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Optical devices play a vital role in modern technology facilitating communication, sensing, imaging, and energy conversion across various disciplines. These devices rely on strong interaction between light and the underlying materials to achieve control of light at nanoscale. The conventional dielectric materials, including Si and InP, although widely used, are limited by their optical material properties and can hardly further induce stronger light-matter interactions for enabling better device performance.Emergent layered van der Waals materials exhibit unique optical properties and thus can offer opportunities for building more advanced photonic devices. In particular, light can efficiently couple with the intrinsic material resonances to form half-light, half-material hybridized states, yielding a greatly enhanced light-matter interaction. My dissertation project focuses on understanding how the coupling between light and material resonances modifies the optical responses in nano-structures made of van der Waals materials, especially transition metal dichalcogenides and hexagonal boron nitrides, and the profound implications it brings to the device performance.By combining theoretical analysis with experimental characterization techniques, in this thesis we examine the dynamics of light-matter interactions within optical nanostructures made of bulk TMDCs and hBN, especially in the vicinity of intrinsic material excitations. We provide a numerical modelling of key integrated photonic components made of bulk form of layered van der Waals semiconductors, paving the way for using a more advanced material platform to complement today’s Si and InP photonics technologies. Moreover, we also theoretically examine the limits of optical confinement in van der Waals materials. Due to the coupling between light and intrinsic excitons, these materials possess strongly modified dielectric permittivity, allowing light to be efficiently confined to very small spots; well beyond diffraction limit with greatly concentrated optical fields. These results are of paramount interest for creating more advanced integrated optical systems with higher integration density and more efficient electro-optic modulation.In addition to theoretical analysis, we also adopt state-of-the-art material characterization, nanofabrication, and optical measurement techniques to experimentally demonstrate the functional nanophotonic devices made of multilayer van der Waals materials in both near- and mid-infrared bands. The nanofabrication processes we develop allows creating compact, deeply subwavelength nanostructures out of exfoliated van der Waals material thin films with smooth, less tilted sidewalls compared to previously reported work. Furthermore, we use far- and near-field optical characterization techniques to examine the functionalities realized by these novel van der Waals photonic devices.The findings in this dissertation work can provide a guideline for designing, fabricating, and characterizing advanced optical systems made of novel van der Waals materials, paving the way to achieving high precision optical sensing, nanoscale opto-mechanics, small footprint optical interconnects, and tunable thermal radiation and imaging.

Micro-light-emitting diode (microLED, micro-LED, μLED) is an emerging display technology based upon high-density arrays of micron-sized III-nitride (or III-V) LEDs. Small form factor displays, including wearables (smartwatches) and AR (augmented reality) glasses will benefit most from microLED. The paradigm shift from LEDs for solid-state lighting to microLEDs for displays introduces new research challenges. MicroLEDs become difficult to process as dimensions reduce. Mesa diameters as small as 1 μm are desirable for maximum resolution and small sizes in AR displays. However, layer-to-layer alignment errors during lithography prevented researchers from fabricating microLEDs below 5 μm when using standard LED processing approaches. This thesis presents a self-aligned processing technique developed to circumvent these issues. The result is the smallest microLED ever published at 1 μm diameter. This milestone enables new microLED device research at the size ranges most targeted for AR displays. Due to the different usage and requirements for microLEDs in displays versus LEDs in lighting, new measurement systems for characterizing device performance were constructed. Key considerations when characterizing on-chip microLED electroluminescence are discussed. As dimensions are reduced in microLEDs, new size-dependent physical effects appear. It is well known that microLED efficiency decreases as the diameter shrinks due to increased non-radiative surface recombination. However, results comparing red, green, and blue microLEDs show increasing the indium fraction in InGaN limits surface recombination through enhanced carrier localization. An added effect in small microLEDs is improved extraction efficiency from sidewall reflections redirecting photons within the extraction window. To separate competing size-dependent contributions from sidewall recombination and sidewall reflections, optical models are necessary. A combined approach uses FDTD to model interactions with near-wavelength-sized patterned sapphire features beneath the mesa.

Thermal infrared radiation, conventionally defined as the band from 7-14um, is an important waveband for a variety of applications in radiative cooling and thermal imaging. Due to its broadband character, traditional resonant photonic phenomena's typically extremely narrow band, high quality factor behaviors are not generally applied to the thermal wavelengths. Here we show a variety of ways in which conventionally high quality factor, narrow band resonances can be augmented for effective use over broadband regimes. This theme is demonstrated in the cases of relativistic lightsails, thermal infrared optics, and solar cell module glass. I show how conventional nanophotonic structures can be applied to these systems to derive benefits such as improved radiative cooling performance, increased interfacial transmission efficiency, and increased photovoltaic module power generation efficiency and longevity.

The study of optical materials for extreme environments is a rapidly growing field, with a number of possible applications from space science to energy efficiency to defense. In order to meet the growing demand for optical devices which can operate at very high temperatures and in different environments, we must first characterize the optical properties of different materials which might be used in the design of such devices at these elevated temperatures. Given their high-temperature stability, this makes refractory materials the ideal candidates for such a study. Refractory materials have previously been analyzed in great detail for their high-temperature structural and chemical stability. However, their usage as optical materials has been comparably limited.In this thesis, I begin to analyze the optical properties of multiple classes of refractory materials: refractory metals and their oxides, refractory nitrides, and refractory carbides. In addition to analyzing their optical and chemical responses to high-temperature treatment, I present in situ high-temperature data up to 1,500°C, which are used to analyze their stability–or lack thereof–at very high temperatures. This thesis will also study multiple possible applications of each structure, including structural color, superabsorption, and thermophotovoltaics.Structural color is used in a variety of applications, from conformal coatings for spacecraft, to sensing, to anticounterfeit technology. However, in many cases, the structural color systems proposed are either too complicated to fabricate making them difficult to scale for large-scale production, or would not be able to survive in the extreme environments required by some of these applications. We propose a simple multi- layer structure composed of a refractory metal and its oxide on silicon, in which the oxide is grown via high-temperature treatment in an oxygen-rich environment. During this high-temperature treatment (up to 600C) we measure the samples’ optical properties in situ using ellipsometry, which can give us very precise control over the thickness of the generated oxide layer and thus the generated color of the structure. In addition to our experimental results for three refractory metals (Ru, Ta, and W), we present simulated results demonstrating the full range of possible colors achievable with these metals and their oxides.Many technological applications in photonics require devices to function reliably under extreme conditions, including high temperatures. To this end, materials and structures with thermally stable optical properties are indispensable. State-of-the-art thermal photonic devices based on nanostructures suffer from severe surface diffusion-induced degradation, and the operational temperatures are often restricted. Here we report on a thermo-optically stable superabsorber composed of bilayer refractory dielectric materials. The device features an average absorptivity 90% over >500 nm bandwidth in the near-infrared (NIR) regime, with minimal temperature dependence up to 1,500°C. Our results demonstrate an alternative pathway to achieve high-temperature thermo-optically stable photonic devices.As the impacts of extended fossil fuel use are felt by our environment, economy, and public health, it becomes increasingly crucial to minimize our dependence on fossil fuels in favor of reusable, clean energy generation techniques. This requires the optimization of efficiency for each technique, thus requiring the minimization of waste heat. One method through which we can recover this waste heat is using thermophotovoltaics (TPV): converting the waste heat into usable electricity utilizing a thermal emitter and a photovoltaic cell. We propose a simple bi-layer SiC/AlN structure as a broadband thermal emitter for thermophotovoltaic applications, demonstrating ex situ and in situ its optical stability in temperatures up to 1,200°C in air and 1,500°C in inert environments. Our results demonstrate a scalable, broadband thermal emitter which can be applied for waste heat recovery across a myriad of high-temperature energy generation processes.

