ELSEVIER Physica B 219&220 (1996) 681 683 Picosecond ultrasonics study of metallic multilayers B. Perrin*, B. Bonello, J.-C. Jeannet, E. Romatet Dkpartement de Recherches Physiques, UniversitbPierre & Marie Curie, T22-E4, 4 Place Jussieu, F-75252 Paris cedex 05, France Abstract A picosecond ultrasonics study of Cu/W superlattices in periods between 5 and 20 nm has been performed. Determina- tion of acoustic echoes, observed over 200 ps, showed a softening of the longitudinal elastic constant for the shortest periods. High frequency oscillations were also detected over the first 20 ps. These oscillations, with frequencies extending up to 0.810THz, are tentatively attributed to localized surface modes in the first zone center frequency gap. 0. Introduction Picosecond ultrasonics has been proved very useful to study acoustic properties of multilayer structures [1 5]. In this paper, we report such a study performed on immiscible Cu/W metallic multilayers. 1. Theory The dispersion relation in alternated layered media for waves propagating along the stacking axis is given by COs(qd) = cos((gdl~ cos(°)d2~ \C1 ,/ \C2,] ~sin sin|--/, (1) \ (-:2 ,1 where q is the wave vector, d = d 1 ± d e the superperiod, and 1 ( plCI +p2c2~ = ~ \P2C2 plClJ * Corresponding author. is related to the acoustic contrast between the two layers; dl and d 2 are the thicknesses, Pl, P2 the densities, and cl, c2 the sound velocities of layers 1 and 2, respectively. A long wavelength expansion (qd---, 0) of this relation gives the effective velocity Cetf of the multilayer: (d~ ~d 2 2~d~d2~½ cerf = d + c~ + C1C2 / (2) This expression can also be written as d/Cef f = dl/C1 + d2/C2 and expresses the fact that the effective elastic constant Cetf is the harmonic mean of the elastic con- stants C1 and C2 of the layers 1 and 2 respectively; c:Z-r ~gives the slope of the dispersion curve in the Brillouin zone center as shown on Fig. 1. For semi-infinite layers, modes vibrating at frequencies V~o¢ [1,5] such that tan (2rrvl°cd2)+ plcl tan (27zv~ocd~ = 0 (3) \ (?2 / P2C2 \ C1 fl lying within the gaps and localized within the very first layers, may exist if the upper layer has the smallest acoustic impedance. Thus, the creation of a picosecond strain, by optical absorption of a short laser pulse in the near surface of a multilayer structure, will generate acoustic echoes bouncing back and forth in the 0921-4526/96/$t5.00 .C 1."95 Elsevier Science B.V. All rights reserved SSDI 0921-4526(95)00852-7