Nanoscale organization of luminescent materials and their polarization properties investigated by two-dimensional polarization imaging

Semiconductor materials (e.g., conjugated polymers, metal halide perovskites) have been widely used in solar cells, light-emitting diodes, and photodetectors. Organic conjugated systems have high mechanical flexibility and low costs for production. Metal halide perovskites have the advantage of strong light absorption, long charge-carrier diffusion lengths, and low intrinsic surface recombination.Polarization-sensitive single-molecule methods have been extensively used to study the chromophore organization and excitation energy transfer (EET) process. Our novel polarization technique, two-dimensional polarization imaging (2D POLIM) is designed to simultaneously measure and control both the excitation and emission polarization characteristics of an individual object. A model based on single funnel approximation (SFA) is applied to fit the 2D polarization portrait obtained from 2D POLIM measurements. 2D POLIM in combination with the SFA model allows the quantitative characterization of EET efficiency. Overall, A large number of polarization parameters, e.g., modulation depths, phases, luminescence shift, fluorescence anisotropy, energy funneling efficiency, and properties of the EET-emitter, can be extracted from 2D polarization portraits. They give a full picture of chromophores’ organization and a quantitative measure of the EET process.In this thesis, we applied the 2D POLIM technique to investigate the fundamental optoelectronic process in different types of luminescent materials. H-aggregates forming in spin-cast conjugated films are visualized by modulation depth and phase imaging contrast. Light-harvesting efficiency shows the efficient ET within the amorphous phase and poor ET between H-aggregates due to the less overlap between absorption and emission spectra. Together with single-molecule spectroscopy and scanning electron microscope, we studied the polarization property of individual MAPbBr3 aggregates, which shows the well-known dielectric screening effect cannot fully explain the absorption polarization from weakly elongated objects (even with irregular shapes). We propose that power dependent quantum yield can further increase the modulation depth of excitation. 2D POLIM was also applied to explore the aggregation state of proteins in the biological system. Furthermore, we did a series of computer experiments to examine and improve the SFA model. We break the limit of energy funneling efficiency and propose an asymmetric three-dipole model, which is more applicable for multi-chromophore systems. In the future, quantitative phase-contrast imaging and time-resolved 2D POLIM might be further develope

Breaking Symmetry: A Study of Novel Phenomena in Asymmetric Nanoplasmonic Systems

Offering tailorable optical properties not achievable with symmetric or periodic optical materials, chiral, weakly disordered, deterministic aperiodic, quasiperiodic and random structures make up a new wave of asymmetric optical systems demonstrating unprecedented control of light compared to their periodic counterparts in areas such as random lasing, imaging, and bio-sensing. The governing physics of asymmetric systems is, however, not as analytically intuitive and computationally straightforward as periodic or highly symmetric systems, and thus the availability of simple analytic and computational design tools has made periodic systems an attractive option for many optical applications. For example, plasmonic systems consisting of periodic arrays of achiral metallic sub-wavelength scatterers, referred to as metasurfaces, can manipulate the phase front of light waves over nanometer scale distances. This is possible due to the plasmonic confinement of light to sub-wavelength dimensions. In Part I of this work, a novel class of plasmonic aperiodic metasurfaces is introduced exhibiting novel functionalities not possible in their periodic counterparts. Freeing the design process from time costly FDTD simulations, the development of an analytically intuitive model describing interference at a slit-aperture between directly incident light and surface plasmon polaritons arriving from nearby illuminated grooves has enabled the speedy design, fabrication, and experimental characterization of aperiodic slit-grooved plasmonic devices with easily tunable angle-dependent multi-spectral responses. These devices, constituting part of a new and novel class of aperiodic systems referred to as aperiodic-by-design, have lateral dimensions ≤ 10 μm and consist of a sub-wavelength slit (circular) aperture surrounded by grooves (semi-annular rings) on an opaque metal film. Each groove is individually optimized for position, width, and depth in order to achieve a specific desired multi-spectral response. Part II of this work explores the chiroptical (CO) response of optical media. The potential several-orders of magnitude plasmonic enhancement of the weak circular dichroism (CD) response of natural molecules has generated a plethora of research interest and publications describing the so-called CD response of plasmonic systems. However, this work demonstrates, through the development of a generalized coupled-oscillator (GCO) model, the presence of other CO responses not related to CD. Closed-form analytic expressions for various CO response types are developed within the GCO model, and characteristics of each type are highlighted. This work both demonstrates the necessity of careful interpretation of CO measurements and provides tools for distinguishing between the response types. The GCO model unifies, for the first time, many of the separately observed chiral-optical phenomena into a single theoretical framework. The results presented in this dissertation testify to the novel and seemingly exotic behaviors of asymmetric plasmonic systems. The in-depth analysis of the systems provided in this work emphasizes the fundamental origins of these behaviors, providing a clear roadmap towards the development of a new generation of optical devices with functionalities extending beyond the existing state-of-the-art technologies

Multiparametric Optical Characterization of Biological Nanoparticles using Evanescent Field Sensing

