Optical second harmonic generation in radio-frequency sputter-deposited AlN thin films has been studied for harmonic wavelengths from ultraviolet to near infrared. The effective second-order nonlinearity χ(2)(ω) was determined to have a nonresonant background value of ∼5×10−9 esu for second harmonic wavelengths longer than 400 nm, and it increases dramatically as the second harmonic frequency approaches the bandgap of 6.2 eV. This is likely due to resonance of the second harmonic frequency with the critical point transition associated with the direct bandgap of AlN.

Polarized second harmonic generation measurements were performed on AlN films deposited on (100) sapphire substrates by the reactive rf sputtering technique. The bulk effective second order nonlinearity observed in these films is typically about 6×10−9 esu at 1.06 μm, several times larger than that of quartz or KTP. The tensorial properties of the nonlinearity are consistent with the crystal symmetry of AlN and the microcrystallinity of these films.

We present a new type of Josephson transmission line (JTL) in which discrete multilayer Josephson junctions are employed instead of the standard single tri-layer junction technology. It is shown via numerical simulation that such a structure supports the propagation of multiple flux quantum solitons, which are represented in this system by a 2πn kink in the phase difference across a multilayer junction of n superconductor-insulator-superconductor layers. As a practical aspect, parallel and series connected multilayer junctions in a JTL can significantly reduce the need for microstrip line inductance, thereby providing the optimal conditions for large scale integration of JTL-based logic and memory circuitry. In this article we investigate in detail through numerical simulation the properties of JTL designs with multilayer junctions, incorporating expected variations in junction characteristics. A brief description of the current fabrication of multilayer junctions for flux quantum circuitry will be provided.

A high resolution transmission electron microscopy study of multilayer films prepared by magnetron sputtering shows that the morphology of the growing interface in a-Si/Nb and a-SiN/NbN multilayers is remarkably uniform and smooth; this is in contrast to the polycrystalline AlN/NbN multilayers grown under similar conditions, which exhibit columnar grain structure with rough interfaces. For proper sputtering parameters, the amorphous layers seem to periodically restore a relatively smooth initial interface condition for the successive Nb (or NbN) layer growth, consequently interrupting the tendency toward increased roughness due to mechanisms such as columnar growth. Artificial multilayers having very flat interfaces could stimulate applications based on multilayer Josephson junctions.

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