Polycrystalline materials have shown promise in advanced optical applications becauseof their unique optical and mechanical properties. As transparent polycrystalline materials becomemore important in optical applications, evaluation of their transmission across a wide range ofwavelengths is crucial to device design. Most polycrystalline materials have non-uniform opticalproperties due to residual porosity, secondary phases, and/or crystalline anisotropies (e.g.birefringence). To better understand and describe transmission in these materials, an in-linetransmission model is developed that incorporates reflection, scattering, and absorption losses.The model was fit onto transmission data from three polycrystalline material systems: ruby, yttriastabilized zirconia, and terbia. Parameters extracted from these fits can be used to describewavelength dependent transmission with one simple analytical expression. The fit can also beused to decouple absorption form scattering, allowing for the extraction of important propertiessuch as absorption coefficients. Another unique challenge to modeling transmission by transparent ceramics is modelingthe scattering losses from birefringence. Even in the case of a single-phase polycrystal withnegligible porosity, birefringence scattering will always be present whenever the crystal isanisotropic (non-cubic). Multiple models for birefringence scattering have been suggested in theliterature that are generally successful in describing scattering loss in specific material systems.However, these models do not readily extrapolate to new material systems and their connectionto birefringence scattering are never fully justified. We remedy this by deriving from Maxwell’smicroscopic equations a first-principles model of birefringent scattering that is applicable to anysingle-phase, unaligned transparent polycrystal. This is achieved by first developing a frameworkfor describing light propagation in dielectric solids by perturbing Maxwell’s equationsThese expressions are then used to develop a birefringence scattering model in ceramics.The derivation culminates in an equation which describes the birefringence scattering coefficientspectra using the single-crystal refractive index tensor and a representative polished surfacemicrograph. This model should be useful for designing materials and characterization. Forexample, by generating hypothetical transmission for a polycrystal with varying grain size or bycharacterizing average grain size using a transmission measurement.

In the past few years, there has been a growing requirement for the development and innovation of MicroLED displays that can outperform Liquid Crystal Displays (LCDs) or Organic Light-Emitting Diodes (OLEDs) in terms of dynamic color range, contrast ratios, and brightness. This increased demand is mainly driven by the emergence of new application areas, such as wearable Virtual Reality (VR), Augmented Reality (AR) devices, Head-Up Displays (HUDs), and advancement in Optogenetics. Various companies have presented prototype MicroLED displays primarily in the form of large displays and smartwatches. However, there are still several challenges that need to be overcome before these displays can be commercially available. One of the technical challenges faced by the MicroLED displays is optical crosstalk. In this study, a method has been developed to quantify the optical crosstalk in the Quantum Dot-based flexible MicroLED display. The effect of features such as the gap between the MicroLED and the Q.D. layers, pixel dimensions, pitch length, and misalignment distance on the overall optical crosstalk of the display has been analyzed. Experimental analysis of optical crosstalk of MicroLED display with different features has been done to validate the ray-tracing model developed. Comprehensive analytical and experimental studies of the optical crosstalk have been done in this work.

In recent years, Augmented Reality (AR) glasses have emerged as groundbreaking wearable devices, enabling a harmonious integration of virtual information with the real world. However, challenges remain in enhancing the optical systems that underpin AR glasses, including resolution and field of view limitations. This research focuses on leveraging metasurfaces, composed of subwavelength nanostructures, to design and optimize advanced optical components for AR glasses. By tailoring the properties of metasurfaces, such as amplitude, phase, and polarization control, we aim to overcome limitations and enhance the resolution, field of view, and overall visual performance of AR glasses. To achieve truly immersive AR experiences, improving the optical performance is essential. This involves minimizing aberrations, enhancing contrast, reducing glare, and optimizing light transmission efficiency. Metasurfaces offer unique capabilities for precise manipulation of light at the nanoscale, enabling the creation of highly efficient and compact optical components. By integrating metasurface-based elements, such as gratings, lenses, beam splitters, and polarization controllers, we can enhance resolution, expand the field of view, and improve overall optical performance.Furthermore, metasurfaces offer multifunctionality and the potential for dynamic control. By combining multiple functionalities within a single metasurface-based element, the complexity of optical systems can be reduced, leading to more compact and lightweight AR glasses. Additionally, incorporating tunable or switchable elements within metasurfaces enables real-time control of optical properties, allowing for adaptation to changing environmental conditions and advanced functionalities like adaptive focus and depth perception.This research also explores the challenges of small eyebox in Maxwellian view systems, which restricts user head movement and comfort. Innovative methods utilizing input angle modulation at the metasurface optical element (MOE) are proposed to increase the eyebox size while maintaining optical performance. The study includes the design and optimization of metasurface gratings, algorithm development for optimizing optical elements, implementation of a high-resolution full-color prototype, and investigation of imaging techniques like ultrafast light field tomography (LIFT).By combining these research efforts, this study aims to significantly advance the design and optimization of metasurface-based diffractive waveguide systems for AR applications. The outcomes have the potential to revolutionize the field of augmented reality, pushing the boundaries of optical resolution, addressing limitations, and enhancing the overall user experience. The findings contribute to the advancement of AR glasses, enabling new possibilities in industries such as education, entertainment, healthcare, and beyond.

