Large anisotropy in the optical reflectance of Ag(110) single crystals: Experiment and theory

Abstract

We report the first observation of a large (more than 10%) reflectance anisotropy on a Ag(l 10) single crystal. We reproduce the experimental data fairly well by use of a theoretical model, based on the quasistatic approximation, which takes into account both the actual geometry of the crystal surface and the interband and intraband transitions. This demonstrates that the main effect is the screened surface local-field effect on resonant dipolar oscillations localized near the surface.PACS numbers: 78.66. Bz, 73.20.Mf, 78.20.Ci The symmetry of crystal surfaces has been shown to have a great influence on the physical and chemical properties of materials [1][2][3]. It is therefore of crucial interest to reach the surface properties of crystals by means of nondestructive techniques which can be applied in environments other than vacuum, such as gas or liquids. Reflectance difference spectroscopy (RDS) is a powerful tool for studying semiconductor surfaces [4,5] and monitoring semiconductor crystal growth, even in a gas atmosphere [6]. However, up to now, metal surfaces have not been investigated by RDS, although anisotropy effects have been observed on (110) surfaces of single Ag crystals by electroreflectance [7,8] and by electron energy loss spectroscopy experiments [9], and the dispersion relation of surface plasmons has been shown to depend on the direction of propagation along the surface [8,9].In this Letter we present an experimental and theoretical investigation of the optical reflectance of Ag(l 10). In particular, we report the first experimental observation by RDS of an anisotropy in the optical reflectance of the Ag(l 10) surface, the size of which is larger than that of semiconductor surfaces by 1 to 2 orders of magnitude [4,10,11]. We also reproduce fairly well the experimental reflectance anisotropy (RA) with a theoretical model which takes into account both d and s-p electron transitions, as well as the crystalline geometry of the (110) surface [12]. This large optical anisotropy originates from resonances in the surface conductivities, which are found to be different for the two principal symmetry directions of the (110) surface.The Ag single crystals were disks of 8 mm diameter and 2 mm thickness, cut from the same crystal rod by electroerosion. They were oriented by Laue x-ray back diffraction and mechanically polished to a mirrorlike finish using successively finer grades of diamond polishing suspension down to 0.25 jirn. Then they were chemically polished using a well-established chromium trioxide/hydrochloric acid etching procedure described previously [13]. This chemical treatment results in the selective dissolution of the cold-rolling layer formed during the mechanical polishing and was shown to provide welldefined single crystal Ag surfaces by capacitance and scanning tunneling microscopy studies [13]. The real and imaginary parts of the RAAr/r -(/•[,j 0 ] -r[ooi])/nooiJ were measured with a commercial RDS instrument, developed by ISA Jobin-Yvon Company (France). /*[n 0 ] and nooi] are t...

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