Fabian Videla

Silver-silver oxide core-shell nanoparticles by femtosecond laser ablation: core and shell sizing by extinction spectroscopy

The generation of small silver metal nanoparticles (Nps) by ultrashort pulsed laser ablation has been an active area of research in recent years due to their interest in several fields of applied research such as biotechnology and material research, in particular those with sizes smaller than 10 nm. In general, laser ablation tends to produce environmentally clean metal Nps compared with wet chemical methods. However, since silver may be oxidized in the presence of water or ethanol, core–shell silver–silver oxide (Ag–Ag2O) Nps can be formed, whose size and thickness must be determined and characterized for functionalization related to future applications. This work analyses the size characteristics of core–shell Ag–Ag2O colloid nanostructures (smaller than 10 nm) obtained by femtosecond laser ablation of solid silver targets in different liquid media (water or ethanol) through the study of their optical extinction spectra. A fit of full experimental spectrum using Mie theory allows the determination of core size and shell thickness distributions as a function of fluence. The red-shift of the plasmon peak wavelength with respect to the bare-core peak wavelength at 400 nm, produced by the oxide shell, may be easily measured even for very small thicknesses. It was found that the dominant Ag2O effective thickness is inversely proportional to the fluence, reaching a maximum of 0.2 nm for a fluence of 60 J cm−2 and a minimum of 0.04 nm for a fluence of 1000 J cm−2.

Analysis of the main optical mechanisms responsible for fragmentation of gold nanoparticles by femtosecond laser radiation

Studies of fragmentation process of gold nanoparticles (Nps) in deionized water after generation by femtosecond laser ablation were performed. To analyze the fragmentation process, direct IR ultrafast pulses or super-continuum (SC) radiation focused in the colloidal solution were used in separate steps. IR pulses and SC generated externally in a sapphire crystal or directly inside the water were applied under low fluence regime. In the latter cases, to evaluate the effect on fragmentation of the different spectral bands present in the SC, we have determined different efficiency regions characterized by means of the product between the spectral response and the optical extinction spectrum corresponding to the initial Nps solution. From the analysis of this product function, we can conclude that the main fragmentation mechanism is due to linear absorption in the visible region. Likewise, the SC generated in water resulted more efficient than the SC obtained externally by a sapphire crystal. This fact may be...

High-order harmonic generation driven by chirped laser pulses induced by linear and non linear phenomena

Abstract We present a theoretical study of high-order harmonic generation (HHG) driven by ultrashort optical pulses with different kind of chirps. The goal of the present work is to perform a detailed study to clarify the relevant parameters in the chirped pulses to achieve a noticeable cut-off extensions in HHG. These chirped pulses are generated using both linear and nonlinear dispersive media. The description of the physical mechanisms origin responsible for this extension is, however, not usually reported with enough detail in the literature. The study of the behaviour of the harmonic cut-off with this kind of pulses is carried out in the classical context, by the integration of the Newton-Lorentz equation complemented with the quantum approach, based on the integration of the time dependent Schrödinger equation in full dimensions (TDSE-3D). Graphical abstract

Alternative method for concentration retrieval in differential optical absorption spectroscopy atmospheric-gas pollutant measurements

Differential optical absorption spectroscopy is a widely used technique for open-column atmospheric-gas pollution monitoring. The concentration retrieval is based on the fitting of the measured differential absorbance through the Lambert-Beer law. We present an alternative method for calculating the gas concentration on the basis of the proportionality between differential absorbance and differential absorption cross section of the gas under study. The method can be used on its own for single-component analysis or as a complement to the standard technique in multicomponent cases. The performance of the method for the case of cross interference between two gases is analyzed. The procedure can be used with differential absorption cross sections measured in the laboratory or taken from the literature. In addition, the method provides a criterion to discriminate against different species having absorption features in the same wavelength range.

Size-Dependent Optical Properties of Metallic Nanostructures

Metallic nanostructures are a key component of current and future nanotechnology devices since their individual properties convey the appropriate characteristics for applications in several fields of science and technology. At the nanoscale size, optical properties of metal structures depend not only on the type of material but also on the dimensions and geometry of the structure, suggesting the possibility of tuning optical resonances through appropriate engineering. In this chapter, we will describe methods for calculation of size-dependent optical properties of metal nanostructures and show the successful use of extinction spectroscopy technique to determine the size of nanoparticles (Nps).

