We theoretically analyze a method for matching group velocities of fundamental and second harmonic femtosecond pulses during phase matched frequncy doubling by predispersing the fundamental pulse with a prism. The method permits improved conversion efficiency by allowing crystal lengths of several millimeters without sacrificing second harmonic pulse duration. Second harmonic pulse energy and duration are analyzed for beta-BaB(2)O(4), and limiting experimental factors are discussed. The results show that the method is most advantageous for incident pulses between 0.1- and 1.0-ps duration and microjoule and higher energies and that second harmonic pulse duration and conversion efficiency are not highly sensitive to optical misalignments of the order of 1 degrees .
The time-resolved, above-gap optical response of optically thick Si1−xGex alloys to carrier injection by a femtosecond pump pulse is measured across the entire compositional range (0≤x≤1) using a novel femtosecond ellipsometric technique which clearly distinguishes the real and imaginary parts of the time-varying dielectric function ε1(t)+iε2(t). The results are modeled microscopically in terms of the Drude contribution from a diffusing hot electron-hole plasma, augmented by transient-induced absorption from hot-carrier-induced band renormalization. Further corrections from thermal band-gap shrinkage, intervalley scattering, and transient interband absorption saturation are also discussed.
We measured the dielectric constants of strained In0.7Ga0.3AsyP1−y (y=0.2, 0.4, 0.8, 1.0) and lattice-matched 1.32 μm In1−xGaxAsyP1−y thin films grown on InP substrates by metalorganic chemical vapor deposition. Measurements were performed by phase-modulated spectroscopic ellipsometry in the range of 0.76–4.9 eV. Our data bridge the gap between literature data in the near-infrared region and those in the visible-ultraviolet region. The critical point energies of strained In0.7Ga0.3AsyP1−y were compared with unstrained counterparts and were found to be shifted in accordance with the theory, which predicts that the compositional shift is compensated. Thus, the critical point energies of strained In1−xGaxAsyP1−y thin films of arbitrary composition can be estimated accurately and, conversely, the composition of strained In1−xGaxAsyP1−y thin films can be estimated by measuring their critical point energies, as for unstrained materials.
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