Type of presentation: Poster 

IT-8-P-2148 Sub-picosencond electron beam and femtosecond optical pump systemin spin-polarized TEM 

Kuwahara M.1, Nambo Y.1, Saitoh K.2, Ujihara T.1, Asano H.1, Takeda Y.3, Tanaka N.2 1Graduate School of Engineering, Nagoya University Nagoya, 464-8603, Japan, 2EcoTopiaScience Institute, Nagoya University, Nagoya 464-8603, Japan, 3Nagoya Industrial ScienceResearch Institute, Nagoya 464-0819, Japan 

Email of the presenting author: kuwahara@esi.nagoya-u.ac.jp Time-resolved measurements at nanometer spatial resolutions are very important forinvestigating relaxation processes, catalyzed reactions and phase-transition phenomena. It ispossible to carry out such time-resolved analysis using transmission electron microscopy (TEM)by using a pulsed electron beam as a probe beam. Such an approach has been applied indynamic TEM (DTEM) and ultra-fast electron microscopy (UEM), which use metals and LaB6 fora photoemission source driven by a pulsed laser. These methods have led to the possibility offour-dimensional electron microscopy with high spatial and temporal resolutions.Spin-polarized transmission electron microscopy (SP-TEM) can satisfy two abilities ofspin-resolved imaging and pulsed electron gun operation simultaneously, because theinstrument consists of a laser-driven polarized electron source (PES) and a conventional TEMsystem [1].Spin-polarized electron can be generated by photoemission from III–V semiconductors with anegative electron affinity (NEA) surface. Several beam parameters of a PES are vastly superiorto those for conventional thermal electron beams. A high spin-polarization of 92% and a highquantum efficiency of 0.5% have been simultaneously realized using a GaAs–GaAsPstrained-layer-superlattice photocathode. In addition, such a photocathode has the ability togenerate a sub-picosecond multibunch beam [2]. In order to realize a pump-probe methodusing the spin-polarized pulse electron beam, we have developed a synchronizing system anddemonstrated a phase-locked TEM image of wobbling probe beam by using a pulse electronbeam [3]. TEM images were already acquired with a pulsed electron beam with a 1.4-ns pulseduration. Now we have constructed a new optical system, which can provide a sub-picosecondpulse laser and femtosecond pulse simultaneously, to realize an ultrafast temporal resolution.The figure 1 (b) and (c) show the photograph and the schematic diagram, respectively. Thesub-picosecond pulse laser is used to drive the electron gun. Another femtosecond laser istransferred to pump a specimen to create an excited state. The sub-picosecond pulse isgenerated by narrowing a bandwidth of a seed laser which is emitted from a mode-lockTi:Sapphire laser. Figure 2 shows the femtosecond and picosecond pulse. The sub-picosecondpulse laser is necessary to keep the high polarization. These results suggest the possibility ofpump-probe measurements in SP-TEM using the pulsed electron beam as a probe, allowingnanometer-scale time-resolved spin mapping.[1] M. Kuwahara et al., Appl. Phys. Lett. 101 (2012) 03310[2] Y. Honda, et al., Jpn. J. Appl. Phys. 52, 086401-086407(2013).[3] M. Kuwahara et al., Microscopy 62, 607-614 (2013). 

Acknowledgement: The authors thank Drs. H. Shinada, M. Koguchi and M. Tomita of HitachiCentral Research Laboratory for fruitful discussions and encouragement. This research wassupported by MEXT KAKENHI Grant Number 51996964, 24651123, 25706031.

 Fig. 1:  (a) Photograph of the spin-polarized TEM, (b) Photograph of pulse laser system and (c) the schematics. 

 Fig. 2: Auto-correlation amplitudes of pulse lasers as a function of correlation time. (a) a correlation amplitude of afemtosecond pulse laser for excitation of a specimen. (b) a correlation amplitude of picosecond pulse laser for drivingan electron source.