23P176-B
Study on THz Wave Frequency Up-conversion by Flash Ionization
フラッシュ電離を用いたTHz波の周波数上昇に関する研究
Akinori Nishida1,2, Masahiro Nakata3, Takafumi Oba3, Takamitsu Otsuka3, Fuminori Suzuki3,Syohei Ono3, Takeshi Higashiguchi2,3, Noboru Yugami2,3, Yasuhiko Sentoku4
and Ryosuke Kodama1,2
西田明憲, 仲田真大, 大場貴文, 大塚崇光, 鈴木史典, 小野勝平,
東口武史, 湯上登, 千徳靖彦, 兒玉了祐
1

Graduate School of Engineering, Osaka University,2-1 Yamada-oka, Suita-shi, Osaka, 565-0871, Japan2
Japan Science and Technology Agency, CREST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012 Japan3
Department of Advanced Interdisciplinary Sciences, and Center for Optical Research & Education (CORE)
Utsunomiya University, Yoto 7-1-2, Utsunomiya, Tochigi 321-8585, Japan
4
Department of Physics, University of Nevada, Mail Stop 220, Reno, Nevada 89506, USA
A frequency up-shifted from a source THz pulse is experimentally generated by the over-dense plasma thatis rapidly created by ultra-intense femtosecond pulse laser. The center frequency of the source pulse is 0.35
THz is propagating a ZnSe crystal which is irradiated by short pulse laser. The magnitude of the upshifting
frequency was controlled by laser intensity. We observed frequency up-conversion from 0.35 THz to 3.3
THz by the irradiance of the Ti:sapphire laser in ZnSe crystal.

1. Introduction

intensity.

Over-dense plasma can be easily created byultra-short intense pulse laser. Electromagnetic (EM)wave generated from such plasmas has a broad range offrequency which includes terahertz (THz) whose
frequency is between traditional photonics andmicrowave frequency. THz region has many applications
i.e, imaging, security, information-communication [1, 2].
Until now, high power tunable THz radiation source
is using free electron laser [3]. But the system size isinconvenient for use in general applications [4, 5].
Development of compact and tunable THz source has
been an open problem which we believe can be realized
by using flash ionization. It is a physical phenomenon
arising out of interaction between electromagnetic (EM)wave and over-dense plasma [6]. If an electromagnetic
wave propagates in a homogeneous plasma, the wave
suffers frequency up-shifting. The dispersion relation forangular frequency of up-shifting plasma f is given by

2. Experimental Setup
Fig.1 schematically illustrates the experimental
setup of a pump and probe system. The Ti:sapphire
chirped-pulse amplification laser system delivered
an maximum energy 1mJ at a center wavelength of
800nm, and 1kHz repetition rate. The laser beam
was split into three pulses. One is a pumping pulse
to create electron carrier in a ZnSe crystal. Second
is the chopped pump beam that was passed throughthe half wave plate and focused on ZnTe crystal togenerate source THz pulse that is placed at centerof two lenses. The angle of incidence pump laser tothe ZnSe is 68° (Brewster's angle). Third is for
detecting up-shifted THz wave by photoconductive(PC) antenna. The source THz pulse is focused in
the ZnSe crystal by the off-axis parabola (OAP)
mirror and lens pair which effective focal length is76.2 mm and 150 mm, respectively. The output
THz pulse is collected by another OAP and lenspair, and finally focused the PC antenna to detect
the THz signal. We measured this small current by
chopping the 0.5 kHz optical pulse train thattriggers the emitter and using a lock-in-amplifier.
The THz spectrum in the range to 4THz wasobtained by applying a first Fourier transform.

 2f  k 2 c 2  P2  02  P2 ,(1)

where k0 is the wave number, c is the speed of light, p is
plasma angular frequency and 0 is angular frequency ofthe source electromagnetic wave. Therefore, themagnitude of up-conversion frequency is represented by  02  P2  0 ,(2)

The up-shift frequency is controllable by changing theflash plasma density. Until now, the several experiments
had done in microwave region [7-9]. In this letter we
performed the frequency up-conversion with a ZnSe
crystal irradiated by ultra-short pulse laser. We confirmed
dependency of frequency up-conversion with laser

4. Conclusion

Fig.1. Experimental setup

3. Experimental Results
Fig. 2 shows the spectrum of the source THz wave
and its up-shift. A solid line indicates the upshifted
experimental data and dashed line is the originalspectrum without the pump laser irradiation. The central
frequency of source wave is 0.35 THz with a frequency
broadening of 0.4 THz (FWHM). The upshifted
spectrum is observed at 3.4 THz, when the laser intensity
is 4.2 GW/cm2.
Fig. 3 shows the dependency of amount of frequency
up-conversion on laser intensity. We obtain frequency
up-conversion from 0 THz to 3.0 THz.

Fig.2. Observed spectrum at the laser intensity
I0 = 4.2 GW/cm2.

Fig.3. Dependency of amount of frequency
up-conversion on laser intensity.

In conclusion, we performed the proof and principle
experiment of the flash ionization with thesemiconductor crystal of ZnSe. The plasma creation time
is much shorter than the period of the source
electromagnetic wave and plasma size is much larger
than THz wave. We have demonstrated frequency
up-shift up to 3.5 THz. Various frequencies can beobserved by changing pumping laser intensity irradiatedon the ZnSe crystal. We confirmed relationship between
electric field strength of up-shifted THz pulse andintensity of it. This result has possibilities of tunable highpower radiation source in THz region.

Acknowledgement
The authors are grateful to Kashiwazaki Hiroaki,and Jun Miyazawa for their unparalleled technicalsupport. This work was supported by the Japan
Society for the Promotion of Science Global
Centers of Excellence program (GCOE). This work
was also supported by Core Research for
Evolutional Science and Technology (CREST) ofthe Japan Science and Technology Agency (JST). A
part of this work was performed under the auspicesof MEXT (Ministry of Education, Culture, Scienceand Technology, Japan). We also are grateful to theCooperative Research Center and the Venture
Business Laboratory (VBL) of Utsunomiya
University for providing the laser system.
References[1] B. Ferguson and X.-C. Zhang, Nature materials 1,(2002) 26.
[2] M. Tonouchi, Nature Photonics 1, (2007) 97.
[3] G. P. Williams, Rev. Sci. Instrum. 73, (2002)
1461.
[4] D. M. Mittleman, R. H. Jacobsen, and M. C.
Nuss, IEEE J. Sel. Top. Quantum Electron. 2,(1996) 679.
[5] Fischer, B. M. et al., B. M. et al., Opt. Express
13, (2005) 5205.
[6] S.C.Wilks, J.M.Dawson, and W.B. Mori,Phys.Rev. Letter. 61, (1988) 337.
[7] S.P.Kou, Phys.Rev.Lett. 65, (1990) 1000.
[8] C.J.Joshi, C.E.Clayton, K.Marsh,
D.B.Hopkins, A.Sessler, and D.Whittum,
IEEE Trans. Plasma Sci. 18, (1990) 814.
[9] N.Yugami, et.al., Phys.Rev. E,65, (2002)
036505.