Ultrafast hot-carrier dynamics at chemically modified Ge interfaces probed by SHG

Time-resolved second-harmonic generation (SHG) was used to study the hot-carrier dynamics and nonlinear optical properties of S-terminated and Cl-terminated Ge(111) interfaces on the femtosecond time scale. The hot-carrier second-order nonlinear optical susceptibilities were determined to be 720 +/- 50 times greater than the valence-band second-order nonlinear optical susceptibilities for the Ge(111)-S system and 880 +/- 100 times greater in the Ge(111)-Cl system. Furthermore, the ground- and excited-state second-order nonlinear optical susceptibilities are suggested to be out of phase for Ge(111)-S and Ge(111)-Cl systems, leading to a pump-induced decrease in the SHG signal as opposed to the increase in the SHG signal observed in the Ge(111)-GeO2 system. Although the SHG response reaches a steady state in 415 +/- 90 fs in the Ge(111)-GeO2 system, a faster response is observed in the Ge(111)-S system, 220 +/- 85 fs, and in the Ge(111)-Cl system, 172 +/- 50 fs. This suggests significantly faster carrier cooling at the Ge(111)-Cl and Ge(111)-S interfaces, with significant implications for hot-carrier mediated device degradation, and migration to high-K dielectrics.     

Compensation of strong thermal lensing in high-optical-power cavities

In an experiment to simulate the conditions in high optical power advanced gravitational wave detectors, we show for the first time that the time evolution of strong thermal lenses follows the predicted infinite sum of exponentials (approximated by a double exponential), and that such lenses can be compensated using an intracavity compensation plate heated on its cylindrical surface. We show that high finesse approximately 1400 can be achieved in cavities with internal compensation plates, and that mode matching can be maintained. The experiment achieves a wave front distortion similar to that expected for the input test mass substrate in the Advanced Laser Interferometer Gravitational Wave Observatory, and shows that thermal compensation schemes are viable. It is also shown that the measurements allow a direct measurement of substrate optical absorption in the test mass and the compensation plate.     

Two-nucleon pion absorption in 3,4He

Results are presented for π⁺NN → Np yields in ^3,4He relative to πd → pp from (π, p) measurements at energies between 50 and 295 MeV and estimates are given for total cross sections. These yields are nearly proportional to the number of target NN pairs, which is not true for ⁶Li and heavier nuclei. This is believed to indicate that πNN → Np is sensitive to the NN short-range behavior which is deuteron-like for ^3,4He but not always so for NN pairs in other nuclei.     

The C(α, X)11C cross sections at 1.59 and 4.19 GeV

The absolute cross sections for the production of ¹¹C by 1.59 GeV and 4.19 GeV α-particles incident on natural carbon have been measured to be 46.4 ± 1.3 mb and 42.5 ± 1.1 mb respectively. These results, together with data reported at other energies, indicate that the C(α, X)¹¹C cross section becomes approximately constant at a value of about 43 mb for energies above 3 GeV (750 MeV/n). A similar energy dependence is exhibited by the C(p, X)¹¹C reaction whose cross section has been measured previously over an extensive energy range. The C(α, X)¹¹C cross sections are found to be in good agreement with predictions of a semi-empirical model developed to describe nuclear fragmentation.     

Linearised tuning of a cw spin-flip Raman laser using anti-reflection coated cavities

By reducing the electron concentration of the InSb cavity and anti-reflection coating its ends, the frequency tuning of a cw spin-flip Raman laser has been linearised to ?99% of the spontaneous tuning rate, and the magnitude of mode jumps decreased to ? 30 MHz (0.001 cm^-1).     

Analysis of collisional alignment and orientation studied by scattering of spin-polarized electrons from laser excited atoms

A general framework using density matrices is developed for the analysis of atomic excitation by spin-polarized electrons. This framework is applied to the specific case of the 3S _1/2→3P _3/2 transition in Na, as studied by the time-reversed, superelastic process. The scattering is characterized in terms of physical parameters describing the collisionally excitedp-state, i.e., its angular momentum (L _⊥), linear polarization (P _lin), and alignment angle (γ), with these parameters defined separately for singlet and triplet excitation. An expression for the scattering intensity is derived which is valid for arbitrary electron polarization and atomic state preparation. Specific examples are discussed with a view toward complete determination of the relevant scattering amplitudes and phases. Recent experimental results are reevaluated for comparison with theoretical calculations, and suggestions are made for future experiments.