The Official Site of Hideo Takeuchi, Ph. D.,

Research description

Theme II. Coherent phonons: Ultrafast optical pump-probe measurements

Ultrafast spectroscopy of coherent phonons in superlattices

1 : Summary of outline, objectives and outcomes

The progress of ultrafast spectroscopy, which is based on femtosecond pulse laser technologies, enables the generation of coherent phonons and plasmons, the in-phase collective motions of the atoms and carriers, respectively. The coherent phonons are detected as the time-domain signals of modulated optical constants. The coherent phonons not only provide a clue to elucidate the ultrafast phenomena in semiconductors but also lead to THz radiation. When I started the investigation of coherent phonons, coherent phonons in superlattices (SLs) were attracting much at-tention; however, the detection/generation mechanisms were not clear in detail. To clarify these mechanisms, I directed my attention to the spatial hierarchy of phonons in SLs. Three kinds of coherent phonons were investi-gated: (a) coherent longitudinal optical (LO) phonons in GaSb/AlSb SLs confined in each constituent layer, (b) coherent folded longitudinal acoustic (FLA) phonons in GaAs/AlAs SLs extending to the whole SL, and (c) coherent LO phonon-plasmon coupled (LOPC) mode in InAs/GaAs SLs with a electron miniband structure.

In this research, a reflection-type pump-probe measurement system was constructed using a femtosecond pulse laser (~70 fs). In addition, the time-domain signal analysis method using the Cole-Cole plot was developed in or-der to estimate the phase of the coherent phonon related to the generation mechanism.

At first, investigated are the generation mechanisms of the coherent LO phonons in the GaSb/AlSb SLs. It was found that laser pulses producing carriers only in the GaSb well layer can simultaneously generate the co-herent AlSb-like LO phonons in the barrier layer as well as the coherent GaSb-like LO phonons in the well layer. The Cole-Cole plot clarified the difference in the phase between the coherent AlSb-like and GaSb-like LO pho-nons; namely, the generation mechanisms of the two coherent LO phonon modes are different each other.

Next, the coherent FLA phonons in the GaAs/AlAs SLs with limited periodicity were investigated. It was found that finite size effects originating from limited periodicity relax the selection rules both for the phonon wave vector and for the mode symmetry. In addition, it was revealed that the observed phonon mode depends on whether the laser pulses excite the whole SL structure or only the surface layer.

The coherent LOPC mode in the InAs/GaAs SLs was also investigated. It was demonstrated that the laser pulses, of which photon energy is almost equal to fundamental transition energy of each SL but smaller than that of GaAs, can generate the coherent plasmon coupled with the LO phonon confined in the GaAs barrier layer. It was clarified from the dependence of the LOPC mode on the detection energy that the detection mechanism of the LOPC mode is dominated by the resonant effects arising from the miniband structures of the SLs. The research activities on PR brought the following 8 papers. I would like to emphasize that the experience obtained in this research field leads to the successful results of Thema IV, terahertz electromagnetic waves.

2 : List of outcomes

_Publications

1 "Coherent longitudinal optical phonons in GaSb/AlSb superlattices".
H. Takeuchi, K. Mizoguchi, M. Nakayama, H. Nishimura, K. Kuroyanagi, and T. Aida.
Special Issue, Memoirs of The Institute of Scientific and Industrial Research, Osaka University vol.57, pp.292-293 (2000).
-Proceedings of the Third SANKEN Int. Symp. "Advanced Nanoelectronics: Devices, Materials, and Computing", Osaka, Japan, (2000).
The manuscript is here.

2 "Phase difference between coherent GaSb-like and AlSb-like LO phonons in GaSb/AlSb superlattices".
H. Takeuchi, K. Mizoguchi, M. Nakayama, K. Kuroyanagi, and T. Aida.
Springer Proceedings in Physics vol.87, pp.451-452 (2001).
-Proceedings of the 25th International Conference on the Physics of Semiconductors, Osaka, Japan, (2000).
# Link to the publisher's site.

3 "Simultaneous observation of coherent GaSb-like and AlSb-like longitudinal optical phonons in GaSb/AlSb superlattices".
H. Takeuchi, K. Mizoguchi, M. Nakayama, K. Kuroyanagi, T. Aida, M. Nakajima, and H. Harima.
Journal of the Physical Society of Japan vol.70, pp.2598-2602 (2001).
# Link to the online-journal site.

