Detection of monolayer gratings of adsorbates by linear optical diffractions

We report the detection of linear optical diffraction of a He-Ne laser probe beam from a monolayer grating of molecular adsorbates up to the fifth order with no electronic or electromagnetic enhancement. The advantages of using linear diffraction to probe surface diffusion of adsorbates are discussed.     

Reversible Vertical Manipulations of Single Pt Adatom on Pt（111） Surface with a Triple-Apex Tip

With a triple-apex tip, we investigate theoretically the vertical manipulation of single Pt adatom on the Pt（111） surface. The adatom adsorbed on the f cc site of the flat Pt（111） surface can be transferred vertically to the tip by adjusting the tip height properly. Moreover, based on the strong vertical trapping ability and the relatively weak lateral trapping ability of the tip, we propose a simple method to realize a reversible vertical manipulation of the Pt adatom from the highly coordinated sites, the kink and the step sites, of the stepped Pt（111） surface. All the vertical manipulations are completed using only the atomic force between the tip and the adatom, without the electric field.     

Nanostructuring with laser radiation in the nearfield of a tip from a scanning force microscope

Platinum-carbon multilayer mirrors with a bilayer spacing of 50 Å were fabricated in an ultrahigh vacuum electron beam evaporator. The thermal stability of these multilayers was studied under vacuum annealing using X-ray reflectivity and X-ray diffraction. Up to 450°C, the bilayer spacing increases monotonically accompanied by a gradual increase in crystallite size and grain texture. At 500°C multilayer reflection vanishes, platinum crystallites grow abruptly, and there is a strong texture of platinum in the [220] -plane. Possible reasons for thermally induced structural modifications in these multilayers are discussed.     

In situ monitoring of laser-induced silicon oxidation

A low cost, computer-controlled, in situ monitor of laser-induced oxidation of c-Si is described. Both oxide thickness and processing temperature can be determined, simultaneously and with high spatial resolution, from the sample reflectivity and a knowledge of the temperature dependence of the optical constants of silicon and silicon dioxide (available in the literature).     

Proposal of a frequency-synthesis chain between the microwave and optical frequencies of the Ca intercombination line at 657 nm using diode lasers

We propose a frequency synthesis chain which can directly connect a microwave atomic clock with a visible laser. We design this chain for the frequency measurement of a visible laser locked on the intercombination transition of Ca at 657 nm. The proposed chain is based on both an optical difference frequency divider and an optical frequency comb generator, and it is designed to use nine visible and near-infrared diode lasers. We discuss the technical requirements to realize the frequency measurement accuracy level of 10^–14.     

Surface modification of SnO2 nanoparticles containing Mg or Fe: Effects on sintering

Controlling the surface chemistry of oxide systems has emerged an effective tool to obtain desirable nanostructures and macro properties. A relatively simple way to achieve this is by using dopants that are prone to segregate to the surfaces of the powders. In this work, we delineate the effect of Mg and Fe on SnO₂ nanopowders focusing on the surface modifications caused by surface segregation. The effects of increasing the temperature of calcinations are particularly addressed to evaluate the surface modifications at high temperatures. The powders were studied by infrared spectroscopy, zeta potential measurements, X-ray diffraction, and specific surface area measurements. Since sintering is a high-temperature process strongly dependent on surface characteristics, we drawn a relationship between the final densities after sintering and the surface chemistry of the doped powders. Doped SnO₂ pellets were sintered to over 95% of the theoretical density within a few seconds (fast firing) when significant surface modifications were observed.     

Expulsion of carriers from the double-barrier quantum well and investigation of its spectral consequences

We propose a simple procedure for evaluating the spontaneous (in absence of bias) expulsion of carriers from a nanostructure and exemplify it by computing the distribution of the net charge in the DBQW. We calculate the corresponding correction to the spectrum of this nanostructure and discuss lifting of degeneracy in narrow DBQWs. Numerical examples illustrate these effects in a wide range of temperatures, carrier densities, and widths of the quantum well.     

Resonant reflectance dips induced by coupled surface plasmon polaritons in thin metal-film/dielectric superlattices

The optical reflectance of metal films changes dramatically as the film thickness becomes thinner than the electron mean free path. We have developed a transfermatrix formalism for deducing the dispersion relations of the electromagnetic waves in infinite and semi-infinite metal-dielectric superlattices by taking into account the presence of the size effect and coupled plasmon waves. This work shows that the resonance frequency occurring at the reflecting dip increases while the bandwidth decreases as the thickness of the dielectric films increases. Reducing the values of p and q shifts the resonance frequency upward and yields multiple numbers of minimum reflectivity.     

Oxidation of Si(1 1 1) promoted by K multilayers: K and SiO2 islands

In the context of the alkali-metal promotion of Si oxidation, high-pressure oxidation of potassium multilayers on cooled Si(111) has been investigated using SEM, micro-AES and AFM. The oxidation process at high pressure turns the potassium islands observed at high coverage into potassium oxides islands. A subsequent potassium desorption at moderate temperature (900 K) yields SiO₂ islands (height of 20 nm and lateral dimensions of 10 m) surrounded by a thin continuous SiO₂ overlayer. Other conditions such as potassium multilayer coverage exposed to a low O₂ pressure, potassium monolayer or simultaneous potassium/oxygen adsorption yield a uniform SiO₂ overlayer.     

