Rozprawy doktorskie na temat „Micro-reflectance”

Le rendu de micro-reflets, utile pour simuler l'apparence de matériaux pailletés, de métal brossé ou de plastique rayé, est un défi théorique et technique en informatique graphique. Il implique l'utilisation de fonctions de distribution de réflectance bidirectionnelles surfaciques (P-BRDFs) hautes fréquences et qui varient spatialement. Dans cette thèse, nous proposons deux nouvelles P-BRDFs basées sur des cartes de normales presque parfaitement spéculaires. La première empêche toute création d'énergie grâce à une normalisation dépendante de l'empreinte du rayon, contrairement à la méthode précédente [YHMR16]. Cette normalisation est possible grâce à une nouvelle représentation d'une carte de normales en une mixture de NDFs de Beckmann décentrées et non-alignées sur les axes. La deuxième méthode améliore la première et empêche, pour la première fois, toute création et perte d'énergie, en simulant du multi-rebonds dans la micro-géométrie du matériau. Elle permet donc un rendu sans artefacts de surfaces opaques possédant des micro-reflets. De plus, nous donnons un algorithme d'échantillonnage optimal, utilisant la visibilité des normales. L'idée clé de cette méthode est la définition d'une V-cavité en chaque point de la surface. Pour simuler le multi-rebonds à l'intérieur, nous compensons l'énergie perdue par une modélisation simple rebond, en la réintégrant à l'aide d'une BRDF de compensation d'énergie. Nos méthodes ont le même ordre de grandeur que la méthode précédente en matière de temps de rendu et d'empreinte mémoire
Glint rendering, useful for simulating the appearance of glittery materials, brushed metal or scratched plastic, is a theoretical and technical challenge in computer graphics. It involves the use of spatially varying patch bidirectional reflectance distribution functions (P-BRDFs) with high frequencies. In this thesis we propose two new P-BRDFs based on specular normal maps. Unlike the previous method [YHMR16], our first BRDF prevents any creation of energy through footprint-dependent normalisation. This normalisation is possible thanks to a new representation of the normal map based on a mixture of non-centered and non-axis aligned Beckmann NDFs. The second method improves the first one and prevents, for the first time, any creation and loss of energy, by simulating multiple scattering in the microgeometry. It enables artifact-free rendering of opaque and sparkling surfaces. In addition, we provide an optimal sampling algorithm using the visibility information of the normals. The key idea of this method is the definition of a V-cavity for each point of the surface. To simulate multiple scattering inside it, we compensate for the energy lost by a single scattering model, by reintegrating lost energy with an energy compensation BRDF. The rendering time and memory footprint of our methods are in the same order of magnitude than previous methods

Cette thèse présente quelques avancées sur la représentation efficace de l'apparence matérielle dans une simulation de l'éclairage. Nous présentons deux contributions : un algorithme pratique de simulation interactive pour rendre la réflectance mesurée avec une géométrie dynamique en utilisant une analyse fréquentielle du transport de l'énergie lumineuse et le shading hiérarchique et sur-échantillonnage dans un contexte deferred shading, et une nouvelle fonction de distribution pour le modèle de BRDF de Cook-Torrance. Dans la première partie, nous présentons une analyse fréquentielle de transport de l'éclairage en temps réel. La bande passante et la variance sont fonction de l'éclairage incident, de la distance parcourue par la lumière, de la BRDF et de la texture, et de la configuration de la géométrie (la courbure). Nous utilisons ces informations pour sous-échantillonner l'image en utilisant un nombre adaptatif d'échantillons. Nous calculons l'éclairage de façon hiérarchique, en un seul passage. Notre algorithme est implémenté dans un cadre de deferred shading, et fonctionne avec des fonctions de réflectance quelconques, y compris mesurées. Nous proposons deux extensions : pré-convolution de l'éclairage incident pour plus d'efficacité, et anti-aliasing utilisant l'information de fréquence. Dans la deuxième partie, nous nous intéressons aux fonction de réflectance a base de micro-facette, comme le modèle de Cook-Torrance. En nous basant sur les réflectances mesurées, nous proposons une nouvelle distribution des micro-facettes. Cette distribution, Shifted Gamma Distribution, s'adapte aux donnée avec plus de précision. Nous montrons également comment calculer la fonction d'ombrage et de masquage pour cette distribution. Dans un deuxième temps, nous observons que pour certains matériaux, le coefficient de Fresnel ne suit pas l'approximation de Schlick. Nous proposons une généralisation de cette approximation qui correspond mieux aux données mesurées. Nous proposons par ailleurs une nouvelle technique d'optimisation, canal par canal, en deux étapes. Notre modèle est plus précis que les modèles existants, du diffus au spéculaire.

