J. C. Kieffer

Band-Selective Measurements of Electron Dynamics in VO2 Using Femtosecond Near-Edge X-Ray Absorption

We report on the first demonstration of femtosecond x-ray absorption spectroscopy, made uniquely possible by the use of broadly tunable bending-magnet radiation from laser-sliced electron bunches within a synchrotron storage ring. We measure the femtosecond electronic rearrangements that occur during the photoinduced insulator-metal phase transition in VO2. Symmetry- and element-specific x-ray absorption from V2p and O1s core levels (near 500 eV) separately measures the filling dynamics of differently hybridized V3d-O2p electronic bands near the Fermi level.

Hardness and Young's modulus of amorphous a -SiC thin films determined by nanoindentation and bulge tests

Due to its interesting mechanical properties, silicon carbide is an excellent material for many applications. In this paper, we report on the mechanical properties of amorphous hydrogenated or hydrogen-free silicon carbide thin films deposited by using different deposition techniques, namely plasma enhanced chemical vapor deposition (PECVD), laser ablation deposition (LAD), and triode sputtering deposition (TSD). a -Si x C 1− x : H PECVD, a -SiC LAD, and a -SiC TSD thin films and corresponding free-standing membranes were mechanically investigated by using nanoindentation and bulge techniques, respectively. Hardness ( H ), Youngs modulus ( E ), and Poissons ratio ( v ) of the studied silicon carbide thin films were determined. It is shown that for hydrogenated a -Si x C 1− x : H PECVD films, both hardness and Youngs modulus are dependent on the film composition. The nearly stoichiometric a -SiC: H films present higher H and E values than the Si-rich a -Si x C 1−x : H films. For hydrogen-free a -SiC films, the hardness and Youngs modulus were as high as about 30 GPa and 240 GPa, respectively. Hydrogen-free a -SiC films present both hardness and Youngs modulus values higher by about 50% than those of hydrogenated a -SiC: H PECVD films. By using the FTIR absorption spectroscopy, we estimated the Si-C bond densities ( N SiC ) from the Si-C stretching absorption band (centered around 780 cm −1 ), and were thus able to correlate the observed mechanical behavior of a -SiC films to their microstructure. We indeed point out a constant-plus-linear variation of the hardness and Youngs modulus upon the Si-C bond density, over the N SiC investigated range [(4–18) × 10 22 bond · cm −3 ], regardless of the film composition or the deposition technique.

Effect of rapid thermal annealing on both the stress and the bonding states of a-SiC:H films

The stress evolution of plasma enhanced chemical vapor deposition a‐SiC:H films was studied by increasing the annealing temperature from 300 to 850u2009°C. A large stress range from −1 GPa compressive to 1 GPa tensile was investigated. Infrared absorption, x‐ray photoelectron spectroscopy, and elastic recoil detection analysis techniques were used to follow the Si‐C, Si‐H, and C‐H absorption band evolutions, the Si2p and C1s chemical bondings, and the a‐SiC:H film hydrogen content variations with the annealing temperatures, respectively. It is pointed out that the compressive stress relaxation is due to the hydrogenated bond (Si—H and C—H) dissociation, whereas the tensile stress is caused by additional Si—C bond formation. At high annealing temperatures, a total hydrogen content decrease is clearly observed. This total hydrogen loss is interpreted in terms of hydrogen molecule formation and outerdiffusion. The results are discussed and a quantitative model correlating the intrinsic stress variation to the Si...

Nonlinear ultrafast modulation of the optical absorption of intense few-cycle terahertz pulses in n-doped semiconductors

We use an open-aperture

Terahertz pulse induced intervalley scattering in photoexcited GaAs

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Laser beam wavefront correction for ultra high intensities with the 200 TW laser system at the Advanced Laser Light Source

scan technique to show how intense few-cycle terahertz pulses can experience a nonlinear bleaching of absorption in an

Pedestal cleaning for high laser pulse contrast ratio with a 100 TW class laser system.

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X-ray mask development based on SiC membrane and W absorber

-doped semiconductor due to terahertz-electric-field-driven intervalley scattering of electrons in the conduction band. Coherent detection of the transmitted terahertz pulse wave form also allows the nonlinear conductivity dynamics to be followed with subpicosecond time resolution. Both the

Time-Resolved Terahertz Spectroscopy of Free Carrier Nonlinear Dynamics in Semiconductors

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Investigation of the thermally induced laser beam distortion associated with vacuum compressor gratings in high energy and high average power femtosecond laser systems

scan and time-domain results are found to be in agreement with our theoretical analysis.