Measurement of the thermal boundary conductance (TBC) by use of a nondestructive optical technique, transient thermoreflectance (TTR), is presented. A simple thermal model for the TTR is presented with a discussion of its applicability and sensitivity. A specially prepared sample series of Cr, Al, Au, and Pt on four different substrates (Si, sapphire, GaN, and AlN) were tested at room temperature and the TTR signal fitted to the thermal model. The resulting TBC values vary by more than a factor of 3 0.71×108-2.3×108 W/m2 K. It is shown that the diffuse mismatch model (DMM) tended to overpredict the TBC of interfaces with materials having similar phonon spectra, while underpredicting the TBC for interfaces with dissimilar phonon spectra. The DMM only accounts for diffuse elastic scattering. Other scattering mechanisms are discussed which may explain the failure of the DMM at room temperature.

The p42 and p44 mitogen-activated protein kinases (MAPKs), also called Erk2 and Erk1, respectively, have been implicated in proliferation as well as in differentiation programs. The specific role of the p44 MAPK isoform in the whole animal was evaluated by generation of p44 MAPK-deficient mice by homologous recombination in embryonic stem cells. The p44 MAPK-/- mice were viable, fertile, and of normal size. Thus, p44 MAPK is apparently dispensable and p42 MAPK (Erk2) may compensate for its loss. However, in p44 MAPK-/- mice, thymocyte maturation beyond the CD4+CD8+ stage was reduced by half, with a similar diminution in the thymocyte subpopulation expressing high levels of T cell receptor (CD3high). In p44 MAPK-/- thymocytes, proliferation in response to activation with a monoclonal antibody to the T cell receptor in the presence of phorbol myristate acetate was severely reduced even though activation of p42 MAPK was more sustained in these cells. The p44 MAPK apparently has a specific role in thymocyte development.

Femtosecond thermoreflectance data for thin films and bulk quantities of Au, Cr, and Al are compared with the parabolic two-step thermal diffusion model for the purpose of determining the electron-phonon coupling factor. The thin films were evaporated and sputtered onto different substrates to produce films that vary structurally. The measurement of the electron-phonon coupling factor is shown to be sensitive to grain size and film thickness. The thin-film thermoreflectance data are compared with that of the corresponding bulk material and to a theoretical model relating the coupling rate to the grain-boundary scattering and size effects on the mean free path of the relevant energy carrier.

Ultrashort-pulsed lasers have been demonstrated as effective tools for the nondestructive examination ͑NDE͒ of energy transport properties in thin films. After the instantaneous heating of the surface of a 100 nm metal film, it will take ϳ100 ps for the influence of the substrate to affect the surface temperature profile. Therefore, direct measurement of energy transport in a thin film sample requires a technique with picosecond temporal resolution. The pump-probe experimental technique is able to monitor the change in reflectance or transmittance of the sample surface as a function of time on a subpicosecond time scale. Changes in reflectance and transmittance can then be used to determine properties of the film. In the case of metals, the change in reflectance is related to changes in temperature and strain. The transient temperature profile at the surface is then used to determine the rate of coupling between the electron and phonon systems as well as the thermal conductivity of the material. In the case of semiconductors, the change in reflectance and transmittance is related to changes in the local electronic states and temperature. Transient thermotransmission experiments have been used extensively to observe electron-hole recombination phenomena and thermalization of hot electrons. Application of the transient thermoreflectance ͑TTR͒ and transient thermotransmittance ͑TTT͒ technique to the study of picosecond phenomena in metals and semiconductors will be discussed. The pump-probe experimental setup will be described, along with the details of the experimental apparatus in use at the University of Virginia. The thermal model applicable to ultrashort-pulsed laser heating of metals will be presented along with a discussion of the limitations of this model. Details of the data acquisition and interpretation of the experimental results will be given, including a discussion of the reflectance models used to relate the measured changes in reflectance to calculated changes in temperature. Finally, experimental results will be presented that demonstrate the use of the TTR technique for measuring the electron-phonon coupling factor and the thermal conductivity of thin metallic films. The use of the TTT technique to distinguish between different levels of doping and alloying in thin film samples of hydrogenated amorphous silicon will also be discussed briefly.

Ultrashort pulsed lasers have repeatedly been demonstrated as effective tools for the observation of transport properties on atomistic time and length scales. Transient thermoreflectance scans of a Au thin film taken using a 200 fs pulsed laser are compared to the parabolic two step model using both the assumption of a linear relationship between reflectance and temperature and using an intraband reflectance model. In this study, the incident photon energy was less than the interband transition energy, therefore the thermoreflectance response is primarily due to intraband transitions. The intraband transitions are influenced by the electron and lattice temperatures through the electron collisional frequency.During the ultrashort pulsed laser heating of metals the radiant energy is initially absorbed by the electrons. 1 The subsequent rate of energy exchange between the electrons and lattice can be related to an electron-phonon coupling factor and the temperature difference between the two systems. 2 The electron-phonon coupling factor is typically measured using a transient thermoreflectance technique with thin reflective films; however, a relationship between the change in reflectance of the sample surface and the electron and lattice temperatures is required. 3-5 A number of studies have also been performed on metallic nanoparticles 6,7 observing the transient changes in the absorptance. Whether probing thin films or nanoparticles, the importance of intraband transitions must be considered.In this investigation we observe the thermoreflectance response of Au thin films at probe photon energies far below the interband transition energy. The Drude model is used to estimate the complex dielectric function; 8 therefore the thermal dependence of the dielectric function arises due to thermal effects on electron collisions. Typically, the electron collisional frequency is dominated by electron-phonon collisions at room temperature. 9 However, in the case of ultrashort pulsed laser heating, the electron system can easily experience changes in temperature one to two orders of magnitude greater than the lattice. Electron-electron collisions can then have a significant effect on the reflectance response. In the current investigation, a Au film is heated and probed with a 200 fs pulsed laser at a probe photon energy of 1.55 eV, which is significantly less than the first interband transition energy of 2.45 eV. 10 When the probe energy is near an interband transition, the generally accepted model for calculating the thermomodulation is given by Rosie and Lynch. 11 The experimental setup for this investigation was a classic pump-probe technique utilizing a 200 fs Ti:sapphire Coherent MIRA. The details of the experimental setup are described by Hostetler et al. 5 Figure 1 shows the experimental transient thermoreflectance response of a 23 nm Au film excited with different pump fluences while the probe fluence was held constant. The Au samples were deposited on glass substrates at a pressure of 7ϫ10 Ϫ7 Torr using a Temescal B...