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Radiation Physics and Chemistry 77 (2008) 1131– 1135

Contents lists available at ScienceDirect

Radiation Physics and Chemistry

0969-80

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/radphyschem

Development of high power THz-TDS system based on S-band compactelectron 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, Japanb National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japanc Sumitomo Heavy Industries, Ltd. (SHI), 2-1-1 Yatocho, Nishitokyo, Tokyo 188-8585, Japan

a r t i c l e i n f o

Keywords:

Terahertz

Time domain spectroscopy

THz-TDS

Linac

Electron beam

Laser

6X/$ - see front matter & 2008 Elsevier Ltd. A

016/j.radphyschem.2008.05.009

esponding author. Tel.: +8129 8615104; fax:

ail address: ryu-kuroda@aist.go.jp (R. Kuroda)

a b s t r a c t

The high power terahertz (THz)-time domain spectroscopy (TDS) system has been designed based on S-

band 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 onbiomedical and material studies (Mittleman et al., 1996).Especially, THz-time domain spectroscopy (THz-TDS) has recentlyemerged as a powerful probe of charge transport in materials,owing to the fact that it provides a probe of the complexconductivity in a wide frequency range with sub-picosecond timeresolution (Kawayama et al., 2002). However, even if theconventional laser-driven THz source has a high repetition rateabout 100 MHz, its power is quite low about 1 mW whichcorresponds to pulse energy of about 10�17 J/pulse (10 aJ/pulse)with pulse length of about 1 ps. Typical peak power of THz pulse isabout 10 mW, so that the investigation of the sample that haslarge absorption in the THz region is not practical. The high peakpower THz source is required instead of the laser-based THzsource. Especially, it is difficult to investigate liquid materialsusing the low power THz-TDS because water is opaque to THz.Consequently, the high power THz-TDS is necessary to measurethe complex refractive index of the un-researched electronicmaterials such as conductive polymers in liquid state and toinvestigate the spectral fingerprints of the biological samples andso on. While optical rectification has long provided an accessiblemeans to generate terahertz pulses (Chang et al., 2006; Reimann

ll rights reserved.

+8129 8615683.

.

et al., 2003), their energies have been well under 100 nJ. The freeelectron laser sources have been able to generate high powerterahertz pulses that have at least 1mJ of energy (Knippels et al.,1999). The generation of terahertz pulses with 1.5mJ of single-cycle terahertz pulses using about 50 mJ/pulse Ti:sapphire laser(Blanchard et al., 2007), but the total system is quite large. Thegeneration of single-cycle terahertz pulses via four-wave mixingof the fundamental and the second harmonic of 25 fs pulses froma Ti:sapphire amplifier in air plasma were recently reported(Bartel et al., 2005). Until recently, such high power THz source isnot applied to the TDS.

On the other hand, coherent synchrotron radiation (CSR)-basedTHz source, which has high peak power of kW-order has beendesigned for the high power THz-TDS system using the S-bandcompact electron linac at National Institute of Advanced IndustrialScience and Technology (AIST) in Japan. The CSR THz pulse can begenerated in a wavelength between 150mm 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 systemshould be compact and installed in a room of middle size about

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Fig. 2. Enhancement factor of CSR as a function of frequency by changing electron

rms bunch length (500, 300, 100 fs, incoherent radiation).

R. Kuroda et al. / Radiation Physics and Chemistry 77 (2008) 1131–11351132

10 m�10 m including all components. The 40 MeV compactS-band linac has a electron injector, an achromatic arc sectionfor the bunch compressor, a 901 bending magnet and lasersystems. The electron injector consists of a laser photocathode rfgun which has the BNL-type S-band 1.6 cell cavity with Cs2Tephotocathode load-lock system and a solenoid magnet for theemittance compensation. It can generate the low-emittanceelectron beam with more than 1 nC. The electron beam canbe accelerated up to about 40 MeV using S-band linac and therf source of a 20 MW klystron. The electron beam is compresseddown to 300 fs using the magnetic bunch compressor. The CSR ofTHz region is generated from the ultra short and high chargeelectron bunch at the 901 bending magnet located after Q-tripletdownstream from the bunch compressor. The THz CSR pulse isextracted from the z-cut quartz window for the THz-TDS. Fig. 1shows a top view of the THz-TDS system based on S-band compactelectron linac.

