A. Oliver

Controlled anisotropic deformation of Ag nanoparticles by Si ion irradiation

The shape and alignment of silver nanoparticles embedded in a glass matrix is controlled using silicon ion irradiation. Symmetric silver nanoparticles are transformed into anisotropic particles whose larger axis is along the ion beam. Upon irradiation, the surface plasmon resonance of symmetric particles splits into two resonances whose separation depends on the fluence of the ion irradiation. Simulations of the optical absorbance unambiguously show that the anisotropy is caused by the deformation and alignment of the nanoparticles, and that both properties are controlled with the irradiation fluence.

Anisotropic linear and nonlinear optical properties from anisotropy-controlled metallic nanocomposites

High-energy metallic ions were implanted in silica matrices, obtaining spherical-like metallic nanoparticles (NPs) after a proper thermal treatment. These NPs were then deformed by irradiation with Si ions, obtaining an anisotropic metallic nanocomposite. An average large birefringence of 0.06 was measured for these materials in the 300-800 nm region. Besides, their third order nonlinear optical response was measured using self-diffraction and P-scan techniques at 532 nm with 26 ps pulses. By adjusting the incident lights polarization and the angular position of the nanocomposite, the measurements could be directly related to, at least, two of the three linear independent components of its third order susceptibility tensor, finding a large, but anisotropic, response of around 10(-7) esu with respect to other isotropic metallic systems. For the nonlinear optical absorption, we were able to shift from saturable to reverse saturable absorption depending on probing the Au NPs major or minor axes, respectively. This fact could be related to local field calculations and NPs electronic properties. For the nonlinear optical refraction, we passed from self-focusing to self-defocusing, when changing from Ag to Au.

Large optical birefringence by anisotropic silver nanocomposites

A large optical birefringence of oriented Ag nanoellipsoids embedded in silica was measured using an ellipsometric technique. The two main surface plasmon resonances associated with the axes of the ellipsoid were tuned, allowing us to quantify the light transmission through the samples when placed and rotated between crossed and parallel polarizers. This birefringence can be physically associated with the selective optical absorption of one component of the linear polarization of the incident light with respect to the anisotropic axis of the sample, depending on the wavelength used to perform the measurement.

Optical properties of Ir2+-implanted silica glass

Abstract High-purity silica samples (OH content less than 1 ppm, impurity content less than 20 ppm) were implanted with 2 MeV Ir2+ ions at doses ranging from 0.6 to 7×10 16 ions/cm 2 , and annealed in air at 300°C, 600°C and 900°C for 1 h. An optical absorption band at 248 nm, associated with the presence of B2 defects, appears in the spectra of all the Ir-implanted samples before the annealing, but these B2 defects disappear for heat treatments at T⩾600°C. It is important to stress that no surface plasmon resonance associated with Ir ions was observed in the optical absorption spectra of the Ir-implanted silica samples. Photoluminescence (PL) spectra show emission bands at 310, 415 and 620 nm, which can be associated with oxygen vacancies (B2 defects), O–O weak bonds and non-bridging oxygen hole centers (NBOHC), respectively. In this work we present the first optical studies on Ir-implanted silica glass and discuss the effect of the ion-induced structural defects on the absorption and emission spectra after heat treatments in air.

Size-and shape-dependent nonlinear optical response of Au nanoparticles embedded in sapphire

Nonlinear optical response of Au metallic nanoparticles, synthesized and embedded in sapphire by using ion implantation, as a function of their size and shape is studied. The size of the Au NPs was varied by controlling the annealing time of the gold-irradiated sapphire in a reducing atmosphere. Their shape was changed from approximately spherical to prolate by swift heavy-ion irradiation using Si3+, obtaining an anisotropic composite consisting in deformed NPs, all oriented in the direction of the Si beam irradiation. At 532 nm and 26 ps pulses, the isotropic system shows negative nonlinear absorption increasing with size, and positive nonlinear refraction. On the other hand, prolate nanoparticles show negative (null) absorption and null (positive) refraction for the minor (major) axis. This kind of system also shows figures of merit and relaxing times in the order of the picoseconds, appropriate for all-optical switching applications.

