Evanescent wave scattering by aggregates of clusters – application to optical near-field microscopy

Extended Mie-theory is used to investigate scattering and extinction of evanescent waves by aggregates of clusters. In an application to apertureless near-field optical microscopy involving total internal reflection at the surface substrate–air, the variation of the scattered power is calculated when a silicon particle is scanned across single clusters or aggregates of clusters in the evanescent field. Metallic, dielectric, and semiconducting particles are taken into consideration, and the dependence on sizes, materials, and the wavelength is discussed. Received: 27 August 1999 / Revised version: 8 November 1999 / Published online: 1 March 2000     

Optical extinction of an assembly of spherical particles in an absorbing medium: Application to silver clusters in absorbing organic materials

The theory presented by Gerardy and Ausloos for the calculation of the linear optical response of aggregates of spherical particles is analytically continued for absorbing embedding media. The method is based on the calculation of the extinction rate by a single particle embedded in an absorbing matrix. Explicit expressions for the extinction and scattering cross-sections are given. The method is applied to calculate the energy losses in several organic matrices with embedded silver clusters. Comparison with experimental data shows a very good agreement. Received: 21 December 1998     

Broadening of resonances in yttrium nanoparticle optical spectra

The dielectric function of yttrium in the range between 0.2 μm and 2 μm is composed of a harmonic oscillator contribution due to a discrete interband transition and the contribution of free electrons. Hence, it is possible to discuss surface plasmon polaritons as well as other electronic resonances in the optical extinction spectra of yttrium nanoparticles. For the latter, we discuss the broadening of the resonance caused by the aggregation of particles. When particles are lumped into aggregates, the color of the particle system also changes. Aggregation also affects the surface plasmon resonance in yttrium nanoparticles in a way comparable to silver or gold nanoparticle aggregates. Comparison is made with the first experimental results on yttriumnanoparticles, showing that aggregation is the dominant effect for the broad resonance in the measured extinction spectra. Received: 2 July 2001 / Revised version: 10 September 2001 / Published online: 15 October 2001     

Optical extinction resonance of Au and Ag clusters formed by ion irradiation in SiO2 and Al2O3

Three methods based on ion-beam irradiation were used to fabricate Ag and Au colloids in silica and alumina. Their surface-plasmon resonance in the visible was characterised by transmittance measurements and interpreted on the basis of transmission electron microscope observations. Despite their bimodal size distribution, particles formed by ion-beam mixing of sandwich layers exhibit much narrower resonances than those obtained by ion implantation. This unusual effect of an inhomogeneity in cluster size is ascribed to the spatial organisation of these clusters. Irradiation of supersaturated solid solutions at much lower ion fluences produces colloids with more uniform size and spatial distributions, and equally strong resonances. Received: 17 March 2001 / Accepted: 31 July 2001 / Published online: 11 February 2002     

Resonance Absorption of Metallic Plate with Subwavelength Hole Array

The optical reflectance by a metallic plate arranged with array consisting of subwavelength periodic square hole is investigated by using the three-dimensional finite-difference time-domain method （3D-FDTD）. There are dips in the reflectivity spectra, which indicate the absorption peaks. The absorption peaks behave differently according to the ratio of hole width and the period of the hole array. Combined with the near fields of the absorption peaks, it is found that the surface plasmon （SP） resonance on the surface of plate and localized SP in the hole play a major role for the two absorptions.     

Optical functions of tris (8-hydroxyquinoline) aluminum (Alq3) by spectroscopic ellipsometry

Optical functions of tris (8-hydroxyquinoline) aluminum (Alq₃) have been studied in the spectral range from 1.55 eV to 5 eV using spectroscopic ellipsometry. The samples have been deposited by thermal evaporation on glass substrates. Optical functions of Alq₃ deposited on unheated substrates and on substrates kept at 100 °C have been determined. The optical functions have been modeled using point-to-point fitting, with the conventional oscillator model and modified oscillator model. It has been found that point-to-point fitting gives the best agreement with the experimental data, and that the modified oscillator model yields better agreement with the experimental data than the conventional oscillator model. Received: 3 September 2001 / Accepted: 22 March 2002 / Published online: 5 July 2002 RID="*" ID="*"Corresponding author. Fax: +852-2559/8738, E-mail: dalek@eee.hku.hk     

Optical anisotropy in nonspherical quantum dots

Received: 10 April 1997/Accepted: 12 June 1997     

Local fields close to the surface of nanoparticles and aggregates of nanoparticles

