AbstractCharacterisation of chiral molecules and nanoparticles, i.e., molecules and nanoparticles that lack mirror symmetry, has long relied on linear chiral optical methods, such as optical rotation and circular dichroism. While these methods are well-established, the measured effects are inherently weak.
Recently, a new nonlinear chiral optical effect was observed for the first time - optical activity in hyper-Rayleigh scattering. The effect presents itself as a difference in intensity of light scattered from chiral nanoparticles/molecules at the second-harmonic frequency of the incident light, depending on the direction of circular polarisation of the incident light. It has been shown to be stronger than linear chiral optical effects and to have great potential for chiral optical characterisation in small volumes.
In this work, we present our investigations of optical activity in (second-harmonic) hyper-Rayleigh scattering, as well as of chiral optical effects in higher-order scattering.
In the first part of this thesis, the relevant theoretical background, including optical resonances in nanoparticles, the basics of nonlinear optics, and chirality and chiral optical effects, is presented. Subsequently, we discuss the details of our experimental setups. The remaining chapters include some of the results obtained during this PhD project. Specifically, we present measurements of hyper-Rayleigh scattering originating from a microscopic volume (approximately 30 um3). We observe optical activity and demonstrate that our measurements can distinguish between two chiral versions (enantiomorphs) of gold cuboids dispersed in a solvent. Based on the size of the illumination volume and the concentration of the gold cuboids, we calculate that the measurements are done on a single nanoparticle level. Next, we present the first experimental observation of optical activity in third-harmonic Rayleigh scattering. In the final chapter, we focus on nonlinear scattering by dielectric scatterers with size comparable to the wavelength of the illuminating light. In this regime, scattering is highly directional. Hence, we call it third-harmonic Mie scattering. Circular intensity difference in the signal allows for distinguishing between enantiomorphs of CdTe helices. We demonstrate the scalability of this chiral optical characterisation method and discuss potential applications.
|Date of Award||25 May 2022|
|Supervisor||Ventsislav Valev (Supervisor) & Simon Bending (Supervisor)|