Symmetry and light coupling to magnetic excitations in SrCu2(BO3)2

We present a T = 3 K Raman scattering study in the spin-dimer compound SrCu$_{2}$(BO$_{3})_{2}$. The symmetry of the elementary triplets and magnetic bound states in the 0 - 60 cm$^{-1}$ energy range are experimentally determined. A 4-spin cluster approximation is able to reproduce the symmetry and the anisotropic dispersions in external magnetic fields of the spin gap modes. We propose a light coupling mechanism induced by intra-dimer antisymmetric interactions and discuss the observation of a distinct Raman resonance behavior of the elementary and composite magnetic modes.