7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Generation of high-order harmonics radiations around 13.5 nm by usinga T-shaped He gas tube
K. Mizushima1, J. Seres2, E. Seres2. C. Serrat3, T.-H. Dinh4, N. Hasegawa4, M. Ishino4, T.
Niinuma5，M. Kume5, T. Higashiguchi5, K. Yamasaki1, and S. Namba1
1
Graduate School of Advanced Science and Engineering, Hiroshima University, 2Institute of Atomic
and Subatomic Physics, Vienna University of Technology, 3 Department of Physics, Polytechnic
University of Catalonia, 4 Kansai Photon Science Institute, National Institutes for Quantum Scienceand Technology (QST), 5Department of Electrical and Electronic Engineering, Utsunomiya
Universitye-mail (speaker): m221326@hiroshima-u.ac.jp

40, 80 mm long tubes
TiS laserHe gas

He gas

He gasFIG.1 T-shaped gas tube for HH generation.

pulse energy: 50 mJ). The laser was focused onto Hegas target with a lens (focal length: 4 m, Rayleighlength: 40 mm). The He gas target was fed by using
a fast solenoid valve into T-shaped Mo tube of 40,and 80 mm in length and their inner diameter was 2.9mm, in which the laser interacted with the He gas togenerate HH emission. Figure 1 shows a photographof the T-shaped He gas tube. The HH spectra weremeasured with a grazing incident spectrometerequipped with a spherical mirror, flatfield grating of1200 grvs/mm, X-ray CCD. Fundamental laser light
was blocked by a 0.5 μm thin Zr filter.
8CCD count (arb. unit)

When an ultrashort high intensity laser pulse isfocused onto rare gases, various order harmonics
with frequencies with odd multiples in terms of thefundamental laser wavelength are generated. It wasin 1987 such HH generation was discovered [1].
Since then, various attempts have been made toincrease the photon energy that already reaches thekeV region [2].
Material processing by lasers is indispensable as
one of the recent processing methods. The shorter thewavelength of the laser light used for processing, themore precise processing can be performed, so that
extreme ultraviolet light is more suitable for finer
processing than ones by visible/UV lights. HHs
generated in this study are in extreme ultravioletwavelengths. The current issues are: (1) higher order
HH generation by using longer wavelength pumplaser, (2) single attosecond pulse, which provides
powerful tools to understand the X-ray and matterinteraction, (3) increase of the HH photon number.
Among these, we have focused on the issue (3),especially, 13.5 nm (~59th order at 800 nm drive
laser), where the HH can contribute to fundamental
technology of efficient extreme ultraviolet (EUV)
lithography.
The experiment was carried out at QST Kansai.
We used a Ti:S laser (wavelength: 800 nm, pulse
duration: 65 fs, beam diameter: 22 mm, maximum

59th (~13.5 nm)

765432
10500
1000
1500
CCD position (pixel)

2000

FIG.2 HH signal observed with 80 mm gas tube.

Figure 2 shows the highest HH signal observedby using 80 mm gas tube at 0.45 bar, 40-mJ pulse
energy. With increasing the gas pressure, the HH
intensities drops. Similarly, with increasing laser
energy, the signal is decreased.
The parameter dependences of HH intensity andits distribution on laser energy, gas pressure, nozzle
length will be reported in details.
References[1] A. McPherson et al., J. Opt. Soc. Am. B4, 595(1987).
[2] T. Popmimtchev et al, Science 336, pp.12871291 (2012).