Enhancement of nonlinear processes near rough nanometer-structured surfaces obtained by deposition of fractal colloidal sliver aggregates on a plain substrate

In this paper we extend our previous results concerning the enhancement of nonlinear optical responses near rough self-affine metal surfaces to differ ent objects in a wider spectral range and also beyond the quasistatic limit . In our previous work [E. Poliakov et al, Phys. Rev. B 57, 14901 (1998)] w e studied local and averaged enhancement factors for the second- and third- harmonic generation and degenerate four-wave mixing for nonlinear molecules adsorbed on nanostructured self-affine metal surfaces in the quasistatic a pproximation in the spectral range from 0.4 to 1.8 mu m In the present work , we study the same nonlinear effects for surfaces formed by deposition on a plane substrate of fractal clusters rather than individual nanoparticles. The spectral range is extended to 10 mu m. We have calculated the enhancem ent factors for the above nonlinear process with the account of retardation and performed numerical calculations for significantly larger samples. Our numerical results indicate that the enhancement factors increase with the wavelength up to lambda = 10 mu m. The enhancement of the degenerate four-w ave mixing can reach, for example, the value of 10(22) in the far-infrared spectral region (the maximum enhancement reported by us earlier for this pr ocess was similar to 10(16) near lambda = 1.8 mu m). [S0163-1829(99)10539-3 ].