Dissertations / Theses on the topic 'Near-resonant'

Two compact planar crossed-dipole antennas loaded with near-field resonant parasitic (NFRP) elements are reported. The NFRP and crossed-dipole elements are designed for the desired circularly polarized (CP) radiation. By placing the NFRP element over the driven element at angles of 0 degrees and 45 degrees, respectively, dual-band and broadband CP antennas are realized. All radiating elements of antennas are 35 mm x 35 mm x 0.508 mm (0.187 lambda(0) x 0.187 lambda(0) x 0.0027 lambda(0) at 1.6 GHz) in size. The dual-band CP antenna has a measured vertical bar S-11 vertical bar < -10-dB bandwidth of 226 MHz (1.473-1.699 GHz) and measured 3-dB axial ratio (AR) bandwidths of 12 MHz (1.530-1.542 GHz) and 35 MHz (1.580-1.615 GHz) with minimum AR CP frequencies of 1.535 GHz (AR = 0.26 dB) and 1.595 GHz (AR = 2.08 dB), respectively. The broadband CP antenna has a measured vertical bar S-11 vertical bar< -10-dB bandwidth of 218 MHz (1.491-1.709 GHz) and a 3-dB AR bandwidth of 145 MHz (1.490-1.635 GHz). These compact antennas yield bidirectional electromagnetic fields with high radiation efficiency across their operational bandwidths.

Thesis (Ph. D.)--University of North Carolina at Chapel Hill, 2008.
Title from electronic title page (viewed Feb. 17, 2009). "... in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Marine Sciences." Discipline: Marine Sciences; Department/School: Marine Sciences.

Compared with the detector devices made of inorganic semiconductors, organic photodetectors are granted with additional strengths, such as flexibility, high scalability and bio-compatibility. However, in the family of organic optoelectronic devices, the detectors that are capable of detecting photons at two or multiple specific wavelengths are still missing. Such photodetectors are highly interesting because they could identify the target objects or materials much more precisely by detecting the reflected, transmitted or emitted photons at two or multiple characteristic wavelengths. In this thesis project, the optical simulations using Transfer Matrix Method (TMM) were performed on the organic devices to achieve the dual wavelength narrowband detection in the near-infrared (NIR) spectral range of 700 ~ 1100 nm. The devices use the fact that, at the interface of the blended organic electron donating and accepting materials, the charge-transfer (CT) states with the transition energies that are lower than the optical gap of the neat materials are formed. Combined with a Fabry-Perot resonant cavity, the CT absorption can be dramatically enhanced at certain wavelengths. The simulation results show that the two detection wavelengths can be tuned independently from 650 to 1100 nm. The spectral resolution (full with at half maximum - FWHM) of the detection bands varies between 10 and 30 nm. The simulated external quantum efficiency (EQE) is ~35% at 700 nm and ~10% at 1000 nm, respectively. A possible application of such photodetectors is for example moisture detection, where two of the characteristic absorption peaks of water are located at around 750 and 960 nm. By optimizing the thickness of the two photo-absorbing layers in a tandem device structure, the detection bands can be tuned to match with those two wavelengths for simultaneous and precise detection.

Plasmon resonance associated with nanoparticles of gold can enable photothermal ablation of tissues or controlled drug release with exquisite temporal and spatial control. These technologies may support many applications of precision medicine. However, clinical implementations of these technologies will require new methods of intraoperative imaging and guidance. Near-infrared laser surgery is a prime example that relies on improved image guidance. Here we set forth applications of augmented microscopy in guiding surgical procedures employing plasmon resonant gold-coated liposomes. Absorption of near-infrared laser light is the first step in activation of various diagnostic and therapeutic functions of these novel functional nanoparticles. Therefore, we demonstrate examples of near-infrared visualization of the laser beam and gold-coated liposomes. The augmented microscope proves to be a promisingimage guidance platform for a range of image-guided medical procedures.