Sizing particles by backscattering spectroscopy and Fourier analysis

Optical single backscattering spectroscopy can be used for sizing particles suspended in aqueous solution. In this work, we present results of backscattering spectroscopy applied to the determination of radii of calibrated spherical latex microparticles when a beam of white light is incident on the sample. From Mie calculations and Fourier analysis, we can determine the radius of particles covering the range between 0.5 and 12 µm for monomodal samples, with a mean error of 0.7 µm. To improve the accuracy, a correlation algorithm is applied that reduces the uncertainty in more than an order of magnitude and compares to the traceable error given by the manufacturer. The method can be also applied to bimodal and trimodal samples, allowing the separation of different size components.

Design, development and characterization of a DFB (distributed feedback) laser diode control system

The main goal of this work deals on the design and implementation of a programmable controller that allows the operation of a DFB within certain restrictions. This type of laser diode must work under strict stability requirements of both temperature and current. The systems that are part of the controller were implemented independently following different control strategies. Their parameters program were autonomously configured either with a console attached to the board or through an external interface. The programming flexibility leads to an innovative possibility to work with almost any DFB device. Furthermore, considering its small size, it could be suitable to be used inside of an optical sensor system. These characteristics are very important because currently there are no controllers with such versatility. The control system performance was evaluated using an OFRR (Optical Fiber Ring Resonator). The results obtained in this high-sensibility optical test, has allowed characterizing the optical response of the system in relation to perturbations introduced with temperature variations.

Bending waveguides made in x-cut lithium niobate crystals for technological applications

In this paper we analyse the performance of several designs of integrated optical deviators made in x-cut lithium niobate crystals by means of femtosecond laser writing using the double line approach. Straight and bent guiding structures have been designed and implemented using this technique. Well-confined propagation modes at communication wavelengths (1.55 μm) were conducted in these structures with acceptable overall losses (less than 2 dB cm−1). Further, a discussion about the optical propagation losses for curved and straight deviators devices is included in this work. At a low aperture angle (less than 0.2°), as expected, low losses were determined for both structures; however, a weak output light was observed for large angles (greater than 0.2°) in the straight optical circuits. In contrast, a smooth variation of the output was measured for the bent structures. The results presented in this paper support the possibility of the technological implementation of integrated optical circuits for optical communications fabricated with ultrashort laser writing in lithium niobate crystals. In addition, some hypotheses of loss mechanisms that are normally not considered are discussed in order to explain the differences between the measured values and predictions obtained by calculating with the usual models.

Plasmonic properties and sizing of core-shell Cu-Cu2O nanoparticles fabricated by femtosecond laser ablation in liquids

The synthesis and study of optical properties of copper nanoparticles are of great interest since they are applicable to different areas such as catalysis, lubrication, conductive thin films and nanofluids. Their optical properties are governed by the characteristics of the dielectric function of the metal, its size and environment. The study of the dielectric function with radius is carried out through the contribution of free and bound electrons. The first one is corrected for size using the modification of the damping constant. The second one takes into consideration the contribution of the interband transitions from the d-band to the conduction band, considering the larger spacing between electronic energy levels as the particle decreases in size below 2 nm. Taking into account these specific modifications, it was possible to fit the bulk complex dielectric function, and consequently, determine optical parameters and band energy values such as the coefficient for bound electron contribution Qbulk = 2 x 1024, gap energy Eg = 1.95 eV, Fermi energy EF = 2.15 eV and damping constant for bound electrons γb = 1.15 x 1014 Hz. The fit of the experimental extinction spectra of the colloidal suspensions obtained by 500 μJ ultrashort pulse laser ablation of solid target in water and acetone, reveals that the nanometric and subnanometric particles have a Cu- Cu2O structure due to an oxidation reaction during the fabrication. The results were compared with those obtained by AFM, observing a very good agreement between the two techniques, showing that Optical Extinction Spectroscopy (OES) is a good complementary technique to standard electron microscopy.

Role of supercontinuum in the fragmentation of colloidal gold nanoparticles in solution

In this work we have studied the fragmentation of gold nanoparticles (NPs) after generation by femtosecond laser ablation of a solid target in deionized water. The fragmentation process was carried out using two different types of radiation: direct ultra-fast pulses and super-continuum radiation focused in the colloidal solution. In the former case, IR pulses were applied both in low and high fluences regime, while in the latter, super-continuum was generated by an external sapphire crystal. In this last case, to assess the effects of the different spectral bands present in the super-continuum for fragmentation, we have determined different efficiency regions. From the analysis of optical extinction spectra and Transmission Electron Microscopy (TEM) histograms we can conclude that the main mechanism is linear absorption in the visible region. Likewise, the super-continuum generated in water during fragmentation resulted more efficient than that obtained externally by the sapphire crystal. This fact can be attributed to the broadening of the water continuum band originated due to large intensity used for generation. TEM and Small Angle X-ray Scattering (SAXS) measurements support the results found from optical extinction spectroscopy.