Modulation spectroscopy under uniaxial stress

During the past several years the combination of modulation spectroscopy and static uniaxial stress has been developed into a powerful tool for the investigation of the relationship between the optical properties of semiconductors and their electronic energy bands. Studies of the stress-induced splittings and shifts of energy levels and oscillator strengths as well as the dependence of the induced fine structure on polarization direction and strain configuration have produced a wealth of information concerning the intrinsic properties of the undeformed crystal such as symmetries of interband optical transitions, deformation potentials, spin-exchange interaction of excitons, etc. Symmetry assignments are of considerable value for comparison with band structure calculations while the latter parameters are of significance for comparison with theories based on model calculations and systematic trends such as ionicity. This paper will review experiments and theories dealing with the effects of static uniaxial stress on those optical properties of semiconductors related to the intrinsic properties of the material with special emphasis on the modulated optical spectra.     

Modulation spectroscopy on anisotropic systems

Results of modulation spectroscopy on the anisotropic materials trigonal Se and the layer structure compound MoS₂ are presented. Particularly in the case of selenium, the application of modulation spectroscopy leads to considerably better understanding of the band structure. In both materials, strong electric field induced changes of the optical properties are observed which cannot be interpreted by an interaction of the external field with the electronic states but which result indirectly from an interaction of the external field with the lattice.     

Modulation cancellation method in laser spectroscopy

A novel spectroscopic technique based on modulation spectroscopy with two excitation sources and quartz enhanced photoacoustic spectroscopy is described. We demonstrated two potential applications of this detection technique. First, we investigated the measurement of small temperature differences in a gas mixture. In this case, a sensitivity of 30 mK in 17 sec was achieved for a C₂H₂/N₂ gas mixture with a 0.5% C₂H₂ concentration. Second, we demonstrated the detection of broadband absorbing chemical species, for which we selected hydrazine as the target molecule and achieved a detection limit of ∼1 part per million in 1 sec. In both cases, the measurements were performed with near-IR laser diodes and overtone transitions.     

Cavity ring-down spectroscopy of metallic gold nanoparticles

The optical properties of supported gold nanoparticles with sizes of 1.3 nm, 1.6 nm, 2.5 nm, and 2.9 nm have been studies by using cavity ring-down spectroscopy in the photon energy range between 1.8 eV and 3.0 eV. The obtained results show the possibility to obtain optical information of nanoassembled materials with high sensitivity. The experimental findings are compared to calculations using Mie-Drude theory. Whereas the broadening of the surface plasmon resonance with decreasing size is well described by this model, the observed blue-shift of the surface plasmon resonance contradicts the predictions of the Mie-Drude theory. The latter effect can be explained by the presence of a skin region with decreased polarizability typical for coinage metal particles. Furthermore, it is found that the supported gold nanoparticles are robust under ambient conditions, an important issue when using these materials for optical applications.     

Reflectance spectroscopy of gold nanoshells: computational predictions and experimental measurements

Gold nanoshells are concentric spherical constructs that possess highly desirable optical responses in the near infrared. Gold nanoshells consist of a thin outer gold shell and a silica core and can be used for both diagnostic and therapeutic purposes by tuning the optical response through changing the core–shell ratio as well as the overall size. Although optical properties of gold nanoshells have already been well documented, the reflectance characteristics are not well understood and have not yet been elucidated by experimental measurements. Yet, in order to use gold nanoshells as an optical contrast agent for scattering-based optical methods such as reflectance spectroscopy, it is critical to characterize the reflectance behavior. With this in mind, we used a fiber-optic-based spectrometer to measure diffuse reflectance of gold nanoshell suspensions from 500 nm to 900 nm. Experimental results show that gold nanoshells cause a significant increase in the measured reflectance. Spectral features associated with scattering from large angles (~180°) were observed at low nanoshell concentrations. Monte Carlo modeling of gold nanoshells reflectance demonstrated the efficacy of using such methods to predict diffuse reflectance. Our studies suggest that gold nanoshells are an excellent candidate as optical contrast agents and that Monte Carlo methods are a useful tool for optimizing nanoshells best suited for scattering-based optical methods.     

