Am. B 18, 823–831. Kawayama, I., Kotani, K., Tonouchi, M., 2002. Time-domain terahertz spectroscopy of SrBi2Ta2O9 thin films on MgO substrates. Jpn. J. Appl. Phys. Part 1 41, 6803–6805. Knippels, G.M.H., Yan, X., MacLeod, A.M., Gillespie, W.A., Yasumoto, M., Oepts, D., Meer, A.F.G.van der, 1999. Generation and complete electric-field characterization of intense ultrashort tunable far-infrared laser pulses. Phys. Rev. Lett. 83, 1578–1581. Mittleman, D.M., Jacobsen, R.H., Nuss, M.C., 1996. T-ray imaging. IEEE J. Sel. Top. Quantum Electron. 2, 679–692. Nahata, A., Welling, A.S., Heinz, T.F., 1996. A wideband coherent terahertz spectroscopy system using optical rectification and electro-optic sampling. Appl. Phys. Lett. 69 (116), 2321–2323. Reimann, K., Smith, R.P., Weiner, A.M., Elsaesser, T., Woerner, M., 2003. Direct fieldresolved detection of terahertz transients with amplitudes of megavolts per centimeter. Opt. Lett. 28, 471–473. Sakai, F., Ito, S., Yanagida, T., Yang, J., Yorozu, M., 2003. Status of laser-compton X-ray generation project for FESTA. In: Proceeding of SPIE. 48th Annual Meeting, San Diego, pp. 156–162.
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Development of high power THz-TDS system based on S-band compact electron linac R. Kuroda a,, N. Sei a, T. Oka b, M. Yasumoto a, H. Toyokawa a, H. Ogawa a, M. Koike a, K. Yamada a, F. Sakai c a
Research Institute of Instrumentation Frontier, National Institute of Advanced Industrial Science and Technology (AIST), Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan c Sumitomo Heavy Industries, Ltd. (SHI), 2-1-1 Yatocho, Nishitokyo, Tokyo 188-8585, Japan b
a r t i c l e in fo
Keywords: Terahertz Time domain spectroscopy THz-TDS Linac Electron beam Laser
abstract The high power terahertz (THz)-time domain spectroscopy (TDS) system has been designed based on Sband compact electron linac at Advanced Industrial Science and Technology (AIST). The THz pulse is expected to have the peak power of about 25 kW with frequency range 0.1–2 THz using the 40 MeV electron beam which has about 1 nC bunch charge with 300 fs bunch length (rms). The aptitude discussion of the EO sampling method with ZnTe crystal was accomplished to apply to our THz-TDS system. The preliminary experiment of the absorption measurements of P-PPV on the Si wafer has been successfully demonstrated using the 0.1 THz coherent synchrotron radiation (CSR) pulse and W-band rf detector. It is confirmed that the intense of the THz pulse is enough to perform the THz-TDS analysis of the sample on the Si wafer. In near future, the investigation of the un-researched materials will be started in the frequency range 0.1–2 THz with our high power THz-TDS system. & 2008 Elsevier Ltd. All rights reserved.
1. Introduction The terahertz (THz) radiation is a useful tool for progressing on biomedical and material studies (Mittleman et al., 1996). Especially, THz-time domain spectroscopy (THz-TDS) has recently emerged as a powerful probe of charge transport in materials, owing to the fact that it provides a probe of the complex conductivity in a wide frequency range with sub-picosecond time resolution (Kawayama et al., 2002). However, even if the conventional laser-driven THz source has a high repetition rate about 100 MHz, its power is quite low about 1 mW which corresponds to pulse energy of about 1017 J/pulse (10 aJ/pulse) with pulse length of about 1 ps. Typical peak power of THz pulse is about 10 mW, so that the investigation of the sample that has large absorption in the THz region is not practical. The high peak power THz source is required instead of the laser-based THz source. Especially, it is difficult to investigate liquid materials using the low power THz-TDS because water is opaque to THz. Consequently, the high power THz-TDS is necessary to measure the complex refractive index of the un-researched electronic materials such as conductive polymers in liquid state and to investigate the spectral fingerprints of the biological samples and so on. While optical rectification has long provided an accessible means to generate terahertz pulses (Chang et al., 2006; Reimann Corresponding author. Tel.: +81 29 861 5104; fax: +81 29 861 5683.