Photoreflectance characterization of CdTe thin films grown by Molecular Beam Epitaxy on GaAs(100)

CdTe films have been grown on top of GaAs(100) by means of Molecular Beam Epitaxy (MBE) at 300 °C substrate temperature. Different procedures for the CdTe growth and for the preparation of the GaAs substrates resulted in diverse crystalline qualities of the CdTe films. We present the results obtained from PhotoReflectance (PR) measurements of these films employing HeNe and Ar-ion lasers as modulating excitation. For Ar excitation, the ratio of CdTe to GaAs signal strength for the E ₀ transition is enhanced, allowing a differentiation of the contributions from film and substrate. Both the PR line shape and intensity are correlated to the structural quality of the CdTe films. One of the samples presented a below-band-gap transition of the GaAs substrate around 30±5 meV from E ₀ which is attributed to donor states produced by Te atoms diffused in the interface; this result demonstrates the high sensitivity of the photoreflectance technique to the structural properties of interfaces.     

Raman-spectroscopy study of PbTiO3 thin films grown on Si substrates by metalorganic chemical vapor deposition

Raman spectra have been investigated in PbTiO₃ thin films grown on Si by metalorganic chemical vapor deposition. A large grazing-angle scattering technique was taken to measure the temperature dependence of Raman spectra below room temperature. All Raman modes in the thin films are assigned and compared with those in the bulk single crystal, a newA ₁(TO) soft mode at 104 cm^–1 was recorded which satisfies the Curie-Weiss relation ² =A(T _c –T). Intensities of theA ₁(1TO) andE(1TO) modes were anomalously strengthened with increasing temperature. Raman modes for the thin films exhibit remarkable frequency downshift and upshift which is related to the effect of internal stress.     

Optical characterization of oxynitride films in the visible-ultraviolet range

Oxynitride optical properties in the visible-ultraviolet spectral range are very interesting, due to their use in electronic device manufacturing. This paper presents spectra of refractive index and extinction coefficient of oxynitride films deposited on silicon with different composition, as derived from spectroscopic ellipsometry measurements on the basis of an effective medium approach. These data evidence the presence of a Si-rich layer on the oxynitride/silicon interface. Electronic polarizability and energy gap of all compounds were evaluated. Moreover, absolute reflectance of the samples was derived from optical functions and compared with the measured value.     

Vision-based system for automatic stabilization of mode,beam steer,and output power of a high-power laser

As advancements in laser technology result in higher-power devices, increased thermal loads experienced by resonator optical components generate instabilities in the output beam. Consequently, methods for maintaining and optimizing output-beam parameters are necessary for efficient long-term operation. This study examines a vision-based feedback control system which monitors a sampled cross-sectional image of the laser's output, and regulates both cavity mirrors, as well as the excitation level, to stabilize beam uniformity, beam steer, and output-power level.The system optimizes the output-power distribution achievable from a given laser, while significantly reducing beam steering effects. Direction of beam propagation is restricted to within 60 rad of the desired setpoint, while the output power is stabilized to within 4%. Results clearly document a considerable improvement in laser performance through maximization of beam uniformity, minimization of beam steering, and stabilization of total output power. As such, an important degree of consistency and repeatability in output-beam parameters, essential to precision laser applications, is easily achieved. Valuable diagnostic capabilities pertaining to output beam intensity and energy profiles are also incorporated into the control system. These data may be monitored on-line or stored for subsequent analysis.     

Electrochemically intercalated MxC60 thin films in a solid state cell (M = Li,K): Optical and photoelectrochemical characterization

Solid-state electrochemical cells have been prepared with C₆₀ vacuum-evaporated thin films, a Li- or K-source counter electrode and a polymer PEO-LiClO₄ (PEO-KClO₄) electrolyte. The electrochemical intercalation in C₆₀ of Li⁺ (or K⁺) ions has been performed under constant current conditions up to a formal stoichiometry of the fulleride film equal to Li₁₂C₆₀ (K₅C₆₀). A complete charge-transfer pocess from the intercalated alkali to the alkali-metal compound has been assumed. Several quasi-equilibrium potential plateaux were observed during intercalation, that we associate with the coexistence of phases with different intercalant concentration. The electrochemical intercalation process is irreversible to a large extent. Optical and photoelectrochemical spectroscopy of the fulleride films was done in-situ at different moments of the intercalation reaction by illuminating the film electrodes through the transparent and conducting glass substrates. The photo electrochemical spectral response agrees well with the optical absorption spectra, both indicating a bandgap of 2.2 eV. The photoelectrochemical response shows a minimum forx = 3 (in the K_xC₆₀ compound) and a maximum atx = 4 (in both K_xC₆₀ andLi_xC₆₀ compounds), in agreement with previous conductivity results. NIR diffuse reflectance spectra of the solid-state cell show absorption bands in the fulleride films at a wavelength of 1100 nm. A band-energy diagram has been proposed for the M_xC₆₀/PEO-MClO₄/M electrochemical cell.On leave from Instituto de Fisica, Facultad de Ingenleria, C.C. 30, 11000 Montevideo, Uruguay     