碩士
國立臺灣科技大學
電子工程系
105
There are two parts in the study. One study is about the micro/nano- scale composite structure effected to 6-inch silicon wafer. We used silicon pyramid structure then we sputtering silver film. For the Dewetting phenomenon, it can change the silver morphology to ball-like shape. Due to the tension of silver film and the coefficient of heat expansion difference between substructure and film. By changing annealing temperature, we get different size mask of ball-like silver. Metal assisted electroless chemical etching (MAEE) was used to fabricate hole structures which had advantage for anti-reflection. The benefit of this method is that it can fabricate large area, simple to product, low cost. It is an effective way to reduce reflection and increasing the ability of capturing photons. For the diameter of 100 nm and 100 nm depth hole, the diffuse reflection rate of whole wavelength(250-1100nm) decreasing to 3.3 % however, the carrier lifetime not obviously decreased. The second part of study is about the Indium nanoparticle characteristic of LSPR (Localized Surface Plasmon Resonance). When light interacting to metal, the electric field and magnetic field were periodically changed with time and space. It influence the distribution of electron in metal. The effect cause to the changed of charge density, energy level transition and polarization. The electric field was produces by these effects interact coupling to the electric field induce non by itself showed different optical and electronic phenomenon as scattering, absorption, refraction and dispersion. In this study it help to absorption of ultraviolent wavelength. When the hole diameter of 200 nm and 100 nm depth, deposition 2.5 min indium film then undergo thermal annealing, the diffuse reflection rate decreacing to 1.9 % and total reflection rate decreasing to 2.2% in ultraviolent wavelength(250-400 nm). Theses phenomenon improve the external quantum efficiency and improve the minority carriers lifetime by localized surface electric field. Moreover it can increasing the solar cell device power conversion efficiency.