E-mail address: [email protected] (R. Kuroda). 0969-806X/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2008.05.009
et al., 2003), their energies have been well under 100 nJ. The free electron laser sources have been able to generate high power terahertz pulses that have at least 1 mJ of energy (Knippels et al., 1999). The generation of terahertz pulses with 1.5 mJ of singlecycle terahertz pulses using about 50 mJ/pulse Ti:sapphire laser (Blanchard et al., 2007), but the total system is quite large. The generation of single-cycle terahertz pulses via four-wave mixing of the fundamental and the second harmonic of 25 fs pulses from a Ti:sapphire amplifier in air plasma were recently reported (Bartel et al., 2005). Until recently, such high power THz source is not applied to the TDS. On the other hand, coherent synchrotron radiation (CSR)-based THz source, which has high peak power of kW-order has been designed for the high power THz-TDS system using the S-band compact electron linac at National Institute of Advanced Industrial Science and Technology (AIST) in Japan. The CSR THz pulse can be generated in a wavelength between 150 mm and 3 mm (0.1–2 THz) using ultra short electron bunch with a bunch length of 300 fs (rms) and energy of 40 MeV.
2. Feasibility study 2.1. System design of THz-TDS based on S-band compact linac In our concept design for the high power THz-TDS, total system should be compact and installed in a room of middle size about
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10 m 10 m including all components. The 40 MeV compact S-band linac has a electron injector, an achromatic arc section for the bunch compressor, a 901 bending magnet and laser systems. The electron injector consists of a laser photocathode rf gun which has the BNL-type S-band 1.6 cell cavity with Cs2Te photocathode load-lock system and a solenoid magnet for the emittance compensation. It can generate the low-emittance electron beam with more than 1 nC. The electron beam can be accelerated up to about 40 MeV using S-band linac and the rf source of a 20 MW klystron. The electron beam is compressed down to 300 fs using the magnetic bunch compressor. The CSR of THz region is generated from the ultra short and high charge electron bunch at the 901 bending magnet located after Q-triplet downstream from the bunch compressor. The THz CSR pulse is extracted from the z-cut quartz window for the THz-TDS. Fig. 1 shows a top view of the THz-TDS system based on S-band compact electron linac. 2.2. Theoretical and experimental THz CSR generation Synchrotron radiation less than critical frequency oc is coherently emitted from a ultra short electron bunch (sz). Its frequency is expressed by oc ¼ pc=sz .
Fig. 2. Enhancement factor of CSR as a function of frequency by changing electron rms bunch length (500, 300, 100 fs, incoherent radiation).
(1)
The total photons (Itot) with both of incoherent and coherent radiation are derived from equations Itot ¼ Iinc ð1 þ ðN 1Þf ðoÞÞ
(2)
and 2
f ðoÞ ¼ eðosz Þ
=2
.
(3)
Here, Iinc is the photons of incoherent radiation, N is the number of electrons in the bunch and f(o) is the Fourier transform of the longitudinal electron density for Gaussian bunches with bunch length sz (Blum et al., 1991). In Fig. 2, the enhancement factor as a function of frequency, Itot/Iinc, was calculated by changing the electron bunch length from 100 to 500 fs with 1 nC and 40 MeV against the in CSR yield of about 0.1 THz which is normalized to 1.
Fig. 3. Detected signal of W-band detector.
In Fig. 3, about 0.1 THz CSR radiation generated from 40 MeV (g ¼ 78) electron bunch with less than 500 fs and 1 nC was estimated to be about 2.5 pJ/mm2/pulse at 60 cm down stream from a radiation point by a W-band rf detector (WiseWave FAS-10SF-01) which has a sensitive area of 1 mm 2 mm, the sensitive range 0.075–0.11 THz and its signal of 500 mV corresponds to 1 mW. As a result of Fig. 2, the total energy of extracted THz CSR with a range 0.1–2 THz was estimated to be about 5 nJ/pulse within area of about 200 mm2 at 60 cm because the synchrotron radiation has divergence of 1/g. In case of 300 fs electron bunch, we can obtain the total energy of about 65 nJ with range 0.1–2 THz and its peak power is estimated about 25 kW.
