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  • 1
    Electronic Resource
    Electronic Resource
    Real time optical diagnostics of the plume dynamics during laser ablation of germanium in an oxygen environment (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 2472-2476 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The dynamics of the gas phase induced by excimer laser ablation of Ge is investigated by analyzing the light emitted by the plume. Space and time-resolved optical spectroscopy measurements indicate the presence of both, neutral (Gei) and ionized (Geii) species. Two populations of neutrals with different velocities, which are related to the neutral atoms directly ejected from the target and those which are produced by recombination of ions, are observed. The velocities of the species remain unchanged for oxygen pressures up to 1 mbar, which suggest that the expansion of the plume occurs without further collisions with the foreign gas in this pressure range. The spectral emission characteristics are consistent with plume dynamics initiated by charged species.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.353106
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  • 2
    Electronic Resource
    Electronic Resource
    Excimer laser melting of GaAs: Real-time optical study (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 71 (1992), S. 1032-1034 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In situ real-time reflectivity measurements have been made in GaAs under ArF excimer laser (λ=193 nm) irradiation. The results obtained provide a reflectivity value for the solid material at the melting temperature of 0.44 and for the liquid of 0.63, both at 633 nm. The reflectivity values obtained for fluences just above the melting threshold (E=225 mJ/cm2) show that melting proceeds inhomogeneously, the near-surface region being formed by a mixture of solid and liquid phases. The comparison of these results to those obtained previously for irradiation of single-crystalline Si and Ge shows that inhomogeneous melting can be a general phenomenon at least in semiconductors. Higher fluences (E(approximately-greater-than)300 mJ/cm2) are necessary to induce an homogeneous melted layer on the surface of the irradiated material.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.350391
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  • 3
    Electronic Resource
    Electronic Resource
    Early stages of melting in Si under nanosecond laser pulse irradiation: A time-resolved study (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 2105-2111 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Time-resolved reflectivity (TRR) measurements are performed in crystalline Si under UV and visible wavelength irradiation. The former are carried out with ArF excimer laser pulses whereas the latter are performed in micron-sized areas irradiated with Ar+ laser pulses by means of a novel experimental setup. It is the first time that TRR measurements in the nanosecond regime are performed in micron-sized irradiated areas although they are very suitable to characterize processes in phase change optical storage and microelectronics applications. The energy density melting thresholds at both Ar+ and ArF laser wavelengths are determined. The reflectivity values obtained for pulse fluences just above the melting threshold show that melting proceeds inhomogeneously being the near-surface region formed by a mixture of solid and liquid phases without a well-defined interface. The comparison of the results obtained with uv and visible irradiation indicates that inhomogeneous melting is a general phenomenon which does not depend on the irradiation wavelength. It is present in the early stages of the melting process and its origin is related to the phase nucleation process itself. As the laser fluence is increased, the evolution of the melt duration exhibits a "different'' behavior which is related to the formation of a homogeneous molten layer on top of the surface.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.348968
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  • 4
    Electronic Resource
    Electronic Resource
    Evidence of a nonthermal mechanism for ejection of ions and neutrals during excimer laser ablation of Ge (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 74 (1993), S. 4271-4273 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Resonance ionization spectroscopy combined with time of flight mass spectrometry is used to analyze the ejection of ions and neutrals from the surface of a Ge target irradiated with ultraviolet XeCl excimer laser pulses. The ejection of ions and neutrals has been observed to occur at temperatures well below the melting point of the target and, therefore, through a nonthermal mechanism that may be dominant for fluences below the threshold for visible plasma formation. Within this regime, the velocities of the ejected neutrals in the ground state are in the order of 3×104 cm/s.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.354389
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  • 5
    Electronic Resource
    Electronic Resource
    Melting kinetics of Sb under nanosecond UV laser irradiation (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 3099-3101 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Real time reflectivity measurements at the HeNe laser wavelength (λ=632.8 nm) have been performed in crystalline Sb under pulsed UV laser irradiation (λ=193 nm). For energy densities above 100 mJ cm−2, a transient low-reflectivity phase is observed, which is related to surface melting. The change of reflectivity of the solid material upon heating is small, and a maximum variation of −0.02 relative to the room-temperature value (RSb,rt) is determined for the solid material at the melting point. The value of the reflectivity of the liquid material is determined to be 0.94RSb,rt(632.8 nm)=0.67, which is lower than that of the solid and it is associated with a decrease of the absorption coefficient upon phase change. The analysis of reflectivity transients at energy densities just above the melting threshold suggests that melting nucleation proceeds inhomogeneously.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.352996
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  • 6
    Electronic Resource
    Electronic Resource