Comparative study of ethanol sensor based on gold nanoparticles: ZnO nanostructure and gold: ZnO nanostructure

Gold colloid:ZnO nanostructures were prepared from Zn powder by using thermal oxidation technique on alumina substrates, then it was impregnated by gold colloid for comparative study. The gold colloid is the solution prepared by chemical reduction technique; it appeared red color for gold nanoparticle solution and yellow color for gold solution. The heating temperature and sintering time of thermal oxidation were 700 °C and 24 h, respectively under oxygen atmosphere. The structural characteristics of gold colloid:ZnO nanostructures and pure ZnO nanostructures were studied using filed emission scanning electron microscope (FE-SEM). From FE-SEM images, the diameter and length of gold colloid:ZnO nanostructures and ZnO nanostructures were in the ranges of 100-500 nm and 2.0-7.0 μm, respectively. The ethanol sensing characteristics of gold colloid:ZnO nanostructures and ZnO nanostructures were observed from the resistance alteration under ethanol vapor atmosphere at concentrations of 50, 100, 200, 500, and 1000 ppm with the operating temperature of 260-360 °C. It was found that the sensitivity of sensor depends on the operating temperature and ethanol vapor concentrations. The sensitivity of gold colloid:ZnO nanostructures were improved with comparative pure ZnO nanostructures, while the optimum operating temperature was 300 °C. The mechanism analysis of sensor revealed that the oxygen species on the surface was O²⁻.     

Modulation spectroscopy on metamorphic InAs quantum dots

The modulation spectroscopy on 1.45 μm metamorphic InAs quantum dots (QDs) with In_0.3Ga_0.7As capping layer grown on GaAs substrate by molecular beam epitaxy (MBE) has been investigated by differential absorption (Δα), electro-reflectance (ER), and photo-reflectance (PR) spectra at different reverse bias. The optical transitions of the ground state, excited states, and wetting layers were identified and discussed. The micro-structure characterization was also analyzed by TEM and AFM. The variation of refractive index spectra (Δn) by calculating Δα spectra through Kramers–Kronig transform is obtained to study the electro-absorption behaviors. Additionally, a simple physical model is proposed to explain the experimental values between the Δn and ΔR spectra performed by two different modulation spectroscopies (Δα and ER). The built-in electric field of metamorphic InAs QDs structure was determined to analyze the Franz–Keldysh Oscillation (FKO) extreme in PR spectra with different bias.     

Modulation spectroscopy using the beam-foil light source

When a beam of ions passes through a thin exciter foil, certain radiation emitted by the beam particles can exhibit time-periodic intensity variations. These variations can be induced by external E and H fields, or they can be the result of the field-free atomic structure itself.Intensity modulations observed so far in beam-foil spectroscopy can be divided into three classes: (1) Quantum mechanical interference of fine structure levels. This is a QM resonance arising from time-dependent populations of emitting states having different transition probabilities. The resonance is induced by external constant electric fields. (2) Initial coherent superpositions of radiating states. This results from the creation of M_L alignment at the instant of excitation by the foil. The modulations are field free and are observed in polarized light. (3) Rotating electric dipole in a magnetic field. When alignment occurs, the intensity of the beam radiation after excitation satisfies the relation I(t) = = I₀[1+Acos(2γHt)] e^?αt. The modulation will be a function of the magnetic field H and the gyromagnetic ratio γ.These effects can be used to study Lamb shifts, g-values, fine structure levels, and interaction processes.     

Plasma desorption mass spectroscopy of thiol-passivated gold nanoparticles

Plasma desorption mass spectrometry has been applied to characterization of dodecanthiol-passivated gold nanoparticles. An overview of the experimental set-up and mass analyses for the nanoparticles prepared in different conditions are shown. Mass distributions were found to shift to higher mass regions with increasing reaction temperature and reaction period. The results are consistent with those of transmission electron microscopy observations, UV-visible absorption spectra and also with a reported laser desorption mass spectrometry.     

Vibration spectroscopy of water on stepped gold surfaces

The vibration spectrum of H₂O (ice) adsorbed at low temperatures on Au(1 0 0), Au(1 11 1), and Au(1 1 5) is studied using electron energy loss spectroscopy. On the Au(1 0 0) surface, the spectra show the presence of the typical H-bonded network of water molecules for all coverages from the submonolayer into the multilayer range. The absence of a non-H-bonded OH-stretching mode is indicative for the “H-down bilayer”. On stepped surfaces, on the other hand, a considerable fraction of the H-atoms remains in the non-H-bonded state; surprisingly even in the multilayer range, and even after annealing. The fraction of non-H-bonded hydrogen atoms scales with the step density. Spectral features of water adsorbed at step-sites are isolated after annealing a surface exposed to small doses of H₂O. The results are discussed in the context of recent theoretical studies as well as in conceivable relation to the experimentally found reduction of the Helmholtz-capacitance on stepped Au(1 1 n) electrodes.     