Interface reactions in [Fe/B]<Subscript>n</Subscript> multilayers: a way to tune from crystalline/amorphous layer sequences to homogeneous amorphous Fe<Subscript>x</Subscript>B<Subscript>100-x</Subscript> films

[Fe/B]_{n ≥2} multilayers were prepared by thermal evaporation, ion-beam sputtering and laser ablation. By applying in situ electron spectroscopies (UPS, XPS) and monitoring the electrical resistance during layer growth, evidence could be provided for the occurrence of interface reactions within the range of studied deposition temperatures (77 K ≤T ≤300 K). These reactions result in amorphous Fe_xB_100-x phases, which are spatially restricted to a width of less than 3 nm at the original interface. The amorphicity of the reacted interlayers was unequivocally proven by additional high-resolution electron microscopy (HRTEM) and their characteristically changed magnetic properties. Due to the well-defined width of the interface reaction, homogeneous amorphous Fe_xB_100-x films can be obtained by reducing the individual Fe and B layer thicknesses to below the above reaction depth, while for larger thicknesses layer sequences of the crystalline/amorphous/crystalline type will result. Received: 30 January 2002 / Accepted: 31 January 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +49-731/502-2963, E-mail: hans-gerd.boyen@physik.uni-ulm.de     

Beam radiated from quasi-homogeneous uniformly polarized electromagnetic source scattering on quasi-homogeneous media

We study the coherence properties of the field generated by beam radiated from quasi-homogeneous (QH) electromagnetic source scattering on QH media. Formulas for the spectral density and spectral degree of coherence of the three dimensional scattered field are derived. The results show under assumption that the diagonal correlation coefficients of the source are proportional to each other, the far field of the scattered light satisfy two reciprocity relations analogous to that in the scalar case, that, the spectral density is proportional to the convolution of the spectral density of the source and the spatial Fourier transform of the correlation coefficient of the scattering potential; the spectral degree of coherence is proportional to the convolution of the diagonal correlation coefficients and the strength of the scattering potential.     

Influence of cubic nonlinearity on laser radiation transmission in a photonic crystal with spatially modified media properties

The transfer matrix method was modified to explore the Kerr nonlinearity influence on laser radiation propagation in a one‐dimensional photonic crystal. The suggested spatial distribution of the refractive index allows to remove minibands and to make the transmission curve much steeper. Plain and steep photonic band gap edges are effective in creation of transmission anisotropy of powerful laser radiation. The investigated photonic crystal has a strong anisotropic optical transmission and acts as an optical analog of the electronic diode. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Femtosecond Kerr ellipsometry in polydiacetylene solutions: Two-photon effects

We describe a pump-probe femtosecond Kerr experiment in polydiacetylene solutions (4BCMU, yellow form). The Kerr ellipsometry technique permits a separate determination of the real and of the imaginary part of anisotropic index changes. The wavelength (620 nm) of the pump pulses lies in the transparency region of polydiacetylene while the probe pulses have a broad white spectrum. The largest contribution to the Kerr signal is a quasi-resonant stimulated anti-Stokes Raman absorption. A flat two-photon transition spectrum with 4×10^–3 cm/MW is observed close to 4 eV. The two-photon excited states have a life time close to 12 ps and they present a broad absorption band in the 2 eV range.     

Modulated photoreflectance characterization of ion-implanted semiconductor wafers

Modulated PhotoReflectance (MPR) measurements on semiconductor wafers implanted with boron or silicon ions in the dose range 5×10¹⁰–5×10¹⁵ ions/cm² are presented. Correspondingly, a one-dimensional theoretical multilayer model is established. In the theory, as the implant dose is lower than a critical value, the variation of the MPR signal is contributed mainly by the implanted defects and damages. However, when the dose is above the critical dose, the change of the MPR signal is chiefly due to the formation and growth of an amorphous layer. The theoretical results are in good agreement with those of experiments.     

Frictional and atomic-scale study of C60 thin films by scanning force microscopy

Scanning force microscopy (SFM) was employed to characterize C₆₀ island films in an ultra-high vacuum (UHV). The initial growth stage of C₆₀ on NaCl cleavage faces and nanotribological properties of this solid lubricant are investigated. In comparison to the NaCl(001) face, higher friction is measured on the C₆₀ islands, resulting in a ratio of friction of 13 for NaClC₆₀. The friction coefficient of the (111) oriented C₆₀ island is determined to be 0.15±0.05. High-resolution SFM images reveal the hexagonal lattice of the unreconstructed (111) top surfaces and the overgrowth relationships of the C₆₀ islands.