Abstract
We have determined a refractive index along the crystal long axis of an orthorhombic thiophene/phenylene co-oligomer (TPCO) crystal. From the space group symmetry of the crystal, the three crystallographic axes are identical to the principal axes of the refractive indices. We estimated the index to be 3.76 on the basis of the mode intervals of the equally-spaced longitudinal multimode laser oscillation which is highly polarized along the crystal long axis (i.e., the a-axis). This index is much larger than those associated with the other axes. These experimental indices were compared to the semiempirically calculated ones, in qualitatively good agreement with them.