Extreme light-matter interaction results in a variety of fascinating nonlinear optical effects, including the generation of new light frequencies, optical wave mixing, the optical Kerr effect (OKE), the formation of optical solitons, and the generation of entangled photon pairs, among others. However, conventional materials typically exhibit weak nonlinear optical responses. Longer light-matter interaction lengths are often required to observe significant nonlinear optical properties, resulting in bulky devices. However, the current trend in nonlinear optics is to create ultra-compact and energy-efficient nonlinear optical devices for practical applications, which would necessitate the discovery of a nano-scale optical material with an exceptionally high nonlinear optical response. Therefore, the ultimate goal is to develop an ultrathin material with extraordinarily high nonlinearity and tunability.Recently, highly doped transparent conducting oxides with vanishing real parts of electric permittivity, or epsilon-near-zero (ENZ) materials, have shown strong optical field confinement in subwavelength dimensions. This extreme field confinement is possible due to the excitation of plasmon-polariton modes, known as ENZ modes. Extreme light-matter interactions in these nanoscale ENZ thin films lead to enhanced nonlinearity with ultrafast (sub-picosecond) recovery time. The main goal of this thesis is to study the effect of the excitation of ENZ modes on the nonlinearity of ENZ thin films.This work seeks to understand the nonlinearity of ENZ thin films by nano-engineering their linear optical properties. By precisely controlling the linear optical properties and thickness of ENZ thin films, I could optimize field confinement within the ENZ materials. Then, the nonlinear mechanisms were investigated by examining the abnormally enhanced higher-order nonlinear effects on the refractive index, which prior studies have neglected. By implementing an advanced hydrodynamic model, I investigated the intensity-dependent absorption and its ultrafast nature in ENZ thin films. I demonstrated and explained the effect of the saturable behavior of the nonlinear absorption coefficient. Finally, I investigated the ultrafast nature of the nonlinearity and its dependence on ENZ mode excitation.This study provides insight into the illusive behavior of epsilon-near-zero nonlinearity. The results presented in this thesis will aid in developing and integrating epsilon-near-zero materials in sophisticated, ultra-compact, energy-efficient, ultrafast nonlinear optical devices.

Condensates are a hallmark of emergence in quantum materials with superconductors and charge density waves as prominent examples. Excitonic insulators (EI) present an intriguing addition to this library, exhibiting spontaneous condensation of electron-hole pairs in the vicinity of an insulator-semiconductor phase transition. Across this library, canonical condensate observables can be obscured through parasitic coupling to various degrees of freedom. Broadband terahertz (THz) spectroscopy can disentangle such obscurants through temperature and fluence dependent measurements of the quantum dynamics. We target Ta_2 NiSe_5 (TNS) a putative room-temperature EI where electron-lattice coupling complicates the identification of excitonic correlations. The first three chapters cover a concise background of strongly correlated materials, the power of ultrafast spectroscopy as a tool to unveil novel physics of condensed matter systems, properties of TNS as an EI, experimental techniques built and developed by myself. In the following chapter, I report the optical signatures of exciton condensate dynamics in TNS where a pronounced increase in the THz reflectivity manifests following photoexcitation showing a BEC-like temperature dependence. This occurs well below the structural transition temperature (326K), suggesting a novel approach to monitor exciton condensate dynamics stripped of residual static interactions. Nonetheless, dynamic condensate-phonon coupling remains as evidenced by peaks in the enhanced reflectivity spectrum at select infrared active phonon frequencies. Indeed, the origin of the parametric reflectivity enhancement arises from phonon squeezing upon condensate perturbation, validated using Fresnel-Floquet theory and DFT calculations. Our results highlight that coherent order parameter dynamics can, in the presence of signal photons, drive parametric stimulated emission with concomitant possibilities that includes novel entangled photon sources at terahertz frequencies.

In the fascinating family of two-dimensional (2D) materials, layers of atoms are bonded strongly into sheets that are held together by weak van der Waals bonds. Consequently, atomically-thin layers of various 2D materials can be isolated into monolayers and stacked together to form synthetic materials with emergent electronic and optical properties. The behavior of a 2D stack is determined by the behavior of electrons within each layer and interactions between the layers, which provides remarkable opportunities for creativity and control in electronic material design. In the past few years, a unique and exciting prospect for 2D stacks has emerged: the ability to tune the twist angle between atomic layers. With a small twist angle or lattice mismatch, the interference of two atomic lattices produces a large periodic pattern, called a moiré superlattice, where the alignment between atoms varies periodically across the system. Recently, it has become clear that moiré superlattices can dramatically modify the electronic properties of a 2D stack, resulting in novel states of matter, modified light-matter interactions, and more. In this dissertation, I focus on moiré superlattices formed from WSe2 and WS2 monolayers, which are both members of the semiconducting transition metal dichalcogenide (TMD) family known for their strong light-matter interactions and promise for optoelectronic applications. I begin with an introduction to semiconducting TMDs, which guides our understanding of TMD stacks and, in particular, moiré superlattices. I then describe several experimental studies highlighting how a simple interference pattern can produce fascinating electronic and optical behavior. First, I consider how a moiré superlattice modifies the excitonic properties of TMDs, where our combined experimental and theoretical investigations reveal exciton states unique to superlattices. From there, I describe how moiré superlattices can lead to novel correlated states of matter by shifting the balance between the kinetic energy and interaction energy of electrons. Using a new experimental approach, we reveal the presence of generalized Wigner crystal states in TMD moiré superlattices thanks to the enhanced influence of electron-electron interactions. Finally, I demonstrate an optical technique for measuring spin transport in TMD moiré superlattices that provides insight into how spin behave in correlated 2D materials.

A central challenge in contemporary materials and photonics research is understanding how intrinsic materials properties can be optimally combined with nano- or micro-scale structuring to deliver a target functionality. By leveraging subwavelength nanostructures and the intrinsic dispersion of constituent materials, tailored changes in the amplitude and phase of incident wavefronts can be precisely engineered, along with desired spectral characteristics. However, our ability to meet increasing demands in the performance of photonic structures faces roadblocks due to the complexity of the materials and structural design spaces that are currently accessible. Conventional optimization methods, which rely on numerical simulations that solve Maxwell’s equations, have shown remarkable capabilities in designing nanophotonic structures and are now commonly used. However, they can be computationally costly and are often intractable for large-scale designs or high-dimensional design spaces. As a result, data-driven approaches based on machine learning (ML) have been extensively explored in order to tackle challenging photonics design problems. To this end, this work explores the application of various advance deep learning methods for the design and characterization of nanophotonic materials and structures.

