Ultrafast Dynamics of Coherent Phonons in Thin Films and Free-Standing Membranes

Femtosecond pump-probe spectroscopy is an important tool for investigating ultrafast dynamics of coherent phonons in sub-μm-thick thin films and free-standing membranes. The absorption of the pump pulses generates a strain pulse which consists of highfrequency coherent acoustic phonons propagating into sample. As the strain pulse propagates it changes the local optical constants of the sample, which in turn leads to a reflectivity change. The relative reflectivity change is then recorded in the time domain by a time-delayed probe pulse train. In this dissertation, gigahertz to terahertz coherent phonons in sub-μm-thick skutterudites, zinc-blende (cubic) GaN, and GaAs are investigated by femtosecond pump-probe spectroscopy based on high-speed asynchronous optical sampling. Firstly, the phononic properties of thin films of CoSb3, FeSb3, and partially filled YbxCo4Sb12 skutterudites are studied. Skutterudites are considered as interesting material for thermoelectric applications. Filling foreign atoms into the cage-like structure of a CoSb3 skutterudite is beneficial to its thermoelectric properties by increasing phonon scattering while maintaining the electrical conductivity. We demonstrate the generation and detection of coherent acoustic phonons in thin films of CoSb3 and partially filled YbxCo4Sb12 skutterudites using femtosecond pump-probe spectroscopy. By using a pulse-echo method, the longitudinal sound velocity of amorphous and polycrystalline CoSb3 thin films is obtained. For partially filled YbxCo4Sb12 thin films, an obvious decrease of the longitudinal sound velocity is observed at high filling fractions. Concomitantly, the high frequency acoustic phonon modes are strongly damped as the Yb filling fraction increases, which gives direct evidence for an increase of acoustic phonon scattering processes. It is shown that the reduction of the lattice thermal conductivity after Yb filling is mainly achieved by a strong scattering of acoustic phonons. In addition, coherent optical phonons are excited and detected in thin films of YbxCo4Sb12 (x = 0.57 and 0.68) and FeyCo1−ySb3 (y = 0.49 and 1). The observation of the optical phonons are both related to their enlarged lattice constants after filling or substitution. Secondly, two sub-μm-thick semiconductor films, cubic GaN and GaAs, are studied by femtosecond pump-probe spectroscopy. Their phononic properties are thoroughly investigated by employing a free-standing membrane geometry, where the free-standing membranes are obtained through etching away the substrates. In the membranes, the absorption depth of the pump light, which has a wavelength of ∼800 nm for the GaAs membrane and ∼400 nm for the c-GaN membrane, is much larger than the membrane thickness, thus the strain is generated throughout the membrane. The optoexcited strain leads to a period thickness oscillation at gigahertz frequencies. A theoretical model is proposed to describe the ultrafast thickness oscillation of the membrane. Using this i model the phonon lifetime can be extracted despite of the limitation of the measurement window. Additionally, by using a frequency-doubled light, the excitation of the GaAs membrane generates a strain pulse which travels back and forth in the GaAs membrane. The detected echo signal has a double-peak structure in the frequency domain. Our analysis indicate that this is basically caused by the double-peak structure of the spectral transformation function of the acousto-optic conversion, KAO(ω). This function determines the effectiveness of different acoustic frequencies to be transformed into the spectrum of the reflected probe intensity variation in the detection region of the probe light.