  We have studied spin excitation spectra in the Shastry-Sutherland model
compound SrCu$_2$(BO$_3$)$_2$ in magnetic fields using far-infrared Fourier
spectroscopy. The transitions from the ground singlet state to the triplet
state at 24 cm$^{-1}$ and to several bound triplet states are induced by the
electric field component of the far-infrared light. To explain the light
absorption in the spin system we invoke a dynamic Dzyaloshinskii-Moriya (DM)
mechanism where light couples to a phonon mode, allowing the DM interaction.
Two optical phonons couple light to the singlet to triplet transition in
SrCu$_2$(BO$_3$)$_2$. One is $a$-polarized and creates an intra-dimer dynamic
DM along the c axis. The other is $c$-polarized and creates an intra-dimer
dynamic DM interaction, it is in the $(ab)$ plane and perpendicular to the
dimer axis. Singlet levels at 21.5 and 28.6 cm$^{-1}$ anti-cross with the first
triplet as is seen in far-infrared spectra. We used a cluster of two dimers
with a periodic boundary condition to perform a model calculation with scaled
intra- and inter-dimer exchange interactions. Two static DM interactions are
sufficient to describe the observed triplet state spectra. The static
inter-dimer DM in the c-direction $d_1=0.7$ cm$^{-1}$ splits the triplet state
sub-levels in zero field [C\'{e}pas et al., Phys. Rev. Lett. \textbf{87},
167205 (2001)]. The static intra-dimer DM in the $(ab)$ plane (perpendicular to
the dimer axis) $d_2=1.8$ cm$^{-1}$, allowed by the buckling of CuBO$_3$
planes, couples the triplet state to the 28.6 cm$^{-1}$ singlet as is seen from
the avoided crossing.