2.3. THz-TDS system with EO sampling method
Fig. 1. THz-TDS system based on S-band linac.
In our design of the THz-TDS system (Fig. 4), generated THz pulse is extracted from the z-cut quartz window and focused to a sample using off-axis parabolic mirrors. Transmitted THz pulse is
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Fig. 4. THz-TDS scheme.
detected by EO sampling method using the femtosecond Ti:Sa laser and EO crystal, and the THz pulse temporal waveform can be measured by the pump–probe method. The spectrum and the phase information of the sample is obtained by the Fourier transform of the waveform. The EO crystal should have highspeed time response against such ultra short THz pulse. Nahata et al. had already measured about 270 fs (FWHM) THz pulse by EO sampling method with a [11 0]-oriented ZnTe crystal in 1996 so that its time response was enough high speed and the ZnTe crystal could be applied for our design. A fs-Ti:sapphire laser is used as a probe, which is chirped by a grating stretcher, amplified by a regenerative amplifier and compressed to about 50 fs by a grating compressor. This probe pulse passes through an optical delay stage and Gran Laser Prism (GLP) for high linearly polarization. The linearly polarized probe pulse and the THz pulse are co-propagated through a [11 0]oriented ZnTe crystal. The transmitted probe pulse passes through a quarter wave plate and a polarizer analyzer. The ZnTe crystal as EO transceiver for THz wave was established by Chen et al. (2001). The probe transmission of the polarizer is T ¼ (1+sin G/2), where G is magnitude of induced phase retardation expressed by G¼
2pL 3 pETHz L n g41 ETHz ¼ . l V l=2
(4)
Here, l is the probe wavelength, L is the crystal length, n is the probe refractive index, g41 is the EO coefficient involved in the Pockels effect, ETHz is the THz electric field, and Vl/2 is the halfwave voltage of about 3 kV at 800 nm. In this equation, the phase retardation is increased with the crystal length L, but the length is limited by the phase matching and the coherence length between the THz pulse and the probe pulse (Nahata et al., 1996). The phase matching condition for the optical rectification process in the EO crystal is given by Dk ¼ kðo þ oTHz Þ kðoÞ kðoTHz Þ ¼ 0,
(5)
where o and oTHz are the probe and THz frequencies, respectively, and o and (o+oTHz) lie within the spectrum of the optical pulse. An equivalent equation can be written for EO sampling method and the coherence length lc ( ¼ p/Dk) is expressed with the dispersion in the optical spectral range by lc ¼
pc pc ¼ oTHz neff nTHz . dn oTHz n l nTHz dl
Fig. 5. Coherence length of ZnTe crystal as a function of THz frequency w/ and w/o probe dispersion.
where f (oTHz/2p) is THz frequency in THz unit. The coherence length as a function of THz frequency for ZnTe crystal with and without the probe dispersion is shown in Fig. 5. The crystal length should be less than about 2.7 mm for the 0.1–2 THz detection. In our system, the maximum phase retardation of probe pulse is estimated about p/2 with THz pulse (0.1–2 THz) of about 25 kW peak power corresponding to about 100 kW/cm2 peak power density which is obtained by focusing the probe pulse to about 3 mm radius on the ZnTe crystal of about 2.7 mm length. The limited detected spectrum is related with the time resolution in the time domain and less than 500 fs time resolution should be required for the 0.1–2 THz frequency region, but it is easily achieved by our system. 2.4. Synchronization between THz pulse and probe pulse The temporal synchronization and quite low jitter between the THz pulse and the probe pulse are strictly required for the accelerator-based THz-TDS because the source deference between THz and probe laser comparing with the laser-based THz source. The synchronization has been accomplished by the synchronization between the driving laser for the rf gun and the probe laser. The driving laser for the electron beam generation and the probe laser are the same mode-lock frequency of 79.3 MHz, which is 1/36 of 2856 MHz for the electron accelerator. Two lasers were synchronized to the master with 36th harmonics signal of their repetition frequencies (79.3 MHz) by a phase-locked loop (PLL) feedback control against the 2856 MHz accelerator frequency. Consequently, the relative timing jitter have been obtained less than 10 fs for the 150 fs laser Compton X-ray generation (Sakai et al., 2003) and it is also enough for the THz-TDS. 2.5. Numerical calculation sample model of THz-TDS
(6)
Here, c is the speed of light, nTHz is THz refractive index, and neff is the effective refractive index of probe pulse, which is estimated to be about 3.22. The THz refractive index is obtained from qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 nTHz ¼ ð289:27 6f Þ=ð29:16 f Þ, (7)
To estimate the dynamic range and the resolution of the EO sampling method, we assumed the sample, which has the absorption in around 0.5 THz and the reference THz spectrum generated from the electron bunch with 40 MeV, 1 nC, 300 fs in Fig. 2. Fig. 6 shows the reference and transmission THz spectra in a range 0.01–2 THz. Fig. 7 shows the temporal waveform of the reference and transmission THz pulses calculated by the Fourier
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Fig. 8. Setup of the absorption measurement with 0.1 THz CSR pulse using quarts window, parabolic antenna, W-band wave guide and detector.
