  The anisotropic optical response of the layered, nodal-line semimetal ZrSiS
at ambient and high pressure is investigated by frequency-dependent
reflectivity measurements for the polarization along and perpendicular to the
layers. The highly anisotropic optical conductivity is in very good agreement
with results from density functional theory calculations and confirms the
anisotropic character of ZrSiS. Whereas the in-plane optical conductivity shows
only modest pressure-induced changes, we found strong effects on the
out-of-plane optical conductivity spectrum of ZrSiS, with the appearance of two
prominent excitations. These pronounced pressure-induced effects can neither be
attributed to a structural phase transition according to our single-crystal
x-ray diffraction measurements, nor can they be explained by electronic
correlation and electron-hole pairing effects, as revealed by theoretical
calculations. Our findings are discussed in the context of the recently
proposed excitonic insulator phase in ZrSiS.
