Studies of optical second-harmonic generation in single- and multilayer films based on aluminum nitride.

摘要

AlN is a second-order nonlinear optical material and it was synthesized in thin-film form by reactive direct-current magnetron sputtering in a mixture of Ar and N2 gases. The films exhibited optical second-harmonic (SH) generation and were studied by the Maker-hinge technique. Additionally, as measured by spectroscopic ellipsometry, they showed low optical absorption in the visible region of the spectrum. A film was used as a slab optical waveguide and with a tunable laser source, phasematched SH generation was observed in it for a set of discrete optical wavelengths. These wavelengths agreed with those predicted by a computational simulation. To study second-harmonic generation in finite periodic thin-film structures, a computational technique based on a transfer-matrix and Green function formalism was implemented. A nonlinear optical stack was designed to be phase-matched and doubly Bragg-resonant in the presence of dispersion in the refractive indices of the two materials used. The scaling of the phase-matched SH power was found to go asymptotically as the eighth power of the number of periods for the range 40 to 320 periods. An AlN/SiO2 periodic stack was fabricated and its linear and second-order nonlinear optical characteristics studied. The stack showed an optical stop-band with a reflectance maximum at 528 nm at normal incidence. The second-order nonlinear optical characteristics of the individual aluminum nitride layers were found to be similar to those of single AlN films on glass substrates. Additionally, the data support the hypothesis that aluminum nitride films as-deposited possess a preferred crystalline polarity.