First Demonstration of Pump and Probe Experiment for Wide-gap Semiconductors Using Free Electron Laser and Synchronously-operated Femtosecond Laser

Transcription

1 First Demonstration of Pump and Probe Experiment for Wide-gap Semiconductors Using Free Electron Laser and Synchronously-operated Femtosecond Laser Nobuhiko Sarukura1, Kohei Yamanoi1, Kohei Sakai1, Tomoharu Nakazato1, Marilou Cadatal-Raduban1, Toshihiko Shimizu1, Masataka Kano2, Akira Wakamiya2, Tsuguo Fukuda3, Mitsuru Nagasono4, Tadashi Togashi4, 5, Takahiro Sato4, Atsushi Higashiya4, Makina Yabashi4, Tetsuya Ishikawa4, Haruhiko Ohashi4, 5, Hiroaki Kimura4, 5 1Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita, Osaka , Japan 2Daishinku Corporation, 1389 Shinzaike, Hiraoka-cho, Kakogawa, Hyogo , Japan 3WPI Advanced Institute for Materials Research Tohoku University,2-1-1 Katahira,Aoba-ku,Sendai , Japan 4RIKEN XFEL Project Head Office, 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo , Japan 5Japan Synchrotron Radiation Research Institute, 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo Japan

2 Outline XFEL Project in the world Necessity for fast scintillator Hydrothermal method-grown ZnO crystal as scintillator Improvement of ZnO scintillator by impurity doping Pump and probe experiments using ZnO crystal Summary

3 XFEL projects DESY in Europe SACLA in Japan SLAC in USA New scientific fields have already opened.

4 Newest X-ray Free Electron Laser (XFEL) Facility June 7, 2011: = 0.12 nm June 10, 2011: = 0.1 nm Shortest in the world Expected peak brightness: 1x10 8 times compared to SPring8 Expected : <10 fs

5 X-ray structure analysis by XFEL XFEL has characteristics of lasers and synchrotron radiations. Femtosecond pulse duration and high peak brightness in the X-ray region. Structural analysis of a protein molecule Direct observation of atomic motion during photo chemical reaction Research of new materials

6 Outline XFEL Project in the world Necessity for fast scintillator Hydrothermal method-grown ZnO crystal as scintillator Improvement of ZnO scintillator by impurity doping Pump and probe experiments using ZnO crystal Summary

7 Why is a fast scintillator needed for XFEL facilities? High accuracy temporal overlap with optical pulse is required for pump and probe experiments FEL and optical pulse should be synchronized within 0.1 psec accuracy. The response time of conventional scintillator (CeYAG) is about 50 ns

8 Outline XFEL Project in the world Necessity for fast scintillator Hydrothermal method-grown ZnO crystal as scintillator Improvement of ZnO scintillator by impurity doping Pump and probe experiments using ZnO crystal Summary

9 ZnO crystal grown by the Hydrothermal method Short fluorescence decay time of ~1 ns. Useful emission wavelength (Transparent for glass). Large-sized single crystal of up to 3 inch-diameter can be grown. Pumped by He-Cd laser at 325 nm Fluorescence peak at 380 nm (3.26 ev) ZnO crystal grown by hydrothermal method E. Ohshima, et.al. J. Crystal Growth 260 (2004) 166

10 Large-sized ZnO crystals

11

12 Fluorescence properties of Conventional ZnO crystal EUV(13.9nm) excitation UV(351nm) excitation Excitation laser pulse Double exponential decay A = 1 ns, B = 3 ns The fluorescence behavior is similar in both cases. τ A =1.1ns τ A =0.91ns τ B =3.5ns τ B =3.2ns ZnO crystal is a promising scintillation material. Tanaka et.al., APL. 91, (2007) Need to improve response time

13 Outline XFEL Project in the world Necessity for fast scintillator Hydrothermal method-grown ZnO crystal as scintillator Improvement of ZnO scintillator by impurity doping Pump and probe experiments using ZnO crystal Summary

