  We present heterodyne detected transient grating (HD-TG) measurements on 1D
surface corrugated Phononic Crystal (PC) with characteristic band edge wave
vector of $\pi/5$ $\mu$m$^{-1}$. This experimental investigation enables both
the direct band diagram characterization of surface waves and a direct
measurement of the group velocity dispersion. The experimental data are
compared to the simulations performed with the structural-mechanic module of a
finite element method software. In this work we show the presence of two band
gaps, observe distinct types of surface waves and, for the first time to our
knowledge, measure extremely slow waves in correspondence of the band edge wave
vector.