Determination of the size distribution of metallic nanoparticles by optical extinction spectroscopy

A method is proposed to estimate the size distribution of nearly spherical metallic nanoparticles (NPs) from optical extinction spectroscopy (OES) measurements based on Mies theory and an optimization algorithm. The described method is compared against two of the most widely used techniques for the task: transmission electron microscopy (TEM) and small-angle x-ray scattering (SAXS). The size distribution of Au and Cu NPs, obtained by ion implantation in silica and a subsequent thermal annealing in air, was determined by TEM, grazing-incidence SAXS (GISAXS) geometry, and our method, and the average radius obtained by all the three techniques was almost the same for the two studied metals. Concerning the radius dispersion (RD), OES and GISAXS give very similar results, while TEM considerably underestimates the RD of the distribution.

Optical third-order nonlinearity by nanosecond and picosecond pulses in Cu nanoparticles in ion-implanted silica

We report a study of the Kerr effect and nonlinear optical absorption in a high-purity silica sample containing Cu nanoparticles prepared by ion implantation. With a vectorial self-diffraction experiment, we measured the tensorial components of the third-order nonlinear optical susceptibility response at 532 nm with pulses of 7 ns and 26 ps. We identified thermal effect as the main mechanism responsible for the nonlinear refraction in the nanosecond regime and electronic polarization for the picosecond regime. We observed saturable optical absorption in both regimes studied. We also measured the ablation threshold for this material, finding that the contribution of the linear optical absorption to the ablation threshold in each case can be different due to the presence of hot electrons.

Modification of the nonlinear optical absorption and optical Kerr response exhibited by nc-Si embedded in a silicon-nitride film

We studied the absorptive and refractive nonlinearities at 532 nm and 26 ps pulses for silicon-nitride films containing silicon nanoclusters (nc-Si) prepared by remote plasma-enhanced chemical vapor deposition (RPECVD). Using a self-diffraction technique, we measured for the as-grown sample beta=7.7x10(-9)m/W, n(2)=1.8x10(-16)m(2)/W, and /chi(3)1111/ = 4.6x10(-10)esu; meanwhile, when the sample was exposed to an annealing process at 1000 degrees C during one hour in a nitrogen atmosphere, we obtained beta=-5x10(-10)m/W, n2=9x10(-17)m(2)/W, and /chi(3)1111/=1.1x10(-10)esu. A pure electronic nonlinear refraction was identified and a large threshold ablation of 41 J/cm(-2) was found for our films. By fitting nonlinear optical transmittance measurements, we were able to estimate that the annealed sample exhibits a response time close to 1 fs. We report an enhancement in the photoluminescence (PL) signal after the annealing process, as well as a red-shift due to an increment in size of the nc-Si during the thermal process.

Measurement of L X-ray production cross sections by 400–700 keV proton impact on rare earth elements

Abstract The L-shell X-ray production cross sections of rare earth elements Dy, Ho, Er, Tm, Yb, and Lu by the impact of protons with energies between 400 and 700 keV have been measured. Thin films of the rare earth fluorides have been employed as targets. A comparison with the predictions given by the ECPSSR theory of Brandt and Lapicki, following the algortihm developed by Smit for their calculation, shows that this theory underestimates the measured cross sections. In order to obtain cross sections values which are more reliable, a compilation of most of the existing La line production cross sections (for elements with atomic numbers 45–92) was done. A scaling of the ratio of measured to ECPSSR values, with the reduced velocity parameter with relativistic correction ξ L R defined as ξ L R ≡ ( ξ L 1 R + ξ L 2 R + 2 ξ L 3 R )/4, is then proposed and the results present a better agreement with the cross sections measured in this work. It is also found that more studies at low proton energies and heavy targets are required in order to check the validity of the ECPSSR theory for small values of ξ L R .

Uses of PIXE at low proton energies

Abstract A review of proton-induced PIXE at low energies ( E p ≤ 1 MeV) is presented. A discussion of applicable stopping powers, ionization cross sections and background processes is given, as well as their effects on quantitative analysis. The use of low-energy PIXE for thin-film thickness determination, one of its most useful applications, is also discussed.