Advances in life sciences in recent decades have revolutionized our understandingof biochemical and biophysical interactions associated with diseases and disorders of thehuman body. This newly acquired knowledge has fueled intense interest in a range ofbiotechnological strategies that can improve health outcomes, spanning from biosensors todrug development to tissue engineering. There is thus an increasing need for even betterunderstanding of biomolecular interactions at nanoscale to explore new medical frontiers,and for powerful analytical tools of increasing complexity and diversity in multiplexedbioanalysis. Surface plasmon resonance (SPR) is a core optical spectroscopic principle inthe bioanalytical sphere, and its label-free methodology for bioassays has been broadlyapplied in drug discovery, medical diagnosis, and environmental monitoring. Advances inmaterials sciences, however, have provided new opportunities for re-invention of thetechnique and expansion of the range of analyses by SPR. The aim of this dissertation is todevelop and improve the fundamental technological diversity of SPR based techniques forenhanced biosensing applications.The main strategy for the development takes the form of integrating novelmethodologies and new materials to the SPR bioanalytical workflows. First, an orthogonalanalytical platform was developed by combining SPR/SPR imaging with matrix-assistedlaser desorption ionization mass spectrometry (MALDI-MS). A multistep functionalizedplasmonic microarray was developed into a new mode (SPR-MALDI) for the sensitivedetection of bacterial toxin proteins in complex environmental matrices. The combinationof the techniques allowed for both quantitative determination and unambiguous qualitativeidentification of biological identity of the target. Second, SPR techniques were integratedwith three-dimensional (3D) printing for enhancing analytical performance. A novel hybrid3D printing and PDMS molding process was developed that overcomes fundamentalresolution limits of the 3D printed optical components for spectroscopy. Prisms of multiplegeometries were fabricated that demonstrated surface roughness comparable tocommercial, glass-based components, providing economical alternative while yieldinghigh sensitivity towards SPR biosensing of protein targets. Finally, we have developed ahigh performing SPR platform based on a more fundamental shift, switching the plasmonicmaterial from gold to aluminum. Al thin films under Kretschmann configurationdemonstrated a 60 % higher optical sensitivity in imaging mode and reduced surfacefouling by 75 %. They proved excellent substrates for array-based chemical surfacemodifications by ionic polymers that were further employed for successful analysis ofurine-based chemokine biomarkers. The work presented here should pave the way for morecomplex modalities in developing the next generation of biotechnology.

Quantum materials are a growing field of interest in condensed matter physics and materials science due to their manifestation of quantum mechanical effects on the macroscale. Interactions between the charge, spin, lattice, and orbital degrees of freedom, combined with topology and often reduced dimensionality lead to the emergence of electronic ground states which defy understanding in terms of classical physics. Within the field of quantum materials, particular interest is placed on unconventional superconductors as possible candidates for use in quantum information science. Considerable inquiry is focused on the pairing mechanism in unconventional superconductors, as well as interplay between superconducting states and adjacent low-temperature states such as charge density wave (CDW) order and ferroelectricity. This thesis focuses on optical characterization of the quantum materials SrTiO3 and CsV3Sb5, both unconventional superconductors. I will discuss the methods used to characterize these materials: second harmonic generation (SHG) and pump-probe spectroscopy. The results I have obtained clarified interesting and previously ambiguous phenomena in these materials: SHG techniques supported a connection between ferroelectricity and superconductivity in SrTiO3, pump-probe spectroscopy explained the origin of CDW order in CsV3Sb5.

Rapid growth of plasmonics and nonlinear optics in recent decades has driven a new demand for materials having simultaneously large optical responses and excellent robustness to extreme environments. Adding to this challenge, useful applications of plasmonic and nonlinear optical devices for nanophotonics require that not only the previous criteria be met, but highly controllable materials fabrication over minute length scales must be achievable as well. These increasingly stringent requirements can be met with low loss metallic nanofilms and nanocomposites of some noble metals like Ag and Au, and some alternative plasmonic materials such as transition metal mononitrides (TMNs) like TiN and ZrN. Desire for the attainment of new plasmonic devices with low losses and large nonlinear responses has shifted interest into the quantum-size regime. This dissertation explores means of manipulating the chemistry, defects, geometry, and dimensionality of these quantum-size metals in order to achieve materials with desirable, tunable properties. In these explorations, optical, structural, and chemical characterizations are jointly employed with advanced fabrication techniques to innovate and optimize new materials, and to fill in contemporary and technologically-important gaps. Pushing the physically-fabricable limits of the quantum materials and the limits of their crystalline order is the transdimensional regime where film properties are massively tunable with small changes to a film's minute thickness. Passing some kinetic limitations on the fabrication of quantum-size Au films, TMNs can be grown down to the atomic scale and not only their crystal structures but their dimensionality as well can be controlled with choice of growth conditions, substrate, and environment. The investigations given in this dissertation represent efforts to push the boundaries of what real materials can do by combining simulation and experimentation to predict and demonstrate how optical properties in metallic nano- and quantum-sized materials can be tuned with asymmetric structuring, dimensionality, chemistry, defect engineering, and chemistry. The nano- and quantum-sized metals realized in this work reduce barriers to attaining gigantic, efficient, and low-loss light-matter interactions and present new means of achieving such.

Introduction: Functional near-infrared spectroscopy (fNIRS) is an imaging method in which a light source and detector are installed on the head; consequently, the re-emission of light from human skin contains information about cerebral hemodynamic alteration. The spatial probability distribution profile of photons penetrating tissue at a source spot, scattering into the tissue, and being released at an appropriate detector position, represents the spatial sensitivity. Methods: Modeling light propagation in a human head is essential for quantitative near-infrared spectroscopy and optical imaging. The specific form of the distribution of light is obtained using the theory of perturbation. An analytical solution of the perturbative diffusion equation (DE) and finite element method (FEM) in a Slab media (similar to the human head) makes it possible to study light propagation due to absorption and scattering of brain tissue. Results: The simulation result indicates that sensitivity is slowly decreasing in the deep area, and the sensitivity below the source and detector is the highest. The depth sensitivity and computation time of both analytical and FEM methods are compared. The simulation time of the analytical approach is four times larger than the FEM. Conclusion: In this paper, an analytical solution and the performance of FEM methods when applied to the diffusion equation for heterogeneous media with a single spherical defect are compared. The depth sensitivity along with the computation time of simulation has been investigated for both methods. For simple and Slab modes of the human brain, the analytical solution is the right candidate. Whenever the brain model is sophisticated, it is possible to use FEM methods, but it costs a higher computation time.

