Wavelength Dependence of Total and Depolarized Back-Scattered Laser Light from Rough Metallic Surfaces

The wavelength dependence of both the total back-scatter cross section σ_T and the depolarized back-scatter cross section σ_D for rough metallic surfaces of known statistical characteristics has been determined experimentally by comparing data at 0.63-, 3.39- and 10.6 μ wavelengths. The rms height from the mean, h, and the mean scale size l of the two surfaces used in the experiment are: h≈1 μ, l≈10 μ; h≈7 μ, l≈50 μ. At or near normal incidence, results show that the total cross section per beam area σ_T/A₀ is independent of wavelength λ, provided that h/λ is greater than approximately $\frac{1}{4}$. When $h/\mathrm{\lambda }\le \frac{1}{4}$, σ_T/A₀ increases rapidly with decreasing h/λ. Previous microwave data suggest that the metallic surface reflects nearly as a perfectly smooth surface without significant scattering losses when h/λ≲1/40. At or near normal incidence the ratio of σ_D to σ_T varies as (h/l)⁴λ/4πδ for all values of h/λ studied, where δ is the skin depth of the metallic surface. For incident angles ψ in the range from 20° to 80° and $h/\mathrm{\lambda }>\frac{1}{4}$, both σ_T/A₀ and σ_D/A₀ vary as λ⁻ⁿ where n increases with increasing ψ. n has a value of 0.40 (±0.2) at 4ψ=20° and 0.8 (±0.2) at 80°.