Elasto-Raman scattering: Arsenic optical phonon as a probe of nematicity in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Ba</mml:mi><mml:msub><mml:mi>Fe</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>As</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>

We report a Raman scattering study of nematic degrees freedom in the iron-based superconductor parent compound BaFe$_2$As$_2$ under tunable uniaxial strain. demonstrate that polarization resolved arsenic (As) phonon intensity can be used to monitor order parameter as function both temperature and At low ordered phase we use it track continuous reversible orientation domains variable higher temperature, evolution As strain reflects an enhanced susceptibility close transition $T_S$. Its dependence strong follows qualitatively expected behavior Ising symmetry breaking field. Our elasto-Raman illustrates interest combining selective anisotropic with probe like scattering. Elasto-Raman applied wide variety quantum materials where tunes electronic orders.