Optical spectroscopy of surfaces: Reflectance studies of chemisorption

A high precision technique has been used to study the small changes in near-normal incidence reflectance R caused by chemisorption of gases on an atomically clean metal surface maintained in ultrahigh vacuum. Examples are given for O₂, CO, and H₂ on Mo(100) in the photon energy range 1.9 <?ω < 4.8eV. The relative reflectance change $\text{ΔR}\text{R}$ is negative and reaches as much as 1%. Structure that appears in ΔR(e), where e=exposure (pressure × time), is attributed to different binding configurations of the adatoms as observed, e.g., in LEED experiments. The dependence of $\text{ΔR}\text{R}$ on ?ω suggests the presence of adsorbate-induced surface states within a few eV of the Fermi energy. Possible approaches to determining the dielectric function of the surface region from the data are discussed.     

Raman spectroscopy and polarization: Selected case studies

We show, through several selected case studies, the potential benefits that can be obtained by controlling the polarization states of the exciting and scattered radiations in a Raman scattering experiment. When coupled with polarization control, Raman spectroscopy is thus capable of providing extra information on the structural properties of the materials under investigation. The experimental examples presented in this work are taken from the area of both conventional, i.e., far-field, as well as from near-field Raman spectroscopy. They cover topics such as the stress tensor measurement in strained semiconductor structures, the vibration mode assignment in pentacene thin films and the Raman scattering tensor determination from near-field measurements on azobenzene monolayers. The basic theory necessary for modelling the far- and near-field polarized Raman responses is also given and the model efficiency is illustrated on the experimental data.     

Near-field spectroscopy of surface plasmons in flat gold nanoparticles

We use near-field interference spectroscopy with a broadband femtosecond, white-light probe to study local surface plasmon resonances in flat gold nanoparticles (FGNPs). Depending on nanoparticle dimensions, local near-field extinction spectra exhibit none, one, or two resonances in the range of visible wavelengths (1.6-2.6 eV). The measured spectra can be accurately described in terms of interference between the field emitted by the probe aperture and the field reradiated by driven FGNP surface plasmon oscillations. The measured resonances are in good agreement with those predicted by calculations using discrete dipole approximation. We observe that the amplitudes of these resonances are dependent upon the spatial position of the near-field probe, which indicates the possibility of spatially selective excitation of specific plasmon modes.     

Raman spectroscopy of gold nanoparticles in polycrystalline LiF film

Results of the Raman spectroscopy analysis of a new composite material based on a thin polycrystalline LiF film containing gold nanoparticles are presented. The formation of spherical gold nanoparticles in the film has been confirmed by the X-ray structural analysis and observation of the optical plasmon resonance absorption spectrum with a maximum at 534 nm. The obtained composite layers have been subjected to annealing by ruby laser (λ = 694 nm) in the spectral region on a descending long-wavelength wing of the plasmon absorption band of gold nanoparticles. Raman spectroscopy has been applied for the first time to the investigation of the modification of the shape of gold nanoparticles in LiF during laser annealing. The experimental Raman spectra are compared with calculated modes of in-phase bending vibrations generated in gold nanoparticles.     

High-resolution wavelength-dispersive spectroscopy of K-shell transitions in hydrogen-like gold

We present a measurement of K-shell transitions in H-like gold (Au⁷⁸⁺) using specially developed transmission type crystal spectrometers combined with Ge(i) microstrip detectors. The experiment has been carried out at the Experimental Storage Ring at GSI in Darmstadt. This is a first high-resolution wavelength-dispersive measurement of a K-shell transition in a high-Z H-like ion, thus representing an important milestone in this field. Ideas on possible future improvements are discussed as well.     

Devaluation of the gold standard in x-ray photoelectron spectroscopy

The value of vapor-deposited gold as a standard in x-ray photo-electron spectroscopy has been studied. During the deposition gold may react with the sample giving consequent shifts and/or broadening of the Au 4f peaks. Shifts to higher binding energies have been observed with KCN and NACl and to lower energies with Na₂S₂O₃ and copper phthalocyanine. The An 4f peaks have been investigated over the temperature range ?50 to 200°C. At higher temperatures peaks due to metallic gold are observed. Anomalous effects have been observed in the XPS spectrum of gold plated copper phthalocyanine in the binding energy range 650–950 eV.