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Herein, we describe a momentum-resolved optical reflectometry technique for precisely quantifying absorption anisotropies, with a particular interest in its ability to estimate distinct out-of-plane dipole strengths in solution-processable semiconductors. We demonstrate major advantages over conventional techniques, e.g., variable-angle spectroscopic ellipsometry, and subsequently show how to merge the strengths of the two techniques. We interrogate two distinct material systems: organic semiconductor thin films and two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs). In organic thin films, we resolve molecular reorientations due to processing conditions. In 2D HOIPs, we adopt a layered effective medium model to show that strong optical anisotropies arise predominantly from classical electromagnetics effects (i.e., dielectric inhomogeneity) rather than anisotropies in the quantum-mechanical matrix elements. Finally, we demonstrate unexpected multipolar light-matter interactions in 2D HOIPs, revealed by a highly polarized and oblique emission sideband. Electromagnetic and quantum-mechanical analyses indicate that this emission originates from an out-of-plane magnetic dipole transition arising from the 2D character of electronic states. The techniques described herein are materials agnostic and may provide insight into fundamental optoelectronic processes and processing-dependent structure-function relationships in a wide variety of interrogated materials.

Biofuel produced from photosynthetic microorganisms offers a green and sustainable alternative to fossil fuels and bioethanol for next generation transportation fuels. Their simple cell structures enable them to be more efficient in photosynthesis than higher order plants. However, large scale cultivation of these microorganisms is typically hampered by the poor light utilization of photobioreactor systems. In order to increase biomass productivities, the light transfer in photobioreactors must be characterized and optimized. The radiation characteristics of photosynthetic microorganisms are essential parameters in this analysis. This study aims to (1) experimentally measure the radiation characteristics of various photosynthetic microorganisms of different morphologies and (2) to develop models that can predict these radiation characteristics.First, the spectral complex indices of refraction of unicellular spheroidal green algae Botryococcus braunii, Chlorella sp., and Chlorococcum littorale were retrieved from their experimentally measured average absorption and scattering cross-sections. Next, the temporal evolution of the scattering and absorbing cross-sections of marine eustigmatophycease Nannochloropsis oculata grown in flat-plate photobioreactor (PBR) was reported. The variations in cross-sections were found to be directly related to the up- and down-regulations of pigments and other intracellular components and vary significantly with time in response to changes in light and nutrients availability. Furthermore, this study demonstrates that the light transfer in the PBR could be predicted using constant radiation characteristics measured during the exponential growth phase with reasonable accuracy provided that the cultures were not nitrogen limited. In addition, this study presents experimental measurements of the absorption and scattering cross-sections and the spectral complex index of refraction of filamentous heterocystous cyanobacterium Anabaena cylindrica. Its filaments, consisting of long chains of polydisperse cells, were modeled as infinitely long and randomly oriented volume-equivalent cylinders. Finally, it was demonstrated that the absorption and scattering cross-sections and asymmetry factor of randomly oriented and optically soft bispheres, quadspheres, and circular rings of spheres, with either monodisperse or polydisperse monomers, can be approximated by an equivalent coated sphere with identical volume and average projected area. Based on this approximation, the spectral complex index of refraction of unicellular dumbbell-shaped cyanobacterium Synechocystis sp. was retrieved from experimental measurements of its average absorption and scattering cross-sections and size distribution.

A new sensor technology based on optical diffraction of visible light shows promise for sensing metal ions and other species that employ chemically-responsive metal oxide and conducting polymer grating elements. These materials undergo reversible redox processes upon interaction with a chemical analyte that subsequently induces changes in the materials refractive index. The two key design parameters of this sensing technique involve preparation of micropatterned sensor elements and the evaluation of appropriate wavelengths for detection of diffracted light. Much of the ability to “tune” a desired sensing response is dictated by the understanding of how factors of size, dimension, crystallinity, morphology, porosity, and heterogeneity influence analyte/sensor interactions (i.e., adsorption, binding, and transport). The effect of composition, structure, and morphology of MoO₃, WO₃, Moₓ W₁₋ₓO₃, IrOₓ and polyaniline grating materials on chemical, electrochemical and optical properties of these systems will be examined by a range of spectroscopic and electrochemical techniques. Comprehensive evaluation and correlation of materials’ optical properties to diffraction-based detection will advance understanding of the capabilities and limitations for the diffraction-based sensing methodology. This information can then used to determine optimal sensing parameters to improve detection limits, enhance sensitivity and increase the dynamic range for detection of model analytes.

A new sensor technology based on optical diffraction of visible light shows promise for sensing metal ions and other species that employ chemically-responsive metal oxide and conducting polymer grating elements. These materials undergo reversible redox processes upon interaction with a chemical analyte that subsequently induces changes in the materials refractive index. The two key design parameters of this sensing technique involve preparation of micropatterned sensor elements and the evaluation of appropriate wavelengths for detection of diffracted light. Much of the ability to “tune” a desired sensing response is dictated by the understanding of how factors of size, dimension, crystallinity, morphology, porosity, and heterogeneity influence analyte/sensor interactions (i.e., adsorption, binding, and transport). The effect of composition, structure, and morphology of MoO₃, WO₃, Moₓ W₁₋ₓO₃, IrOₓ and polyaniline grating materials on chemical, electrochemical and optical properties of these systems will be examined by a range of spectroscopic and electrochemical techniques. Comprehensive evaluation and correlation of materials’ optical properties to diffraction-based detection will advance understanding of the capabilities and limitations for the diffraction-based sensing methodology. This information can then used to determine optimal sensing parameters to improve detection limits, enhance sensitivity and increase the dynamic range for detection of model analytes.

