BL19LXU

Yoshihito Tanaka,Oct. 2013

Text for the class,
Pump-Probe Technique for Picosecond Time-resolved X-ray Diffraction
at Cheiron School
Abstract
The pulsed time structure of synchrotron radiation (SR) and the synchronization
technique between SR and a femtosecond pulsed laser have offered the opportunity tomake pump-probe measurements with 40 ps-time resolution. Application of thetechnique to x-ray diffraction brings out the investigation on fast structural dynamics in
sub-nanosecond time scale. Furthermore, the recently-developed x-ray free electron
laser, “SACLA”, which generates intense femtosecond pulsed x-rays, approaches thefemtosecond time-resolution (Ref. 1). The course will show the timing control andmonitoring techniques of the SR and laser pulses, and demonstration of the picosecond
time-resolved x-ray diffraction measurement for lattice dynamics of a single
semiconductor crystal at BL19LXU.
1. Introduction
1-1. Purpose
Optical laser pump-SR probe measurements have been available forinvestigation of fast dynamics. In the class, I will show the scheme of thetime-resolved technique including timing control, and demonstrate a pump-probe
method for x-ray diffraction to investigate the lattice motion of a single
semiconductor crystal. The course will help you with starting the picosecond
time-resolved experiments at your station.

Fig. 1

Pump-probe method for time-resolved x-ray diffraction1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 at the SPring-8 undulator beamline.

Fig. 2 Fast lattice expansion of a GaAs single crystal
induced by femtosecond pulsed laser irradiation.

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 anintense hard X-ray beam. The SR beam generated by the 25 m-long undulator isguided to the experimental hutches through a Si 111 double-crystal
monochromator (See Ref. 2). The X-ray time-structure is determined by theelectron bunches in the storage ring. The electron bunches have the pulse width ofabout 40 ps at full width at half maximum (FWHM), with a repetition rate of 1
MHz to 509 MHz. The repetition rate can be controlled by the ring operation withpreferable filling pattern. At the beamline practials, the filling pattern operated isslightly complicated, which is called “hybrid mode”, to gain the bunch current forparticular electron bunches as keeping a total current of 100 mA for the otherusers.

2Fig. 3

Fig.4

Time structure of the SPring-8 SR pulses.

Examples of the filling patterns of electron bunches in the storage ring.

Fig. 5

Overview of the RIKEN beamline of BL19LXU.

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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 laser
pulses with a pulse width of 130 fs and a pulse energy of about 1mJ/pulse. Thelaser pulses are synchronized with the master oscillator of the storage ring
controlling the acceleration timing of the electron bunches. (See Ref. 3-5)

Fig. 6

Synchronization system of femtosecond laser and SR pulses.

4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.

Fig. 7

Experimental setup for time-resolved measurement offast lattice expansion of a GaAs wafer.

(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 sensitive 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.

Fig. 8 Time chart of laser and SR pulses, together with electric gate signals.

5(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. Then you can obtain the diffraction intensity
dependent on the delay timing (Ref. 6).

4. Expected results
The x-ray diffraction intensity becomes large
with a response time of about a few hundred psat the smaller angle side of the Bragg diffraction
profile, due to the Bragg peak shift. (see Refs. of7 -9)

5. References(1) Y. Tanaka et al., Journal of the Ceramic Society of Japan, 121 pp.283-286(2013). “Time-resolved Bragg coherent X-ray diffraction revealing ultrafast
lattice dynamics in nano-thickness crystal layer using X-ray free electron laser”(2) http://www.spring8.or.jp/wkg/BL19LXU/instrument/lang-en/INS-0000000361/i

nstrument_summary_view(3) 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”(4) Y. Tanaka et al., Jpn. J. Appl. Phys., 48, p.03A001, (2009). “Development ofpicoseconds time-resolved microbeam x-ray diffraction technique forinvestigation of optical recording process”(5) N. Yasuda et al., Rev. Sci. Instrum. 84, 063902 (2013). “System of laser pumpand synchrotron radiation probe microdiffraction to investigate opticalrecording process”
6(6) 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”(7) Y. Tanaka et al., J. Synchrotron Rad., 9, p.96 (2002). “Optical-switching of
X-rays using laser-induced lattice expansion”(8) Y. Hayashi et al., Phys. Rev. Lett., 96, p.115505 (2006). “Acoustic pulse echoes

probed with time-resolved x-ray triple-crystal diffractometry”(9) Y. Tanaka et al., J. Phys. Conf. Ser., 278, p.012018 (2011). “Time-resolved

X-ray diffraction studies of laser induced acoustic pulse generation insemiconductors using synchrotron radiation”

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