Tailoring Van – der – Waal materials for nonlinear nanophotonics

Abstract

Van der Waals or two-dimensional (2D) material are emerging nanomaterials which has shown tremendous potential in building next-generation nonlinear optical devices. Monolayer and multilayer 2D materials exhibit strong nonlinear optical response owing to their highly ordered crystalline structure, layer-tunable bandgap and symmetry, and polarization dependence, and which offer the possibility of incorporating electrical tunability due to their unique layer-dependent, electric and strain-tunable electrical and optical properties. The unique properties of these van der Waal materials have led to their proposed use in various applications such as wavelength conversion, saturable absorption, optical limiting, optical modulation, and parametric down-conversion. For instance, ultra-thin active photonic devices made from 2D materials could potentially provide significant advantages such as smaller device size, higher efficiency, and faster response time when compared to traditional photonic devices. Therefore, characterizing the strength of nonlinear response and boosting it further by augmenting it with a resonant photonic structure is of utmost importance. The thesis aims to characterize the nonlinear response of emerging 2D materials and will discuss optimally designed nanophotonic structures to enhance the nonlinear response.

In the first part of the work, we report strong second-harmonic generation (SHG) from the 2H polytype of multilayer Tin diselenide (SnSe2) for fundamental excitation close to its indirect band-edge in the absence of excitonic resonances. Rapid oscillations in signal strength for slight changes in flake thickness have been observed which are in good agreement with a nonlinear wave propagation model considering nonlinear polarization with alternating signs from each monolayer. We observed enhanced SHG at 1040 nm compared to 1550nm which is attributed to the enhanced nonlinear optical response for fundamental excitation close to the indirect band-edge. We also studied SHG from heterostructures of monolayer MoS2/multilayer SnSe2 and found the SHG signal and any interference effect in the overlap region to be dominated by the SnSe2 layer for the excitation wavelengths considered. The comparison of SHG from SnSe2 and MoS2 underscores that the choice of the 2D material for a particular nonlinear optical application is contextual and requires consideration of the wavelength range of interest and its optical properties at those wavelengths. This work further highlights the usefulness of near-band-edge enhancement of nonlinear processes in emerging 2D materials towards realizing useful nanophotonic devices.

Furthermore, we report up-conversion of 1550 nm incident light using third harmonic generation (THG) in multi-layered tin di-selenide (SnSe2) and study its thickness dependence by simultaneously acquiring spatially-resolved THG images in the forward and backward propagation direction. We find good agreement between the experimental measurements and a coupled-wave equation model we have developed when including the effect of Fabry-Perot interference between the SnSe2 layer and the surrounding medium. We extract the magnitude of the third-order electronic nonlinear optical susceptibility of SnSe2, for the first time to our knowledge, by comparing its nonlinear response with a glass substrate and find this to be ∼1500 times higher than that of glass. The large nonlinear optical susceptibility of multi-layer SnSe2 makes it a promising material for studying nonlinear optical effects.

The next part of this thesis focused on building nanophotonic structures for enhanced nonlinear optical response. Here in the first part, we report hybrid-genetic optimization (HGA) based design and experimental demonstration of second harmonic generation (SHG) enhancement from Fabry–Perot structures of single and double multilayer gallium selenide (GaSe) flakes with bottom silicon dioxide and index-matched polymethyl methacrylate spacer/encapsulation layers. The HGA technique utilized here speeds up the multilayer cavity design by 8.8 and 89 times for the single and double GaSe structures when compared to the full parameter-sweep, with measured SHG enhancement of 128- and 400-times, respectively, when compared to a reference sample composed of GaSe layer of optimized thickness on 300 nm silicon dioxide layer. SHG conversion efficiencies obtained from the HGA structures are 1–2 orders of magnitude higher than previous reports on 2D material integrated resonant metasurfaces or Bragg cavities.

In the final section of this thesis, we leverage the high refractive index exhibited by transition metal dichalcogenides (TMDCs) to create isolated Mie-resonant optical structures that facilitate strong nonlinear optical interaction. To achieve this, we performed scattering cross-sectional simulations to determine the appropriate dimensions for the MoS2 disk to enable higher-order anapole modes within our desired signal wavelength range of 1400-1700nm. We then fabricated the MoS2 disks by patterning a dry-transferred MoS2 flake on a 2.2-micron thick thermal SiO2 layer deposited on a silicon wafer. Our experiments show that the fabricated isolated MoS2 disks do exhibit higher-order anapole resonances, leading to enhanced second harmonic generation. Specifically, we observed a maximum experimental SHG enhancement of 96 times at 1470nm from the MoS2 disk compared to the un-patterned MoS2 flake region. We also performed detailed nonlinear wave propagation simulations, which were in good agreement with our experimental data. These results demonstrate that optical nanostructures based on layered materials, such as MoS2, have the potential to serve as efficient wavelength converters across widely separated wavelength bands.