Abstract
Femtosecond pump-probe experiments with a ∼6.4 fs time-resolution were performed to investigate the coherent phonon dynamics in a c-plane sapphire crystal before and after intense 800 nm femtosecond laser irradiation. The intense femtosecond laser induced defect/distortion and even re-crystallization of crystalline structures, which result in the appearance of new peaks and relative intensity change in coherent phonon and Raman spectra. The combination of these two spectra was found to be beneficial to evidence the variation of crystalline structure and further to differentiate the origins of new Raman peaks after irradiation. Further analysis of time-dependent differential absorbance with damped cosine function fitting and Fourier transfer calculation yields the vibrational parameters, including periods, damping times and initial phases, before and after irradiation. With these parameters, the defect-effects on damping time and the mechanism of coherent phonon generation were addressed.
Highlights
Sapphire has been intensively studied as substrate materials due to its excellent mechanical, physical and chemical properties [1,2,3,4], such as high transmittance, high hardness, high melting point, strong abrasion resistance and corrosion resistance, and so on
coherent phonon (CP) spectra before and after irradiation were obtained by performing Fourier transfer (FT) of ∆A spectra, as depicted in Figs. 2(c) and (d), respectively
The intense irradiation induced defect/distortion and even recrystallization of crystalline structures, leading to appearance of new peaks and relative intensity change in CP and Raman spectra. The combination of these two spectra is proved to be beneficial to differentiate the origins of new peaks, i.e., to distinguish whether they are from Raman active modes or fluorescent impurity ions
Summary
Sapphire has been intensively studied as substrate materials due to its excellent mechanical, physical and chemical properties [1,2,3,4], such as high transmittance, high hardness, high melting point, strong abrasion resistance and corrosion resistance, and so on. The laser pulse duration in most of the work on ultrafast spectroscopy is hitherto around 100 fs or even longer It is only suitable for the study of low-frequency vibrations since the CP generation requires the pulse duration much shorter than the phonon periods [14,19]. It was reported that typical Raman spectrum of sapphire crystal includes at least 7 active phonon modes (2A1g + 5Eg) [10], further studies necessitate much shorter pulse duration to resolve more details of the CP dynamics. The intense fs-laser induced defect/distortion of crystalline structures, which brought about appearance of relative intensity change and even new peaks in CP and Raman spectra. The defect-effects on damping time and the mechanism of CP generation were further discussed
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