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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - Chi2-Nano-Oxides (Second-Order Nano-Oxides for Enhanced Nonlinear Photonics)

Teaser

Summary

Nonlinear optics started one year after the invention of laser in 1961 with a first experiment of light conversion from one color into another, a phenomenon called second-harmonic generation (SHG). Many applications ranging from microsurgery light sources or boosting signals in telecommunication fibers have emerged in the last 50 years using nonlinearities of bulk materials. However, applying nonlinear effects at the nanoscale generates very tiny signal and finding materials with high enough nonlinearities is still an open challenge to avoid using high power and large interaction length within the material.
Here, I propose to overcome low nonlinear signal by using multiple Mie scattering resonances in a broad light window spanning from near ultraviolet to the near infrared. I am focusing on materials that were rarely used at the nanoscale, namely quadratic (Chi2) nano-oxides as barium titanate and lithium niobate.
Many studies focus on third order nonlinearities, since every material exhibit a Ï‡(3) tensor like semiconductors as silicon nanowires. Nevertheless, the current limitations of semiconductors (small bandgap thus limited transparency range), or metals (low efficiencies for surface SHG, losses) lead me to focus on non centrosymmetric nano-oxides for their large transparency range, reasonably high second-order tensor coefficients, refractive index above 2, and colloidal suspension for easy fabrication process.
I will demonstrate that those quadratic materials can be used in a solution process combined with imprint lithography to overcome the complicated fabrication of ultrafast nonlinear photonic crystal cavities. And finally, I will utilize single high aspect ratio nanowaveguides as cavity to enhance wave mixing effects and manage their dispersion for developing on chip optical parametric oscillator and spectrometer.
The impact of these quadratic materials is interdisciplinary since it involves material sciences, photonics and nanotechnology. Applications in integrated optics and information technology can be expected. They may range from modulators for telecommunication or spectrometer for sensing applications, as well as compact nonlinear light sources for quantum computers.

The key idea is to demonstrate original strategies to enhance the optical nonlinearities of Chi2 nano-oxides, with the material itself and without involving any hybrid structure such as metals by addressing the following questions:
(1) How is it possible to enhance SHG signal within a single Chi2 nanoparticle?
(2) How to fabricate nonlinear photonic crystal cavity with Chi2 nano-oxides that are very difficult to etch?
(3) How to realize (even commercialize) compact devices with Chi2 nanomaterials?

Work performed

During this period of the project, we have made progress in every work packages (WP) that are related to the three questions mentioned above.
(1) How is it possible to enhance SHG signal within a single Chi2 nanoparticle? (WP1)
We demonstrate multiple Mie resonances in different Chi2 materials, namely: lithium niobate nanocubes, III-V nanostructures and assembly of barium titanate with gold nanoparticles. For each type of samples, we mastered the fabrication and the full linear and nonlinear optical characterization of the system. Besides the material knowledge, it shows that we can control the position and the optical properties at the nanoscale in order to obtain strong Mie resonances that can be further exploited for nonlinear optics.
(2) How to fabricate nonlinear photonic crystal cavity with Chi2 nano-oxides that are very difficult to etch? (WP2)
Up to now, we elaborate a viable protocol for the dispersion of the nanoparticles that are not standard as compared to previous work. Moreover, we designed and fabricated by nanoimprint lithography 2D and 3D photonic crystal structures. We also performed the linear and nonlinear optical characterization and the first results are soon to be sent for review.
(3) How to realize (even commercialize) compact devices with Chi2 nanomaterials? (WP3)

After the training to master the nanotechnology process flow of typical material, the PhD student looked for the fabrication parameters fitting our material, lithium niobate. We managed to obtain a very low footprint modulator with high speed. Moreover, in collaboration with two other PhD students in the group, an integrated spectrometer with broadband wavelength range and no moving part was demonstrated in the near infrared range.

Final results

In the respective WPs, we reached very original results that are summarized below, together with possible perspectives:
In WP1, the demonstration of Mie resonances down to the ultraviolet (UV), with a material system (lithium niobate nanocubes) that was never used before is opening the possibility to develop a UV resonator using a compact and cost-effective material. The first demonstration of anapoles in III-V nanowires is also very promising for the further development of antenna for beam steering.
In WP2, the progress in mastering the material for nanoimprint lithography is a major step for the realization of large surface area photonic structure at a low cost, and even disposable. Beyond the passive structure, we are going to design and fabricate active photonic structure using electro-optic effect that can very efficiently tune the optical path in such material.
In WP3, the development of a viable process flow for the lithium niobate waveguides fabrication was key to start fabricating devices. The modulator and the spectrometer performances are outstanding and beyond the current state-of-the-art. Moreover, with this advanced fabrication skills, we can now design and produce many other devices with low losses and electro-optic properties that can be further exploited.