Yoshihito Tanaka & Yoshiki Kohmura

Oct. 6-7, 2008

Brief text for the class,“Pump and probe technique for picosecond time-resolved x-raydiffraction”
at the Cheiron School
1. Introduction
1-1. Purpose
The pulsed nature of the synchrotron radiation (SR) and the synchronization
technique between SR pulse and ultrashort laser pulse enable us to make pumpand probe measurements with 40 ps time resolution. The time resolved x-ray
diffraction experiments will be demonstrated for the lattice dynamics of a single
semiconductor crystal. The course may help you with starting the picosecond
time-resolved experiments at your stations for investigation on fast structuraldynamics.
1-2. Objectives
When a surface of GaAs single crystal is irradiated by a femtosecond pulse laser,
the lattice expands with a response time of a few hundreds of ps. To observe thefast lattice response, a stroboscopic method in X-ray diffraction , i.e., laser-pumpand SR-probe method is performed in the SPring-8 beamline.
2. Experimental apparatus and equipments prepared2-1. The hard X-ray beamline BL19LXU
The course will be opened at the beamline BL19LXU which can produce themost intense X-ray beam in the world at present. The SR beam generated by the25 m-long undulator is guided to the experimental hutches through a Si 111
double-crystal monochromator (See ref. 1). The X-ray time-structure isdetermined by the electron bunches in the storage ring. The electron bunches havethe pulse width of about 40 ps at full width at half maximum (FWHM), with arepetition rate of 1 MHz to 509 MHz. The repetition rate can be controlled by thering operation with preferable filling pattern. In the class, the filling patternoperated is slightly complicated, which is called “hybrid mode”, to gain the bunch
current for particular electron bunches as keeping a total current of 100 mA for theother users.
2-2. Femtosecond pulsed laser system
A femtosecond laser system for irradiation of a GaAs crystal surface has been
installed in the experimental hutch of the beamline. The laser system is composedof the mode-locked laser oscillator, and a regenerative amplifier to produce laser1pulses with a pulse width of 130 fs and a pulse energy of about 1mJ/pulse. The
laser pulses are synchronized with the master oscillator of the storage ring
controlling the acceleration timing of the electron bunches. (See ref. 2)
3. Experimental procedure(1) Set up for diffraction measurement of GaAs 400.
In order to observe the 400-Bragg reflection of a GaAs wafer, set the sample on
the goniometer, and find the Bragg reflection with an ion chamber.
(2) Alignment of a femtosecond laser beam onto the sample.
Guide the femtosecond laser beam onto the sample surface so as to achieve thespatial overlap with the SR beam whose footprint is observed by a burned paper.
(3) Monitor of the synchronization between the laser pulse and the SR pulse.
An avalanche photodiode is used to monitor the timing of the laser and the SRpulses. Tune the pulse timing of the laser to be close to the target SR pulse asmonitoring with an oscilloscope. Then, adjust the timing of electric gate to cover thetarget SR pulse signal.
(4) Control of the timing delay between the pulses.
Scan the timing of the trigger signal for the femtosecond laser by an electronic
circuit including a phase shifter. (See ref. 3) Then you can obtain the diffraction
intensity dependent on the delay timing.
4. Expected results
The diffracted X-ray intensity becomes large with a response time of about a few
hundred ps at the smaller angle side of the Bragg diffraction profile, due to theBragg peak shift. (See refs. of 4 and 5)
5. References(1) http://www.spring8.or.jp/wkg/BL19LXU/instrument/lang-en/INS-0000000361/i
nstrument_summary_view(2) Y. Tanaka et al., Rev. Sci. Instrum., 71, p.1268 (2000), “Timing control of an
intense picosecond laser to the SPring-8 synchrotron radiation pulses”(3) Y. Fukuyama et al., Rev. Sci. Instrum., 79, p.045107 (2008),“Ultra-high-precision time control system over any long time delay for laser
pump and synchrotron x-ray probe experiment”(4) Y. Tanaka et al., J. Synchrotron Rad., 9, p.96 (2002), “Optical-switching ofX-rays using laser-induced lattice expansion”(5) Y. Hayashi et al., Phys. Rev. Lett., 96, p.115505 (2006), “Acoustic pulse echoes

probed with time-resolved x-ray triple-crystal diffractometry”
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