Linear optics at surfaces and interfaces

Abstract

Conventional surface science techniques have, over the last forty years, advanced our understanding of solid surfaces and adsorbed monolayers dramatically, but there has been less progress in understanding solid-gas and condensed matter interfaces, mainly because these are inaccessible to conventional surface science techniques. Optical probes, with their large penetration depth, offer the possibility of characterizing such interfaces, provided the interface response can be distinguished from the normally dominant bulk response.Sophisticated optical techniques (`epioptics') have been developed, which are capable of delivering unique information about the crystallographic, electronic, vibrational, and spin structure of these interfacial regions. Novel approaches attract healthy scepticism, which has led to detailed experimental studies of the optical response of well-characterized surfaces under ultra-high vacuum conditions, and to the development of effective ab initio approaches to calculating the surface optical response. The availability of high-performance supercomputers, together with efficient and parallel algorithms, has resulted in optical response calculations of the surface that are now approaching the accuracy of bulk calculations, within the framework of many-body perturbation theory. Contributions due to many-particle effects, such as the electron self-energy and the electron-hole attraction, which only a few years ago were shown to be essential in calculating the bulk dielectric function of silicon, can now be included in the surface optical response. The good understanding of the optical response of semiconductor surfaces, which we are now achieving, allows these epioptic techniques to be extended, with confidence, to other surfaces and condensed matter interfaces. This special section in Journal of Physics: Condensed Matter presents a collection of fourteen invited papers on linear optics at surfaces and interfaces. The first paper by Cricenti reviews the use of surface differential reflectance (SDR) to characterize semiconductor surface states, while the second paper by Arzate reviews the formalism and application of polarizable dipole models to calculate reflectance anisotropy spectra (RAS) of semiconductor and metal surfaces, and thin organic films. The paper by Roseburgh compares the surface sensitivity of different linear optical spectroscopies, while Goletti shows that optical sum rules, applied to SDR and RAS data, are useful in interpreting the surface optical response. The next seven papers address semiconductor surfaces, particularly Si(001), and show the detailed information available from state-of-the-art experiment and theory. Borensztein compares the optical response of the clean and di-hydrogenated vicinal Si(001), measured using RAS and SDR, with previous ab initio calculations, and stresses the complementary information available from the two techniques. Palummo shows how the first-principles theoretical framework of the Bethe--Salpeter equation can now be used to determine self-energy, local field and excitonic effects in the surface optical response: the RAS response of Si(100) monohydride is calculated and compared with experiment. Schmidt shows the potential of RAS for studying the oxidation of Si(001) and the functionalization of the Si surface with organic molecules, by comparing experiment with ab initio calculations. Hinrichs shows that, by extending spectroscopic ellipsometry into the mid-IR region, new information can be obtained about the initial stages of oxidation of the passivated Si(001) surface. Jalochowski uses differential reflectance measurements in the visible and near IR region to characterize phase transformations of ultrathin Pb films, grown on the Si(111)-7 x 7 surface at low temperatures. Pulci presents calculated and measured RAS data from cleaved GaAs(110) surfaces covered with one monolayer of As and Sb, while Fleischer shows that the formation of Cs wires on cleaved III-V surfaces leads to characteristic changes in the RAS response.The remaining papers show that linear optics is now advancing into the area of metallic and organic interfaces. Martin investigates the temperature dependence of the RAS signal from the Au(110)-1 x 2 surface and identifies transitions involving surface-modified bulk bands. A solid--liquid interface is characterized by Smith using RAS: the adsorption of 2,2´-bipyridine and 4,4´-bipyridine onto Au(110) in an electrochemical cell results in ordered isomeric structures, which can be distinguished using RAS. Finally, Goletti uses RAS to monitor, in situ and in real time, the growth of thin organic oligothiophene layers on potassium hydrogen phthalate in ultra-high vacuum.The editor is very grateful to all the invited authors for their timely contributions to this special section of Journal of Physics: Condensed Matter.