23P172-B
Characteristics of THz radiation from air plasma generated by two-color laser
二色レーザー励起大気プラズマからのテラヘルツ電磁波発生の諸特性
Junghun Shin1, Alexei Zhidkov2, Zhan Jin2, Tomonao Hosokai2, Ryosuke Kodama1, 2
申定訓1, Zhidkov Alexei2, 金展2, 細貝知直2, 兒玉了祐1, 2
1Graduate School of Engineering, Osaka University, 2-1, Yamadaoka, Suita, Osaka 565-0871, Japan2
Photon Pioneers Center, Osaka University, 2-1, Yamadaoka, Suita, Osaka 565-0871, Japan1
大阪大学大学院工学研究科 〒565-0871 大阪府吹田市山田丘2-1
2
大阪大学光科学センター 〒565-0871 大阪府吹田市山田丘2-1

Characteristics of THz radiation from air plasma generated by two-color laser, such as power, spectrum
and angular dependency have been studied with particle-in-cell simulation. At low laser intensity, THzpower increased nonlinearly against the laser intensity. As the laser intensity become higher than 10 15
W/cm2, it becomes slower than linear. Strong radiation at around 30 degree is found in addition to theradiation along laser propagation axis.

1. Introduction
Generation of intense THz electromagnetic
wave has been of interest since its realization
would allow new studies and applications in thisdirection. One of the promising approaches is touse plasma as the radiation source, especiallyusing two-color laser [1-4], where a femtosecond
laser pulse consisting of a fundamental frequency
ω and its second harmonic 2ω which is usuallyobtained by second harmonic generation withnonlinear crystals such as type-I β-BaB2O4 (BBO)
are focused to generate plasma in gas, which,during interaction with laser field radiates THzwave.
So far, most studies concerned with the method
uses relatively low laser intensity at around 10 14
W/cm2. One obvious way to increase the THzpower with this highly effective method is toincrease the incident laser power. However, as thelaser intensity increases, interaction between laser
and plasma cannot be neglected.
At higher laser intensity region, there are anumber of physical processes that would result in
THz radiation, e.g., the processes of frequency
up-shift in the front of a pulse and k vector
downshift in its rear resulting in beat waves;pulse modulation owing to the refraction on theelectron density ramp; steep plasma boundary, etc.
It is therefore necessary to look into theinteraction between femtosecond laser pulses andunderdense plasma with kinetic simulations that
self-consistently include plasma and atomicphysics [5-8].
In this presentation, spectrum and power ofbroadband THz radiation from plasma generated
by co-propagating laser pulses consisting of two

frequency components, i.e., fundamental andsecond harmonics in air are studied for peak laser
intensity of 1014, 1015 and 1016 W/cm2.
2. Simulations
To study characteristics of THz radiation, we
perform PIC simulations using FPLaser2D [9, 10]with a moving window for a linearly polarized laser
pulse propagating in air under normal pressure. The
2D calculations are performed for the laser intensityfrom 1014 (which is about the threshold intensity forair) to 1016 W/cm2 and 90 fs FHWM duration. The
calculation are performed both for a single pulse λ =0.8 μm and for two laser pulses λ = 0.8 μm and λ1 =0.4 μm with total energy in the second harmonic ~0.3 times the energy of fundamental harmonic. The
focus spot of fundamental harmonic is chosen toprovide the Rayleigh length on the order of 1 mm;
the focus spot of the second harmonic is same asthat of fundamental harmonic. We use a 320 μm x
150 μm window and λ1 / 10 spatial resolution; thekinetic cell is twice as large as the PIC one. The
kinetic simulation was simplified by including only
nitrogen molecules; the laser pulse is set as
Gaussian.
Figure 1 shows the spectrum of radiation for thelaser intensity at 1016 W/cm2. There is a strong
radiation at around 30o, which is not seen in the
spectra of radiation at lower intensities. Those
spectra for lower intensity will be shown in thepresentation.
Figure 2 shows the increase of THz power along
laser propagation axis against the change infundamental laser intensity. It is visible that there
are two different slopes in the curves: up to the laser
intensity I ~ 1015 W/cm2, the power growth is fasterthan linear, while after the growth is slower thanlinear. An approximation function can be presented

as following: W  AI / B  e B / I with α = 0.7, A= 9 x 10-3, and B = (2 - 3) x 1014 cm2.

lines indicate linear growth of THz power against thelaser intensity.

4. Conclusion
In conclusion, we have studied numerically theradiation spectrum and the dependence of power ofTHz radiation from air plasma generated byfemtosecond laser pulse consisting of fundamental
and second harmonic components. In numericalsimulations, we have found strong non-linear
dependence of the THz power on the laser intensity.
The power growth is faster than linear up to laser
intensities on the order of I ~ 1015 W/cm2, then, thegrowth becomes slower than the linear; therefore
the conversion efficiency reaches its maximum at
I~1015 W/cm2. This slowing in the growth rate may
partly be attributed to the increase in the waist ofbeam, due to the refraction of the laser pulse by theinduced plasma during its focusing in atmosphere.
Acknowledgments
Authors are grateful for the support of thecomputer room of ILE and the cybermedia center at
Osaka University. This study was supported by
Japan Science and Technology CREST and Global
COE program in Ministry of Education, Culture,Sports, Science and Technology (ElectronicDevices Innovation).

Fig. 1. 2D spectrum of p-polarized radiation in plasma
and its angular distribution with the peak laser intensityof ω at 1016 W/cm2. The label of inset indicatescorresponding frequency in THz.

Fig. 2. The dependence of power of THz radiation on thelaser pulse intensity at the emission angles at 0o. Dotted

References[1] D. J. Cook and R. M. Hochstrasser: Opt. Lett. 25(2000) 1210.
[2] T. Bartel et al.: Optics Letters 30 (2005) 2807.
[3] X. Xie, J. Dai and X.-C. Zhang: Phys. Rev. Lett. 96(2006) 075005.
[4] K. Y. Kim, A. J. Taylor, J. H. Glownia and G.
Rodriguez: Nat. Photonics: 2 (2008) 605.
[5] M. Chen, A. Pukhov, X. Y. Peng, O. Willi: Phys. Rev.
E 78 (2008). 046406.
[6] Z.-M. Sheng, H.-C. Wu et al.: Commun. Comput.
Phys. 4 (2008) 1258.
[7] H.-C. Wu, J. M. Vehn, Z.-M. Sheng: New J. Phys. 10(2008) 043001.
[8] N. A. Zharova, V. A. Mironov, D. A. Fadeev: Phys.
Rev. E 82 (2010) 056409.
[9] A. Zhidkov et al.: Phys. Rev. Lett. 103 (2009)
215003.
[10] A. Zhidkov et al.: Phys. Rev. E 69 (2004) 066408.