HYDRODYNAMIC MODEL FOR SUM AND DIFFERENCE-FREQUENCY-GENERATION AT METAL-SURFACES

We develop a hydrodynamic model for the calculation of sum and differe nce frequency generation (SFG/DFG) at the surface of nonlocal conducto rs with arbitrary equilibrium electronic density profiles n(0). We app ly our model to simple profiles and calculate the nonlinear surface su sceptibility tensor chi(zzz)(s)(omega(1),omega(2)) and the radiated ef ficiency R(omega(3) = omega(1) +/- omega(2)) as a function of the pump frequencies omega(1) and omega(1). R is strongly enhanced due to the excitation of the dipolar surface plasmon characterized by a resonant frequency omega(d); it displays ridges whenever omega(1), omega 2, Or omega(3) approximate to 5 omega(d) , an additional ridge at the bulk p lasma frequency omega(3) approximate to omega(b) , and very large doub le resonance peaks whenever two ridges cross each other. These results suggest that SFG/DFG spectroscopy might be a useful probe of surface collective modes. [S0163-1829(98)03403-1].