Nonlinear optics at nanoscale: frequency conversion at interraces

Laura Rodríguez i Suñé, Presentation date: 19th June 2023

Author: Laura Rodríguez i Suñé
Title: Nonlinear optics at nanoscale: frequency conversion at interraces 

Supervisors: Cojocaru, Crina; Trull Silvestre, José Francisco

Presentation date: 19th June , 2023

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Abstract: The use of semiconductors, metals, or ordinary dielectrics in the process of fabrication of nanodevices is at the front edge of nowadays technology. In the last decade an impressive technological progress has been made towards the miniaturization process, giving birth to the field of nanotechnology. Currently, nanostructures are routinely fabricated and integrated in different photonic devices for a variety of purposes and applications. At the nanoscale, light-matter interaction can display new phenomena, different from those occurring in homogeneous materials or even micrometer-scale optical structures and devices. This scenario makes conventional approximations to the dynamics of light-matter interactions to break down and new strategies must be sought in order to study, understand, and ultimately harness the performance of subwavelength nonlinear optical materials. This is the case of nonlinear interactions and in particular, of nonlinear frequency conversion, a fundamental physical process that lies on the basis of many modern disciplines, from bioimaging in nanomedicine to material characterization in material science and nanotechnology. Nonlinear photonics also holds great promise in laser physics with applications in information technology for optical signal processing and in the development of novel coherent light sources. Thus, a deep understanding of the specific aspects of light-matter interaction at the nanoscale is crucial if one is to properly engineer nanodevices. In this thesis we report comparative experimental and theoretical studies of nonlinear frequency conversion in different strategic materials for photonics having nanoscale dimensions. We start our study with homogeneous layers and project our results to nanostructures, where second and third harmonic conversion efficiencies drastically decrease compared to typical nonlinear optics working conditions. We have developed novel experimental set-ups capable of measuring second and third harmonic generation efficiencies arising from semicondutors, conductive oxides and metal nanolayers and nanostructures. Our experimental approach allows us to estimate very low conversion efficiencies, and it is designed to perform an exhaustive study of harmonic generation by analyzing the nonlinear signals as a function of incident angle, wavelength and polarization, important parameters that determine and distinguish the origin of the nonlinear process. At the nanoscale phase matching conditions and even absorption no longer play a primary or significant role, and new linear and nonlinear sources become relevant, including magnetic dipole and electric quadrupole (surface) nonlinearities arising from both free and bound electrons, as well as nonlocal effects, convection, and hot electrons nonlinearities, associated with free electron dynamics, pump depletion, and phase-locking. We have performed numerical simulations based on a unique microscopic hydrodynamic model that considers all these contributions to the nonlinear polarization. By comparing experimental results with numerical simulations we are able to identify and distinguish the different mechanisms that trigger the harmonic generated signals at visible and UV wavelengths, while extracting basic physical properties of the material. With this knowledge we are able to make a step forward and predict conversion efficiencies in complex structures which are specifically designed to enhance harmonic generation. The capability to efficiently generate harmonics at the nanoscale will have an enormous impact in the fields of nanomedicine and nanotechnology, since it would allow one to realize much more compact devices and to interrogate matter in extremely confined volumes.