Ricardo R. B. Correia

Pump-probe and pump-deplete-probe spectroscopies on carotenoids with N=9–15 conjugated bonds

A series of all-trans-carotenoids with N=9, 13, and 15 conjugated bonds has been studied by pump-probe and pump-deplete-probe spectroscopies to obtain a systematic analysis of the energy flow between the different electronic states. The ultrafast dynamics in the carotenoids are initialized by excitation to the S2 state and subsequently manipulated by an additional depletion pulse in the near-IR spectral range. The changes in the dynamics after depletion of the excited state population allowed differentiation of the excited state absorption into two components, a major one corresponding to the well known S1 state and the small contribution on the red wing of the S0-S2 absorption band originating from the hot ground state. We found no evidence for an additional electronically excited state, usually called S*. Instead, a deactivation mechanism that includes the hot ground state supports the observed results nicely in the framework of a simple three state model (S2, S1, and S0).

First hyperpolarizability in proton-transfer benzoxazoles: computer-aided design, synthesis and study of a new model compound

Abstract With regard to second-order nonlinear optics (NLO) applications, a new class of 2-(2′-hydroxyphenyl)benzoxazoles (HBO) was designed for a combination of high first hyperpolarizability, β, with good photothermal stability, in association with a fast excited state intramolecular proton transfer (ESIPT) mechanism. Semi-empirical optimization of molecular structures and ab initio calculations of dipole moments were performed. Clear evidence was found that conditions such as conjugation efficiency and electron donor/acceptor strength cannot be evaluated separately, due to structural changes in molecular spatial distribution. Experimentally, a new fluorescent molecule of the HBO family, 2-(2′-hydroxy-4′-aminophenyl)-6-nitrobenzoxazole (BO6), was synthesized, purified and characterized, including solvent environments of distinct polarities. Hyper-Rayleigh scattering, UV–Vis absorption and emission spectroscopy, differential scanning calorimetry and thermogravimetric analysis of BO6 show a significant β (213.4±25.7×10−30 esu in acetone, at 1064 nm) and thermal stability up to 270 °C. Such results, in this first study of ESIPT dyes for second-order NLO to our best knowledge, indicate that the HBO family well deserves further attention towards promising application materials.

Time-resolved optical Kerr-effect investigation on CS2/polystyrene mixtures.

The relaxation dynamics of carbon disulfide are investigated in mixtures with polystyrene (PS) using the time-resolved optical heterodyne-detected optical Kerr effect (OHD-OKE). The data are analyzed using both the model-dependent approach, which assumes four distinct temporal responses, and the model-independent Fourier transform approach, which generates a spectral response that can be compared with results obtained by depolarized Rayleigh scattering. A slow dynamics is observed for the OHD-OKE transient decaying exponentially with a time constant that varies from 1.68 ps for neat CS2 to 3.76 ps for the most concentrated CS2PS mixture. The increase of this time constant accompanies an increase in the viscosity of the mixture, so we can associate this component with the diffusive reorientation process of the induced polarizability anisotropy of the carbon disulfide in the mixture. The short-time nuclear response is characterized in the frequency domain by a broad band that peaks around 30 cm(-1) for neat carbon disulfide, and is associated with a complex relaxation pattern. The vibrational distribution shifts to higher frequencies when the PS concentration is increased in the mixture. This result is discussed in terms of an increase in the interaction strength between the PS phenyl rings and the carbon disulfide molecules.

Synthesis and Characterisation of Fluorescent Carbon Nanodots Produced in Ionic Liquids by Laser Ablation

Carbon nanodots (C-dots) with an average size of 1.5 and 3.0 nm were produced by laser ablation in different imidazolium ionic liquids (ILs), namely, 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI.BF4 ), 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMI.NTf2 ) and 1-n-octyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (OMI.NTf2 ). The mean size of the nanoparticles is influenced by the imidazolium alkyl side chain but not by the nature of the anion. However, by varying the anion (BF4 vs. NTf2 ) it was possible to detect a significant modification of the fluorescence properties. The C-dots are much probably stabilised by an electrostatic layer of the IL and this interaction has played an important role with regard to the formation, stabilisation and photoluminescence properties of the nanodots. A tuneable broadband fluorescence emission from the colloidal suspension was observed under ultraviolet/visible excitation with fluorescence lifetimes fitted by a multi-exponential decay with average values around 7 ns.

