U. Zammit

High‐resolution simultaneous photothermal measurements of thermal parameters at a phase transition with the photopyroelectric technique

Photothermal techniques allow, under particular experimental conditions, the simultaneous determination of specific heat, thermal conductivity, and thermal diffusivity. A comparison has been made between measurements performed with the gas‐microphone (P) configuration and the photopyroelectric (PPE) one at a liquid‐crystal second‐order phase transition. An increase in the sensitivity with respect to thermal parameter variation has been observed in the case of PPE.

Simultaneous determination of specific heat, thermal conductivity and thermal diffusivity at low temperature via the photopyroelectric technique

The photopyroelectric effect has been used to measure simultaneously specific heat (c), thermal conductivity (k) and thermal diffusivity (α) at low temperatures. A calibration procedure which allows the use of a pyroelectric transducer at low temperatures is described. Simultaneous measurements of c, k, and α over a high Tc superconducting phase transition are reported.

Simultaneous absolute measurements of the thermal diffusivity and the thermal effusivity in solids and liquids using photopyroelectric calorimetry

We report on the simultaneous “absolute” measurements of the thermal diffusivity and of the thermal effusivity in solid and liquid samples by using only the phase of the so called self-normalized Photopyroelectric signal, performing no other normalization/calibration procedure than that carried out during the actual measurements. Measurements, performed as a function of temperature, are characterized by a temperature resolution adequate to study phase transitions. The obtained results are in excellent agreement with those obtained in the previous studies.

Ion dose effect in subgap absorption spectra of defects in ion implanted GaAs and Si

Subgap optical absorption investigations have been used to study the influence of increasing ion dose on the properties of ion implanted layers of Si and GaAs. Beside the spectral region corresponding to the band edge region, a second region at lower energy, also exhibiting an exponential behavior but with a substantially larger value of the inverse logarithmic slope, has been observed in all the investigated samples. A common trend has been observed for ion implanted Si and GaAs in the dependence of the values of the lower energy region inverse logarithmic slope as a function of the implantation dose, and it depended on the presence or not of amorphous material in the implanted layer. The results are discussed in terms of a possible evolution of gap states responsible for the observed features.

Photoacoustic optical and thermal characterization of Si and GaAs ion implanted layers

The values of the thermal conductivity and of the optical absorption coefficient of ion implanted Si and GaAs have been measured in a nondestructive way by photoacoustic measurements. The values obtained for Si are in good agreement with those found in literature, while no data have been found for ion implanted GaAs as a function of the implanted ions dose. The thermal conductivity value for the implanted layers is more than two orders of magnitude lower than the ones for the respective crystalline material. The implanted ion dose dependence of the two parameters is reported for GaAs.

SUB-GAP OPTICAL PROPERTIES OF ION IMPLANTED SIC

〈0001〉 6HSiC was implanted with B+ ions at room temperature. The relative defect concentration ndat was determined by Rutherford backscattering spectrometry, and the optical properties in the photon energy range 0.7 eV ≤ hω ≤ 3.2 eV were analysed using transmission and reflection measurements. For ndat ≲ 0.15 the layers contain mainly point defects and point defect complexes and the occurrence of a refractive index change can be largely excluded. The measured transmission can be simulated assuming an exponential sub-gap absorption tail which is superimposed on a broad absorption band in the range 1.3 eV ≤ hω ≤ 2.4 eV. Amorphous SiC is characterized by a refractive index change of 30% with respect to the unimplanted substrate and by an optical gap of 1.3 eV. In the case of medium damage (0.15 < ndat < 1), analysis of the transmission spectra indicates the existence of amorphous zones and of point defects and point defect complexes in the crystalline regions between the amorphous zones in the implanted SiC layers.

