Optical excitation of a coherent transverse optical phonon in a polycrystalline Zr metal film

Optical pump-probe measurements of transient reflectivity were made on a sputtered polycrystalline thin film of hcp Zr. On top of a slow multiexponential decay, an oscillatory signal was observed with frequency varying from the initial value of f₀=2.24±0.06 THz at zero time delay to a value of 2.56±0.12 THz after 2 ps, which is similar to that of a transverse optical (TO) phonon near the Γ point of the Brillouin zone. From the data, the TO phonon relaxation time was determined to be 0.67 ps. The excitation of the coherent phonon was identified as being mediated by resonant coherent Raman scattering.