4 "Coherent phonon-plasmon coupled modes in (InAs)1/(GaAs)m strained-layer superlattices".
H. Takeuchi, K. Mizoguchi, and M. Nakayama.
IPAP Conference Series 2 (CS2), pp.207-209 (2001).
-Proceedings of the 10th International Conference on Narrow Gap Semiconductors and Related Small Energy Phenomena, Ishikawa, Japan, ( 2001).
# Link to the online-journal site.

5 "Finite-size effects on coherent folded acoustic phonons in GaAs/AlAs superlattices".
K. Mizoguchi, H. Takeuchi, T. Hino and M. Nakayama.
Journal of Physics: Condensed Matter vol.14, pp.L103-L109 (2002).
# Link to the online-journal site.

6 "Effects of a miniband structure on coherent LO phonon-plasmon coupled modes in an (InAs)1/(GaAs)30 strained-layer superlattice".
H. Takeuchi, K. Mizoguchi, T. Aida and M. Nakayama.
Physica B: Physics of Condensed Matter vol.314, pp.422-426 (2002).
-Proceedings of the 12th International Conference on Nonequilibrium Carrier Dynamics in Semiconductors, Sante Fe, New Mexico, U.S.A., >(2001).
# Link to the online-journal site.

7 "Coherent folded acoustic phonons in GaAs/AlAs superlattices with limited periodicity".
H. Takeuchi, K. Mizoguchi, T. Hino and M. Nakayama.
Physica B: Physics of Condensed Matter vol.316-317, pp.308-310 (2002).
-Proceedings of the 10th International Conference on Phonon Scattering in Condensed Matter, New Hampshire, U.S.A., (2001).
# Link to the online-journal site.

8 "Dynamical properties of coherent plasmons coupled with LO phonons in an InAs/GaAs strained superlattice".
K. Mizoguchi, H. Takeuchi, and M. Nakayama.
Phase Transitions vol. 75, pp. 895-902 (2002).
-Proceedings of the International Conference on Photoinduced Phase Transitions, their Dynamics and Precursor, Tsukuba, Japan, (2001).
# Link to the online-journal site.

3 : Posters and viewgraphs

1 "Coherent longitudinal optical phonons in GaSb/AlSb superlattices".
H. Takeuchi, K. Mizoguchi, M. Nakayama, H. Nishimura, K. Kuroyanagi, and T. Aida.
The Third SANKEN International Symposium, "Advanced Nanoelectronics: Devices, Materials, and Computing", March 15, 2000, SANKEN, Osaka University, Osaka, Japan.
The symposium proceeding is here.

2 "Initial phase difference between coherent GaSb-like and AlSb-like LO phonons in GaSb/AlSb superlattices".
H. Takeuchi, K. Mizoguchi, M. Nakayama, K. Kuroyanagi, and T. Aida.
The 25th International Conference on the Physics of Semiconductors, September 18, 2000, Osaka International Convention Center, Osaka, Japan.
The poster presented at the conference is here.

3 "Coherent phonon-plasmon coupled modes in (InAs)1/(GaAs)m strained-layer superlattices".
H. Takeuchi, K. Mizoguchi, and M. Nakayama.
The 10th International Conference on Narrow Gap Semiconductors and Related Small Energy Phenomena, Physics and Applications , May 27-31, 2001, Ishikawa, Japan.
The poster presented at the conference is here.

4 "Coherent folded acoustic phonons in GaAs/AlAs superlattices with limited periodicity".
H. Takeuchi, K. Mizoguchi, T. Hino, and M. Nakayama.
The 10th International Conference on Phonon Scattering in Condensed Matter (Phonon 2001), August 12-17, 2001, New Hampshire, U.S.A.
The poster presented at the conference is here.

5 "Effects of a miniband structure on coherent LO phonon-plasmon coupled modes in an (InAs)1/(GaAs)30 strained-layer superlattice".
H. Takeuchi, K. Mizoguchi, T. Aida, and M. Nakayama.
The 12th International Conference on Nonequilibrium Carrier Dynamics in Semiconductors (HCIS 12), August 27-31, Sante Fe, New Mexico, U.S.A.
The poster presented at the conference is here.



Copyright 2013 Hideo Takeuchi.