Resonant mode engineering in silicon compatible multilayer guided-mode resonance structures under gaussian beam excitation condition

In this work we present detailed electromagnetic design, fabrication, and experimental study of silicon-compatible, step-index, multilayer guided-mode resonance (GMR) structures. The proposed multilayer stack consists of an amorphous silicon overlayer deposited above a silicon nitride layer on top of one-dimensional sub-wavelength silicon dioxide grating structures. The silicon nitride layer combined with the underlying silicon dioxide gratings define the high-quality factor resonances in the 1500-1600 nm wavelength range. The addition of the amorphous silicon overlayer with varying thickness further red-shifts the resonance combined with the ability to engineer the resonant field profile and its evanescent field fraction. Realistic Gaussian beam excitation with increasing angular spread exciting finite extent gratings results in reduced contrast and quality factor of the designed resonances are found to decrease. We experimentally study the linear transmission spectra for the fabricated structures with amorphous silicon overlayer thickness of 10, 50 and 100 nm. The measured optical resonances are at 1580, 1813 and 2104 nm respectively, showing good agreement with Gaussian-beam excitation studies. As an application of the proposed structures, we demonstrate third-harmonic generation (THG) enhancement from the 10 nm amorphous silicon overlayer showing 19 and 178-times enhancement in THG for fundamental excitation with 11o and 4o angular spread respectively. The present work utilizes the thinnest amorphous silicon layer to achieve optical resonances in the near-infrared wavelength and consequently ensures minimum absorption of the generated THG signal when used for nonlinear optical enhancement studies.