In light of the increasingly realized dependence of many biological functions on nanoscopic supramolecular assemblies, also including novel biotechnological applications, there is a need for advanced analysis methods capable of accurately quantifying different characteristics of these elusive entities. The prime aim of this thesis is the development and utilization of surface-based bioanalytical sensing methods for quantitative characterization of biological nanoparticles. The possibility to construct and use a waveguide-based evanescent light scattering microscopy instrument for investigation of various nanoparticle properties is explored through the study of liposomes and mRNA-containing lipid nanoparticles as well as polystyrene and silica nanoparticles. It is shown that through analysis of scattered light from such particles, single-particle-resolved information on their size, refractive index and interactions with surrounding protein solutions is obtainable, thus providing multiparametric characterization beyond the ensemble average. Additionally, this is combined with information gained from fluorescent labeling of certain biomolecular components, allowing nanoparticle content to be correlated with the other particle properties. The aforementioned systems were additionally investigated using a range of complementary methods, including nanoparticle tracking analysis, surface plasmon resonance sensing, and quartz crystal microbalance with dissipation monitoring. It was concluded that the waveguide microscopy method provides quantitative information in good agreement with established methods, but offers certain key advantages, such as the possibility to provide single-particle resolved label-free information on protein binding kinetics combined with simultaneous evanescent light fluorescence microscopy measurements, thus providing new insights regarding nanoparticle heterogeneity

Roadmap for Optical Tweezers 2023

Optical tweezers are tools made of light that enable contactless pushing, trapping, and manipulation of objects ranging from atoms to space light sails. Since the pioneering work by Arthur Ashkin in the 1970s, optical tweezers have evolved into sophisticated instruments and have been employed in a broad range of applications in life sciences, physics, and engineering. These include accurate force and torque measurement at the femtonewton level, microrheology of complex fluids, single micro- and nanoparticle spectroscopy, single-cell analysis, and statistical-physics experiments. This roadmap provides insights into current investigations involving optical forces and optical tweezers from their theoretical foundations to designs and setups. It also offers perspectives for applications to a wide range of research fields, from biophysics to space exploration

Open-ended response theory with polarizable embedding:Multiphoton absorption in biomolecular systems

We present the theory and implementation of an open-ended framework for electric response properties at the level of Hartree–Fock and Kohn–Sham density functional theory that includes effects from the molecular environment modeled by the polarizable embedding (PE) model. With this new state-of-the-art multiscale functionality, electric response properties to any order can be calculated for molecules embedded in polarizable atomistic molecular environments ranging from solvents to complex heterogeneous macromolecules such as proteins. In addition, environmental effects on multiphoton absorption (MPA) properties can be studied by evaluating single residues of the response functions. The PE approach includes mutual polarization effects between the quantum and classical parts of the system through induced dipoles that are determined self-consistently with respect to the electronic density. The applicability of our approach is demonstrated by calculating MPA strengths up to four-photon absorption for the green fluorescent protein. We show how the size of the quantum region, as well as the treatment of the border between the quantum and classical regions, is crucial in order to obtain reliable MPA predictions

Label-free protein quantitation using liquid crystal-enhanced optofluidic biosensor

Protein detection plays an important role in the medical research. Liquid crystals (LCs), as a class of sensitive materials, exhibit a promising ability in the biosensing field. Herein, we exploited an ultrasensitive biosensor for protein detection, employing the whispering-gallery-mode (WGM) from the LC-amplified optofluidic micro-resonator. The biomolecules can trigger both light-matter interactions and the orientation transitions of LC molecules. The WGM spectral wavelength shift was recorded as the sensing indicator, and a detection limit of 15 fM for bovine serum albumin (BSA) was achieved. Our LC-amplified optofluidic biosensor provides a new solution for the ultrasensitive, real-time, and stable biological detection

In silico characterization and prediction of global protein–mRNA interactions in yeast

Post-transcriptional gene regulation is mediated through complex networks of protein–RNA interactions. The targets of only a few RNA binding proteins (RBPs) are known, even in the well-characterized budding yeast. In silico prediction of protein–RNA interactions is therefore useful to guide experiments and to provide insight into regulatory networks. Computational approaches have identified RBP targets based on sequence binding preferences. We investigate here to what extent RBP–RNA interactions can be predicted based on RBP and mRNA features other than sequence motifs. We analyze global relationships between gene and protein properties in general and between selected RBPs and known mRNA targets in particular. Highly translated RBPs tend to bind to shorter transcripts, and transcripts bound by the same RBP show high expression correlation across different biological conditions. Surprisingly, a given RBP preferentially binds to mRNAs that encode interaction partners for this RBP, suggesting coordinated post-transcriptional auto-regulation of protein complexes. We apply a machine-learning approach to predict specific RBP targets in yeast. Although this approach performs well for RBPs with known targets, predictions for uncharacterized RBPs remain challenging due to limiting experimental data. We also predict targets of fission yeast RBPs, indicating that the suggested framework could be applied to other species once more experimental data are available

UNDERSTANDING AND EVALUATING CRYSTAL POLYMORPHISM BY SECOND HARMONIC GENERATION MICROSCOPY

The crystalline form of a solid can profoundly affect its physical and chemical properties, with both potentially stable and metastable crystal polymorphs are accessible during crystal formation. Conventional methods limit the detection of rare nucleation and rapid phase transitioning events due to their lack of selectivity and sensitivity. Inkjet printing of a solution confines the nucleation event in a few micrometer volumes within the droplet, and furthermore rapid desolvation favors the kinetic factor to trap the rare metastable polymorphs. Second harmonic generation microscopy (SHG) possesses enough sensitivity to detect sub-micrometer size chiral crystals selectively and has the potential for use in crystal nucleation studies