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Spectroscopy

Femtosecond ultrasonic spectroscopy

Appl. Phys. Lett. 99, 051913 (2011)

Acoustic waves in the gigahertz–terahertz range are useful for studying lattice dynamics and manipulating electron–phonon and photon–phonon coupling in condensed matter. However, performing broadband ultrasonic experiments at frequencies of up to 1 THz is still a challenge. Now, Yu-Chieh Wen and co-workers from National Taiwan University, Academia Sinica and National Cheng Kung University in Taiwan have developed ultrabroadband ultrasonic spectroscopy. The researchers measured the acoustic attenuation coefficient of a 22-nm-thick vitreous SiO2 film deposited on a 7-nm-thick GaN layer by plasma-enhanced chemical vapour deposition. They grew a 3-nm-thick In0.14Ga0.86N/GaN single quantum well (SQW) under the GaN layer by metal–organic chemical vapour deposition. The indium content of the SQW was chosen to reduce reflections of acoustic pulses at the boundary between the SQW and GaN. The atomically flat boundaries of each layer also reduced roughness-induced phonon scattering. Illuminating the surface with laser pulses (wavelength 410 nm and pulse width 200 fs) caused the generation of acoustic pulses through the piezoelectric effect. The temporal spectrum of the transmission exhibited two characteristic dips at 3.3 ps and 11.2 ps, which were acoustic echoes reflected from the two interfaces of the SiO2 film. The researchers analysed these echoes to obtain an acoustic attenuation coefficient of 180–650 GHz.

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Horiuchi, N. Femtosecond ultrasonic spectroscopy. Nature Photon 5, 644 (2011). https://doi.org/10.1038/nphoton.2011.276

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