Cancer is a world menace. After years of endeavor seeking the end of it, people started to realize that no matter how powerful the therapy could be, detection at early stage is always a cheaper, easier and more successful solution compared with curative methods for cancer developed onto its advanced stage. However, relatively few early-detection approaches have proven sufficiently effective and practical for mass use as a point-of-care tool. An early-cancer screening tool integrating the desired features of sensitive, informative, portable, and cost-effective is in need for the doctors. The progress in optical imaging and Micro-electro-mechanical system (MEMS) technology offers a promise for an innovative cancer screening alternative that is non-invasive, radiation-free, portable and potentially cost-effective. This dissertation investigates handheld instrumentation as multi-modalities of miniature imaging probes with various designs of MEMS devices, to obtain real-time images of epithelial tissue optical and physiological properties, combining the quantitative advantages of spectral analysis with the qualitative benefits of imaging to distinguish early cancer. This dissertation in sequence presents the handheld instruments in the fashions of Laser-scanning confocal microscopy (LSCM), optical diffuse reflectance imaging, nonlinear optical imaging modalities with their subsequent image-guided managements in oral cancer, skin cancer detection, circulating tumor cell (CTC) imaging, and imaging guided surgeries. One of the main challenges facing miniaturization lies in the mechanism of beam deflection across the sample. This dissertation introduces two generations of MEMS devices desgined, fabricated and incorporated in the imaging probes. A two-axis vertical comb driven silicon micromirror was used in the development of a handheld LSCM for oral cancer detection. Though obtaining numerous advantages, this first generation silicon MEMS micromirror suffers from small aperture size and high voltage requirement for actuation, which result in low collection efficiency in fluorescence imaging and medial safety concerns, respectively. Therefore a stainless steel scanner compatible with electrical discharge machining (EDM) process was fabricated with simplified process, low-voltage magnetic actuation and large fluorescence collection efficiency, with its capability demonstrated in the incorporation and embodiment of a handheld hyperspectral nonlinear imaging probe. Besides, software and controlling innovations for handheld imaging modalities are presented. A feedback controlling system for MEMS scanning status monitoring was developed for stabilized imaging rendering. For the sake of further improved imaging stability in handheld imaging and to enable on-site mosaic for large field viewing, a handheld mosaic system was developed and presented.
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Layered transition metal dichalcogenides (TMDCs) host a variety of strongly bound exciton complexes that control the optical properties in these materials. Apart from spin and valley, layer index provides an additional degree of freedom in a few-layer-thick lm. While in the 1H monolayer TMD inversion symmetry is broken, and the reflection symmetry is maintained but, in the bilayer, it is reversed. Trions are excitonic species with a positive or negative charge, and thus, unlike neutral excitons, the flow of trions can generate a net detectable charge current. Trions under favourable doping conditions can be created in a coherent manner using resonant excitation. The neutral biexciton (bound state of two excitons) can assemble further to create a charged state with another electron or hole. Generally, in W-based TMDs these ve-particle quinton states dominate the population density and this can also be engineered to produce photocurrent at cryogenic temperature. In the firrst work, we show that in a few-layer TMDC lm, the wave functions of the conduction and valence-band-edge states contributing to the K(K0) valley are spatially con ned in the alternate layers - giving rise to direct (quasi-)intralayer bright exciton and lower-energy interlayer dark excitons. Depending on the spin and valley con figuration, the bright-exciton state is further found to be a coherent superposition of two layer- induced states, one (E type) distributed in the even layers and the other (O type) in the odd layers. The intralayer nature of the bright exciton manifests as a relatively weak dependence of the exciton binding energy on the thickness of the few-layer lm, and the binding energy is maintained up to 50 meV in the bulk limit - which is an order of magnitude higher than conventional semiconductors. Fast Stokes energy transfer from the intralayer bright state to the interlayer dark states provides a clear signature in the layer-dependent broadening of the photoluminescence peak and plays a key role in the suppression of the photoluminescence intensity observed in TMDCs with thickness beyond a monolayer. In the second work, we show that bilayer WS2 exhibits a quantum con ned Stark effect (QCSE) that is linear with the applied out-of-plane electric field, in contrast to a quadratic one for a monolayer because of the contrasting symmetries between monolayer and bilayer. The interplay between the unique layer degree of freedom in the bilayer and the field-driven partial interconversion between intralayer and interlayer excitons generates a giant tunability of the exciton oscillator strength. This makes bilayer WS2 a promising candidate for an atomically thin, tuneable electro-absorption modulator at the exciton resonance, particularly when stacked on top of a graphene layer that provides an ultrafast nonradiative relaxation channel. By tweaking the biasing confi guration, we further show that the excitonic response can be largely tuned through electrostatic doping, by efficiently transferring the oscillator strength from neutral to charged exciton. In the third and fourth work, we demonstrate interlayer charge transport from top few-layer graphene to bottom monolayer graphene, mediated by a coherently formed trion state using a few-layer graphene/monolayer WS2/monolayer graphene vertical het- erojunction. This is achieved by using a resonant excitation and varying the sample temperature. The resulting change in the WS2 bandgap allows us to scan the excitation around the exciton-trion spectral overlap with high spectral resolution. By correlating the vertical photocurrent and in situ photoluminescence features at the heterojunction as a function of the spectral position of the excitation, we show that (1) trions are anoma- lously stable at the junction even up to 463 K due to enhanced doping, and (2) the photocurrent results from the ultrafast formation of a trion through exciton-trion coher- ent coupling, followed by its fast interlayer transport. Further, the resonant photocurrent thus generated can be effectively controlled by a back gate voltage applied through the incomplete screening of the bottom monolayer graphene, and the photocurrent strongly correlates with the gate dependent trion intensity, while the non-resonant photocurrent exhibits only a weak gate dependence. We estimate a sub-100 fs switching time of the device. In the final work, we have used the pulsed laser excitation to create the quinton states in monolayer WS2 while resonantly exciting the exciton and trion states at low temperature. Strong light absorption by the charged biexciton under spectral resonance, coupled with its charged nature, makes it intriguing for photodetection - an area that is hitherto unexplored. Using the high built-in vertical electric eld in an asymmetrically designed few-layer graphene encapsulated 1L-WS2 heterostructure, here we report, for the rst time, a large, highly nonlinear photocurrent arising from the strong absorption by two charged biexciton species under zero external bias (self-powered mode). Time- resolved measurement reveals that the photoresponse is ultra-fast, on the order of sub-5 ps. By using single- and two-color photoluminescence excitation spectroscopy, we show that the two biexcitonic peaks originate from bright-dark and bright-bright exciton-trion combinations. The possibility of electrical manipulation and detection of a charged exciton (trion) before its radiative recombination makes it promising for excitonic devices. The demon- stration of coherent formation, high stabilization, vertical transportation, and electrical detection of trions marks a step toward room-temperature trionics. Following the same the ve-particle charged quinton can also be efficiently generated and electrically de- tected. They can be used in electrical detection of constituting bright and dark states and quantum manipulation of the coupled spin-valley physics. Such innate nonlinearity in the photocurrent due to its biexcitonic origin, coupled with the ultra-fast response due to swift inter-layer charge transfer exempli fies the promise of manipulating many- body effects in monolayers. Also, the findings are prospective toward highly tunable, atomically thin, compact, and light on chip, re-confi gurable components and promising for several applications such as higher harmonics generation of the modulating signal, receiver design in microwave photonics and visible light communication, square-law cir- cuits, and also in nonlinear next generation optoelectronics.

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Excavations in the 1920s and 1970s at the Sculptor’s Cave, North-East Scotland, revealed that the site was used for mortuary rituals during the Late Bronze Age (c. 1100–800 BC) and Roman Iron Age (late first to fourth centuries AD), whilst a series of Pictish symbols carved into its entrance walls suggest that the cave’s importance continued into the Early Medieval Period. A new programme of analysis has utilised advanced 3D digital documentation and 3D metrology (specifically, 3D laser scanning) to enable this inaccessible site to be appreciated by wider audiences and analysed remotely. Detailed in situ recording of the Pictish symbols was undertaken using macro-level structured light scanning and the high-fidelity digital models blended with terrestrial laser scan data of the cave interior to show the location and detail of the carvings. This chapter examines the value of emerging digital approaches in the analysis, presentation and management of the Sculptor’s Cave, from the elucidation of additional carved details and the monitoring of surface degradation, to the dissemination of this difficult-to-access site to the wider public via online platforms.
Historic Environment Scotland provided funding for scanning work. Collaborators Visualising Heritage and Fragmented Heritage at the University of Bradford, funded by HEIF (via the University of Bradford) and the Arts and Humanities Research Council (AH/L00688X/1), respectively.