27,28 However, the rapid development of ultrafast diffraction methods now allows direct recording of ultrafast lattice dynamics through diffraction patterns using femtosecond electron or x-ray pulses. 26 These methods typically require theoretical modeling to connect femtosecond reflection and transmission data with microscopic lattice dynamics. Other all-optical ultrafast characterization methods include transient transmittance measurements 25 and transient absorption spectroscopy. Ultrafast transient reflectivity has been widely employed to create and investigate coherent phonons, 22–24 accompanied by strains that induce observable changes in dielectric constants. 20,21Įxperimental observation of the sequential vibration period of coherent phonons often requires the use of ultrafast time-resolved pump–probe configurations to reveal terahertz-picosecond dynamics. 19 Generally, coherent longitudinal acoustic phonons (CLAPs) are more common in solids, while generating transverse coherent acoustic phonons requires specific crystal geometry selection. 18 Coherent acoustic phonons are classified into longitudinal modes that propagate as longitudinal strain pulses in crystals and transversal modes visualized as the propagation of shear strains. They can be generated via impulsive stimulated Brillouin scattering (ISBS) in transparent media, 15 thermoelastic effects in metals, 16 deformation potential coupling in semiconductors, 17 and piezoelectric effects in piezoelectric materials. 14 Conversely, coherent acoustic phonons have a broad spectrum ranging from GHz to a few THz frequencies. Coherent optical phonons, typically within the THz frequency range, are generated through mechanisms such as impulsive stimulated Raman scattering (ISRS), 12 displacive excitation of coherent phonons (DECP), 13 and resonant ISRS. Our findings demonstrate that acoustic impedance can serve as an effective means to control coherent phonons in nanometer-thin films and may also play a crucial role in phonon engineering at interfaces or heterostructures.Ĭoherent phonons encompass both optical and acoustic branches, corresponding to different frequencies and excitation mechanisms. However, interestingly, when examining L a 0.7 C a 0.175 S r 0.125 Mn O 3 / B a 0.5 S r 0.5 Ti O 3 bilayers, no oscillations are observed due to the favorable impedance matching between the layers. In contrast, in ultrafast x-ray diffraction experiments, we discover that CLAPs are partially confined within an Au (111) thin film due to the mismatch of acoustic impedance with the substrates, leading to an oscillation period of 122 ps. We observe that CLAPs can efficiently propagate from a LaMnO 3 thin-film to its SrTi O 3 substrate due to the matching of their acoustic impedance, and the oscillation period increases from 54 to 105 GHz. In this study, we present our findings on the propagation of laser-induced CLAPs at thin-film interfaces and heterojunctions using ultrafast optical reflectivity and ultrafast x-ray diffraction measurements. The manipulation of CLAPs' behavior is thus of significant interest for potential applications, particularly in achieving ultrafast modulations of material properties. Femtosecond laser excitation of crystal materials can produce coherent longitudinal acoustic phonons (CLAPs), which possess the capability to interact with various quasiparticles and influence their dynamics.
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