Photon Factory Activity Report 2004 #22 Part B (2005)
Instrumentation and Technique

16B/2004G213

Observation of non-linear effect in soft X-ray region with GaAs thin film
Yasuhiro TAKAYAMA*1, Tetsuo Yoshida1, Satoshi NAKAMURA1
Naoya SASAKI1, Tsuneaki MIYAHARA1, Jun OKABAYASHI2, Shigeru YAMAMOTO3
1
Tokyo Metropolitan Univ., Hachioji-shi, Tokyo 192-0397, Japan2
Univ. of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan
3
KEK-PF, Tsukuba, Ibaraki 305-0801, Japan
Introduction
The non-linear (NL) effect in the x-ray region is apromising study to explore a new science utilizing theintense x-ray such as the FEL. Some attempts inobserving the NL effect with the SR has been performedin the hard x-ray region [1]. However, the NL effects inthe soft x-ray region have not been observed, and it isimportant to establish the technique to observe the NL
effect for future earnest experiments. For this purpose, wehave developed a new apparatus in observing the secondharmonic generation in the soft x-ray region.
Result and Discussion
The experiment was performed at an undulator
beamline BL-16B. A brief layout of the instrument in thevacuum chamber is shown in Fig. 1. The
monochromatized beam is incident to a plane mirror. Thehalf of the beam is reflected by the plane mirror and theother half goes through the mirror. The divided beams arereflected by two toroidal mirrors and are focused on aGaAs thin film, respectively. The beamsize on the GaAs
film was about 200 µm. The GaAs film was prepared by
the vacuum-evaporating method. The material of thesubstrate of the film was polycarbonate and the thicknessof the GaAs film was about 1000 Å.

signal due to the NL effect. The probability of the secondharmonic generation is expected to be extremely small.
Therefore, we have modulated the position of one beamreflected by the upper toroidal mirror with about 1 Hz,
and measured the PMT signal with a lock-in amplifier(LA). We have performed the experiment in two
conditions as shown in Fig. 1. At first, we adjusted thebeam position on the GaAs film so that two beams havean overlap during the modulation (on-condition). Next,
we changed the position of the beams so that two beams
have no overlap during the modulation (off-condition).
Since the NL effect will occur only in “on-condition”, theoutput of the LA is expected to change only in thiscondition as the photon energy changes.
The right figure in Fig. 2 shows the absorption
spectrum of the GaAs film near the absorption edge of As.
We determined the photon energies for the measurementof the NL effect as indicated in the figure. The left figurein Fig. 2 show output of the LA for the different photonenergies. The output of the LA in “off-condition” has no
photon-energy dependence, and this shows that the NL
effect does not occur in this condition. The output of theLA in “on-condition”, on the other hand, has clear
photon-energy dependence. This shows that the second
harmonic was generated due to the x-ray absorption by
the GaAs film.
A B
C-0.3

A: 142.0 eV
B: 143.25 eV
C: 144.0 eV

C

-0.4

0.06

143.25 eV
0.062
0.064

144.0 eV
142.0 eV

0.066

-0.5

0.068

B-0.6

0.07

A-0.7

1.2

1.4

1.6

1.8

2

2.2

2.4

2.60.072

140

2.8142

144

146

148

150

Photon Energy (eV)X average (Arb. Units)

Figure 1: Layout of the optical system and the beam
position on the GaAs film.
We can modulate the focal position of one beam byvibrating the upper toroidal mirror with a piezo actuator.
The second-harmonic generation occurs only when thetwo beams have an overlap on the GaAs film, and thebeam corresponding to the second harmonic, which ismeasured by a PMT, proceeds in the direction of the sumof two direction vectors of the incident beams. The output
current of the PMT is proportional to the magnitude of the260

Users' Report

Figure 2: Output of the LA for the different photonenergies near the As absorption edge (left). The closed(open) marks indicate the results measured in “oncondition (off-condition)”. The right figure shows theabsorption spectrum of GaAs near the As absorption edge.
References[1] Y. Yoda et al., J. Synchrotron Rad. 5, 967 (1998).
* takayama@phys.metro-u.ac.jp