14 Impurity doping by the Hydrothermal method Doping method 1 Basic aqueous solution (3mol/l KOH + 1mol/l LiOH) Dopant powder Dissolved or mixed precursor (sinterd ZnO) Basic aqueous solution (3mol/l KOH + 1mol/l LiOH) Low LiOH precursor (sinterd ZnO) Sintered In:ZnO Doping method 2 E. Ohsima et al., J. Cryst. Growth, 260,166 (2004)

15 ZnO crystals undoped In-doped Other dopant

16 Result of lifetime measurements = 15 ps undoped Fe-doped In-doped with few LiOH Yamanoi et.al. Opt. Mat. 32 (2010)

17 Result of lifetime and light yield measurements Ref.1 Ref.2 Ref.1 : Tanaka et.al., APL. 91, (2007) Ref.2 : Shimizu et.al.,rsi. 81, (2010) 17

18 Outline XFEL Project in the world Necessity for fast scintillator Hydrothermal method-grown ZnO crystal as scintillator Improvement of ZnO scintillator by impurity doping Pump and probe experiments using ZnO crystal Summary

19 SCSS (SPring-8 Compact SASE Source) test accelerator SACLA T. Shintake, et al., Nature Photonics 2, 555 (2008) from the pamphlet of XFEL at SPring-8 (2008)

20 Experimental Setup Injector SCSS test accelerator C-band accelerator 61 Hz 10 J, 200 fs Undulator ZnO sample Dump Dump Focusing mirror Synchronizer Synchrolock-AP Ti:sapphire laser oscillator 800 Hz 30 fs Ti:sapphire regenerative amplifier Tripler 266 nm Streak camera Filter Hydrothermal method grown ZnO crystal

21 Streak images for different @+200ps Delay

22 Analysis: dependence of amplification on delay time Intensity increase ー FEL excitation S f ー 266nm(UV) excitation S 266 ー Simultaneous irradiation S all ー S f +S 266 Fitting S fit S fit = A S f + B S 266 Fitting parameters A and B were defined as S f multiplied by A and S 266 multiplied by B, and are equal to S all.

23 Results: dependence of amplification on delay time A,B as a function of delay time Delayed signal amplified up to 1.4 times Leading signal not amplified UV leading FEL leading (=3ω signal increasing) (=FEL signal increasing) Intensity reversal not at delay 0? Delay (ps) Delayed signal was amplified with a longer decay time

24 Comparison between undoped and Fe-doped ZnO undoped Fe-doped 3ω FEL 3ω FEL Delay time dependence of amplification is different for undoped and Fe-doped ZnO. This could be related to the emission lifetime.

25 Fe-doped ZnO FEL 3ω I(FEL)/I(3w)~1 I(FEL)/I(3w)~3 Difference in delay time dependence of amplification for different intensity ratio is not clear. (Larger FEL intensity, larger amplification?)

26 Delay time dependence of 3ω excitation luminescence Luminescence intensity of undoped ZnO for different intensity ratios between pump and probe light w/o Fe dopeing w/i Fe doping Difference of two excitation beam(ps) Difference of two excitation beam(ps) Luminescence originating from 3ω excitation was amplified by FEL excitation. This phenomena depend on the delay time in pump and probe light and the intensity ratio of the two excitation sources. Hypothesis: Overflow in trap level by intense excitation?

27 Mechanism of amplification influenced by delay time E E Trapped excitons (~ps) Impurity level (~ms) Trapped excitons (~ps) Impurity level (~ms) Γ k Γ k After pump laser After probe laser

28 Summary Pump and probe experiment for ZnO crystal was performed using EUV-FEL(first experiment for solid material) Luminescence intensity depends on delay time between pump and probe laser. Amplification of luminescence originating from probe light was observed. Procedure of pump and probe experiment with FEL was established. Future plan: VUV wavelength-emitting material