  With infrared and terahertz ellipsometry we investigated the anisotropy of
the infrared active phonon modes in SrTiO3 (110) single crystals in the
tetragonal state below the so-called antiferrodistortive transition at T* = 105
K. In particular, we show that the anisotropy of the oscillator strength of the
so-called R-mode, which becomes weakly infrared active below T*, is a valuable
indicator for the orientation of the structural domains. Our results reveal
that a mono-domain state with the tetragonal axis (c-axis) parallel to the
[001] direction can be created by applying a moderate uniaxial stress of about
2.3 MPa along the [1-10] direction (with a mechanical clamp). The resulting,
intrinsic splitting of the infrared-active phonon modes is reported.
