Paramagnetic gold in a highly disordered Au-Ni-O alloy.

Magnetic materials are usually classified into a distinct category such as diamagnets, paramagnets or ferromagnets. The enormous progress in materials science allows one nowadays, however, to change the magnetic nature of an element in a material. Gold, in bulk form, is traditionally a diamagnet. But in a ferromagnetic environment, it can adopt an induced ferromagnetic moment. Moreover, the growth of gold under certain conditions may lead to a spontaneous ferromagnetic or paramagnetic response. Here, we report on paramagnetic gold in a highly disordered Au-Ni-O alloy and focus on the unusual magnetic response. Such materials are mainly considered for plasmonic applications. Thin films containing Au, Ni and NiO are fabricated by co-deposition of Ni and Au in a medium vacuum of 2 × 10-2 mbar. As a result, Au is in a fully disordered state forming in some cases isolated nanocrystallites of up to 4 nm in diameter as revealed by high resolution transmission electron microscopy. The disorder and the environment, which is rich in oxygen, lead to remarkable magnetic properties of Au: an induced ferromagnetic and a paramagnetic state. This can be proven by measuring the x-ray magnetic circular dichroism. Our experiments show a way to establish and monitor Au paramagnetism in alloys.

Waveguide saturable absorbers at 1.55 µm based on intraband transitions in GaN/AlN QDs.

We report on the design, fabrication and optical characterization of GaN/AlN quantum-dot-based waveguides for all-optical switching via intraband absorption saturation at 1.55 µm. The transmittance of the TM-polarized light increases with the incident optical power due to the saturation of the s-p(z) intraband absorption in the QDs. Single-mode waveguides with a ridge width of 2 µm and a length of 1.5 mm display 10 dB absorption saturation of the TM-polarized light for an input pulse energy of 8 pJ and 150 fs.