The spectral linewidth and intensity of narrowband terahertz pulses generated by photomixing with beating of nonlinearly chirped laser pulse pairs are studied. The direct relationship between the dispersion of the pump pulse stretcher and the chirp of the terahertz pulse is shown. Simple analytical expressions for the linewidth and intensity of the terahertz spectrum are obtained and their validity is confirmed by numerical simulations. Generation of narrowband terahertz pulses with minimal chirp is demonstrated.

Defect reduction methods for III-nitride heteroepitaxial films grown along nonpolar and semipolar orientations P Vennéguès Defects in semipolar (11bar2bar2) ZnO grown on (112) LaAlO3/(La, Sr)(Al, Ta)O3 substrate by pulsed laser depositionWe have investigated the position dependence of crystalline quality and defect distribution in a semipolar ð2021Þ hydride vapor phase epitaxy (HVPE)-GaN film grown on a ð2243Þ patterned sapphire substrate (PSS). Position-dependent X-ray microdiffraction (XRMD) measurement clearly revealed the periodic fluctuation of the 20 21 lattice plane tilting in HVPE-GaN films. This correlated with the periodic distribution of (a + c)-type dislocations owing to the patterning pitch of the PSS as confirmed by transmission electron microscopy (TEM). In the three-dimensional reciprocal lattice space map, the diffuse streak exactly along the c-axis can be clearly detected, indicating the presence of basal plane stacking faults in HVPE-GaN films. Furthermore, we have quantitatively estimated the defect densities from the results of XRMD and TEM measurements. From the obtained results of XRMD and TEM measurements, the fluctuation of the lattice plane tilting and the defect distribution in ð2021Þ HVPE-GaN films grown on two types of metalorganic vapor phase epitaxy-GaN templates will be discussed in detail.

We demonstrate a method for controlling the chirp of the frequency-tunable narrowband terahertz pulses that are generated by photomixing with nonlinearly chirped laser pulse pairs. We find that in a grating-based laser-pulse stretcher, the frequency sweep rates of the generated terahertz pulses can be controlled by simply changing the incident angle. This method is also applicable to other mechanisms of terahertz pulse generation.

Three-dimensional (3D) lattice plane microstructures were investigated at local regions in an epitaxial AlN thick film grown on a trench-patterned AlN/sapphire template. A 3D reciprocal lattice space mapping technique combined with cross-sectional X-ray microdiffraction using an appropriate Bragg reflection quantitatively revealed the inhomogeneity of the lattice structures in the AlN film without loss of spatial resolution. The results showed a strong correlation of the lattice plane tilt/twist and variations with respect to the void configuration, the patterning structure of the template, and the dislocation morphologies confirmed by transmission electron microscopy.

We theoretically show that it is possible to generate chirp-free terahertz (THz) pulses with a chirped-pulse beating method by using an optical fiber as a pulse stretcher. Proper choices of the core radius and the dopant fraction of the core material of a step-index single-mode optical fiber eliminate the third-order spectral phase of the fiber, thus giving the pump laser pulse a purely linear chirp. We also show that even a standard commercial single-mode optical fiber can give THz pulses of lower chirp than the lower limit for a grating pair. We perform experiments to verify our theory.