An analytic approach to modeling the optical response of anisotropic nanoparticle arrays at surfaces and interfaces.

Anisotropic nanoparticle (NP) arrays with useful optical properties, such as localized plasmon resonances (LPRs), can be grown by self-assembly on substrates. However, these systems often have significant dispersion in NP dimensions and distribution, which makes a numerical approach to modeling the LPRs very difficult. An improved analytic approach to this problem is discussed in detail and applied successfully to NP arrays from three systems that differ in NP metal, shape and distribution, and in substrate and capping layer. The materials and anisotropic NP structures that will produce LPRs in desired spectral regions can be determined using this approach.

Controlled in situ growth of tunable plasmonic self-assembled nanoparticle arrays.

Self-assembled silver nanoparticle (NP) arrays were produced by deposition at glancing angles on transparent stepped Al2O3 templates. The evolution of the plasmonic resonances has been monitored using reflection anisotropy spectroscopy (RAS) during growth. It is demonstrated that the morphology of the array can be tailored by changing the template structure, resulting in a large tunability of the optical resonances. In order to extract detailed information on the origin of the measured dichroic response of the system, a model based on dipolar interactions has been developed and the effect of tarnishing and morphological dispersion addressed.

Probing surface and interface structure using optics.

Optical techniques for probing surface and interface structure are introduced and recent developments in the field are discussed. These techniques offer significant advantages over conventional surface probes: all pressure ranges of gas-condensed matter interfaces are accessible and liquid-liquid, liquid-solid and solid-solid interfaces can be probed, due to the large penetration depth of the optical radiation. Sensitivity and discrimination from the bulk are the two challenges facing optical techniques in probing surface and interface structure. Where instrumental improvements have resulted in enhanced sensitivity, conventional optical techniques can be used to characterize heterogeneous adsorbed layers on a substrate, often with sub-monolayer resolution. Nanoscale lateral resolution is possible using scanning near-field optics. A separate class of techniques, which includes reflection anisotropy spectroscopy, and nonlinear optical probes such as second-harmonic and sum-frequency generation, uses the difference in symmetry between the bulk and the surface or interface to suppress the bulk contribution. A perspective is presented of likely future developments in this rapidly expanding field.

Structure of si(111)-in nanowires determined from the midinfrared optical response.

The anisotropic optical response of Si(111)-(4x1)/(8x2)-In in the midinfrared, where ab initio studies predict significant changes in the band structure between competing models of this important quasi-1D system, has been measured using infrared spectroscopic ellipsometry (IRSE) and reflection anisotropy spectroscopy (RAS). Both IRSE and RAS of the (8x2) phase show that the anisotropic Drude tail of the (4x1) phase is replaced by two peaks at 0.50 and 0.72 eV, which appear in ab initio optical response calculations for the hexagon model of the (8x2) structure, but not the trimer model.

Using surface and interface optics to probe the capping, with amorphous Si, of Au atom chains grown on vicinal Si(111).

The distinct optical signatures of aligned single and double Au atom chain structures, grown on vicinal Si(111) substrates, have been identified using reflectance anisotropy spectroscopy (RAS). Deposition of 0.04 monolayers (ML) of amorphous Si (a-Si) at room temperature perturbs the anisotropic optical response of the double chain structure. By one third of a monolayer, no significant optical anisotropy associated with the chains remains. No anisotropic response re-emerges at higher coverages, up to 4.6 nm (14.5 ML) where there is recent evidence that the crystal structure of the double chain phase is maintained under the cap. The RAS results show that the anisotropic properties of the phase are quenched by a-Si adsorption, even though the crystal structure of the capped phase appears to be preserved.

Magnetic second-harmonic generation from the terraces and steps of aligned magnetic nanostructures grown on low symmetry interfaces.

Aligned magnetic nanostructures grown on low symmetry interfaces are generally inhomogeneous, with different magnetic species, such as terrace and step atoms, contributing to the overall magnetic response from the interfacial regions. It is shown that the presence of different magnetic regions can be detected by means of normal incidence (NI) magnetic second-harmonic generation (MSHG). A phenomenological model of NI MSHG at magnetic interfaces of 1m symmetry is developed and a methodology is described for optimizing the signal-to-noise ratio of extracted hysteresis curves by adjusting the input polarization angle. Quadratic terms in the magnetization are properly accounted for, using recently published formulae. It is shown that, where more than one magnetic region is present, the shape of the extracted hysteresis curve, which contains contributions from the different magnetic regions, varies with the input polarization angle. The new approach is used to determine hysteresis loops from the various magnetic regions of Au-capped ultrathin Fe films grown on a vicinal W(110) substrate. The results for 0.75 ML Fe coverage are of particular interest, revealing distinct contributions from terrace and step Fe atoms. This experimental procedure and phenomenology opens up low symmetry magnetic interfaces and aligned nanostructures to characterization by means of MSHG.

Optical reflectance anisotropy of buried Fe nanostructures on vicinal W(110).

The optical anisotropy of Au protected Fe layers grown on a vicinal W(110) surface has been investigated using reflectance anisotropy spectroscopy (RAS). Iron nanostripes formed at submonolayer coverage, as well as Fe layers up to 3 ML coverage, were protected by 12 and 16 nm gold caps and measured ex situ under ambient conditions. The RAS is dominated by structures originating in the interfacial W(110) region, modified by the absorption in the Au cap and possibly by uniaxial strain in the Au cap itself. The Fe nanostructures themselves do not produce a significant RAS signature but, nevertheless, differences with Fe coverage were identified and explained in terms of a simple isotropic Fe absorbing layer, together with strain relief in the W/Fe/Au interfacial region.