C. L. Cesar

Two‐beam photoacoustic phase measurement of the thermal diffusivity of solids

A simple method is demonstrated for obtaining the thermal diffusivity of solids, by measuring the phase lag between a front and rear illumination, at a single chopping frequency. The method is tested using some semiconductor and glass samples.

PROBING OF THE QUANTUM DOT SIZE DISTRIBUTION IN CDTE-DOPED GLASSES BY PHOTOLUMINESCENCE EXCITATION SPECTROSCOPY

We studied confinement effects in CdTe quantum dots by means of photoluminescence excitation spectroscopy. We show that by changing the detection energy we can resolve the spectrum of quantum dots of different sizes inside their much broader size distribution in CdTe‐doped glass. The spectra obtained show several well‐resolved lines. There is excellent agreement between the photoluminescence excitation spectra peak energies and calculations of the confined energy transitions based on a modified multiband envelope function model.

Size effects on the phonon spectra of quantum dots in CdTe‐doped glasses

We studied the confinement effects on the phonon spectra of CdTe quantum dots by means of resonant Raman scattering measurements. The spectra show clearly longitudinal optical phonons, surface phonons and some of their overtone combinations. We show that the scattering due to surface phonons increases as the quantum dot size decreases. The results are obtained by tuning the laser excitation energy to resonance for quantum dots of different sizes inside the broad size distribution in CdTe‐doped glasses.

Ultrafast optical switching with CdTe nanocrystals in a glass matrix

This letter describes a principle demonstration of an ultrafast optical switch operating at 1Tbit∕s using CdTe-quantum-dots-doped glasses. Using a three-beam pump and probe experiment, we showed that thermal effects are responsible for a baseline in the pump and probe graphs and the nonexistence of carrier accumulation effects. After eliminating the thermal effects, we showed that, when two pump pulses are delayed by 1 ps, each pump pulse modulates the probe pulse independently, making this material highly promising for ultrafast all optical switching.

Recombination processes in CdTe quantum-dot-doped glasses

Electron-hole recombination in CdTe quantum dots was studied by photoluminescence and resonant femtosecond pump–probe measurements. A dependence of recombination times with pump pulse intensity was observed and we attribute this to the Auger recombination process. The overall kinetic processes that we observed are a fast decay from the initial excited state to surface trap states, the Auger recombination, the recombination of electrons from the surface states, and a longer time recombination which we attribute to electrons in the deep traps states.

Photoacoustic determination of thermal diffusivity of solids: Application to CdS

A novel and simple approach, which uses a lateral heating source for the sample, is proposed as an alternative method for the photoacoustic determination of the thermal diffusivity of solids. The method is experimentally tested using a CdS sample.

Coherent acoustic phonons in PbTe quantum dots

Femtosecond pump-probe studies at 1.5 μm of PbTe quantum dots in a glass matrix have revealed oscillations due to a spheroidal acoustic mode. The wavelength dependence of the observed frequencies indicates size selectivity. The frequency and damping of the mode as a function of wavelength agree with the predictions of an acoustic continuum model that suggests that the dominant damping mechanism is radiative loss from the dots to the surrounding glass.

Thermal diffusivity of superconducting YBa2Cu3O7−x

Thermal diffusivity results for superconducting YBa2Cu3O7−x systems are reported. Two laser excitation techniques were used: photoacoustic phase‐lag and the flash method. We observed an anomalous jump in the thermal diffusivity at the critical temperature and related it to the electronic specific heat anomaly. This correlation yielded a characteristic phonon frequency of 320 cm−1 in the range of an active Raman mode already observed in a single crystal of the same compound. This result reinforces the current electron‐phonon coupling mechanism as responsible for superconductivity.

SiO2∕PbTe quantum-dot multilayer production and characterization

We report the fabrication of multilayer structures containing layers of PbTe quantum dots (QDs) spaced by 15–20 nm thick SiO2 layers. The QDs were grown by the laser ablation of a PbTe target using the second harmonic of Nd:YAG laser in an argon atmosphere. The SiO2 layers were fabricated by plasma chemical vapor deposition using tetramethoxysilane as a precursor. The influence of the ablation time on the size and size distribution of the QDs is studied by high-resolution transmission electron microscopy. Optical absorption measurements show clearly the QDs confinement effects.

Mechanical and electrical properties of red blood cells using optical tweezers

Optical tweezers are a very sensitive tool, based on photon momentum transfer, for individual, cell by cell, manipulation and measurements, which can be applied to obtain important properties of erythrocytes for clinical and research purposes. Mechanical and electrical properties of erythrocytes are critical parameters for stored cells in transfusion centers, immunohematological tests performed in transfusional routines and in blood diseases. In this work, we showed methods, based on optical tweezers, to study red blood cells and applied them to measure apparent overall elasticity, apparent membrane viscosity, zeta potential, thickness of the double layer of electrical charges and adhesion in red blood cells.