Showing papers by "Oscar E. Martínez published in 2002"

Calibration of subnanometer motion with picometer accuracy.

Abstract: We have developed a method for calibrating subnanometer movements of a piezoelectric actuator with picometer accuracy and for a wide range of frequencies. This range make this calibration useful for scanning probe microscopes, particularly for an apertureless scanning near-field optical microscope in which the tip is dithered to modulate the optical signal. The setup consists of a Michelson interferometer that has a mobile arm capable of moving more than one fringe. The piezoelectric actuator to be calibrated vibrates at the desired frequency in the other arm. Net displacement can be calculated by simultaneous measurement of an interferometric signal and its derivative. Hysteresis of the system can be also measured. It will be shown that the actuator response is linear only for the low-frequency region (in our case as much as approximately 10 kHz). Above that frequency range, higher harmonics appear and cannot be neglected to obtain real displacement. Finally, it will be shown that the use of higher harmonics in calibration or detection schemes (that rely on the linearity of the response) must be validated, and this technique has proved adequate for that purpose.

Optical and structural characterization of GaN/AlN quantum dots grown on Si(111)

Abstract: GaN/AlN-based heterostructures made from stacked GaN quantum dots (QDs) have been studied by means of the cathodoluminescence (CL), photoluminescence (PL), near-field scanning optical microscopy (NSOM) and micro-Raman techniques. The influence of the number of stacked layers (2–85) and of the different electron beam injection conditions on the main optical emissions was studied by means of CL, revealing transitions from 2.5 and 4.4 eV. Power-dependent cross-sectional CL studies revealed a large (87–180 meV) blue-shift only for the optical bands located in the 2.5 and 3.1 eV spectral range. This observation enabled us to assign a zero-dimensional character to those bands. The results were confirmed by PL and NSOM studies. Different values of the blue-shift were found for specimens with different numbers of stacked layers. This suggested the presence of different residual strains inside the structures, as confirmed by micro-Raman studies. An inhomogeneous distribution of the QD emissions was also observed both in the plane and along the growth direction.

Steel Surface Roughening by Transient Self Dewetting After Pulsed Electron Beam Melting

Abstract: Experimental evidence of surface roughening by transient self dewetting of metallic surfaces is presented. Steel surfaces are melted by a pulsed electron gun and the subsequent fast cooling against the substrate gives rise to the formation of characteristic patterns that are attributed to the dewetting of the liquid film. The patterns formed are similar to those obtained by spinodal dewetting, that is, when the dewetting action develops from a non-linear instability on the liquid surface, and not from hole nucleation. The patterns are characterised by the presence of valleys of unmelted substrate surrounded by ridges of resolidified material. Martensitic steels can form ferritic substrates with martensitic ridges. Two possible mechanisms respon sible for the onset of the surface instability are proposed.

Online Measurement of Residual Dirt On Steel Plates by Laser Ablation and Acoustic Detection

Abstract: Cold rolled steel plate emerges from the manufacturing process with a thin layer of residual dirt that has to be kept under control for two main reasons: it may be indicative of deviations from normal functioning in previous manufacturing stages and it may impair the e ffectiveness of downstream operations such as painting and galvanising. Standard measurements of this residual dirt are based on undependable laboratory sample measurements, outside the production line. A method has been developed that allows online determination of the cleanliness for the whole manufactured steel plate bobbin. Based on this method, the first industrial instrument (Elmes I) has been designed, which is now in operation at the final inspection line of the Siderar plant at Ensenada, Buenos Aires, Argentina. SE / S216

Laser instrument for determination of the degree of cleanliness in cold-rolled steel plate manufacturing

Abstract: Cold-rolled steel plate emerges from the manufacturing process with a thin layer of residual dirt that has to be kept under control for two main reasons: It may be indicative of deviations from normal functioning in previous manufacturing stages and it may impair the effectiveness of downstream operations such as painting, galvani-zing, etc. Standard measurements of this residual dirt are based on not trustworthy laboratory sample measurements, outside the production line. We developed a method that allows on-line determina-tions of the cleanliness degree for the whole manufactured steel plate bobbin. Based on this method, we designed the first industrial instrument (ELMES I), which is now in operation at the final inspection line of the Siderar's plant at Ensenada (Buenos Aires).

Testing stability of surfaces and coatings by resonant photoacoustic phase transition detection

Abstract: A technique has been developed for the determination of the phase transition temperature of very small samples or thin films and coatings. The technique relies on the determination of changes in the heat capacity of the surface by resonant piezoelectric detection of the photoacoustic signal. The sample is illuminated by means of a low power diode laser modulated at the resonant frequency of a piezoelectric detector. The detector senses the acoustic signal generated by the expansion of the surface that has been slightly heated (about 1 K) by the laser. The signal is monitored as the sample is heated at an established rate, and the phase transition temperature is determined by the change in the amplitude and phase of the signal. As the penetration of the light in metals is very small (well below 1 μm), the technique is particularly suitable for the study of surface coatings and thin films.

Direct method for phase and amplitude determination of ultra-short light pulses

Abstract: We present a new method for obtaining the phase of an ultra-short light pulse in the frequency domain. This technique is based on measuring the logarithmic derivative of the cross correlation of the pulse with a frequency-filtered version of it. It only requires sweeping the center frequency of the filter, while the time delay of the correlator is held fixed for measuring the group delay of the frequency components of the pulse. It is, to our knowledge, the first one-dimensional method to recover directly the phase information without resorting to a deconvolution or a procedure involving two Fourier transforms. Since it directly measures the delay operator we have called this technique direct group delay operator measurement. We present the theory, experimental results and a brief discussion of the best choice for the fixed setting of the time delay in order to improve the signal to noise ratio in this technique.

Cathodoluminescence and microRaman study of GaN ELO structures

Abstract: Epitaxial Lateral Overgrowth (ELO) is an effective method to reduce dislocations in heterostructures with large lattice mismatch. This method has been widely used to improve the quality of GaN layers. We present herein a study of the properties of ELO GaN layers grown by HVPE from an MOVPE GaN buffer layer. Cathodoluminescence shows a strong enhancement of the luminescence emission in the ELO regions, where TEM has proven the absence of dislocations. Local cathodoluminescence spectra show that this enhancement is mostly due to the yellow luminescence band. Besides Donor-Acceptor Pair recombination bands are observed in the near band gap spectral range. Raman data show that the ELO layers present a good crystalline quality. The Raman spectra did not reveal the presence of free carriers in concentration high enough to allow the presence of Longitudinal Optic Phonon Plasmon Coupled modes. The results are discussed in terms of the incorporation of impurities in the ELO layers together with the formation of compensating deep acceptors, probably V Ga .