Piezoreflectivity of the Noble Metals

The piezoreflectance spectra of evaporated films of copper, silver, and gold have been measured in the range 1.5-5.0 eV. The experimental results were subjected to a Kramers-Kronig analysis in order to determine the corresponding phase changes $\Delta \theta$. In this manner a complete specification of the optical properties of the strained material is made. The conditions which must be fulfilled if this Kramers-Kronig inversion is to be made are discussed. The strain-induced changes in the real and imaginary parts of the dielectric constant tensor are found to be quite similar for all three materials. The over-all structure shows qualitative agreement with the deformation-potential model, and the signs of the deformation potentials of some of the transitions are tentatively assigned. The results for all three metals show additional structure in a region lower in energy than that for which structure has been previously detected. The most reasonable explanation for this is that there are interband transitions possible at lower energy than has previously been presumed. The similarity of this structure in $\Delta {\epsilon }_2$ for all three metals indicates that the same transitions are involved for copper, silver, and gold.