Optical properties of surfaces with supercell ab initio calculations: Local-field effects

TitleOptical properties of surfaces with supercell ab initio calculations: Local-field effects
Publication TypePalaiseau Article

GENCI-IDRIS Grant 090544 and CCRT-DSM Grant 100.

Refereed DesignationRefereed
Tancogne-Dejean, N, Giorgetti, C, Veniard, V
Year of Publication2015
JournalPhys. Rev. B
Keywordslow-dimensional, surface

Surface optical and electronic properties are crucial for material science and have implications in fields as various as nanotechnology, nonlinear optics, and spectroscopies. In particular, the huge variation of electronic density perpendicular to the surface is expected to play a key role in absorption due to local-field effects.
Numerous state-of-the-art theoretical and numerical ab initio formalisms developed for studying these properties
are based on supercell approaches, in reciprocal space, due to their efficiency. In this paper, we show that the
standard scheme fails for the out-of-plane optical response of the surface. This response is interpreted using
the “effective-medium theory” with vacuum and also in terms of interaction between replicas, as the supercell
approach implies a periodicity which is absent in the real system. We propose an alternative formulation, also
based on the supercell, for computing the macroscopic dielectric function. Application to the clean Si(001) 2 × 1
surface allows us to present the effect of the local fields for both peak positions and line shape on the bulk and
surface contributions. It shows how local fields built up for the in-plane and out-of-plane dielectric responses of
the surface. In addition to their conceptual impact, our results explain why the standard approach gives reliable
predictions for the in-plane components, leading to correct reflectance anisotropy spectra. Our scheme can be
further generalized to other low-dimensional geometries, such as clusters or nanowires, and open the way to
nonlinear optics for surfaces.

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