159 research outputs found
Digital speckle pattern interferometry applied to a surface roughness study
Surface roughness determination is of great interest for many applications. Several methods can be found in the literature, but most of them rely on indirect evaluation of the information, such a s photographic techniques. We propose a method to measure surface roughness that takes advantage of digital speckle pattern interferometry for obtaining the data, and of digital image processing for evaluating it. After defining the problem, a theoretical description is presented, and finally it is compared with experimental results, showing good agreement.Facultad de IngenierÃ
Digital speckle pattern interferometry applied to a surface roughness study
Surface roughness determination is of great interest for many applications. Several methods can be found in the literature, but most of them rely on indirect evaluation of the information, such a s photographic techniques. We propose a method to measure surface roughness that takes advantage of digital speckle pattern interferometry for obtaining the data, and of digital image processing for evaluating it. After defining the problem, a theoretical description is presented, and finally it is compared with experimental results, showing good agreement.Facultad de IngenierÃ
Enhancement of photoacoustic detection of inhomogeneities in polymers
We report a series of experiments on laser pulsed photoacoustic excitationin
turbid polymer samples addressed to evaluate the sound speed in the samples and
the presence of inhomogeneities in the bulk. We describe a system which allows
the direct measurement of the speed of the detected waves by engraving the
surface of the piece under study with a fiduciary pattern of black lines. We
also describe how this pattern helps to enhance the sensitivity for the
detection of an inhomogeneity in the bulk. These two facts are useful for
studies in soft matter systems including, perhaps, biological samples. We have
performed an experimental analysis on Grilon(R) samples in different situations
and we show the limitations of the method.Comment: 8 pages, 7 figure
Vertical MEMS Resonators for Real-Time Clock Applications
MEMS resonators are today widely investigated as a desirable alternative to quartz resonators in real-time clock applications, because of their low-cost, integration capability properties. Nevertheless, MEMS resonators performances are still not competitive, especially in terms of frequency stability and device equivalent resistance (and, then, power consumption). We propose a new structure for a MEMS resonator, with a vertical-like transduction mechanism, which exhibits promising features. The vertical resonator can be fabricated with the low-cost, high performance THELMA technology, and it is designed to be efficiently frequency tunable. With respect to the commonly investigated lateral resonators, it is expected to have lower equivalent resistances and improved large-scale repeatability characteristics
Engineering integrated pure narrow-band photon sources
Engineering and controlling well defined states of light for quantum
information applications is of increasing importance as the complexity of
quantum systems grows. For example, in quantum networks high multi-photon
interference visibility requires properly devised single mode sources. In this
paper we propose a spontaneous parametric down conversion source based on an
integrated cavity-waveguide, where single narrow-band, possibly distinct,
spectral modes for the idler and the signal fields can be generated. This mode
selection takes advantage of the clustering effect, due to the intrinsic
dispersion of the nonlinear material. In combination with a CW laser and fast
detection, our approach provides a means to engineer a source that can
efficiently generate pure photons, without filtering, that is compatible with
long distance quantum communication. Furthermore, it is extremely flexible and
could easily be adapted to a wide variety of wavelengths and applications.Comment: 13 pages, 7 figure
Waveguide-based OPO source of entangled photon pairs
In this paper we present a compact source of narrow-band energy-time
entangled photon pairs in the telecom regime based on a Ti-indiffused
Periodically Poled Lithium Niobate (PPLN) waveguide resonator, i.e. a waveguide
with end-face dielectric multi-layer mirrors. This is a monolithic doubly
resonant Optical Parametric Oscillator (OPO) far below threshold, which
generates photon pairs by Spontaneous Parametric Down Conversion (SPDC) at
around 1560nm with a 117MHz (0.91 pm)- bandwidth. A coherence time of 2.7 ns is
estimated by a time correlation measurement and a high quality of the entangled
states is confirmed by a Bell-type experiment. Since highly coherent
energy-time entangled photon pairs in the telecom regime are suitable for long
distance transmission and manipulation, this source is well suited to the
requirements of quantum communication.Comment: 13 page
Enhanced electron-phonon coupling in graphene with periodically distorted lattice
Electron-phonon coupling directly determines the stability of cooperative
order in solids, including superconductivity, charge and spin density waves.
Therefore, the ability to enhance or reduce electron-phonon coupling by optical
driving may open up new possibilities to steer materials' functionalities,
potentially at high speeds. Here we explore the response of bilayer graphene to
dynamical modulation of the lattice, achieved by driving optically-active
in-plane bond stretching vibrations with femtosecond mid-infrared pulses. The
driven state is studied by two different ultrafast spectroscopic techniques.
Firstly, TeraHertz time-domain spectroscopy reveals that the Drude scattering
rate decreases upon driving. Secondly, the relaxation rate of hot
quasi-particles, as measured by time- and angle-resolved photoemission
spectroscopy, increases. These two independent observations are quantitatively
consistent with one another and can be explained by a transient three-fold
enhancement of the electron-phonon coupling constant. The findings reported
here provide useful perspective for related experiments, which reported the
enhancement of superconductivity in alkali-doped fullerites when a similar
phonon mode was driven.Comment: 12 pages, 4 figure
Current mapping of GaN films by conductive atomic force microscopy
Conductive atomic force microscopy has been used to investigate the local conductivity in hydride vapor-phase epitaxy and molecular-beam epitaxyGaN films, focusing on the effect of off-axis facet planes. We investigated two different types of samples, in which the facet planes were either present on the perimeters of as-grown islands, or on the edges of etch pits created by post-growth chemical etching. The results show that crystallographic planes tilted with respect to the c-plane growth direction show a significantly higher conductivity than surrounding areas. The n-type (or p-type) samples required a negative (or positive) sample bias for current conduction, consistent with the formation of a Schottky barrier between the metallized atomic force microscope tip and sample. The time dependence of this enhanced conductivity was different for the two types of samples, possibly indicating different conduction mechanisms
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