Silver migration at the surface of ion-exchange waveguides: a plasmonic template

The formation and evolution of metallic-silver nanoparticles capped with silver oxide, in the surface of Ag-doped waveguides produced by ion-exchange, were characterized. The samples were exposed to air atmosphere for periods lasting until 35 days and their aging process was investigated by optical and Atomic Force Microscopy (AFM) measurements. The results evidence migration of the Ag+ cations from inside the glass to the surface at room temperature, followed by aggregation of the silver nanoparticles (NPs) and oxidation, creating a nanometric-thick layer over the waveguide surface. This layer was employed for surface-enhanced Raman scattering (SERS) signal and for the fabrication of holographic diffraction gratings (HDG), which are presented as application examples of this material as a new plasmonic template.

Simple modeling of plasmon resonances in Ag/SiO2 nanocomposite monolayers.

Normal incidence transmittance and reflectance spectra of sputtered nanocomposite monolayer films of Ag in SiO2, buried and unburied, showed significant redshifted plasmon resonances from 410 to 455 nm, which could be well interpreted with a simple model that starts from the Maxwell Garnett theory and the Kreibig extension of the Drude-Lorentz equation, but with a further extension related to the dipolar interaction between the metal particles distributed on a surface.

Mapping impurity of single-walled carbon nanotubes in bulk samples with multiplex coherent anti-stokes Raman microscopy.

Impurity mapping in a distribution of single-walled carbon nanotube (SWNT) bundles was performed via multiplex coherent anti-Stokes Raman spectroscopy (MCARS). The Dover G-bands in the SWNTs vibrational spectrum are acquired simultaneously and used to quantify the local purity in a spin-coated sample. This technique is faster than conventional Raman microscopy, however it needs the aid of a further data processing called maximum entropy method (MEM) to retrieve the vibrational spectrum. The acquired data set allows mapping of the same spatial region selecting different spectral components. This way, the mapping of purity is performed using the ratio D/G. Additionally, a RGB like construction was performed in order to obtain information about the SWNTs dispersion.

DYE-INDUCED SPECTRAL NARROWING OF STIMULATED SCATTERING IN CS2

Abstract Narrowing of the super-broadened wings of both stimulated Rayleigh and Raman scattering spectral profiles in carbon disulfide (CS 2 ) has been observed in a liquid–core fiber system doped with 10 ppm of a dye (Coumarin 337). This effect is attributed to the light-induced pre-alignment of the anisotropic molecules of CS 2 , caused by the reorientation mechanism of excited dye molecules along the optical pump field. In addition, the enhancement of light-induced anisotropy in the presence of an absorbing dye has been observed in a light-induced birefringence experiment and is also explained in terms of the change in the guest–host interaction induced by optical excitation.

Optical characterization of carbon quantum dots in colloidal suspensions

The remarkable feature of carbon quantum dots is the strong broadband photoluminescence of their colloidal solutions, which is observed on a variety of simple synthetic routes. This article describes the production of carbon nanodots (C-dots) with an average size of 3 nm in poly(ethylene glycol) by laser ablation and the evaluation of the spectral molar absorption coefficient, e(λ), of these particles. In parallel we investigate optical properties as photoluminescence, lifetime and by a thermally managed Eclipse Z-scan we describe the thermo-optical properties of this colloidal suspension.

Characterization of ultrashort pulses by a modified grating-eliminated no-nonsense observation of ultrafast incident laser light E fields (GRENOUILLE) method

The measurement and characterization of ultrashort laser pulses remains an arduous task. The most commonly used pulse-measurement method is known as frequency-resolved optical gating (FROG), and another version with great experimental simplification and low-priced setup is known as grating-eliminated no-nonsense observation of ultrafast incident laser light E fields (GRENOUILLE). Nevertheless, there is interest in elaborating other, more accessible or simpler and cheaper, setups with equal or better assets. We explored modification of the GRENOUILLE method in which we replaced the original Fresnel biprism with a beam splitter and two mirrors and used a cheap webcam to measure the pulse traces. We have evaluated our system, and we propose a method to correct border effects caused by the beam intensitys profile based on the characterization of three pulse classes: Fourier-transform limited, double, and chirped. We compare the recovered electric field with further spectral and second-order correlation data of the corresponding pulses.