| dc.description.abstract | Integrated photonics is a promising technology for realizing miniaturized optical systems with enhanced stability and performance for applications in light-based communication, sensing, imaging, biomedical diagnostics etc. Silicon and allied materials such as silicon nitride are becoming the preferred material platforms for photonic integration due to the compatibility with advanced silicon-based manufacturing, ability to integrate electronic-photonics on the same chip and wide low loss window spanning visible, near infrared and mid infrared to realize low-loss, high efficiency photonic components. Though high-performance passive devices have been realized using silicon and silicon nitride photonic platforms, the lack of efficient light emission, electro-optic effect and free-carrier effect in wide bandgap silicon nitride has been a clear hindrance. The introduction of Two-dimensional (2D) Van der Waal materials both as a stand-alone nanophotonic device or through integration with passive devices is seen as a promising route to realize active functionalities. Such hybrid devices can enable the realization of on-chip light emission, correlated photon pair generation, nonlinear frequency conversion, intensity, phase or polarization modulation and photo-detection. The present work discusses the design, fabrication and experimental demonstration of several active photonic functionalities using both stand-alone patterned and hybrid integrated 2D material for applications in nonlinear and quantum photonics.
First, hybrid multilayer Gallium Selenide (GaSe) integrated silicon nitride (Si3N4) waveguides and ring resonators at 700-800 nm wavelengths were investigated for its linear and nonlinear optical applications. The design, fabrication and experimental measurements of the underlying passive Si3N4 waveguides are presented. Following this, the experimental investigation of single- and two-photon absorption induced optical control in waveguides and ring resonators is discussed. At lower optical powers, significant intensity-dependent saturable absorption is observed, transitioning to two-photon absorption at higher powers in GaSe integrated Si3N4 waveguides (785 nm pulsed laser). The two-photon absorption coefficient of the hybrid waveguide structure is obtained as 0.112 cm/GW. A blue laser incident on the GaSe region is used to induce single-photon absorption in multilayer GaSe on top of Si3N4 ring resonators, enabling all-optical resonance tuning via the free-carrier refraction effect. A significant blue shift of the resonance (12.3 pm/mW) and resonance broadening is observed, which is used to extract free-carrier-induced refractive index and absorption respectively. Two-photon absorption at the guided 785 nm pulsed excitation in GaSe integrated Si3N4 ring resonators also resulted in a measurable blue shift of the resonances. The hybrid integrated 2D material Si3N4 platform with its ability to realize active tuning of optical properties in the shorter near infrared wavelength range can find potential applications in integrated quantum photonics, sensing devices, and biomedical imaging.
Next, the quantum mechanical modelling of correlated photon pair generation using spontaneous four-wave mixing (SFWM) in silicon nitride waveguides and ring resonators is discussed. A time-domain approach is used to calculate the photon pair generation rate and demonstrate its variation with pump power and interaction lengths. The propagation losses in the waveguide are accounted for using a Langevin loss model. This work provides insights into the quantum mechanical aspects of SFWM and its potential applications in optical fiber communication systems. The prospects of extending this to spontaneous parametric down-conversion in the hybrid multilayer Gallium Selenide integrated silicon nitride waveguide will also be discussed.
Furthermore, third harmonic generation (THG) from stand-alone Molybdenum disulfide (MoS2) disks were explored. These disks were fabricated on top of a low-index silicon dioxide layer deposited on a silicon substrate. The sub-wavelength MoS2 disks are designed to excite Mie resonances in the near-infrared wavelength range. In particular, by exciting the anapolar resonance mode within the MoS2 disks in the 1400-1600 nm wavelength range, the resonant THG enhancement was studied while exciting different orders of anapole resonances. The THG signal at the anapole resonance is enhanced by 150-times when compared to the un-patterened MoS2 flake. The experimental study reveals that the polarization response of THG from the disks is heavily influenced by the ellipticity in the structure induced by the fabrication process. Subsequently, altering of THG polar response due to the ellipticity of the disks were theoretically analyzed. These results demonstrate that optical sub-wavelength structures based on layered materials, have the potential to serve as efficient nonlinear photonic devices, in particular for frequency conversion application across widely separated wavelength bands.
Finally, the role of structural asymmetry in the form of a vertical cut introduced to a stand-alone MoS2 disk is studied. Circularly symmetric MoS2 disks supporting second order anapolar resonances are designed in the wavelength range of 1400-1600 nm wavelength range. The addition of the asymmetry inducing cut is found to blue shift the anapolar resonances due to the reduction in the disk size with the resonant response significantly deviating for the incident polarization oriented parallel and perpendicular to the cut direction. The spectral interaction of the first order and second order anapole resonances is also observed as an anti-crossing behaviour as a function of varying asymmetric cut. Interesting whispering gallery type resonances are also observed within the structure with the cut inducing light coupling into the disk. Disks with increasing degree of asymmetry inducing vertical cuts were fabricated on quartz substrates to study the effect of the resonant mode interaction and its effect on the THG response. With increasing vertical cut size, blue shift in the resonant THG response due to second order anapole mode combined with spectral broadening due to the presence of neighbouring resonance features and the appearance of longer wavelength THG peak due to first order anapole resonance are observed in the experimental THG measurements. These observations corroborate well with the theoretical analysis of linear scattering and energy stored spectra within the cut disk structures. The far-field emission patterns from these structures are also studied using back focal plane imaging. The introduction of asymmetry to circularly symmetric sub-wavelength structures provide an interesting scheme to tune the resonance feature, and its interaction with other resonances. | en_US |
