Medium Index Contrast Guided Mode Resonant Structures for Photonic Applications in the Visible-Near Infrared Wavelength Regime
Menon, Sruti Hemachandran Geetha
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Guided mode resonant (GMR) structures are interesting from the point of view of enhanced light-matter interaction and find applications in sensing, filtering, and miniaturized photonic components. The Guided mode resonance (GMR) arises due to the coupling of the incident light into the guiding medium, followed by interference of the leaky modes with the reflected/transmitted light, resulting in high Q resonance features with field enhancement in and around the structure. GMR structures using high refractive index materials like silicon, germanium, gallium arsenide are explored widely due to the high scattering capability of the material. However, such materials are lossy and are not suitable for visible frequency applications. On the other hand, GMR structures based on materials like silicon dioxide, titanium dioxide, polymer, silicon nitride are transparent in the visible frequency regime yet are not widely studied due to its weak refractive index and low scattering capability. In this thesis, silicon nitride and gallium nitride-based GMR with refractive index contrast of ~0.4-1.5 with respect to the substrate are studied for fluorescence and nonlinear enhancement studies. In this first part of the work, the high refractive index contrast gratings are compared with the weak refractive index contrast gratings and the techniques are studied to widen the narrow design space encountered in weak refractive index gratings by altering the design parameters of the gratings. Further, fully etched silicon nitride gratings with 80% duty cycle are studied for resonantly enhancing the absorption and emission of the Rhodamine B ITC dye using TE and TM polarized resonances, respectively. The fluorescence enhancement of 10.8 times obtained experimentally using TE polarized excitation and TM polarized collection is then corroborated with simulations by representing the fluorophores to be an array of dipoles in presence of gratings to model the effect of polarization-sensitive resonances on the enhancement of absorption and fluorescence. The GMR structure discussed in this study proves to be a promising approach to realize highly sensitive fluorescence assays and to probe polarization-sensitive information from 2D materials transferred onto these structures. Next, layered GMR structures consisting of silicon dioxide gratings conformally coated with silicon nitride are studied for resonantly enhancing the third harmonic generation from the 10 nm amorphous silicon layer. The GMR structure is designed to have maximum field interaction with the amorphous silicon layer. In addition, it also behaves as a passive medium with low inherent absorption and nonlinearity, enabling the integration of these structures with other highly nonlinear medium of interest. The GMR structure being sensitive to the incidence angle, the contrast of the resonances and THG enhancement is learned to decrease with an increase in the divergence angle of the incident beam. Experimentally, the backward THG enhancement measured as the ratio of backward THG power in presence of grating and in absence of grating increases from 18 times to 1120 times when the angular spread is reduced from 11 degree to 2.3 degree. This is corroborated with Gaussian beam-based simulations which show good agreement with the experimental results. The structure discussed here enables THG enhancement from the thinnest amorphous silicon layer to the best of our knowledge with minimal absorption at the THG wavelength and provides a scalable platform for nonlinear enhancement and sensing based applications. Furthermore, the gratings being a one-dimensional periodic structure, the angular sensitivity of the resonances reduces in case of full-conical illumination with the projected wave vector parallel to the grating lines as compared to the full-classical illumination with the projected wave vector perpendicular to the grating lines. Leveraging this concept, four orders of forward THG enhancement is reported using a partial conical illumination setup with a rectangular pupil mask placed at the back focal plane of the objective to limit the angles perpendicular to the gratings, while allowing full angular spread supported by the objective along the grating lines. With this setup, improvement in THG enhancement from 2860 to 1.7x10^4 is observed when the angular spread perpendicular to the grating lines is reduced from 2.3 degree to 0.43 degree. The THG enhancement obtained in this work is the best reported so far from 10 nm silicon to the best of our knowledge and the technique discussed here paves way for enhanced nonlinear generation from any angle-sensitive structure. Finally, a simulation and experimental based study of amorphous silicon-Gallium Nitride based heterogeneous structures on sapphire substrate is performed for resonant enhancement of second order harmonic generation (SHG) in 5 𝜇m thick c-Gallium Nitride. Due to the restrictions imposed by the lattice symmetry in c-Gallium Nitride material, it is necessary to have enhanced longitudinal and transverse fundamental field components to ensure the efficient excitation of nonlinear polarization terms and to efficiently radiate the in-plane polarized SHG, thereby enabling enhanced excitation and collection of SHG using low NA optics. With 45 degree polarized fundamental incidence, TE and TM polarized resonances are simultaneously excited in these structures which leads to efficient SHG generation with maximum in-plane polarized component radiated along the optic axis. Experimentally, 1300 times SHG enhancement is demonstrated at 1460 nm, with the measured SHG efficiency of 1.54 x 10^-3 %/W at a smaller peak intensity of 0.11 GW/cm^2. The measured efficiency is comparable to the reported SHG efficiencies in x-cut Lithium Niobate based structures with d33 axis in-plane allowing efficient excitation and collection of SHG and higher as compared to the z-cut Lithium Niobate structures with maximum susceptibility component d33 oriented out-of-plane similar to the c-Gallium Nitride material studied in this work. This work reports the best SHG efficiency value from Gallium Nitride when compared to previous work on GaN based muti-quantum well based metasurfaces.