In the framework of the thermal-spike model the present paper deals with the effect of the electronic stopping power (Se) in metals irradiated by swift heavy ions. Using the strength of the electron-phonon coupling g(z) with the number of valence electrons z as the unique free parameter, the increment of lattice temperature induced by swift-heavy-ion irradiation is calculated. Choosing z=2, the calculated threshold of defect creation by Se for Ti, Zr, Co and Fe is about 11, 27.5, 28 and 41 keV nm-1, in good agreement with experiment. Taking the same z value, the calculation shows that Al, Cu, Nb and Ag are Se insensitive. Moreover, in Fe, the differences in the damage created by U ions of different energies but exhibiting the same value of Se may be interpreted by a velocity effect. Using z=2, other calculations suggest that Be (Se>or=11 keV nm-1), Ga (Se>or=5 keV nm-1) and Ni (Se>or=49 keV nm-1) should be sensitive to Se but Mg should not. These examples put the stress on the effect of the physical parameters governing the electron-phonon coupling constant apart from z determination: the sound velocity linked to the Debye temperature and the lattice thermal conductivity. Furthermore, a simple criterion is proposed in order to predict the Se sensitivity of metals.

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The Se sensitivity of metals under swift-heavy-ion irradiation: a transient thermal process

Over the past 2 decades there has been tremendous advancements in the field of ultrafast carrier dynamics in semiconductors. The driving force behind this movement other than the basic fundamental interest is the direct application of semiconductor devices and the endless need for faster response and faster processing of information. To improve and develop microelectronics devices and address these needs, there must be a basic understanding of the various dynamical processes in the semiconductors which have to be studied in detail. Therefore, the excitation of semiconductors out of their equilibrium and the subsequent relaxation processes with various rates has become a key area of semiconductor research. With the development of lasers that can generate pulses as short as a few femtoseconds the excitation and subsequent probing of semiconductors on an ultrashort timescale have become routine. Processes such as carrier momentum randomization, carrier thermalization, and energy relaxation have been studied in detail using excite-and-probe novel techniques. This article reviews the status of ultrafast carrier and phonon dynamics in semiconductors. Experimental techniques such as excite-and-probe transmission, time-resolved up-conversion luminescence, and pump-probe Raman scattering along with some of the significant experimental findings from probing semiconductors are discussed. Finally, a selfconsistent theoretical model, which correlates the carrier and phonon dynamics in germanium on an ultrashort time scale, is described in detail.

Probing ultrafast carrier and phonon dynamics in semiconductors

We have studied the ultrafast optical response of native-oxide terminated Si(001) with pump-probe reflectivity using 800 nm, 28 fs pulses at an excitation density of $(5.5\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{18}{\mathrm{cm}}^{\ensuremath{-}3}.$ Time-dependent reflectivity changes comprise third-order-response coherent-transient variations arising from anisotropic state filling and linear-response variations arising from excited free carriers, state filling, and lattice heating. A time constant of $32\ifmmode\pm\else\textpm\fi{}5\mathrm{fs}$ associated with momentum relaxation is extracted from the coherent-transient variations. The state-filling and free-carrier responses are sensitive to carrier temperature, allowing an electron-phonon energy relaxation time of $260\ifmmode\pm\else\textpm\fi{}30\mathrm{fs}$ to be measured. The recovery of the reflectivity signal back towards its initial value is largely governed surface recombination: a surface recombination velocity of $(3\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}{10}^{4}\mathrm{cm}\mathrm{}{\mathrm{s}}^{\ensuremath{-}1}$ is deduced for native-oxide terminated Si(001).

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Femtosecond Pump-Probe Reflectivity Study of Silicon Carrier Dynamics

Ion beam induced surface and interface engineering

Ultrafast gain dynamics in GaAlAs diode amplifiers have been studied using 100 fs optical pulses. Pulse propagation through the amplifier resulted in temporal broadening and pulse shaping due to both gain saturation and material dispersion. Pump‐probe experiments indicate the presence of two processes contributing to the gain dynamics but give no evidence of spectral hole burning. A dynamic carrier heating model is presented to explain all of the observed gain nonlinearities, and the implications of our results on the dynamic response of laser diodes are discussed.

Subpicosecond gain dynamics in GaAlAs laser diodes

Femtosecond carrier dynamics in GaAs and Al0.3Ga0.7As are investigated using pump probe measurements of transient absorption saturation. Pulses of 35 fs duration are used both to excite carriers and to investigate their subsequent scattering out of their initial optically excited states. A two‐component ultrafast relaxation is observed. In GaAs the initial rapid relaxation occurs on a time scale of 10–35 fs. Measurements performed in Al0.3Ga0.7As indicate that this initial process slows significantly to 130–170 fs for increasing band gap.

Femtosecond carrier dynamics in GaAs

Femtosecond transient absorption saturation measurements are used to investigate the scattering of optically excited carriers in AlGaAs. With pulses as short as 35 fs at 1.98 eV, scattering times ranging from 13 to 330 fs are observed in samples of AlxGa1−xAs with x=0, 0.1, 0.2, 0.3, and 0.4. A dramatic decrease in the rate of carrier scattering out of the initial optically excited states is observed with increasing Al concentration.

W. Z. Lin

Papers

Femtosecond carrier dynamics in GaAs

Femtosecond dynamics of highly excited carriers in AlxGa1−xAs

Ultrafast Carrier Dynamics in GaAs and AlxGa1−x As