Experimental intensity analysis of second harmonic generation at the Cu(110) surface

We have analyzed second harmonic generation (SHG) intensities from Cu(110) at fundamental wavelengths lambda = 1064 nm and between lambda = 650 and 54 0 nm. Experimentally the light incidence direction was chosen along the two inequivalent mirror planes of the surface lattice unit cell, and the linea r polarizations of both input (fundamental) and output (frequency-doubled) radiation could be varied independently. At lambda=1064nm the relative size s of the different components of the second-order susceptibility tensor are as follows: \chi(zzz)\=2490, \chi(yzy)\=139 and \chi(zyy)\ =33.7. The rema ining elements are below detection threshold: \chi(xzx)\<3 and \chi(zxx)\<3 . This analysis is based on the use of Fresnel coefficients and bulk optica l constants. The results indicate that SHG is dominated by transitions indu ced by the z-components of the incident electric field. However, the situat ion is completely different in the intensity maximum (lambda=600 nm) of a r esonant intersurface band transition occurring around the (Y) over bar-poin t of the surface Brillouin zone: now SHG is dominated by the zyy tensor com ponent, which exceeds both yzy and zzz, while again \chi(zxx)\ and \chi(xzx )\ are negligible. All available data, including temperature-dependent SHG studies of different authors, clearly show that contributions of electronic surface states to SHG intensities may be significant or even dominant. The refore, a detailed quantitative understanding is a necessary condition for any analysis of adsorbate-induced SHG signals, since adsorbates can modify the surface electronic properties considerably. Our results furthermore ind icate that the use of isotropic Fresnel coefficients to model the SHG-activ e electric fields is inadequate for a complete quantitative analysis. (C) 2 000 Published by Elsevier Science B.V. All rights reserved.