Real-Time Detection of Coherent Vibrational Dynamics in TiN Films

Titanium nitride (TiN) has recently gained considerable interest because of its remarkable plasmonic properties and for its strong electron–phonon (e–ph) coupling, leading to extremely fast (<100 fs) electron-lattice cooling. Here, the generation of coherent phonons in TiN films is reported, along with their real-time detection by means of broadband transient reflection spectroscopy with sub-15-fs temporal resolution. The measurements show damped oscillations, superimposed to excited state electronic decay. A coherent vibrational mode is revealed, with ≈10 THz frequency ascribed to defect-activated normal modes, consistent with spontaneous Raman scattering data, and a dephasing time of ≈250 fs. Two π-phase flips are also observed located at photon energies corresponding to interband optical transitions (at 3.2 and 2.5 eV), ascribed to selective coupling of the vibrational mode to these transitions; the energy modulation induced by the vibrational coherence is evaluated. It is shown that the displacive excitation of coherent phonons model describes the coherent response in terms of temporal behavior and of spectral amplitude profile. Overall, a comprehensive and detailed analysis of coherent phonons in TiN films, so far undected, is provided and relevant information on TiN photo-physical properties, potentially useful for its applications, is given.