L. Rodríguez

Thermo-optical properties of gold nanoparticles in colloidal systems

In this work, we report the thermo-optical properties of nanoparticles in colloidal suspensions. Spherical gold nanoparticles obtained by laser ablation in condensed media were characterized using thermal lens spectroscopy pumping at 532 nm with a 10 ns pulse laser-Nd-YAG system. The obtained nanoparticles were stabilized in the time by surfactants (sodium dodecyl sulfate or SDS) in water with different molar concentrations. The morphology and size of the gold nanoparticles were determined by transmission electron microscopy (TEM) and UV–visible techniques. The plasmonic resonance bands in gold nanoparticles are responsible for the light optical absorption, and the positions of the absorption maximum and bandwidth in the UV–visible spectra are given by the morphological characteristics of these systems. The thermo-optical constants such as thermal diffusion, thermal diffusivity, and (dn/dT) are functions of the nanoparticle sizes and the dielectric function of the media. For these reasons, the thermal lens (TL) signal is also dependent on nanoparticle sizes. An analysis of the TL signal of the nanoparticles reveals the existence of an inverse dependence between the thermo-optical functions and the size. This methodology can be used in order to evaluate these systems and characterize nanoparticles in different media. These results are expected to have an impact in bioimaging, biosensors, and other technological applications such as cooling systems.

Sine Sweep Load vs. Impact Excitations and Their Influence on the Damping Coefficients of a Bubbly Oil Squeeze Film Damper

Squeeze Film Dampers (SFD) suppress excessive vibrations and rotordynamic instabilities in turbomachinery. However, air ingestion into the oil film is a pervasive phenomenon that affects their performance, complicating their analysis and design, and demanding careful experimentation. The type of force excitation affects the damping coefficients since the ensuing dynamic journal motions may lead to a rapid expulsion or to coalescence and entrapment of the air within the lubricant matrix. Experimental force coefficients from a small rotor-SFD apparatus operating with controlled mixtures of air and oil, i.e. emulating degrees of air entrainment, are obtatined from the dynamic response to sine sweep forces and impact loads. The parameter identification procedure renders damping coefficients that are sensitive to the type of force excitation. For impact tests, damping coefficients steadily increase for lubricant mixtures up to 50 % in air volume content. For unidirectional sine sweep load excitations, the damping coefficients are nearly constant even for mixtures with larger air volume fractions. The larger and sustained amplitudes of periodic journal motion induced in the sweep sine tests expel rapidly the air from the mixture, thus leaving a lubricant film that generates invariant dumping coefficients. Conversely, in the impact tests, the journal motions are of small amplitude and short duration thus providing larger damping values; the mixture behaves as a nearly incompressible fluid of larger viscosity. Presented at the 56th Annual Meeting Orlando, Florida May 20–24, 2001

Thermo‐optical Properties of Nanofluids

In this work, we report thermo‐optical properties of nanofluids. Spherical gold nanoparticles obtained by laser ablation in condensed media were characterized using thermal lens spectroscopy in SDS‐water solution pumping at 532 nm with a 10 ns pulsed laser‐Nd‐YAG system. Nanoparticles obtained by laser ablation were stabilized in the time by surfactants (Sodium Dodecyl‐Sulfate or SDS) in different molar concentrations. The morphology and size of the gold nanoparticles were determined by transmission electron microscopy (TEM). The plasmonic resonance bands in gold nanoparticles are responsible of the light optical absorption of this wavelength. The position of the absorption maximum and width band in the UV‐Visible spectra is given by the morphological characteristics of these systems. The thermo‐optical constant such as thermal diffusion, thermal conductivity and dn/dT are functions of nanoparticles sizes and dielectric constant of the media. The theoretical model existents do not describe completely this...

Evaluation of photosensitizers singlet oxygen production for photodynamic therapy applications using time resolved thermal lens

We report on the optical characterization and measurement of oxygen singlet quantum yield of Chlorophyll and Chlorin e6 in water-ethanol mixtures by direct observation of thermal relaxation using time resolved thermal lens method. The analysis of the time-resolved curve allows the determination of the quantum yield of singlet oxygen formation. The quantum yield is deduced from the relative magnitudes of the fast and slow components.

Stimulated Raman red-light generation by acetone as a perspective source for photodynamic therapy applications

We report on a new source of coherent red-light with perspective applications in laser photodynamic therapy. The red light is generated through stimulated Raman scattering (SRS) process by acetone of a frequency-doubled Nd:YAG nanosecond laser radiation (532 nm). The Stokes transition is found at 630 nm and with spectral line narrowing similar to the pump one. When pumping using the radiation from a Rhodamine-6G Dye laser (560 - 575 nm), tuning between 640 and 670 nm is demonstrated. Conversion efficiencies up to 50% are reported.

Mode-mismatched differential thermal lensing experiment

Pump-probe thermal lensing (TL) is a well known method for the measurement of small absorption coefficient with high sensitivity and versatility. When measuring the absorption of a complex system such as a solution composed by a high absorbing solvent and a low absorbing solute, the contribution of the solute is usually masked by the absorption of the solvent. To overcome the background contribution differential methods have been proposed [1]. However, these methods involved the focusing of the pump and probe beams onto the sample cell and independently onto the reference cell. Because of that these methods are only effective for short path-length samples. In this communication we present a new scheme for a pump-probe mode-mismatched TL experiment where the probe beam is focused in the presence of a nearly collimated pump beam. The scheme substantially simplifies the realization of a differential TL experiment allowing for the use of large path-length samples.