Photon Factory Activity Report 2005 #23 Part B (2006)
Instrumentation and Technique

16B/2004G213

Observation of the second harmonic generation in the soft x-ray region
Yasuhiro TAKAYAMA*1, Koutatsu SHIBASAKI1, Naoya SASAKI1, Yuji NAKAYAMA1,Keisuke NISHIHATA1, Yuichi YONEDA1, Tetsuo YOSHIDA1, Satoshi NAKAMURA1,Hiroyoshi ISHII1, Tsuneaki MIYAHARA1, Jun OKABAYASHI2, Ken KANAI2,2
3
Masaharu OSHIMA and Shigeru YAMAMOTO
1Tokyo Metropolitan Univ., Hachioji-shi, Tokyo 192-0397, Japan
2
The Univ. of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan3KEK-PF, Tsukuba, Ibaraki 305-0801, JapanIntroduction
Recent developments on the technology of the highbrilliant-light source give us a chance to perform theexperiments of new types. The nonlinear phenomenon
only with the synchrotron radiation (SR) is one of themost interesting topics, but has not been observed up tonow. In order to establish the experimental technique ofthe nonlinear optics, we have constructed a novel
instrument for detecting the second harmonic generation(SHG) in the soft x-ray region. The SHG occurs only in a
crystal without the space-inversion symmetry [1], and wehave selected the GaAs film as a sample and performed
the SHG measurement around the As 3p-absorption edge.
Experimental
The GaAs thin film was prepared by the vacuum
evaporation on the polycarbonate substrate, where thethicknesses of the GaAs film and the substrate are 2000 Åand 5500 Å, respectively. Fig. 1 shows a brief layout ofthe optical system for the SHG measurement. The
monochromatized SR comes from the left side, and theSR is split into two beams by a plane mirror. Each beam
is focused on the GaAs thin film by a toroidal mirror. Theposition of the beam from the upper toroidal mirror canbe modulated by a piezo actuator. The motion of the piezo
actuator is controlled by a function generator which
generates the sine wave with the frequency of about 1 Hz.

outputs of the LA, respectively. In our experimental setup,
X and Y should be positive and zero, respectively, if theSHG signal is detected. By subtracting the X and Y
outputs measured on the condition 2 from those on thecondition 1, the signal of the SHG can be obtained.
Results and Discussion
Fig. 2 shows the transmittance spectrum of the GaAs
film and the outputs of the LA. The measurement of theSHG was performed at the photon energies indicated inthe transmittance spectrum. X1, Y1 and X2, Y2 are outputs ofthe LA on the conditions 1 and 2, respectively. The
differences of these values of ∆X and ∆Y correspond tothe SHG signal. ∆X at the photon energies of 140 eV and141.5 eV is positive, and this indicates that the SHG
signal was observed around the photon energies of the As
3p pre-threshold region. ∆Y also depends on the photon
energy, and this result is not consistent with the expected
output of the SHG signal. The reason of the discrepancy
might come from the incomplete adjustment of the beam
position from the lower toroidal mirror. In this case, thetiming of the pulse current from the PMT due to the SHG
signal changes and, consequently, the Y component also
takes the non-zero value. For the confirmation of thedetection of the SHG signal, more experiments with highprecision and with different samples are required.

Figure 1: Optical system for the SHG measurement.
The second harmonic is generated only when the twobeams on the film has an overlap each other. The signal of
the SHG is detected by a photomultiplier tube (PMT). We
have performed the experiments on the two conditions asshown in Fig. 1, where the SHG signal is expected toappear only on the condition 1. The output current fromthe PMT is analyzed by a digital lock-in amplifier (LA). X
and Y denote the sine and cosine components of theFigure 2: Transmittance spectrum of the GaAs film(left) and the outputs of the LA (center and right).
References[1] Y. R. Shen, The Principles of nonlinear Optics, New
York: John Willey & Sons, Inc., 76 (1984)* takayama@phys.metro-u.ac.jp

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