Nonlinear optical materials (NLOMs) will be key components of optical technologies of the future, thanks to their capability of frequency conversion over an extended optical range, from ultraviolet to near-infrared wavelengths. Nowadays, standard nonlinear optical devices rely on birefringent single-crystal structures, which provide optimal conversion efficiency only through angle or temperature phase-matching, critically increasing the fragility and the cost of the device. The search for alternatives NLOMs, with relaxed phase-matching conditions, have attracted a large interest in the last years. From the nanoscale to the millimeter scale, plasmonic nanostructures, nonlinear photonic crystals and metamaterials have provided alternative mechanisms for nonlinear conversion. However, none of them is expected to satisfy market’s requirements of easy-fabrication, scalability and low-cost, determining a major obstacle to the large-scale application of nonlinear optics in everyday life.
Materials with a tremendous potential in terms of large-scale applicability are disordered NLOMs, thanks to the many advantages they could give for fabrication, scalability and cost. Very generally, they are an ensemble of optically nonlinear single-crystal domains, grains, with random positions, orientations, sizes and shapes. Disordered NLOMs have shown capabilities of broadband conversion with a large acceptance angle and without the need of phase-matching tuning. The nonlinear conversion in this structures relies on the so-called random quasi-phase-matching (RQPM), in which the frequency-converted waves generated by different grains interfere neither constructively nor destructively and the total intensity of the generated wave is the sum of the intensities arising from the single grains.
In this project, we investigated the physics of disordered NLOMs at the micro- and nanoscale, in the transition region where the size of the nonlinear grains gets comparable with or smaller than the wavelength. The toolbox in this research comprised metal-oxide nanoparticles (nano-oxides) and bottom-up assembly techniques, which have been employed to realize miniaturized systems with nano-structured nonlinear disorder and with a perfectly controlled geometry. In this conditions, optical resonances and light scattering play a significant role on the linear optical properties of the disordered structure, providing new degrees of freedom to optimize and control the random quasi-phase-matched nonlinear generation.