In this study, we demonstrate the use of Stokes polarimetry in combination with polarized light microscopy as a powerful tool in the characterization of anisotropic polymeric materials including collagen and poly (ϵ-caprolactone) fibers. Structural anisotropy has been shown to translate to optical anisotropy in the form of birefringence. We demonstrate that polarized light microscopy and Stokes polarimetry can be used to enhance the visualization of fibrous structures that are not ordinarily accessible by conventional microscopy techniques, by leveraging the birefringence of the material. With polarized light microscopy, we show the existence of birefringent Maltese cross structures in collagen synthesized at 22 0C and perform structural analysis of the system using Stokes polarimetry. The highly fibrous microstructural arrangements observed by polarized light microscopy and Stokes polarimetry for collagen synthesized at 40 0C and poly (ϵ-caprolactone) fibers can provide insights into the kinetics of polymerization. We also observe a change in birefringence due to the effect of strain on super elastic poly (ϵ-caprolactone) fibers. Overall, we demonstrate that combining Stokes polarimetry and polarized light microscopy has the potential to provide quantitative, point by point structural analysis of fibrous biomaterials. This study holds potential to provide insights into the kinetics and mechanics of disease progression and polymer synthesis.

Optical absorption driven by intrinsic material loss is often viewed as a challenge to be overcome in photonic and metamaterials-based systems. In the context of applications such as optical communication, reducing absorption remains a key goal to maximize optical transmission. However, for many other applications related to sensing and detection, as well as heat transfer and energy, absorption and emission are essential. The overarching goal of this thesis is about taking advantage and embracing the loss in materials. Sometimes we have to lose to “gain”.In Chapter 1, we leveraged the loss in the epsilon near zero (ENZ) materials to build a broadband directional thermal emitter for the first time. We introduced and experimentally realized gradient epsilon-near-zero (ENZ) materials that enable broad spectrum directional control of thermal emission by supporting leaky electromagnetic modes that couple to free space at fixed angles over a broad bandwidth. We experimentally demonstrated two emitters consisting of multiple metal and semiconductor oxides in a photonic configuration that enable gradient ENZ behavior over long-wave infrared wavelengths. The structures exhibits high average emissivity (> 0.6 and > 0.7) in the p polarization between 7.7 and 11.5�m over an angular range of 70�-85�, and between 10.0 to 14.3�m over an angular range of 60�-75�, respectively. Outside these angular ranges, the emissivity reduces to 0.4 and lower for angles smaller than 50� and 40� respectively. The structures’ broadband thermal beaming capability enables radiative heat transfer only at particular angles and is experimentally verified through direct measurements of thermal emission. By decoupling conventional limitations on angular and spectral response, our approach opens new capabilities for applications such as thermal camouflaging, solar heating, radiative cooling and waste heat recovery.In Chapter 2, we further expanded our gradient ENZ thin film idea using doped III-V semiconductors to realize more continuous and precise control for the directional thermal radiation. Both experimentally and numerically we proved that, by increasing the total thickness of the gradient ENZ thin film made by Si-doped InAs, the high emissivity angle will move towards normal incidence. Also, by changing the doping level of the doped InAs film, we observed the functional bandwidth of the sample changes to a different range, which makes doped III-V semiconductors a more flexible directional thermal emitter candidate.In Chapter 3, we explored the beauty of loss in photonic crystals: in particular non-Hermitian photonic crystal where two materials only have a difference in the imaginary part of the permittivity. We found counter-intuitive behavior in this under-studied class of system, including loss-driven reflection. We characterized the band structure as well as the reflection response in both 1D and 2D non-Hermitian system. By introducing a lossy material in the crystal, a quasi-bandgap purely induced by loss opens up at the band edge. A sharp reflection peak is also observed within the quasi bandgap in both 1D and 2D non-Hermitian photonic crystals we proposed. In the end, we designed a selective reflector which consists a traditional 2D photonic crystal waveguide and a non-Hermitian photonic crystal. This selective reflector can guide light along the waveguide and have the light absorbed by the non-Hermitian part while only the light with certain wavelength can be reflected back, which distinguishes this selective reflector with the traditional reflectors such as a Bragg mirror.Finally in Chapter 4, lossy materials such as conductive polymers can potentially enable a revolution in building energy efficiency by dynamically controlling their thermal emissivity. We proposed a mechanism to decouple the mean radiant temperature from actual temperatures of interior surfaces by dynamically tuning the thermal emissivity of interior building surfaces. We quantitatively evaluated how much impact it will have on building energy saving in a building scale thought out a whole year. We showed that in cold weather, setting the emissivity of interior surfaces to a low value (0.1) can decrease the setpoint as much as 6.5�C from a baseline of 23�C. Conversely, in warm weather, low emissivity interior surfaces result in a 4.5�C cooling setpoint decrease relative to high emissivity (0.9) surfaces. EnergyPlus calculation shows that by implementing the tunable emissivity interior, more than 30% of the cooling and heating energy can be saved year-round in different climates.