The nature of the electronic states of charge carriers and the origin of metal to insulating (MI) transition, in highly interconnected conducting polymer nanostructure network, notably Polyaniline nanofibers (PAN-N), were determined via temperature dependent DC conductivity, optical reflectance (300-50,000 wavenumbers), electron paramagnetic resonance (EPR) and X-ray diffraction probes. The nanostructured network has a room temperature (RT) conductivity of ~ 1 S/cm, similar to that of conventional disordered Polyaniline in the Emeraldine Salt (ES) form, but this value is small when compared to the Mott minimum for metallic conductivity (~ 100 S/cm). Therefore, the signature of the temperature dependent DC conductivity of PAN-N should be dominated by phonon activation, with very little or no metallic contribution. In fact, the signature of the charge transport of PAN-N films shows a large metallic contribution from RT to ~ 235 K, with the change in T-dependent conductivity from RT to the peak of the maximum conductivity ~ 200%. We attribute this large metallic contribution to a “mechanically-induced” crossover from metallic to insulating behavior, due to the “fragile” nature of the conductance at interfiber interfaces. By mechanically modifying the nanostructure morphology via applied pressure, the signature of the charge transport resembles that of conventional disordered Polyaniline, having a broad MI peak and moderate change in conductivity from RT to the peak conductivity. The reduced energy activation of PAN-N has a negative slope at low temperatures, which suggest that the charge carriers are localized by disorder. Similarly, the dielectric functions for all measured temperature reveal that the charge carriers within the network are localized. EPR measurements show a temperature dependent Pauli susceptibility between 300 K and 130 K, and below 130 K, we see the onset of a Curie-like contribution to the magnetic susceptibility. The nanostructured network has a low magnetic susceptibility, dominated by a weak Curie component, with the density of Curie spins of ~ 1 spin per 200 (2-ring) repeat unit. This suggests that a significant fraction of the spins are paired up as bipolarons, implying that most of the charge carriers are localized. Structural studies indicate that the nanostructure films are ~ 50 % crystalline with a coherence length of ~ 2 nm. This coherence length is similar to the values reported earlier for conventional disordered Polyaniline with higher conductivity. This suggests that the nature of conductance within the interfiber interfaces affects the effective conductivity of the network. The data of other charge dynamics including optical, magnetic, and structural probes suggest that the role of interfiber contacts within the network contributes largely to the “metallic” signature of the. We conclude that the MI behavior is due to the “fragile” nature of the conductance at the nanostructure interface, while disorder and localization dominate the charge dynamics.

The optical characteristics of single and multilayer III-V nitride thin film systems deposited by RF magnetron sputtering onto different substrates have measured and analyzed. Spectroscopic Ellipsometry measurements of amorphous (Al, Ga, In) N single layers are carried out at two angles of incidence, 70° and 75°, over the wavelength range 300 – 1400 nm. The measured ellipsometric data are fitted to models consisting of air / a-AlN / c-Si (111), air / roughness / a-GaN/ c-Si (111), and air / roughness / a-InN / c-Si (111). The roughness is modeled using the Bruggeman effective medium approximation, assuming 50 % a-GaN (or a-InN) and 50 % voids. The optical constants and the thicknesses of a-AlN are obtained by analysis of the measured ellipsometric spectra through the Cauchy–Urbach model whereas the optical constants and the thicknesses of a-(Ga, In) N are determined using the Tauc-Lorentz model. Analysis of the absorption coefficient of a-AlN (in the range 200 – 1400 nm) and a-GaN show the optical bandgap to be 5.82 ± 0.05 and 3.38 ± 0.05 eV, respectively. While the analysis of the absorption coefficient of a-InN shows the optical bandgap energy to be 1.68 ± 0.071 eV. These values are confirmed using different optical methods such as spectrophotometry, photoluminescence and polarized absorptivity. From the angle dependence of the p-polarized reflectivity we deduce Brewster and principal angles of the single layers. Measurement of the polarized optical properties reveals a high transmissivity (70 % – 95 %) and low absorptivity (< 18 %) for all three thin films in the visible and near infrared regions. X - ray diffraction analysis verifies the amorphous nature of the (Al, Ga, In) N films. This work also reports the successful growth of sputtered bilayer and multilayer antireflection coatings on quartz using alternating layers of AlN and InN. The designed layers are optimized at 500 nm and 50° – 70° incidence for their performance as a longwave-pass filter and coatings for solar cells with high efficiency by varying the number of periods of AlN / InN multilayer systems. The XRD patterns of these systems reveal a polycrystalline structure with a maximum diffraction peak at 31.2° (InN (002)).