  Excitation of coherent optical phonons in solids provides a pathway for
ultrafast modulation of light on a sub-ps timescale. Here, we report on
efficient 3.6 THz modulation of light reflected from hybrid metal/semiconductor
plasmonic crystals caused by lattice vibrations in a few nm thick layer of
elemental tellurium. We observe that surface plasmon polaritons contribute
significantly to photoinduced formation of this thin layer at the interface
between a telluride-based II-VI semiconductor, such as (Cd,Mg)Te or (Cd,Mn)Te,
and a one-dimensional gold grating. The change in interface composition is
monitored via the excitation and detection of coherent optical tellurium
phonons of $A_1$ symmetry by femtosecond laser pulses in a pump-probe
experiment. The patterning of a plasmonic grating onto the semiconductor
enhances the transient signal which originates from the interface region. This
allows monitoring the layer formation and observing the shift of the phonon
frequency caused by confinement of the lattice vibrations in the nm-thick
segregated layer. Efficient excitation and detection of coherent optical
phonons by means of surface plasmon polaritons are evidenced by the dependence
of the signal strength on polarization of pump and probe pulses and its
spectral distribution.