A refined discussion of the near-field scattering of spherical nanoparticles and the electromagnetic fields close to the particle surface is given. New results for the dependence on the distance from the surface and the angular distribution of the scattered light in the near-field are given. It will be shown that the radial component of the electric field leads to striking differences in the phase functions in the near-field and the far-field. Exemplary computations are presented for Ag and Au particles with different size. In a second part the discussion is extended to assemblies of spherical Ag and Au nanoparticles. It will be shown that large near-fields at wavelengths commonly used in SERS experiments are obtained for aggregates. In the near-field scattering intensity “hot spots” mark regions between particles in the aggregate where the near-field is particularly high. Received: 4 May 2001 / Revised version: 20 July 2001 / Published online: 19 September 2001     

Microgeometry and effective permittivity of discontinuous metal films

This work deals with the investigation of dielectric and optical properties of discontinuous metal films over a wide range of filling factors each side of the percolation threshold. A phenomenological two-parameter representation is used for the effective permittivity &εtilde; of such systems. In order to test the proposed approach, we have taken for gold films on a glass substrate the transmittance spectra in the visible and near-IR regions. It is shown that the representation used for the effective permittivity enables us to adequately describe the experimental transmittance spectra. The parameters entering the above representation are determined and the relationship between them and film microgeometry is discussed. Received: 6 November 2000 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Optical characterization of thermally evaporated thin films of As40S40Se20 chalcogenide glass by reflectance measurements

40 S₄₀Se₂₀, deposited by thermal evaporation, were obtained in the 400 nm to 2200 nm spectral region. The optical constants of this amorphous material were computed using an optical characterization method based mainly on the ideas of Minkov and Swanepoel of utilising the upper and lower envelopes of the spectrum, which allows us to obtain both the real and imaginary parts of the complex refractive index, and the film thickness. Thickness measurements made by a surface-profiling stylus have been carried out to cross-check the results obtained by the optical method. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple–DiDomenico model. The optical band gap has been determined from absorption coefficient data by Tauc’s procedure. Finally, the photo-induced and thermally induced changes in the optical properties of a-As₄₀S₄₀Se₂₀ thin films were also studied, using both transmission and reflection spectra. Received: 11 February 1998/Accepted: 16 February 1998     

Optical multistability in nonlinear negative index Fibonacci multilayers

We have demonstrated numerically that the nonlinear negative index Fibonacci multilayer can exhibit optical multistability and offer a wide variety of switching between states of various contrasts. The stepwise up-switch and one-step or stepwise down-switch operations can take place in such structures. It is also shown that multistable operations and contrasts can be controlled by regulating the width of the negative index slabs.     

Optical characterization of thin thermal oxide films on copper by ellipsometry

A complete optical characterization in the visible region of thin copper oxide films has been performed by ellipsometry. Copper oxide films of various thicknesses were grown on thick copper films by low temperature thermal oxidation at 125 °C in air for different time intervals. The thickness and optical constants of the copper oxide films were determined in the visible region by ellipsometric measurements. It was found that a linear time law is valid for the oxide growth in air at 125 °C. The spectral behaviour of the optical constants and the value of the band gap in the oxide films determined by ellipsometry in this study are in agreement with the behaviour of those of Cu₂O, which have been obtained elsewhere through reflectance and transmittance methods. The band gap of copper oxide, determined from the spectral behaviour of the absorption coefficient was about 2 eV, which is the generally accepted value for Cu₂O. It was therefore concluded that the oxide composition of the surface film grown on copper is in the form of Cu₂O (cuprous oxide). It was also shown that the reflectance spectra of the copper oxide–copper structures exhibit behaviour expected from a single layer antireflection coating of Cu₂O on Cu. Received: 19 July 2001 / Accepted: 27 July 2001 / Published online: 17 October 2001     

Optical determination of interface roughness in multilayered semiconductor structures

Received: 20 September 1998     

The color of finely dispersed nanoparticles

We discuss the dependence of the color of low-concentrated nanoparticle systems on particle size and mass concentration for Ag, Au and TiN nanoparticles, which exhibit a surface plasmon polariton resonance in extinction spectra. Comparison is made with color data obtained for Ag and Au colloidal suspensions. When particles lump into aggregates, the splitting of the surface plasmon resonance into new resonances affects the extinction of light and, hence, the color of the particle assembly. This is demonstrated for aggregated colloidal suspensions of Ag and Au nanoparticles. Finally, for highly concentrated assemblies such as pigment films, we discuss the dependence of the color in diffuse reflectance and transmittance according to Kubelka and Munk (P. Kubelka, F. Munk: Z. Techn. Phys. 12, 593 (1931)), and extend this model by using optical properties of aggregates of spheres. Received: 2 July 2001 / Published online: 10 October 2001     

