Measurement and interpretation of the mid-infrared properties of single crystal and polycrystalline gold

The contribution of electron–phonon scattering and grain boundary scattering to the mid-IR (λ = 3.392 μm) properties of Au has been assessed by examining both bulk, single crystal samples—Au(1 1 1) and Au(1 1 0)—and thin film, polycrystalline Au samples at 300 K and 100 K by means of surface plasmon polariton excitation. The investigation constitutes a stringent test for the in-vacuo Otto-configuration prism coupler used to perform the measurements, illustrating its strengths and limitations. Analysis of the optical response is guided by a physically based interpretation of the Drude model. Relative to the reference case of single crystal Au at 100 K (ε = −568 + i 17.5), raising the temperature to 300 K causes increased electron–phonon scattering that accounts for a reduction of ∼40 nm in the electron mean free path. Comparison of a polycrystalline sample to the reference case determines a mean free path due to grain boundary scattering of ∼17 nm, corresponding to about half the mean grain size as determined from atomic force microscopy and indicating a high reflectance coefficient for the Au grain boundaries. An analysis combining consideration of grain boundary scattering and the inclusion of a small percentage of voids in the polycrystalline film by means of an effective medium model indicates a value for the grain boundary reflection coefficient in the range 0.55–0.71.