Optically induced reorientational birefringence in an artificial anisotropic Kerr medium

Abstract A laser induced optical birefringence has been obtained for the first time in an anisotropic artificial Kerr medium. This consists of an aqueous suspension of dielectric microellipsoids which reorientate under electromagnetic and brownian torques. The measured effective optical Kerr coefficients, n 2 , is 4.0x10 -4 cm 2 /MW while the response time is 2.6 ms. These values agree with the ones obtained from a simple Debye model assuming rigid anisotropic rotors in a statistical environment.

Hidden Text Detection by Infrared Thermography / Anwendung der aktiven Infrarotthermographie (IRT) zur Erfassung von verdeckten Texten / Utilisation de la thermographie infrarouge (IRT) pour détecter des textes cachés

Abstract We present an opto-thermal approach based on the use of active infrared thermography (IRT) for the study of texts hidden inside the bookbinding structure of ancient books. In particular we focus our investigation on the detection and characterization of texts on paper scraps, belonging to earlier handwritten or printed leaves, used for the making of bookbindings and lying between the end papers and the cover. A qualitative description of the physical mechanisms allowing the identification of texts is proposed and a comparative analysis of the results obtained by means of different IRT experimental configurations is presented. The results show that active IRT can be a very useful tool for the detection and the identification of underlying texts whose reading can provide useful information on the specific history of ancient books. Zusammenfassung Die Anwendung von aktiver Infrarotthermographie (IRT) zur Erfassung von Texten, die in Einbandstrukturen verborgen sind, wird vorgestellt. Insbesondere wurden Texte auf Fragmenten, die fur die Buchdeckenherstellung verwendet wurden, beziehungsweise sich zwischen Vorsatz und Buchdeckel befinden. Eine Beschreibung der physikalischen Grundlagen fur die Texterfassung mit IRT wird gegeben ebenso wie die Prasentation ausgewahlter Untersuchungen an historischen Objekten. Die prasentierten Resultate zeigen, dass aktive Infrarotthermographie eine geeignete Methode fur die Erfassung von verdeckten Textfragmenten darstellt und wichtige Informationen zur Geschichte historischer Bucher beziehungsweise Bucheinbande liefern kann. Résumé Nous présentons une approche qui se base sur l’utilisation active de la thermographie infrarouge (IRT) pour l’étude de textes cachés à l’intérieur de la structure reliée des livres anciens. L’examen s’est centre surtout sur la détection et la caractérisation de textes sur les fragments de papier provenant de feuilles écrites auparavant à la main ou imprimées et qui ont servi dans la fabrication des reliures et qui se trouvent entre les dernières feuilles de garde et la couverture des livres. On présentera une description des mécanismes physiques permettant l’identification des textes et une analyse comparative des résultats obtenus au moyen de différentes configurations expérimentales d’IRT. Les résultats présentés démontrent que La thermographie infrarouge active s’avère être un outil très utile dans la détection et la caractérisation de textes sous-jacents dont la lecture peut fournir de précieuses informations sur l’histoire spécifique de livres anciens.

Latent heat investigation by photopyroelectric calorimetry

We report on an approach for photopyroelectric calorimetry enabling a frequency dependence detection of the specific heat and of the latent heat exchanged over first order transitions. It has been applied to the nematic-isotropic transition of 4-n-octyl-4′-cyanobiphenyl liquid crystal confined in a silica nanoparticles network, where the specific heat shows a double peak structure. The larger strain involved with the nematic nucleating over the low temperature peak induces a considerable lower latent heat than the one involved over the higher temperature peak. The frequency dependent measurements have shown a different dynamics of the nematic nucleating over the two peaks.

Surface states and buried interface states studies in semiconductors by photothermal deflection spectroscopy

A method is presented which, through the simultaneous analysis of the photothermal deflection spectroscopy (PDS) signal amplitude and phase spectra, enables to detect surface states and buried interface states and to measure their absorption. A theoretical model for the PDS signal suitable for the present approach has been developed and the measurements were performed on single crystalline GaAs wafers with ion‐implanted layers on the front surface or buried beneath the front surface.