Optical functions of chalcopyrite CuGaxIn1-xSe2 alloys

We report pseudodielectric functions <ε> of the quaternary semiconductor alloys CuGa_xIn_1-xSe₂. Measurements were done in polycrystalline samples from 0.7 to 5.2 eV at room temperature by spectral ellipsometry. Accurate values of refractive indices n and extinction coefficients k representative of the bulk materials are obtained from the data. The structures observed in <ε> are analyzed by fitting the numerically differentiated spectra of d²<ε>(ω)/dω² to analytic line shapes. The obtained energies are related to given inter-band transitions by taking into account the electronic band structures of the ternary end-point compounds. Received: 20 December 2000 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Determination of the dielectric function of porous silicon by high-order laser-harmonic radiation

Porous-silicon reflectance has been determined over a large energy range, from 1 eV to 16 eV, by combining a NIR/visible/UV spectrometer with a new VUV light source as laser-harmonic radiation. The porous-silicon dielectric function was deduced from reflectance measurements by Kramers–Kronig analysis. We point out that, for the first time, laser harmonics have been applied in the optical characterization of materials as a new and suitable alternative to synchrotron radiation. Received: 9 January 2001 / Accepted: 28 April 2001 / Published online: 20 June 2001     

Theoretical study of quantum-confined band-edge shifts and radiative lifetimes in oxidized Si(001) quantum films: comparison with experiment for Si/SiO2 quantum wells

The electronic structure and radiative lifetimes of Si(001) quantum films terminated by SiO₄ tetrahedra, which simulate Si/SiO₂ quantum wells (QWs), are calculated by the extended Hückel-type non-orthogonal tight-binding method. It is found that calculated band-gap widenings and radiative lifetimes account for band-edge shifts and photoluminescence (PL) peak shifts and lifetimes measured in amorphous-Si/SiO₂ QWs, suggesting that quantum confinement effects on the extended band-edge states in the amorphous-Si layer are responsible for the observed results. However, it is shown that band-edge shifts and PL energies and lifetimes observed in crystalline-Si/SiO₂ QWs cannot be reproduced properly by the interface model proposed in this study, implying that further studies are needed on the atomic structure of the crystalline-Si/SiO₂ interface.     

Optical characterisation of CuInSe2 thin films prepared by two-stage process

Optical absorption spectra of CuInSe₂ chalcopyrite semiconductor films prepared using a two-stage technique were investigated. In addition to absorption measurements, energy-dispersive analysis of X-rays (EDAX) and X-ray diffraction measurements (XRD) were also performed. Direct bandgap energy values for the CuInSe₂ films were derived from the variation of (αhν)² with energy. All the measurements were performed on samples with various Cu/In ratios. It was determined from the absorption measurements that the materials have strong absorption at the fundamental band edge. The E_g values showed an increasing trend with decreasing Cu/In ratios. Received: 26 May 2000 / Accepted: 31 October 2000 / Published online: 10 January 2001     

Optical diffuse reflectance spectra of GaSb nanocrystals embedded in SiO2 matrix by radio-frequency magnetron co-sputtering

Nanocrystalline GaSb embedded in SiO₂ films was grown by radio-frequency magnetron co-sputtering. X-ray diffraction pattern and transmission electron microscopy (TEM) confirm the existence of GaSb nanocrystals in the SiO₂ matrix. The average size of GaSb nanoparticles is in the range of 3 to 11 nm. Diffuse reflectance spectra were used to characterize the small change of the band gap of the semiconductor. The diffuse reflectance spectra shows that the absorption peaks have a large blueshift of about 4.0 eV of the absorption relative to that of bulk GaSb. It has been explained by quantum confinement effects. Room temperature optical transmission spectra show that the absorption edge exhibits a very large blueshift of about 2.1 eV with respect to that of bulk GaSb in agreement with quantum confinement. Received: 28 July 1999 / Accepted: 27 October 1999 / Published online: 1 March 2000     

Theory of optical spectra of impurity centres in crystals: general consideration of quadratic vibronic coupling

A method is proposed for the calculation of the spectra of the multiphonon transitions caused by the quadratic vibronic coupling with a phonon continuum. In the method, the time evolution of the final state is described by the path integrals. By applying the Stratonovich-Hubbard identity, the quadratic coupling is presented as the fluctuating linear coupling; the latter is calculated by the Lax method. As a result, the problem is reduced to the calculation of the determinants of matrices, the elements of which are given by the pair correlation functions of the contributing configurational coordinates. The method has been verified for the case of two mixed modes. Numerical calculations have also been made for a modified Debye model.