25G04
Angular Distribution of Intense Terahertz Emission from Argon Clusters
Irradiated by 60 mJ Femtosecond Laser Pulses
60 mJフェムト秒レーザーパルス照射によるアルゴンクラスターから放射する高強度テラヘルツ波強度の角度分布
Fazel Jahangiria,b, Masaki Hashidaa,b, Takeshi Nagashimac, Shigeki Tokitaa,b,Masanori Hangyoc and Shuji Sakabea,ba

Advanced Research Center for Beam Science, ICR, Kyoto University, Kyoto, Japanb
Department of Physics, GSS, Kyoto University, Kyoto, Japanc
Institute of Laser Engineering, Osaka University, Osaka, JapanThe characteristics of THz emission from argon clusters irradiated by intense femtosecond laser pulses
have been investigated. Intense Terahertz pulses have been observed from argon clusters with energies ofalmost three orders of magnitude larger than that from air plasmas. The THz radiation is enhanced by
efficient laser absorption in the clusters. The angular distribution and laser-energy scaling with polarization
characteristics are studied and the mechanism of THz generation is discussed.

1. Introduction
Terahertz (THz) waves are widely used in many
applications such as remote sensing, spectroscopic
analysis of organic and inorganic compounds,
biomedical diagnostics, and threat detection [1]. For
further more applications of THz waves, efficient
high-energy THz sources are in demand. Intense
THz wave pulses with energies in microjoule level
have been generated by using huge free-electron
laser facilities [2] and recently by employing large
nonlinear electro optic crystals [3].
THz radiation from plasmas produced by intense
femtosecond laser pulses has been extensively
studied, since the first demonstration in 1993 [4].
Strong THz emission has been observed from theplasma of solid targets, because of their high laser
absorption. However, this process is companied bydebris generation during the laser plasmainteraction and also considerable reflection atboundary between plasma and vacuum. Therefore,
the plasma of gas targets has attracted attention as
convenient and replenishable THz sources. To
increase the energy of THz radiation from gas
plasmas, a number of artificial schemes such as
electric field biasing of the plasma [5] andfrequency-mixing methods [6] have been employed.
To the best of our knowledge, the highest THz
pulse energy at frequencies below 5.5 THz from gas
targets
hasbeenobtainedthroughafrequency-mixing scheme in air with energy up to570 nJ [7]. Different gases have been also
investigated to find the most efficient gas plasma
THz sources. Recently, Rodriguez et al. showed
that, at laser energies higher than 4 mJ, stronger

THz radiation is generated from argon gas thanfrom air, neon, krypton, and xenon [8].
Atomic clusters can absorb laser light asefficient as solid targets to be, convenient andreplenishable THz sources. We have studied thecompatibility of atomic clusters [9] with gas targets[10] for THz emission under irradiation of intense
femtosecond laser pulses. THz emission from argon
clusters has been studied as function of cluster size.
The enhancement of THz radiation from argon
clusters, comparing to that from argon gas, has been
explained in terms of laser absorption fraction [9].
To investigate the feasibility of atomic clusters
for intense THz source, we have measured theangular distribution of THz waves. Suchmeasurements are important for considering thephysical mechanism of THz wave generation andestimating the total energy of THz pulses.
2. Experiments
Femtosecond laser pulses from a chirped-pulse
amplified Ti:sapphire laser [11] with energy of
10 - 70 mJ, duration of 130 fs, and center
wavelength of 800 nm were focused by a sphericallens with a focal length of 200 mm, onto argon
clusters in a spot with diameter of 17 µm. A
schematic of the experimental setup is shown in Fig.
1. Argon clusters were generated by injecting argon
gas [12] with a backing pressure up to 8 MPa into a
vacuum chamber with diameter and wall thicknessof 100 mm and 5 mm, respectively. The chamber
was made of fused silica glass with refractive index
of 1.95 and transparency of 90% at 0.5 THz. Suchspecial chamber was designed suitable for a precise

measurement of angular distribution of THz waves.
The high-energy ions generated by Coulomb
explosion of argon clusters have been detected in a
separate
experimentusing
time-of-flightmeasurements. The signal of ion detection was usedto confirm cluster production and to synchronizegas injection with the laser pulses.

angular distribution and polarization properties of
THz waves suggest the ponderomotively induced
quadrupole radiation with a contribution of fourwave mixing process as the mechanism of THz
generation in the present experiment.

Fig.1. A schematic of the experimental setup

Fig.2. A typical angular distribution measured at
7 MPa backing pressure.

THz radiation from the argon clusters arecollected by using a polyethylene lens with a focal
length of 150 mm and directly image-relayed on theinput window of a helium-cooled InSb bolometerby using a parabolic mirror with a focal length of
119.2 mm. Wire grids with an extinction ratio of~10-5 at 0.5 THz are used as polarizers in front of
the bolometer to measure the horizontally andvertically polarized components, and as beam
splitters in Martin-Puplett interferometer. Multiple
layers of polystyrene foam and a thin black
polyethylene filter are installed behind the lens andin front of the bolometer, respectively, to exclude
the laser pulses and unwanted lights emitted or
scattered from the plasma (Fig. 1).
3. Results and Discussion
The spectrum and intensity interferogram ofTHz waves were measured using a bolometer with
spectral sensitivity below 2 THz. The maximum
observed frequency was ~1.0 THz with a peak at~0.5 THz. The spectrum of THz waves did notstrongly depend on laser pulse energy, and a nearly
identical spectrum was observed both in theforward and backward directions. The detection
setup was rotated around the center of the glasschamber and the angular distribution of THz waves
was measured. Figure 2 shows a typical angular
distribution measured at 7 MPa backing pressure.
The highest THz power was detected at ±30° and±140° with respect to the laser propagation
direction.
The direction of maximum THz emission wasfound to depend on plasma length. For increasingplasma length, the maximum THz peak was foundat smaller angle. This result agrees with previous
reports [13], which predicts that this angle changes
in proportion to (λ /L) 1/2, where λ and L are thewavelength and length of plasma, respectively. TheAcknowledgments
This work was partially supported by theGrant-in-Aid for the GCOE Program "The Next
Generation of Physics, Spun from Universality andEmergence" from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT)
of Japan.
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