DESIGN AND MODELLING OF NANOPHOTONIC DEVICES

Abstract

This thesis presents a brief overview of the research work carried out on the design and modelling of nanophotonic devices, especially plasmonic switches based on vanadium dioxide (VO2) – a promising phase change material (PCM). Plasmonic switches, which modulate light propagation at the nanoscale, have gained significant attention for applications in integrated photonics, plasmonic logic circuits, and optical communication networks. These devices utilize the tunable plasmonic properties of PCM covered nanostructures under external stimuli — thermal, electrical, or optical — offering ultra-fast switching speeds, wide spectral tunability, compact sizes, and low losses. The thesis also explains the rationale for selecting VO2 and presents an extensive review of VO2 based plasmonic switches developed over the past few decades. Plasmonic switch designs proposed in this thesis are analyzed using Finite Difference Time Domain (FDTD) simulations. Firstly, active broadband plasmonic switches are designed using periodic arrays of metallic nanodisc-dimers with increasing diameters on a gold-coated silicon dioxide (SiO2) substrate, separated by a thin VO2 spacer layer. It is well known that — for a periodic array of circular metallic discs on a substrate, the applied optical field is efficiently coupled into the plasmonic modes of the nanostructure at specific plasmon resonance wavelengths, thus resulting in narrow and strong dips in the reflection spectrum of the nanostructure. Further, there is a red-shift in the plasmon resonance wavelength due to the retardation of the depolarization field when the diameter of the nanodiscs is increased. It can therefore be inferred that by employing an array of dimers of metallic nanodiscs with progressively increasing diameters, multiple wavelengths can be coupled into the plasmonic modes of the nanostructure such that each nanodisc-dimer with a specific value of diameter results in the coupling of incident electromagnetic radiation into the plasmonic modes of the nanostructure at a specific plasmon resonance wavelength. This coupling of incident radiation to plasmonic modes at multiple wavelengths results in an overall broadband resonance dip in the reflection spectrum of the nanostructure, enabling broadband response across the C, L, and U optical communication bands. The proposed designs achieve a broadband extinction ratio (ER) of 5 dB over 650 nm (from 1460 nm to 2110 nm wavelength) and 4 dB over 702 nm (from 1432 nm to 2134 nm wavelength) for a periodic array of five sets of nanodisc-dimers. The trade- off between ER and bandwidth is also explored for design optimization. Next, polarization-independent dual-wavelength switches are proposed using a periodic combination of U and C shaped gold nanostructures on a gold coated SiO2 substrate with a thin VO2 film spacer between the nanostructures and the underlying plasmonic substrate. The U and C shaped nanopillars are placed on the underlying substrate such that there is a spatial offset between them. It is well known that when three nanopillars are arranged in a U shaped nanostructure, two plasmonic modes ⎯ a short wavelength mode and a long wavelength mode ⎯ are generated when the incident light is X-polarized, whereas a single plasmonic mode is generated when the incident light is Y-polarized. For the C shaped nanostructure, this situation becomes inverted, with two plasmonic modes being excited for Y-polarized light and vi one plasmonic mode being excited for X-polarized light. However, when both U and C shaped nanostructures are placed together to form a U-C type plasmonic switch, two plasmonic modes ⎯ a short wavelength mode and a long wavelength mode ⎯ are generated for both X- and Y-polarized incident light. Due to the excitation of two plasmonic modes at both polarization angles of incident light, these U-C type plasmonic nanostructures are employed in this work to realize an efficient polarization-independent multi-wavelength switch by placing them on a VO2 coated plasmonic substrate. The proposed U-C type plasmonic switches exhibit an ER of ~20 dB simultaneously at two wavelengths ⎯ at ~1560 nm and at ~2130 nm ⎯ for a linearly polarized incident light with any polarization angle from 0° to 90°. Further, periodic arrays of identical V-shaped gold nanostructures and variable V-shaped gold nanostructures are designed on top of a gold-coated SiO2 substrate with a thin spacer layer of VO2 to realize multi-wavelength and broadband plasmonic switches, respectively. The periodic array of identical V-shaped nanostructures (IVNSs) with small inter-particle separation leads to coupled interactions of the elementary plasmons of a V-shaped nanostructure (VNS), resulting in a hybridized plasmon response with two longitudinal plasmonic modes in the reflectance spectra of the proposed switches when the incident light is polarized in the x-direction. FDTD modelling is employed to demonstrate that an ER > 12 dB at two wavelengths can be achieved by employing the proposed switches. Further, plasmonic switches based on variable V-shaped nanostructures (VVNSs) — i.e., multiple VNSs with variable arm lengths in one unit cell of a periodic array — are proposed for broadband switching. In the broadband operation mode, an ER > 5 dB over an operational wavelength range > 1400 nm in the near-IR spectral range spanning over all optical communication bands, i.e., the O, E, S, C, L and U bands, is reported. Further, it is also demonstrated that the operational wavelength of these switches can be tuned by adjusting the geometrical parameters of the proposed design. Additionally, the thesis investigates near-field plasmonic switching using pentagon shaped fractal plasmonic nanoantennas placed on a VO2 thin film overlaying a gold-coated SiO2 substrate. These fractal geometries are designed to confine electromagnetic fields to deep sub-wavelength volumes, generating intense field enhancements at the nanogap between the antenna arms. Upon phase transition of the VO2 layer, a significant change in local field distribution is observed, leading to an intensity switching ratio (ISR) exceeding ~2300 for higher fractal orders. The spectral response is shown to be tunable via geometric parameters. These near-field designs hold promise for utilization in areas like surface enhanced Raman spectroscopy (SERS), nonlinear optical phenomena, and nanoscale sensing. In summary, this thesis offers a detailed exploration of VO2 based plasmonic switches, emphasizing their potential for broadband, multi-wavelength, polarization-independent, and near-field switching applications. The proposed designs, supported by rigorous simulations, contribute significantly toward the development of compact and high-performance plasmonic devices for next-generation optical communication and photonic integration.