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  • 1990-1994  (4)
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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 1914-1922 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A theoretical model is presented describing harmonic heat flow in a two layer system heated by a modulated Gaussian laser beam. Amplitude and phase of the modulated temperature rise in the layers, as well as in the backing substrate and adjacent atmosphere, are calculated by solving the three-dimensional heat conduction equation with a source term including exponential absorption of the laser light in one or two layers. Heat conduction is assumed to be isotropic throughout the system, however, a thermal contact resistance between the two layers can be taken into account. Results are presented for single and double layer systems of gold and various dielectric thin film materials on glass substrates. Amplitude and phase of the harmonic temperature variation are calculated either as a function of position in the sample system or at the surface as a function of the laser beam modulation frequency. It is found that both amplitude and phase of the calculated temperature rise exhibit typical thin film features in their frequency dependence, however, the phase is more sensitive to thin film phenomena than the amplitude. The phase shows typical extrema in that frequency region, where the thermal diffusion length in the film is equal to the film thickness. Based on these findings, we demonstrate how these calculations can be utilized for the interpretation of thin film thermal parameter measurements. The influence of thermal wave interference is demonstrated, and it is discussed how the main thermal parameters like conductivity, effusivity, and thermal contact resistance of the thin film system can be extracted from measurements by a fit of theoretical curves to experimental data. Applying a simple one-dimensional thermal expansion model, surface displacements for thin film systems are calculated and the applicability of photothermal surface displacement for thin film conductivity measurements is discussed.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Thermal imaging of hidden electric current distributions with a resolution of several ten micrometers is demonstrated. It is shown that the thermoreflectance technique is capable of monitoring current-induced temperature variations on as well as beneath the surface of thin layered structures. A temperature pattern was generated by Joule heating using an ac current in a 2.5 μm thick structured gold film that was evaporated on a glass substrate and covered by a TiOx layer. The current density distribution in the gold film is revealed by the measured photothermal pattern, provided that both laser beam diameter and thermal diffusion length are smaller than the desired lateral resolution.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1432-0630
    Keywords: 68.60.Dv ; 79.20.Ds
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Single-shot ablation thresholds of nickel and gold films in the thickness range from 50 nm to 7 μm have been measured for 14 ns laser pulses at 248 nm, using photoacoustic shock wave detection in air. The metal films were deposited on fused silica substrates. The ablation threshold was found to increase linearly with film thickness up to the thermal diffusion length of the film. Beyond this point it remains independent of film thickness. The proportionality between threshold fluence and thickness allows the prediction of ablation thresholds of metal films from the knowledge of their optical properties, evaporation enthalpies and thermal diffusivities. Physically it proves that ablation is driven by the energy density determined by the thermal diffusion length. A simple thermodynamic model describes the data well. Thermal diffusivities, an essential input for this model, were measured using the technique of transient thermal gratings. In addition, the substrate dependence of the ablation threshold was investigated for 150 nm Ni films.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1432-0649
    Keywords: 79.20 D
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The kinetic energy of electrons emitted due to laser interaction with a graphite surface was studied with a time-of-flight spectrometer. In addition the yields of carbon atomic and molecular ions were measured as a function of laser pulse energy. Pulse energy thresholds for ion emission are observed to correlate with the observed maximum electron energies. Furthermore, the data suggest that ionic carbon clusters can be dissociated by energetic electrons or photons created in the plasma. We believe that initially photoemitted electrons are accelerated by inverse bremsstrahlung to the energies required for electron impact ionization and dissociation
    Type of Medium: Electronic Resource
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