III-nitride microLEDs have gained much attention as a replacement for organic-LED (OLED) and liquid crystal (LCD)-based displays due to the former's tunable bandgap, high defect tolerance, long lifetimes and superior efficiency. High pixel density, next generation displays also demand efficient and low cost red-green-blue (RGB) pixels with lateral dimensions below 5 µm, for which OLED and LCD platforms become intractable. Unfortunately, nitride-based LEDs become inefficient as device dimensions shrink due to nonradiative surface states, such as point defects and dangling bonds, which are largely introduced during plasma-based device patterning. Due to high surface-area-to-volume ratios, these effects become ever more important for microLEDs. In addition to the surface recombination loss problem, there are challenges related to growing high quality, high In-content InGaN for full color RGB displays, most notably for red emitters. The 11% lattice mismatch between InN and GaN leads to reduced crystal quality. Moreover, large piezoelectric fields in the III-nitrides reduce the radiative efficiency and lead to poor emission characteristics. The goals of this Ph.D. project were to (i) eliminate surface-related nonradiative recombination losses and (ii) develop strategies for mitigating strain in InGaN/GaN LEDs for improved efficiency and emission characteristics. Dielectric passivation and chemical treatments have been used to alleviate surface recombination losses; these approaches, however, have mostly focused on devices with lateral dimensions of 20 µm and larger due to fabrication and optical characterization challenges. A new cleanroom processing scheme and high sensitivity optoelectronic testing system were developed in order to fabricate and test ultrasmall microLEDs in the relevant size ranges. Chemical etching and Al2O3 dielectric passivation were used to minimize nonradiative sidewall defects in InGaN/GaN microLEDs (mesa diameter 2-100 µm). Analysis of the position and shape of EQE curves for all devices suggested size-independent carrier recombination rate coe_cients, suggesting an enhancement in the light extraction e_ciency (LEE). These extraction bene_ts had not been experimentally observed before due to overwhelming surface recombination losses and/or large device dimensions. Ray tracing simulations determined that reducing the mesa diameter increased internal light propagation directionality, leading to fewer total internal reection (TIR) losses, thus confirming the experimental trend of higher EQE for small devices—a trend that is atypical. Given that device performance may actually increase with decreasing mesa diameters when surface losses are minimized, new device applications become viable. One application is sub-micron LEDs as a post-growth, top-down method of strain mitigation, which has been theorized to have many benefits, but in practice has always been limited by surface losses. When mesa dimensions are reduced to the sub-micron scale, the average strain state in InGaN/GaN structures may be relieved. However, the mesa diameter-strain relationship is not well known. To investigate this relationship, strain in InGaN/GaN multiple-quantum well (MQW) light emitters was relaxed through a colloidal lithography and top-down plasma etching platform. The colloidal lithography was performed using Langmuir-Blodgett dip-coating with silica masks (d = 170, 310, 690, 960 nm) and a Cl2/N2 inductively coupled plasma etch to produce nanorod structures. The InGaN/GaN MQW nanorods were characterized using x-ray diffraction (XRD) reciprocal space mapping (RSM) to quantify the degree of relaxation. A peak relaxation of 32% was achieved in the smallest diameter feature (120 nm after etching). Power dependent photoluminescence (PDPL) showed blue-shifted quantum well emission, which suggests via reduced piezoelectric field. The cleanroom process previously discussed for microLEDs (mesa diameter = 2-100 µm) was scaled down to smaller dimensions. A combined electron beam lithography and photolithography approach will be employed to fabricate these devices, which is currently under development. In addition to device processing, optical characterization techniques are being conducted to better understand key properties of sub-micron LEDs. For instance, a reduced piezoelectric field due to strain relaxation is expected to alter the carrier dynamics and ultimately the overall device behavior. In order to isolate the carrier dynamics from overall device EQE characteristics, time-resolved and time-integrated photoluminescence (PL) spectroscopy experiments will be conducted. Specifically, the PL experiments will be used to decouple changes to the radiative and nonradiative recombination rates as the mesa dimensions are reduced. The dielectric passivation and chemical treatments previously developed will continue to be used to minimize surface recombination losses, which not only lower the radiative efficiency but also have the potential to dominate the carrier dynamics. By altering both the temperature and laser pump power (i.e. carrier density) in passivated mesa structures, the various recombination pathway rates may be decoupled from one another.

In recent decades, technological developments in optics, optoelectronics, and wetting phenomena have benefited from the principles behind some of nature’s periodically structured and multifunctional surfaces. Particularly inspiring are the wavelength selectivity of vivid butterfly and bird wings, the light-coupling efficiency of the moth’s eye, and the self-cleaning properties of the lotus leaf. Bridging the technological gaps in structure, function, and design of such functional surfaces requires a simple, geometrically tunable, and high-resolution patterning method to probe the emergent phenomena that arise when materials are heterogeneously arranged at these critical nanoscale dimensions. One compromise between bottom-up scalability and top-down pattern precision comes in the form of close-packed colloidal monolayers, which can be coupled with plasma-based pattern transfer to form highly correlated micron and sub-micron patterns in a variety of substrate materials. This dissertation work focuses on extending this paradigm to hole-array and pillar structures in high-refractive index dielectrics and optoelectronic semiconductors, namely visible-wavelength optical TiO2 nanostructures, micron-scale InGaN-based visible light emitters, and hierarchically structured GaN/Si surfaces for tunable wetting. These were interrogated through a combination of microscopic and spectroscopic experimental methods (SEM, X-ray diffraction, photo/cathodo/electroluminescence, reflectometry, contact angle goniometry) to quantify and elucidate the mechanistic nature of various physical phenomena, which were corroborated and further illuminated by computational work (transfer-matrix and finite-difference time domain [FDTD] methods). First, we computationally investigate the optical response of the micro-periodic, honeycombed glass exoskeleton of the centric diatom microalgae species in its natural aqueous environment. By drying the structure and replacing the top silica slab layer with TiO2 (increasing refractive index contrast), a dramatic increase in electromagnetic field confinement was observed, yielding intense, wavelength-selective reflection at normal incidence owing to strong modal coupling between thin-film and hole-induced interference. The effect of structure geometry (i.e., hole spacing & layer thickness) on optical response was explored and a dimensionless design space was isolated, allowing wavelength-independent design of highly reflective and transmissive surfaces at visible wavelengths. Furthermore, translational pore disorder inherent to the diatom structure transformed this intense, broadband behavior into wavelength-selective, omnidirectional scattering. As a proof of concept, partially suspended TiO2 hole arrays (pitch = 404, 507, and 690 nm) were fabricated with silica colloid deposition, metal masking, and sacrificial Si layer removal. The resulting angle-independent colors spanned the visible spectrum and were in excellent modal agreement with FDTD computations. These optical elements were integrated as hybrid color filters and reflectors in a compact, color-responsive, and quickly recoverable interference-based (Fabry-Pérot microcavity) refractive index sensor. Micron- and nanoscale structuring was also implemented for mitigating problematic defects introduced through lattice mismatch and device processing in long-wavelength III-nitride LED materials. To reduce lattice mismatch between the GaN substrate and InGaN quantum wells (QW), elastic relaxation was encouraged in lattice-expanded crystal growth templates (“buffers”) for subsequent light-emitting layer deposition by etching free boundaries into the material. Accordingly, a In0.06Ga0.94N buffer layer grown with metalorganic chemical vapor deposition (MOCVD) on free-standing (112 ̅2) GaN substrates was patterned with colloidal lithography, forming micro- and nanopillars with aspect ratios of 1:4 and 1:2 (height:diameter), respectively. These patterned buffers showed extensive biaxial relaxation as verified by X-ray diffraction and cathodoluminescence measurements. A suite of microscopic and spectroscopic (SEM, photo/cathodoluminescence) methods were then used to probe the emission quality from subsequently grown QW layers; a longer-wavelength emission profile with orders-of-magnitude defect reduction was observed on patterned buffer structures relative to planar layers.To circumvent non-radiative sidewall defects formed through ion bombardment and implantation during processing of micron-scale LEDs for display and augmented-reality applications, selective-area MOCVD growth of GaN and InGaN layers was conducted on the commercially tractable (0001) c-plane and scientifically interesting (11-22) semipolar orientations. A versatile set of processes was developed for producing colloid-defined SiO2 and Si3N4 growth masks with tunable geometry and fill fraction. After conducting GaN and multi-QW growth out of growth masks with 200 and 2000 nm apertures, the resulting structures were characterized with spatially mapped cathodoluminescence, and emerging crystal facets were assigned based on emission wavelength, facet morphology, and substrate orientation.Finally, drawing inspiration from the waxy, textured surface of the lotus leaf, the dynamic range of wetting states afforded through simple, two-step patterning of GaN and Si surfaces was explored. A critical wetting transition from the impregnated Wenzel state to the hydrophobic Cassie-Baxter state was observed at a 1-μm feature spacing of Si pillars; further sample etching at this transition yielded a unique hydrophobic, all-adhering rose-petal wetting state at an aspect ratio of 4:1, and superhydrophobic lotus-leaf behavior at 10:1 (height:diameter) aspect ratio. Modification of the two plasma etching steps in the patterning process achieved superhydrophobic surfaces with a maximum contact angle of 157°, in line with the limitations set by the Cassie-Baxter model for wetting of chemically heterogeneous surfaces. Finally, Si pillars were patterned with dual (6 μm and then 310 nm) length scales, transforming a nominally Wenzel-wetting hydrophilic surface into a hydrophobic surface robust to atmospheric aging. Similar reversion to higher contact angles were observed when introducing dual length scales in GaN surfaces.

