A. M. Buoncristiani

An analysis of the temperature distribution in finite solid-state laser rods

An expression for the time-dependent temperature distribution in a finite solid-state laser rod, for an arbitrary distribution of pump energy, has been derived. The specific case of end pumping by circular (constant) or Gaussian beams is included. This formulation was used to predict the time evolution of temperature in Ti:sapphire laser rods and in Nd:YAG rods of specific dimensions. >

Excited state dynamics of thulium ions in Yttrium Aluminum Garnets

Abstract The processes that take place in the excited states of a trivalent Thulium (Tm) ion in an Yttrium Aluminum Garnet (YAG) crystal, being relevant to the use of this system for laser applications, have been the object of several studies. We have reexamined this system, focusing our attention on the dynamics of Tm following its excitation in the 3H4 level. Under these conditions the system relaxes through a cross-relaxation process, 3H4→3F4, 3H6→3F4, whose rate depends upon both the concentration of the Tm ion and the temperature of the crystal. The excitation spectrum obtained by monitoring the 1.8 μm emission of Tm (due to the 3F4→3H6 transition) indicates an increase in the contribution to this emission from the 3H4 level relative to the 3H5 level as the Tm concentration increases; this shows the increased role played by the 3H4 level in pumping the infrared emission. Correspondingly the duration of the luminescence originating in the 3H4 level is shortened as the concentration of Tm increases. The concentration quenching of this lifetime can be fit to a model which assumes that the cross-relaxation is due to a dipole-dipole interaction; from this fit the intrinsic Tm lifetime in absence of cross relaxation can be derived. We have used this lifetime to calculate the rate of the cross-relaxation process. We have evaluated this rate as function of temperature and found it to be fastest at 77 K. We have also calculated the microscopic interaction parameters for the cross-relaxation process by using two independent experimental features: (i) the time evolution of the emission from the 3H4 level, and (ii) the spectral overlap between the 3H4→3F4 emission and the 3H6→3F4 absorption. We have also considered the migration of excitation among the Tm ions in the 3F4 level and calculated the relevant microparameter by the use of the relevant spectral overlap. The data are consistent with the model in which the Tm ions, once excited into the 3H4 level decay by cross relaxation to the 3F4 level, and then transfer rapidly their energy to other Tm ions.

Rugged low‐resistance contacts to YBa2Cu3Ox

We have made rugged low‐resistance contacts to the high Tc superconductor YBa2Cu3Ox by melting gold beads onto the surface of the material. After retreating the samples in oxygen, we have measured contact resistance <50 μΩ. This allowed a direct current of ≊5 A to pass through the contacts without heating while the sample remained in the superconducting state at 20 K. In this letter we present results of scanning electron microscopy, and measurements of contact resistance, critical current, and shear strength on these contacts. Such contacts will be of practical use in high current carrying applications of the new high Tc superconductors.

Laser induced fluorescence of dental caries

Significant differences between the optical spectra taken from sound regions of teeth and carious regions have been observed. These differences appear both in absorption and in laser induced fluorescence spectra. Excitation by the 488 nm line of an argon ion laser beam showed a peak in the emission intensity around 553 nm for the sound dental material while the emission peak from the carious region was red-shifted by approximately 40 nm. The relative absorption of carious region was significantly higher at 488 nm; however its fluorescence intensity peak was lower by an order of magnitude compared to the sound tooth. Implications of these results for a safe, reliable and early detection of dental caries are discussed.

Low‐resistance noble metal contacts to high‐temperature superconductors

We have extended our studies of a melting technique for making low‐resistance contacts to high‐temperature superconductors. We have made contacts to both YBa2Cu3O7−x and Bi2BaSr2Cu2O8, and to related superconducting compounds by melting gold or silver pads onto the samples before the final oxygen treatment. Scanning electron microscope studies show that both gold and silver do not diffuse far from the contact area. The surface contact resistivity of the best contacts made by the melting technique has an upper limit value in the 10−8 Ω cm2 range at 77 K. This contact resistivity shows no significant change in its value over a period of 17 months. Furthermore, an electron radiation dose of 5.7×1017 electron/cm2 only doubled the contact resistivity. This method of making low‐resistance contacts to high‐Tc materials can be integrated into the final oxygen treatment of many prospective superconducting elements or devices.

Kinetics and microscopic interaction parameters of Cr to Tm energy transfer in yttrium aluminum garnet crystals

Abstract We have studied the time evolution of the Cr emission in the double-doped samples of Cr, Tm: YAG and measured the spectral overlap between this emission and the Tm 3+ absorption in the 78–350 K temperature region. From these data we have extracted the relevant microscopic interaction parameters that give a measure of the Cr-Tm coupling. The results of our measurements and calculations indicate that, despite the confinement of the Cr emission to a region of the spectrum which is narrower than that in low-crystal-field garnets, the Cr 3+ ion in YAG can be used to sensitize efficiently the Tm 3+ ion.

Comparison of Spectroscopic Properties of Tm and Ho in YAG and YLF Crystals

We compare the cross-relaxation, energy transfer and loss processes in Tm and Ho doped YAG and YLF as function of temperature, Tm concentration and excitation power.

An alternative approach to charge transport in semiconducting electrodes

We analyze the excess‐carrier charge transport through the space‐charge region of a semiconducting electrode using a technique known as the flux method. In this approach reflection and transmission coefficients appropriate for a sheet of uniform semiconducting material are used to describe the transport properties of that material. We give a brief review of the flux method and show that the results obtained using this approach for a semiconductor electrode reduce in a limiting case to those previously found by Gartner if the depletion layer is treated as a perfectly transmitting medium in which scattering and recombination are ignored. Then, in the framework of the flux method we treat the depletion layer more realistically by explicitly taking into account scattering and recombination processes which occur in this region.

High Tc leads for remote sensing applications

Abstract Several NASA programmes designed to monitor the earths atmosphere from space utilize infrared detectors which operate at or below 4.2 K for optimum performance. At present, the detectors are maintained at cryogenic temperatures by a stored volume of liquid helium. These detectors must be electrically linked to amplification electronics and data storage instruments maintained at 80 K. The electrical connections over the temperature gradient account for ≈20% of the total heat load on the Dewar for some systems, accelerating the boil-off of liquid helium cryogen and reducing the operational lifetime of the space-borne instruments. The recent discovery of high temperature superconductors has provided an opportunity to develop electrically conductive, thermally insulating links to bridge this thermal gradient. This paper describes the modelling of the thermal transport properties of thick film, high T c electrical bridges across a 4.2–80 K temperature gradient and the impact of such devices on a space-borne remote sensing system.

Laser Damage Threshold Of Diamond Films

Calculations indicate that the thermal stress resistance for diamond is significantly higher than for other materials, suggesting that diamond films may inhibit damage to optical components in laser systems. To assess this possibility we have begun to study laser-induced damage in diamond films. We have measured laser damage thresholds of free-standing diamond film windows, diamond films deposited on silicon substrates, and bare silicon substrate. Polycrystalline diamond films were deposited using a dc plasma-enhanced chemical vapor deposition process. As expected, the free-standing diamond films showed a high laser damage threshold. Melting or dielectric breakdown induced by laser radiation may be the damage mechanism. The film/substrate combination had a damage threshold lower than the calculated value, which is attributed to film stress and conditions of film deposition.