2.2. Theoretical and experimental THz CSR generation

Synchrotron radiation less than critical frequency oc iscoherently emitted from a ultra short electron bunch (sz). Itsfrequency is expressed by

oc ¼ pc=sz. (1)

The total photons (Itot) with both of incoherent and coherentradiation are derived from equations

Itot ¼ Iincð1þ ðN � 1Þf ðoÞÞ (2)

and

f ðoÞ ¼ e�ðoszÞ2=2. (3)

Here, Iinc is the photons of incoherent radiation, N is the numberof electrons in the bunch and f(o) is the Fourier transform of thelongitudinal electron density for Gaussian bunches with bunchlength sz (Blum et al., 1991). In Fig. 2, the enhancement factor as afunction of frequency, Itot/Iinc, was calculated by changing theelectron bunch length from 100 to 500 fs with 1 nC and 40 MeVagainst the in CSR yield of about 0.1 THz which is normalized to 1.

Fig. 1. THz-TDS system based on S-band linac.

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 wasestimated to be about 2.5 pJ/mm2/pulse at 60 cm down streamfrom a radiation point by a W-band rf detector (WiseWaveFAS-10SF-01) which has a sensitive area of 1 mm�2 mm, thesensitive range 0.075–0.11 THz and its signal of 500 mV corre-sponds to 1 mW. As a result of Fig. 2, the total energy of extractedTHz CSR with a range 0.1–2 THz was estimated to be about5 nJ/pulse within area of about 200 mm2 at 60 cm because thesynchrotron radiation has divergence of 1/g. In case of 300 fselectron bunch, we can obtain the total energy of about 65 nJ withrange 0.1–2 THz and its peak power is estimated about 25 kW.

2.3. THz-TDS system with EO sampling method

In our design of the THz-TDS system (Fig. 4), generated THzpulse is extracted from the z-cut quartz window and focused to asample using off-axis parabolic mirrors. Transmitted THz pulse is

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Fig. 4. THz-TDS scheme.

Fig. 5. Coherence length of ZnTe crystal as a function of THz frequency w/ and w/o

probe dispersion.

R. Kuroda et al. / Radiation Physics and Chemistry 77 (2008) 1131–1135 1133

detected by EO sampling method using the femtosecond Ti:Salaser and EO crystal, and the THz pulse temporal waveform can bemeasured by the pump–probe method. The spectrum and thephase information of the sample is obtained by the Fouriertransform of the waveform. The EO crystal should have high-speed time response against such ultra short THz pulse. Nahataet al. had already measured about 270 fs (FWHM) THz pulse byEO sampling method with a [110]-oriented ZnTe crystal in 1996so that its time response was enough high speed and the ZnTecrystal could be applied for our design.

A fs-Ti:sapphire laser is used as a probe, which is chirped by agrating stretcher, amplified by a regenerative amplifier andcompressed to about 50 fs by a grating compressor. This probepulse passes through an optical delay stage and Gran Laser Prism(GLP) for high linearly polarization. The linearly polarized probepulse and the THz pulse are co-propagated through a [110]-oriented ZnTe crystal. The transmitted probe pulse passes througha quarter wave plate and a polarizer analyzer. The ZnTe crystal asEO transceiver for THz wave was established by Chen et al. (2001).The probe transmission of the polarizer is T ¼ (1+sinG/2), where Gis magnitude of induced phase retardation expressed by

G ¼2pL

ln3g41ETHz ¼

pETHzL

Vl=2. (4)

Here, l is the probe wavelength, L is the crystal length, n is theprobe refractive index, g41 is the EO coefficient involved in thePockels effect, ETHz is the THz electric field, and Vl/2 is the half-wave voltage of about 3 kV at 800 nm. In this equation, the phaseretardation is increased with the crystal length L, but the length islimited by the phase matching and the coherence length betweenthe THz pulse and the probe pulse (Nahata et al., 1996). The phasematching condition for the optical rectification process in the EOcrystal is given by

Dk ¼ kðoþ oTHzÞ � kðoÞ � kðoTHzÞ 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 methodand the coherence length lc ( ¼ p/Dk) is expressed with thedispersion in the optical spectral range by

lc ¼pc

oTHz n� ldn

dl� nTHz

��������¼

pc

oTHz neff � nTHz

�� �� . (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 estimatedto be about 3.22. The THz refractive index is obtained from

nTHz ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið289:27� 6f 2

Þ=ð29:16� f 2Þ

q, (7)

where f (oTHz/2p) is THz frequency in THz unit. The coherencelength as a function of THz frequency for ZnTe crystal with andwithout the probe dispersion is shown in Fig. 5. The crystal lengthshould be less than about 2.7 mm for the 0.1–2 THz detection. Inour system, the maximum phase retardation of probe pulse isestimated about p/2 with THz pulse (0.1–2 THz) of about 25 kWpeak power corresponding to about 100 kW/cm2 peak powerdensity which is obtained by focusing the probe pulse to about3 mm radius on the ZnTe crystal of about 2.7 mm length. Thelimited detected spectrum is related with the time resolution inthe time domain and less than 500 fs time resolution should berequired for the 0.1–2 THz frequency region, but it is easilyachieved by our system.

