  Intentional breaking of the lattice symmetry in solids is a key concept to
alter the properties of materials by modifying their electronic band structure.
However, the correlation of strain-induced effects and breaking of the lattice
symmetry is often indirect, resorting to vibrational spectroscopic techniques
such as Raman scattering. Here, we demonstrate that reflectance anisotropy
spectroscopy (RAS), which directly depends on the complex dielectric function,
enables the direct observation of electronic band structure modulation.
Studying the strain-induced symmetry breaking in copper, we show how uniaxial
strain lifts the degeneracy of states in the proximity of the both L and X
symmetry points, thus altering the matrix element for interband optical
transitions, directly observable in RAS. We corroborate our experimental
results by analysing the strain-induced changes in the electronic structure
based on ab-initio density functional theory calculations. The versatility to
study breaking of the lattice symmetry by simple reflectance measurements opens
up the possibility to gain a direct insight on the band-structure of other
strain-engineered materials, such as graphene and two-dimensional (2D)
transition metal dichalcogenides (TMDCs).
