Studies on bio-inspired disordered photonic structures for multifunctional applications

Abstract

The study of light transport through disordered photonic structures has great significance in understanding the fundamental light-matter interactions as they are crucial in developing various optical functionalities. There are several important phenomena that are induced by the disordered natural photonic nanostructures, such as antireflection, camouflage, antibacterial, and coloration. This has motivated to design the structures with multifunctional optical properties or bio-inspired photonic nanostructures. For instance, the gradient refractive index variation in these nanostructures is a key factor for their antireflection characteristics, whereas the sharp tip of the structures mechanically ruptures the bacterial cell wall result in their bactericidal activities. The certain coloration in natural creatures arises either due to the long-range and short-range order and mimicking such design on an artificial platform provides optimal light scattering features for photonics-and energy-based applications. In this thesis, we have primarily focused on light transport in two types of photonic structures, namely the vertically aligned disordered silicon nanowires and the clusters of monodisperse scatterers. The vertically aligned disordered silicon nanowire arrays synthesized using a facile and cost-effective metal-assisted chemical etching method. We study the broadband omnidirectional anti-reflectivity from vertically aligned disordered SiNWs of large aspect ratio. An aberration-free micro-reflectivity setup equipped with an in-situ optical microscope is designed to measure reflectivity from both the top and cross-sectional surface of the SiNWs samples. We have measured the reflectivity as low as 5% irrespective of spatial-directions and the polarization of incident light in a broad wavelength range. The polarization-and direction-independent near-zero reflection of light constitutes the true two-dimensional omnidirectional anti-reflector. Further, we discuss the estimation of the effective refractive index profile using the spatial- and polarization-dependent reflectivity values along the nanowire length. We have found that the gradient variation of the effective refractive index profile is tunable with nanowire length. The transfer matrix method involving the estimated refractive index profiles is employed to corroborate the measured reflectivity values. We have found that a disordered nanowire sample with an exponential refractive index profile along its length shows enhanced antireflection and light trapping features. Moreover, we have also found the bactericidal activities of the nanowires, which are very effective but lacking cell viability due to the use of hydrofluoric acid during the nanowire fabrication. To overcome this limitation, we have used silicon nanopyramids which are synthesized using a single step wet etching technique. The nanopyramid based antibacterial surface offers better control over bacterial growth, chemical stability, and cell viability. We have also studied the controlled light scattering in three-dimensional correlated disordered photonic structures with short-range order. The samples possess clusters of monodisperse scatterers, and their optical conductance shows a resonant frequency gap which is controllable with the scatterers size and refractive index. The scattering model is designed based on the structural morphology of the samples to validate the tunability of the frequency gap. The light transport parameters such as scattering mean free path and transport free mean path is estimated using static measurements. We have found that the photonic structures with short-range order can actively modify the light scattering parameters in a specific wavelength range. Further, we have obtained an anisotropic scattering regime in the range of frequency gap, which is a much sought-after goal in photonic scattering systems, which is also in complete agreement with the theory.