2.4. Synchronization between THz pulse and probe pulse

The temporal synchronization and quite low jitter between theTHz pulse and the probe pulse are strictly required for theaccelerator-based THz-TDS because the source deference betweenTHz and probe laser comparing with the laser-based THz source.The synchronization has been accomplished by the synchroniza-tion between the driving laser for the rf gun and the probe laser.The driving laser for the electron beam generation and the probelaser are the same mode-lock frequency of 79.3 MHz, which is1/36 of 2856 MHz for the electron accelerator. Two lasers weresynchronized to the master with 36th harmonics signal of theirrepetition 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 lessthan 10 fs for the 150 fs laser Compton X-ray generation (Sakaiet al., 2003) and it is also enough for the THz-TDS.

2.5. Numerical calculation sample model of THz-TDS

To estimate the dynamic range and the resolution of the EOsampling method, we assumed the sample, which has theabsorption in around 0.5 THz and the reference THz spectrumgenerated from the electron bunch with 40 MeV, 1 nC, 300 fs inFig. 2. Fig. 6 shows the reference and transmission THz spectra in arange 0.01–2 THz. Fig. 7 shows the temporal waveform of thereference and transmission THz pulses calculated by the Fourier

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Fig. 6. Reference and sample transmission THz pulse spectra assumed for

calculation.

Fig. 7. Calculated temporal waveform of reference and transmission THz pulse.

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. 9. Signals of W-band detector of THz raw signal, Si wafer and Si wafer with

P-PPV transmission signals.

R. Kuroda et al. / Radiation Physics and Chemistry 77 (2008) 1131–11351134

transform of the spectra. As a result, the dynamic range ofEO sampling detector required is more than 8 bit and the resolutionas a time step of the pomp–probe technique is enough about 500 fscorresponding with about 75mm of the optical delay stage.

3. Preliminary experimental

3.1. Setup for the absorption measurement of 0.1 THz

To demonstrate the absorption measurements of Si wafer andsome layered sample against the THz region wave, the preliminaryexperiment has been performed using 0.1 THz CSR with the W-band detector. Fig. 8 shows the setup of the absorption measure-ment using 0.1 THz CSR. The THz pulse was extracted from a z-cutcrystal quartz window located at 201 direction in the 901 bendingmagnet and collected by the parabolic antenna and passedthrough a W-band wave guide (WR-10), an E-bend and a samplespace. The pulse was guided to the W-band rf detector whose

signal of 500 mV corresponded to 1 mW for about 0.1 THzradiation. In this experiment, Si wafer /10 0S with 0.5 mmthickness the sample is the precursor polyphenylene vinylene(P-PPV) (poly[p-xylene tetrahydro-thiophenium chloride]) of2.5 wt% solution in water and spin-coated about less than100 nm on the Si wafer with 500 rpm, 10 s. The PPV is one of theconductive polymers and the THz-TDS analysis of PPV is expectedfor measurements of its complex refractive index in near future.

3.2. Results

Fig. 9 shows results of the absorption measurements of P-PPVon the Si wafer against the 0.1 THz pulse using the W-banddetector. As a result, the absorption of the Si wafer and samplewere observed about 50% and 5%, respectively. It is found thatthe intensity of the THz pulse is strong enough to perform theTHz-TDS analysis of the sample on the Si wafer.

4. Summary

The design of the high power THz-TDS system based on S-bandcompact electron linac has been accomplished. The THz pulse has

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R. Kuroda et al. / Radiation Physics and Chemistry 77 (2008) 1131–1135 1135

been expected to have the peak power of about 25 kW withfrequency range 0.1–2 THz using the 40 MeV electron beam whichhas about 1 nC bunch charge with 300 fs bunch length (rms). TheEO sampling method with ZnTe crystal is suitable for our THz-TDSsystem and the synchronization between the electron beam andthe probe laser of our system is enough for the low timing jittermeasurements. The preliminary experiment of the absorptionmeasurements of P-PPV on the Si wafer has been successfullydemonstrated 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 performthe THz-TDS analysis of the sample on the Si wafer. In near future,we will complete all components installation for the high powerTHz-TDS system and start the investigation of the un-researchedmaterials in the frequency range 0.1–2 THz.

Acknowledgments

Authors would like to thank KEK-ATF and Waseda Universitymembers for their deep help on development of the Cs2Te rf gunsystem.

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