Fig. 6. Reference and sample transmission THz pulse spectra assumed for calculation.
Fig. 9. Signals of W-band detector of THz raw signal, Si wafer and Si wafer with P-PPV transmission signals.
Fig. 7. Calculated temporal waveform of reference and transmission THz pulse.
transform of the spectra. As a result, the dynamic range of EO sampling detector required is more than 8 bit and the resolution as a time step of the pomp–probe technique is enough about 500 fs corresponding with about 75 mm of the optical delay stage.
signal of 500 mV corresponded to 1 mW for about 0.1 THz radiation. In this experiment, Si wafer /1 0 0S with 0.5 mm thickness the sample is the precursor polyphenylene vinylene (P-PPV) (poly[p-xylene tetrahydro-thiophenium chloride]) of 2.5 wt% solution in water and spin-coated about less than 100 nm on the Si wafer with 500 rpm, 10 s. The PPV is one of the conductive polymers and the THz-TDS analysis of PPV is expected for measurements of its complex refractive index in near future. 3.2. Results
3. Preliminary experimental 3.1. Setup for the absorption measurement of 0.1 THz To demonstrate the absorption measurements of Si wafer and some layered sample against the THz region wave, the preliminary experiment has been performed using 0.1 THz CSR with the Wband detector. Fig. 8 shows the setup of the absorption measurement using 0.1 THz CSR. The THz pulse was extracted from a z-cut crystal quartz window located at 201 direction in the 901 bending magnet and collected by the parabolic antenna and passed through a W-band wave guide (WR-10), an E-bend and a sample space. The pulse was guided to the W-band rf detector whose
Fig. 9 shows results of the absorption measurements of P-PPV on the Si wafer against the 0.1 THz pulse using the W-band detector. As a result, the absorption of the Si wafer and sample were observed about 50% and 5%, respectively. It is found that the intensity of the THz pulse is strong enough to perform the THz-TDS analysis of the sample on the Si wafer.
4. Summary The design of the high power THz-TDS system based on S-band compact electron linac has been accomplished. The THz pulse has
ARTICLE IN PRESS R. Kuroda et al. / Radiation Physics and Chemistry 77 (2008) 1131–1135
been expected to have the peak power of about 25 kW with frequency range 0.1–2 THz using the 40 MeV electron beam which has about 1 nC bunch charge with 300 fs bunch length (rms). The EO sampling method with ZnTe crystal is suitable for our THz-TDS system and the synchronization between the electron beam and the probe laser of our system is enough for the low timing jitter measurements. The preliminary experiment of the absorption measurements of P-PPV on the Si wafer has been successfully demonstrated using the 0.1 THz CSR pulse and W-band rf detector. It is found that the intense of the THz pulse is enough to perform the THz-TDS analysis of the sample on the Si wafer. In near future, we will complete all components installation for the high power THz-TDS system and start the investigation of the un-researched materials in the frequency range 0.1–2 THz.
Acknowledgments Authors would like to thank KEK-ATF and Waseda University members for their deep help on development of the Cs2Te rf gun system. References Bartel, T., Gaal, P., Reimann, K., Woerner, M., Elsaesser, T., 2005. Generation of single-cycle THz transients with high electric-field amplitudes. Opt. Lett. 30, 2805–2807.
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