Multicore microstructure fibers have emerged as promising components in ultrahigh capacity transmissions, biomedical imaging, quantum computing and signal-processing applications like wavelength conversion, and optical amplification. For long-haul transmission they are attractive due to low differential group delay and nonlinear impairment reduction depending on the number of cores. In this thesis we explore the linear-optical and nonlinear-optical properties of single and triple-core microstructure fibers with the goal of using them in parametric devices based on the four-wave mixing (FWM) nonlinearity Although the presence of multiple cores in a fiber produces drawbacks as well as advantages, we experimentally demonstrate that FWM and parametric oscillation are possible in multicore microstructure fibers. One such advantage is the ability to increase the power conversion efficiency of a fiber optical parametric oscillator (FOPO). We also present results for a FOPO built using a single-core microstructure fiber and demonstrate a higher power extraction using a polarization-based output coupling method compared with a polarization-independent method.We discuss measurements of the group velocity dispersion over a wavelength range from 1064-1600 nm. We study the mode coupling among the cores as a function of input power, polarization, wavelength, and spectral broadening. We demonstrate FWM in a triple core microstructure fiber at 1064 nm and study the transition between the case where FWM happens primarily in a single core and the case where FWM is distributed among multiple cores. We find that the effective nonlinear coefficient is reduced by a factor of 3 (the number of cores) when the coupling length is short compared with the nonlinear length, and that this leads to a three-fold reduction in the FWM bandwidth.

There are some factors that play a crucial role in deciding the image quality during the optical as well as the micrographic association with the technique of the shadow casting in which contrast of the image has influenced with obliquely depositing of a thin layer of the metal during the process of the microscopic preparations. Further castingadvantages of the using shadow technique are to be observed during the process if shadow casting itself. Shadow casting posses’ number of the advantages over the other casting methods. The most prominent advantage is that if the height and thickness of an object is not completely flat that can be easily notice by the shadow of that object. The lengths of the shadow of the uneven part are so differentiate from other part that can be easily determined. Shadow casting uses the scattering power or even the optical efficiency to bring out the specimens contours and will easily get notice in order to check the flatness of the surfaces.

A cost-effective stereolithography for medium-scale components is developed to fabricate 3D components with high build speed and resolution from photo-curable resin. The developed SLA utilizes a focused light beam of wavelength range (300 nm–700 nm) coming from the DLP projector and passes through the objective lens and finally imposed on the platform containing a layer of photo-curable resin. After focusing the light beam on the liquid resin layer, the photo-polymerization reaction occurs and the liquid resin becomes solid. Thus, the 3D object is fabricated layer by layer curing of liquid resin. The photopolymer used in this experimentation is polyethylene glycol di-acrylate and Irgacure 784 as photo-initiator. The Creo 3.0 software is used for modelling of 3D objects. A special MATLAB code is developed for slicing of the 3D model and displaying the sliced image one by one through DLP projector. The Arduino microcontroller with stepper motor and ball screw is used to control the motion of Z-stage platform. The Creation workshop software is also used to control motion of the Z-stage and period to display the sliced images through DLP projector. The medium-scale 3D objects with rectangular, square, and circular cross-sections are obtained by curing the aforementioned photo-curable resin. It is observed that the 3D objects are best cured for two seconds curing time with 0.1 mm curing depth along Z-axis.

Computational analysis has been presented in this paper to discuss one of the approaches of optimum synthesis for the design of magnetics lenses (analytical method). This investigation concentrates on finding the optimum design for single-pole piece magnetics lenses magnetically unsaturated to use in construction magnetics optical systems for transmission electron microscopes (TEM). In this investigation, the same computer program (EOD) program is the geometry of the magnetic lens and the axial magnetic field generated and drawing lines of magnetic flux through the lens. The electron beam's trajectory and objective optical characteristics have been calculated, taking the values of axial magnetics field as input data to calculate the objective length of focal and coefficient of aberrations (spherical and chromatic). In present work has been studied the impact of geometrical parameters for magnetics lenses (axial bore diameter). This investigation's importance is the possibility of determining suitable geometry of the magnetics lenses for benefiting from it in making transmission electron microscopes.

