A. Crespo-Sosa

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.

Linear optical response of metallic nanoshells in different dielectric media

Metal nanoshells, which consist of nanometer-scale dielectric cores surrounded by thin metallic shells, have been designed and studied for their linear optical responses. The plasmon resonance of metal nanoshells displays geometric tunability controlled by the ratio of shell thickness either to the core radius or to the total radius of the particle. Using Mie theory the surface plasmon resonance (SPR) of metallic nanoshells (Au, Ag, Cu) is studied for different geometries and physical environments. Considering a final radius of about 20 nm, the SPR peak position can be tuned from 510 nm (2.43 eV) to 660 nm (1.88 eV) for Au, from 360 nm (3.44 eV) to 560 nm (2.21 eV) for Ag, and from 553 nm (2.24 eV) to 655 nm (1.89 eV) for Cu, just by varying the ratio t/RShell and the environments inside and outside. With the decrease of the t/RShell ratio the SPR peak position gets redshifted exponentially and the shift is higher for a higher refractive index surroundings. The plasmon linewidth strongly depends on the surface scattering process and its FWHM increases with the reduction of shell thickness.

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.

Metallic nanoparticle formation in ion-implanted silica after thermal annealing in reducing or oxidizing atmospheres

Abstract High-purity silica samples with OH content less than 1 ppm, were implanted with 2 MeV Cu + , Ag + and Au 2+ ions at fluences of 0.7, 3 and 6×10 16 ions/cm 2 , and annealed in either reducing (70%N 2 +30%H 2 ) or oxidizing (air) atmosphere at 300, 600, 900 and 1100 °C for 1 h. For the three implanted species, after annealing at 900 °C in the reducing atmosphere, the absorption spectra present a broad band at 250 nm (4.9 eV), even for samples in which there are no nanoparticles formed. The nanoparticle formation in both atmospheres is quite different for the three implanted ions.

Ablation and optical third-order nonlinearities in Ag nanoparticles.

The optical damage associated with high intensity laser excitation of silver nanoparticles (NPs) was studied. In order to investigate the mechanisms of optical nonlinearity of a nanocomposite and their relation with its ablation threshold, a high-purity silica sample implanted with Ag ions was exposed to different nanosecond and picosecond laser irradiations. The magnitude and sign of picosecond refractive and absorptive nonlinearities were measured near and far from the surface plasmon resonance (SPR) of the Ag NPs with a self-diffraction technique. Saturable optical absorption and electronic polarization related to self-focusing were identified. Linear absorption is the main process involved in nanosecond laser ablation, but non-linearities are important for ultrashort picosecond pulses when the absorptive process become significantly dependent on the irradiance. We estimated that near the resonance, picosecond intraband transitions allow an expanded distribution of energy among the NPs, in comparison to the energy distribution resulting in a case of far from resonance, when the most important absorption takes place in silica. We measured important differences in the ablation threshold and we estimated that the high selectiveness of the SPR of Ag NPs as well as their corresponding optical nonlinearities can be strongly significant for laser-induced controlled explosions, with potential applications for biomedical photothermal processes.

Thermo-optic effect and optical third order nonlinearity in nc-Si embedded in a silicon-nitride film

Using a self-diffraction experiment with 7ns pulses at 532nm we studied a silicon nitride film containing silicon nanoclusters (nc-Si) of 3.1+/-0.37 nm mean size. The sample was prepared by remote plasma-enhanced chemical vapor deposition (RPECVD), and we found that its nonlinearity consists of a combination of electronic and thermal contributions. By varying the repetition rate of the laser, we discriminated the responsible mechanisms for the nonlinear response. Using this procedure we determined a total /chi((3))1111/ = 3.3x10(-10)esu, n2 = 2.7x10(-16) m(2)/W, beta = 1x10(-9) m/W and dn/dT =1x10(-4) degrees C(-1) for our sample. We also show results for the optical Kerr effect using 80 fs pulses at 820 nm. The purely electronic nonlinearity measured is characterized by /chi((3))1111/=9.5 x10(-11) esu.