Nonlinear light generation in disordered assemblies of nanoparticles

Abstract

The primary source of coherent light at wavelengths not available with common laser sources are nonlinear processes such as second-harmonic generation (SHG), optical parametric oscillation (OPO), and spontaneous parametric down-conversion (SPDC). Noncentrosymmetric crystals are the conventional platform to implement optical frequency conversion. However, their use is limited by optical dispersion, which imposes strict phase-matching conditions to achieve efficient nonlinear conversion. Many methods have been developed for phase-matching control, such as phase-matching in birefringent crystals or quasi-phase-matching in periodically poled materials. These methods produce nonlinear generation that grows quadratically with the volume of the crystal. At the micro- or nanoscale it is also possible to exploit resonant mechanism to enhance the nonlinear light-matter interaction. In all these cases, the optimal nonlinear conversion is achieved in a narrow wavelength range, and it requires to control the temperature and the polarization of the pump beam as well. Disordered photonic materials, consisting of a random assembly of nonlinear optical crystals, provide an alternative platform to bulk crystals. In fact, it is possible to generate broadband coherent nonlinear light with a mechanism called random quasi-phase-matching (RQPM). It allows to circumvent the phase-matching conditions and to generate SHG proportional to the volume of the disordered material. The disadvantage of RQPM is its lower efficiency than more conventional phasematching schemes. RQPM has been mostly implemented in polycrystals with micrometer-sized domains (10-100 µm). The nonlinear generation in micron-size χ⁽²⁾ structures with nanostructured disorder is completely unexplored. In this thesis we present bottom-up assembled microspheres made of nonlinear nanocrystals of barium titanate (BaTiO₃) and lithium niobate (LiNbO₃) as our disordered photonic material. The fabrication ensures that the nanocrystals that constitute the microspheres are randomly placed and oriented in the spherical assembly, and that the volume is known or easy to measure. Our goal is to use them to show second-harmonic generation with the RQPM at the microscale. They can achieve frequency conversion from the nearultraviolet to the infrared ranges, are low-cost, and can cover large surface areas. Moreover, we propose different solutions to enhance the nonlinear emission from the microspheres. In a first part, we combine the Mie resonances stemming from the spherical geometry to increase the SHG from microspheres made of BaTiO₃ nanocrystals of 50 nm of size. The measured second-harmonic generation shows a combination of broadband and resonant wave mixing, in which Mie resonances enhance the second-harmonic generation, while the disorder keeps the phase-matching conditions relaxed. We support our findings with analytical models and simulations. Our assemblies provide new opportunities for tailored phase-matching at the microscale, beyond the coherence length of the bulk crystal. In a second part, we use LiNbO₃ nanocubes of 100 nm to 400 nm as building blocks of disordered microspheres and slabs of variable thickness. Bigger domains are the most direct way to increase the efficiency of the SHG. At the same time, this introduces multiple light scattering in the assemblies. They display a remarkable strong light scattering, evidenced by a subwavelength transport mean free path (l*). We show that RQPM is robust to scattering and that the SHG grows linearly with the thickness of the slabs and the volume of the microspheres. These assemblies represent a promising platform to investigate the interplay between disorder and nonlinear effects. In a third part, we bottom-up assemble spherical dielectric resonators with embedded diamond nanoparticles with nitrogen vacancy centers (NV). Those assemblies can exploit two phenomena: the photonic nanojet to focus the excitation field into a small volume, and the Mie resonances to enhance the emission at the resonant wavelength. We show that we can modulate the fluorescence thanks to the Mie modes and that we can control it with the temperature. This work proposes bottom-up disordered assemblies of nonlinear crystals as a platform for nonlinear light generation. We illustrate the advantages of the scalable fabrication and the flexibility of the nonlinear generation from the disorder. Furthermore, we propose strategies to improve the nonlinear emission and to investigate the physics of a complex nonlinear medium.