25G01
Development of the Plasma Mirror for Contrast Improvement of Ultra ShortIntense Lasers
高強度短パルスレーザー高コントラスト化用プラズマミラーの開発
Kiyoto Kowata, Shigeki Tokita, Shunsuke Inoue, Kazuto Otani, Masaki Hashida, and Shuji Sakabe
木幡清人，時田茂樹，井上峻介，大谷一人，橋田昌樹，阪部周二
Advance Research Center for Beam Science, Institute for Chemical Research, Kyoto University
Gokasho, Uji, Kyoto 611-0011, Japan
Division of Physics and Astronomy, Graduate School of Science, Kyoto University
Kitasirakawa, Sakyoku, Kyoto 606-8502, Japan
京都大学化学研究所，〒611-0011 京都府宇治市五ヶ庄
京都大学大学院理学研究科，〒606-8502 京都市左京区北白川
To develop the plasma mirror to improve the contrast of short pulses generated from a CPA laser system
we have studied the reflectivity from a silica based glass plate. The reflectivity of 80% has been obtained at
the laser intensity of 1015~1016W/cm2 with 10-degree incidence. No much difference between p- ands-polarizations has not been seen.

1. Introduction
A short pulse generated from a chirped-pulseamplification (CPA) laser system is generallycomposed of long (ns-order) tail ahead of the mainpulse (called “pre-pulse”). For the intense lasermatter interaction studies this pre-pulse deforms thematter from its initial state [1-3]. For instance, thelaser-solid interaction is easily reduced to that oflaser-plasma (long-scale expanding plasma). To
quantify the pre-pulse, the contrast ratio is definedas Ip/I0, where the Ip and I0 are the intensities of themain pulse peak and the pre-pulse, respectively.
Ideally (no pre-pulse) the ratio is infinite, but
usually is 107-8. For the pulse with peak intensity of
1020W/cm2, the matter is exposed in the field of
1012-13W/cm2, high enough for the matter to be inplasma state. For technology development ofultra-high intensity lasers, the high contrast pulse
generation is one of the key issues. A method is“plasma mirror”. When a pulse is irradiated on an
optical transparent plate, the pre-pulse parttransmits through the plate, while the main pulse
make the plate surface to plasma and is reflectedfrom the plasma surface, consequently the reflected
pulse can be free from pre-pulse. The plasma mirror
has been studied intensively [4-7] since the intenselasers become available. However, it is not yet
devoted to steady operation for the laser system.
The performance of the plasma mirror is defined by
the contrast improvement and reflectivity. The
improvement of 102-3 has been reported, but thereflectivity is up to 70-80%. The use of two mirrors
to improve the contrast reduces the reflectivity to sosmall 50% [5]. For the plasma mirror we can findno physical reason to limit the reflectivity andcontrast improvement, and therefore we aredeveloping it with the performance of 80%reflectivity and 103-contrast improvement for single
mirror. In this report, we study the reflectivity
characteristics plasma surface produced by prepulses.
2. Experiments
To optimize the laser irradiation conditions forthe plasma mirror, we have measured reflectivity
for different incident angles with changing laser
intensity. Intense short pulses from a CPA
Ti:sapphire laser (duration 150fs, wavelength
800nm, energy 120mJ) were focused by an
off-axis-parabolic mirror (F/5.6) and irradiated ona silica based glass plate with the configuration ofp- and s-polarizations. The energy of the reflected
laser was measured by a pyroelectric energy
meter. The incident angles are 10, 30, 45 and 60
degrees. The laser intensity on the plate ischanged by moving the plate.
3. Results
Figure 1 shows the dependence of reflectivity
on laser intensity for different incident angles.
The smaller the incident angle is, the higher thereflectivity is. For 10-degree incidence themaximum reflectivity of 80% is obtained at theintensity of 10 15~1016 W/cm2. Comparison of pand s- polarizations is shown in Fig. 2. The
incident angle is 10 degrees. No much difference
between p- and s- polarizations can be seen in Fig.
2. The results suggest that the laser light is not10.8

absorbed by resonance absorption but inverseBremsstrahlung, which decrease as the laser
intensity increases.

Reflectivity

10°
30°
45°
60°

0.6
0.4
0.2
0 13
101014

1015
1016
Intensity (W/cm2)

1017

1018

Fig.1. Dependence of reflectivity on laser
intensity for different incidences
10.8

PS

Reflectivity

0.6
0.4
0.2
0 13
101014

1015
1016
Intensity (W/cm2)

1017

1018

Fig.2. Reflectivity for p- and s- polarizations at10-degree incidence

4. Conclusion
We have measured the reflectivity of a silica
based glass plate for intense femtesecond laser
pulses. The dependence of reflectivity on laser
intensity for different incidences has been studied.
To obtain high reflectivity the laser intensity of1015~1016 W/cm2 and the incidence of 10-degree
are optimized for any polarizations.
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