Periodic Reporting for period 1 - SECOONDO (Second Order nano-Oxide Nonlinear Disordered phOtonics)
Berichtszeitraum: 2018-04-01 bis 2020-03-31
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.
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.
The project was developed along three parallel tracks: the material fabrication, the optical characterization and the modelling. We employed barium titanate and litium niobate nanoparticles, which show a second-order nonlinearity thanks to their non-centrosymmetric crystal lattice. They were assembled into larger structures by “emulsion templated assembly” and “evaporation assembly”, obtaining disordered NLOMs with a controllable level of complexity in which nano-oxides are the nonlinear grains. We performed experiments to characterize the linear optical properties of the disordered structures, focusing on their multiple light scattering and optically resonant properties. We entered the nonlinear regime by illuminating the structures with a femtosecond pulsed lasers and investigated their second harmonic generation (SHG), in particular the conversion of (invisible) near-infrared light into (visible) blue light. We developed theoretical and numerical models to account for the presence of geometric optical resonances as well as for a linear birefringence in the quadratic and disordered nonlinearity.
Most significant achievements have been: 1) the realization of disordered micro-assemblies (0.5-50 microns) of oxide nanoparticles having a perfect spherical shape (micro-spheres) and with very different linear scattering properties, from the transparent (barium titanate) to the strongly scattering (litium niobate) regime; 2) the unambiguous observation of RQPM in the strongly scattering regime, obtained with precise control on the number of interacting nanoparticles; 3) the first observation of a Mie-resonant modulation of the SHG in the RQPM regime and the development of a random walk model for its description; 4) the development of a numerical model to simulate RQPM in disordered materials with optical birefringence by explicitly considering the phase propagation and the interference SHG components;
It was also possible to carry on a collaboration on the optical characterization of solution processed barium titanate woodpile photonic structures and to conclude a side project on the statistical properties of bacteria trajectories in complex environment.
Results have been presented at 7 international conferences and 3 international workshops, as well as popularized through 1 non-technical article. They have been continuously highlighted on social media. So far, there have been 2 publications in international peer-reviewed journal and 1 paper is under review. Other 2 manuscripts are under preparations. All publications have or will have open access. This project has funded the organization of the first international workshop on “Complex Materials for Nonlinear Photonics”, a multidisciplinary event gathering international scientists from the fields of disordered and complex photonics, nonlinear optics and material science. The workshop hosted 26 international speakers, 10 posters presenters and more than 50 participants, representing the generation of a scientific community interested in the topics of this project.
Most significant achievements have been: 1) the realization of disordered micro-assemblies (0.5-50 microns) of oxide nanoparticles having a perfect spherical shape (micro-spheres) and with very different linear scattering properties, from the transparent (barium titanate) to the strongly scattering (litium niobate) regime; 2) the unambiguous observation of RQPM in the strongly scattering regime, obtained with precise control on the number of interacting nanoparticles; 3) the first observation of a Mie-resonant modulation of the SHG in the RQPM regime and the development of a random walk model for its description; 4) the development of a numerical model to simulate RQPM in disordered materials with optical birefringence by explicitly considering the phase propagation and the interference SHG components;
It was also possible to carry on a collaboration on the optical characterization of solution processed barium titanate woodpile photonic structures and to conclude a side project on the statistical properties of bacteria trajectories in complex environment.
Results have been presented at 7 international conferences and 3 international workshops, as well as popularized through 1 non-technical article. They have been continuously highlighted on social media. So far, there have been 2 publications in international peer-reviewed journal and 1 paper is under review. Other 2 manuscripts are under preparations. All publications have or will have open access. This project has funded the organization of the first international workshop on “Complex Materials for Nonlinear Photonics”, a multidisciplinary event gathering international scientists from the fields of disordered and complex photonics, nonlinear optics and material science. The workshop hosted 26 international speakers, 10 posters presenters and more than 50 participants, representing the generation of a scientific community interested in the topics of this project.
The project resulted in a significant advancement of the fundamental knowledge on the second-order nonlinear effects in disordered photonic materials. A relevant conclusion is that RQPM can be efficiently implemented in nano-structured quadratic disorder and that it is extremely robust to the multiple scattering of the pump. We discovered an unexplored phase-matching regime, dubbed Mie-driven RQPM, which relies on the coupling of the RQPM with the Mie resonances of the entire disordered structure. This mechanism provides an optimization tool for the frequency conversion that exploits the geometry of the structure and does not rely on the crystal orientation and quality. This was obtained on structures of several micrometers, on a length scale much larger than what was previously achieved. We concluded that barium titanate and litium niobate are promising materials for optical frequency conversion in the RQPM regime, by showing that their birefringence increases the efficiency of the process. These results pave the way for the development of nano-oxides-based nonlinear photonic devices, with a tunable broadband operation, a wide acceptance angle and a cost-effective production, which are expected to be attractive for large-scale technology applications as in up-conversion screens.
Finally, this project gave me the invaluable opportunity to transit from being “a researcher” to being “an independent researcher”, who proposes ideas, takes risks and manage projects on his own. The wealth of experience gathered in the last two years will impact my future as a scientist and drive my choices as a person.
Finally, this project gave me the invaluable opportunity to transit from being “a researcher” to being “an independent researcher”, who proposes ideas, takes risks and manage projects on his own. The wealth of experience gathered in the last two years will impact my future as a scientist and drive my choices as a person.