Nonlinear Optical Enhancement Studies in Silicon-based Resonant Metasurfaces
Metasurfaces are two dimensional arrangements of building blocks called meta-atoms which have been found to be useful to manipulate amplitude, phase, polarization of light at the nanoscale. Using these properties, functional nanoscale devices for light manipulation have been built with wide-ranging applications in the various fields of sensing, metrology, optical communication, quantum computation etc. The subwavelength thick devices along with the possibility of manipulating properties of a light beam at will has made metasurfaces a promising alternative to conventional bulky optical components in various fields. Recently metasurfaces made of dielectric materials have been finding increasing research interest with the excitation of various resonance effects including Mie resonance, guided mode resonance, electromagnetically induced transparency, bound state in the continuum etc. Nonlinear optics is the field of study of light matter interaction where the material response to an incident light beam depends on higher powers of the light field amplitude. The applications of this field of study virtually spans all the optical domains, especially the fields of harmonic generation, wave mixing processes, sensing, switching, quantum optics etc. The enhancement of electric field near the resonances along with relaxed phase matching requirements makes metasurfaces a promising platform to realize these devices. In particular dielectric metasurfaces have been shown to be attractive in realizing these devices due to their field confinement inside the dielectric, high damage threshold, excitation of magnetic resonances etc. Silicon is the preferred dielectric because of its CMOS compatibility along with high third order nonlinear optical susceptibility. However, second order nonlinear optical effect is vanishing in silicon due to centro-symmetry. Due to this research interest has also been directed towards other materials with high second order susceptibilities particularly III-IV materials, two dimensional materials including transition metal dichalcogenides, gallium selenide, hexagonal boron nitride etc. In the first part, resonant third harmonic generation (THG) is demonstrated from amorphous silicon (a-Si) nanodisk arrays arranged in hexagonal lattice. The resonances occurring in such structures can be explained based on isolated Mie resonances and collective guided mode resonances in the array. Resonant THG enhancement ≈ 500 times corresponding to the fundamental resonance wavelength of 1510 nm is obtained compared to un-patterned a-Si film. Along with this we study spatial, spectral and intensity dependence of the THG process in a nonlinear microscopy set up. Intensity dependent reversal of THG image contrast is also demonstrated. Next, large area four wave mixing (FWM) microscopy on a silicon-on-insulator (SOI) based partially etched zero-contrast gratings (ZCG) metasurface is performed. The etch depth is a useful parameter which can be used to tune spectral positions along with Q factor of the resonances. Signal resonance is designed to occur at 1580 nm in sub-wavelength zeroth order diffraction region while the pump wavelength is fixed at 1040 nm higher order diffraction region resulting in the FWM wavelength of 775 nm. Maximum FWM enhancement ≈ 450 times is obtained as the signal beam is scanned through the fundamental resonance wavelength. Such structures can have potential applications in wavelength conversions across widely separated wavelength bands using wave mixing processes. In the next part, polarization-independent resonant enhancement of second harmonic generation (SHG) from multilayer gallium selenide (GaSe) on silicon-based resonant metasurface is performed in the presence of fundamental field depolarization and higher order diffraction effects. Nonlinear wave propagation simulations show that the higher order diffracted SHG exhibit strong polarization dependent enhancement with characteristics very different from the native GaSe layer. In this context, polarization independent enhancement of the second harmonic signal is achieved only for the zeroth order diffracted component. Experimental study of second harmonic generation from the GaSe layer integrated with the silicon metasurface shows maximum nonlinear signal enhancement of ~22 times on-resonance with polarization dependence identical to the native GaSe layer by selectively detecting the zeroth-order diffracted component. In the final part, electromagnetic design and analysis of hybrid metasurfaces composed of multi-layer GaSe coupled to silicon holey disk arrays is presented for achieving high spectral contrast chiral SHG. The silicon holey-disk structures are designed to support EIT-like optical resonances in the 1.5-1.8 µm wavelength range in the vicinity of electric and toroidal dipole scattering modes. The fundamental electric field enhancement above the silicon structures shows RCP and LCP-like characteristics at distinct wavelengths resulting in two prominent peaks for the LCP and RCP resolved SHG from the GaSe layer above the holey-disks. This results in high contrast SHG degree-of-circular-polarization spanning -1 to +1 over the fundamental excitation spectral range considered. The chiral SHG response is also found to be strongly influenced by the unit-cell lattice arrangement with square or hexagonal lattice exhibiting very different chiral SHG response. Finally, circular polarization resolved SHG and THG microscopic studies are also performed on a resonant metasurface of partially etched a-Si nanodisk arrays integrated with multilayer GaSe.