  Optical properties of nodal-line semimetal ZrSiS are studied using
first-principles calculations. Frequency-independent optical conductivity is a
fingerprint of the infrared optical response in ZrSiS. We find that this
characteristic feature is robust with respect to uniaxial compressive strain of
up to 10 GPa, yet with the flat region being narrowed with increasing strain.
Upon uniaxial tensile stress of 2 GPa, the Fermi surface undergoes a Lifshitz
transition accompanied by a weakening of the interband screening, which reduces
the spectral weight of infrared excitations. We also show that the high-energy
region is characterized by low-loss plasma excitations at $\sim$20 eV with
essentially anisotropic dispersion. Strongly anisotropic dielectric properties
suggest the existence of a hyperbolic regime for plasmons in the deep
ultraviolet range. Although the frequencies of high-energy plasmons are
virtually unaffected by external uniaxial deformation, their dispersion can be
effectively tuned by strain.