    Reflectivity and transmission of amorphous thin films at the melting temperature (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 72 (1992), S. 2125-2130 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A method to evaluate the evolution of the optical properties (reflectivity and transmission) of thin solid films as a function of temperature has been developed. A pulsed Ar+ laser focused on the sample surface to a 1/e-beam radius of 4 μm is used as a local heat source while the optical properties of the irradiated surface are monitored in real time by means of another laser focused to 1.7 μm. The use of laser pulses combined with real time optical measurements provides a suitable means to determine accurately the optical properties of thin-film materials for temperatures up to the melting point. In addition, the irradiation pulse can be sufficiently shortened to avoid crystallization prior to melting in the case of amorphous films and therefore the optical properties of amorphous films up to the melting point can be also determined. The analysis of the optical transients with a simple thermal model allows either the determination of the thermal conductivity of a transparent substrate if the film melting temperature is known or the film melting temperature if the thermal properties of the substrate are well characterized. The results obtained in Ge and Si amorphous thin films prove the accuracy of the method and the melting temperature of amorphous Ge is determined to be 987 K.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.351600
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  • 7
    Electronic Resource
    Electronic Resource
    Evidence for surface initiated solidification in Ge films upon picosecond laser pulse irradiation (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 3642-3649 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Amorphous Ge films on Si films have been melted by single 30 ps laser pulses at λ=583 nm and a solidification process has been followed by means of real time reflectivity (RTR) measurements with nanosecond time resolution. Evidence is provided for the occurrence of surface initiated solidification for films with thicknesses in the 80–130 nm range. This process occurs at high fluences following complete melting of the film and of a thin layer of the Si substrate which undergoes mixing with the liquid Ge. The release of the solidification enthalpy of the latter layer together with its lower solidification temperature favored by constitutional undercooling are proposed as the origin of the initial gradient inversion required for such a process. This scenario leads also to the formation of a secondary solidification front counterpropagating towards the film surface, as suggested by optical simulations of the experimental RTR transients. A transition from a surface initiated solidification process towards a bulk solidification process is shown to occur when increasing the film thickness beyond 130 nm. The results further show that, besides the film thickness, the pulse duration has a major influence on the type of solidification process induced. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1347958
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  • 8
    Electronic Resource
    Electronic Resource
    Reflectivity of crystalline Ge and Si at the melting temperature measured in real time with subnanosecond temporal resolution (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 3763-3767 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Real time reflectivity measurements with subnanosecond time resolution have been used to determine the reflectivity at the melting temperature RS(Tm) of single crystalline Ge and Si at 514.5 nm. Due to the excellent time resolution and sensitivity achieved in a single exposure experiment, the reflectivity of the solid just before melting could be measured. Values of RS(Tm)=0.470±0.006 and RS(Tm)=0.440±0.008 for c-Ge and c-Si have, respectively, been determined. These values, together with those determined by heating in vacuum in the range 300–800 K, are compared to those reported earlier in the literature and the differences are discussed. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1350413
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  • 9
    Electronic Resource
    Electronic Resource
    Predictions on runaway current and energy during disruptions in tokamak plasmas (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 3369-3377 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A simple model for a tokamak disruption, taking into account the replacement of the plasma current by the runaway current, is used to evaluate the generation and energy of the runaway population during the current quench phase of a fast disruptive event. The potential efficiency of the ripple resonance and the magnetic fluctuations for runaway current mitigation during plasma disruptions, as well as their dependence on the runaway generation mechanism, are discussed. Predictions are made for the Joint European Torus (JET) [Nucl. Fusion 25, 1011 (1985)] and the projected International Thermonuclear Experimental Reactor (ITER) [ITER EDA Agreement and Protocol 2, International Atomic Energy Agency, Vienna, 1994]. It is shown that the ripple resonance leads to a reduction in the runaway beam energy if the runaway production is dominated by the Dreicer generation process; however, the effect will be negligible if the secondary generation mechanism is included. The effect of anomalous radial runaway losses induced by enhanced magnetic fluctuations is stronger. Large enough levels of magnetic fluctuations, leading to runaway electron loss rates in excess of 103 s−1, can efficiently limit the number and energy of the runaway electrons. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874201
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  • 10
    Electronic Resource
    Electronic Resource
    Effect of magnetic and electrostatic fluctuations on the runaway electron dynamics in tokamak plasmas (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 3925-3933 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A test particle description of the runaway dynamics [J.R. Martín-Solís et al., Phys. Plasmas 5, 2370 (1998)] is extended to investigate the behavior of runaway electrons in the presence of fluctuations of electric and magnetic fields. The interaction with the fluctuations is accounted for via a friction force with an effective "collision" frequency determined by the fluctuation induced radial diffusion coefficient. It is shown that both the runaway generation process and the maximum runaway energy can be noticeably affected by magnetic fluctuations. The test particle model is then used to discuss a proposed runaway control scheme via induced magnetic turbulence, with particular emphasis to situations like major disruptions, where a large number of runaway electrons and high runaway energies are expected. It is found that the efficiency of such scheme can in some cases be jeopardized by drift orbit effects as well as by the coexistence of stochastic magnetic regions with good magnetic surfaces. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873656
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