The monolayer semiconducting transition metal dichalcogenides are a family of 2D crystals that, beginning with MoS2 and soon expanding to include tungsten and other chalcogen variants, has become a focus of much research in 2D materials. Their semiconductor character, with wide direct bandgaps in the visible and near-infrared energy spectrum, make them particular amenable to optoelectronic applications. The low-dimensional character leads the direct gaps to host tightly bound exciton states that are stable at room temperature, allowing for the experiments on exciton physics that were only accessible at low temperatures in classical 2D excitonic system such as GaAs 2D quantum wells. The crystal structure of TMDs further gives the excitons and other quasi-particles valley contrasting physics, which can be discriminated optically with circularly polarized light, and leads to phenomena such as the valley Hall effect. The combination of strong light-matter interactions in TMDs with novel physics makes optical spectroscopy and microscopy powerful tools in the characterization of these materials. However, the native length scales of the excitons, while larger than the lattice constant, are still far below the resolution of optical probes. This resolution is set by the diffraction limit of light, which limits the focusing of propagating electromagnetic fields to roughly half the wavelength. Optical probes will effectively average over large areas (200 x 200 nm2) of the crystal, obscuring the effects of highly localized and heterogeneous perturbations. The need for local experimental probes in the study of TMDs has been highlighted by the recent discovery of single-photon emission from monolayer WSe2. Many far-field experimental works have shown a strong association of the emergence of these emitters and regions inhomogeneous tensile strain, leading to the hypothesis of quantum confinement of the excitons by the strain-created potential wells. However, experimental investigation of this hypothesis is challenging due to the scale mismatch of the localized excitons, which must be on order the exciton Bohr radius of ~ 1 nm, and the ~ 250 nm size of the optical probes. The focus of this work is to bridge the resolution gap of optical microscopy and spectroscopy in the study of excitons in the 2D TMDs using scanning near-field optical microscopy (SNOM), which allows optical resolutions down to a few nanometers. Using this technique to measure spectra of photoluminescence (PL) and scattering from lattice vibrations (Raman scattering), I investigate the nanoscale interplay of strain and the excitons in several of the commonly studied semiconducting TMDs, including WS2, WSe2, and MoSe2. In this thesis, I first discuss methods of performing near-field optical studies on 2D materials, including aperture and apertureless SNOM methods, and demonstrate them by collecting real nanoscale PL (nano-PL) and nano-Raman on WS2 and WSe2. Following that, I show how to estimate the strain in nanobubbles from classical models of plate bending, and show by comparison with shifts in the lattice vibrations that such models on average correctly estimate the strain. Further, by combining high-resolution nano-PL of WSe2 with advanced microscopic theories, I show evidence that strain alone can efficiently localized excitons at room temperature due to formation of atomic scale wrinkles of the crystal. Using gap-mode nano-PL with nanometer resolution, I demonstrate that the spatial location of localized exciton emission in WSe2 occurs along a common axis over several nanobubbles, and that it is highly localized in the wrinkle-like rings in asymmetric nanobubbles. Lastly, investigation of nanobubbles in MoSe2/WSe2 heterostructures shows that similar wrinkle-like ring structures occur commonly in large nanobubbles, and that such structures host strong exciton complexes in energetic regions associated with the interlayer exciton. This emission is unusually strong at room temperature, and has emission peaks that extend passed 1000 nm, which has not been previous described in such heterostructures. The results presented here show the strong and rich interplay of excitons with strain, demonstrating the utility of strain-engineering in future TMD devices, but also represent a frontier in studying the physics of confined excitons both spectrally and spatially.

Intriguing transport phenomena of phonons and photons in nanoporous structures have revolutionized the development of materials in recent decades. The particle- and wave-like transport of phonons have respectively demonstrated significant thermal conductivity reduction in thermoelectrics and backscattering-immune waveguiding in topological insulators; the photon transport in nanoporous composites have demonstrated enhanced solar reflectivity for radiative cooling. This doctoral research focuses on understanding phonon and photon transport in nanoporous structures that will guide optimal designs of thermoelectric energy harvesting systems, future phononic circuits, and hierarchical materials for optical and thermal management. Phonon transport can be viewed in the particle-like picture when the phonon wavelength is much shorter than the structure’s critical dimension. A good example is heat conduction in solids at THz frequencies. Though significant thermal conductivity reduction due to phonon size effect has been observed in nanoporous structures, the presence of various geometric parameters complicates the understanding of governing mechanisms. To investigate phonon-boundary scattering phenomena in Si nanoporous structures with varying pore shapes, alignments, and size distributions, we develop a ray tracing technique that is experimentally validated by bulk Si and Si nanomeshes. The results show that, with identical porosities, asymmetric pore shapes offer a lower thermal conductivity than symmetric ones due to smaller neck sizes that localize heat fluxes; asymmetric pores with fully diffuse surface boundaries show possibilities of realizing a thermal rectification ratio up to 13 by optimally controlling phonon injection angles. Symmetric pore shapes arranged in a hexagonal-lattice provide a lower thermal conductivity than in a squared-lattice due to the limited phonon line of sight; alternating pore size distributions yield a lower thermal conductivity than uniform ones at the same porosity. When phonon wavelength is much longer than the structure’s critical dimension, phonon transport is wave-like. One frequently asked question in condensed matter physics was if the Quantum Spin Hall Effect observed in electrons and photons could be analogously realized for phonons despite their lacking spin-like degrees of freedom and transverse polarizations. Here we numerically demonstrate a six-petal holey Si-based topological insulator, where simple geometric control enables topological edge states for both in- and out-of-plane phonon polarization up to GHz ranges with a submicron periodicity. The unique six-petal geometry induces zone-folding to form a double Dirac cone and breaking discrete translational symmetry leads to the topological phase transition. Our design supports robust backscattering-immune elastic wave transmission up to 90% despite the existence of geometric uncertainties. Though the design has not yet been demonstrated by experiments due to nanofabrication and measurement challenges, the numerical results clearly show the signature of topologically-protected edge transport of phonons, evident by the band inversion and backscattering-immunity. The improved understanding of particle- and like-like phonon transport in Si nanoporous structures regarding phonon-boundary scattering phenomena and phononic topological insulators will pave the way for developing future thermoelectric energy harvesting systems and phononic circuits. As another bosonic particle, photon’s transport can also be modified by nanoporous structures. Here we focus on understanding the enhanced solar reflection realized by dielectric nanoporous composites, which is important for designing selective emitters that are attractive for radiative thermal management. We employ Mie theory and finite-difference time-domain simulations to study the solar reflectivity of SiO2 and TiO2 microspheres in a polydimethylsiloxane (PDMS) matrix. Our analysis shows that hollow microspheres with a thinner shell are more effective in scattering the light, compared solid microspheres, and lead to a higher solar reflectivity. The high scattering efficiency, owing to the large interface density, in hollow microspheres allows low-refractive-index materials to have high solar reflectivity. The uniform- and varying-diameter design of 0.75 µm and 0.5-1 µm provide the highest solar reflectivity of 0.81 and 0.84, respectively. The effect of varying diameter is characterized by strong backscattering in the electric field. The enhanced solar reflectivity and effectiveness of hollow glass microsphere composites for radiative cooling have been experimentally demonstrated in our recent work. The findings in the current work will guide the optimal designs of microsphere composites and hierarchical materials for optical and thermal management systems.