  Nonlinear optical phenomena enable novel photonic and optoelectronic
applications. Especially metallic nanoparticles and thin films with nonlinear
optical properties offer the potential for micro-optical system integration.
For this purpose, new nonlinear materials need to be continuously identified,
investigated, and utilized for nonlinear optical applications. While noble
metal nanoparticles, nanostructures, and thin films of Ag and Au were widely
studied, iridium (Ir) nanoparticles and ultra-thin films have not been
investigated yet. Here, we present a combined theoretical and experimental
study on the linear and nonlinear optical properties of Ir nanoparticles
deposited by atomic layer deposition (ALD). Linear optical constants, i.e., the
effective refractive index n and extinction coefficient k, were evaluated at
different growth stages of nanoparticle formation. Both linear and nonlinear
optical properties of these Ir ALD coatings were calculated theoretically using
Bruggeman and Maxwell-Garnett theories. The third-order susceptibility of Ir
nanoparticle samples was experimentally investigated using the Z-scan
technique. Overall, our studies demonstrate the potential of ultrathin Ir NPs
as an alternative nonlinear optical material at an atomic scale.
