Probing light-matter interactions with differential polarizations: From chiral nanostructures to molecular sensing
Life is chiral and therein lies the true meaning of the universe. Inspired by asymmetry of life, this thesis utilizes plasmonic nanostructures and dark-field scattering to elucidate how differentially polarized light distinguishes between left- and right-handed chiral architectures, and how this discrimination can be utilized for molecular sensing. Leveraging the polarization sensitivity and refractive index sensitivity of localized surface plasmon resonances, this work systematically probes light-matter interactions at the nanoscale. Emphasizing chiral optical phenomena, it demonstrates the fundamental understanding of dichroism at the single‑particle level while unlocking its potential for improved detection of chiral biomolecules. Using DNA-origami templated gold nanorod dimers, we apply single-particle circular differential scattering to probe nanoscale chirality. Supported by electron microscopy and electromagnetic simulations, our study reveals that the presence of a substrate imposes different orientations, each generating unique circular differential scattering line shapes, intensities, and even sign reversals, all from the same enantiomer. These findings highlight how substrate induced orientation profoundly influences single-particle chiroptical measurements and must be considered in both interpretation and design of chiral plasmonic systems. Beyond intrinsic chirality, two-dimensional interfacial dichroism is studied in rotationally symmetric planar chiral pinwheels. Under asymmetric illumination, pinwheels exhibit an orientation-independent linear differential scattering response, forming a linear analogue to optical activity. This work demonstrates that symmetry breaking in the illumination geometry, not the nanostructure itself, can generate chiroptical signals, an approach that broadens chiral sensing modalities. The thesis also presents a biosensing strategy based on plasmon-coupled circular dichroism, utilizing gold nanosphere dimers assembled via chiral bovine hemoglobin. Unlike bulk ensemble measurements, our single-particle dark-field circular differential scattering coupled with electron microscopy reveals analyte specific chiral signatures. These circular differential scattering features originate from hemoglobin’s circular birefringence, which differentially shifts the dimer’s plasmon resonance under left- versus right-circularly polarized light. This study sheds new light on the mechanism of plasmon-coupled circular dichroism by isolating the response of individual nanostructures. Toward scalable sensing platforms, the optical behavior of high quality factor plasmonic lattices is characterized using hyperspectral k-space imaging. Multiple finite lattices fabricated on a single substrate are individually resolved and spectrally analyzed, demonstrating a viable pathway for multiplexed sensing of different analytes or environmental conditions. Collectively, these studies significantly advance the fundamental understanding, rational design, and real-world application of chiral plasmonic systems, establishing a powerful platform for label-free, ultrasensitive, and multiplexed molecular detection at the intersection of nanophotonics, spectroscopy, and biosensing.
Circular dichroism, chirality, circular differential scattering, dark-field scattering spectroscopy, linear dichroism, linear differential scattering, biosensing, plasmon-coupled circular dichroism, plasmonics, k-space imaging, Fourier transformation, extinction, lattices, surface lattice resonance, extrinsic chirality, refractive index sensitivity
University of Illinois - Urbana-Champaign
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