While double negative (DNG) metamaterials (MTMs) were proposed over forty years ago, they have been experimentally demonstrated only in the last decade. The adaptation of a variety of epsilon-negative (ENG), mu-negative (MNG), and double negative (DNG) metamaterials, as well as single ENG, MNG,and DNG MTM unit cells, to achieve antenna systems exhibiting enhanced performance characteristics has received considerable research attention. These include, for example, electrically small antennas, multi-functional antennas, leaky-wave antenna arrays, and higher directivity antennas. Inspired by these metamaterial concepts, several metamaterial-engineered antennas have been investigated in this dissertation to achieve additional enhanced performance characteristics. First, several fabricated and tested variations of the three dimensional (3D) magnetic EZ (easy) antenna at 300 MHz and 100 MHz were examined. The 3D magnetic EZ antenna is composed of an electrically small loop antenna that is coaxially-fed through a finite ground plane and that is integrated with an extruded capacitively loaded loop (CLL) element. This 3D CLL structure is designed to be a near-field resonant parasitic (NFRP) element. With the proper placement of the NFRP element in the very near field of the driven element, it was demonstrated that the overall antenna system achieved complete matching to the source without any external matching network, along with an enhancement of the overall radiation efficiency. Additionally, multi-functional 3D magnetic EZ antennas were designed for wireless communication applications. For instance, by incorporating multiple NFRP elements, several dual-band versions were realized. Second, by properly combining and phasing their effective magnetic dipoles, electrically small multi-band, circular polarized (CP), metamaterial-inspired wire antennas were perfected that are nearly completely matched to a 50 Ω source and have high radiation efficiencies. Again, they were accomplished by incorporating multiple NFRP elements into their designs. Finally, two tri-band-notched UWB antennas were developed and tested successfully. The notched filters were achieved by introducing printed, electrically small CLL resonators into the UWB antenna design. Each CLL element has a high-Q characteristic and a compact size, which made it a very suitable candidate for a band-stop filter function. Like the split ring resonator (SRR) element, the CLL element is self-resonant and has a resonance frequency that is determined primarily by its loop inductance and the capacitances resulting from the cuts which opened the loop. In contrast, the CLL element has a much simpler, more compact design. It was demonstrated that by placing one, two or three CLL elements near the feedline of the UWB antenna and by tuning their sizes, one can control the band-notched frequencies of the radiator, while minimizing their space requirements, to achieve single, dual, and tri-band notched-filter UWB antennas.

Metamaterials have drawn considerable attention because they can exhibit epsilon-negative (ENG) and/or mu-negative (MNG) properties, which in turn can lead to exotic physical effects that can enable interesting, practical applications. For instance, ENG and MNG properties can be engineered to yield double negative (DNG) properties, such as a negative index of refraction, which leads to flat lenses. Similarly, their extreme versions enable cloaking effects. Inspired by such metamaterial properties, a promising methodology has been developed to design electrically small antennas (ESAs). These ESAs use unit cells of metamaterials as their near-field resonant parasitic (NFRP) elements. This new metamaterial-inspired antenna miniaturization method is extended in this dissertation by augmenting the antenna designs with circuits. A rectifying circuit augmentation is used to achieve electrically small, high efficiency rectenna systems. Rectennas are the enabling components of power harvesting and wireless power transmission systems. Electrically small, integrated rectennas have become popular and in demand for several wireless applications including sensor networks and bio-implanted devices. Four global positioning system (GPS) L1 frequency (1.5754 GHz) rectenna systems were designed, fabricated and measured: three resistor-loaded and one supercapacitor-loaded. The simulated and measured results will be described; good agreement between them was obtained. The NFRP ESAs are also augmented with active, non-Foster elements in order to overcome the physical limits of the impedance bandwidth of passive ESA systems. Unlike conventional active external matching network approaches, the non-Foster components are incorporated directly into the NFRP element of the ESA. Three 300 MHz non-Foster circuit-augmented broadband, ESA systems were demonstrated: an Egyptian axe monopole (EAM) antenna, an Egyptian axe dipole (EAD) antenna, and a protractor antenna. The simulated and measured results will be described; reasonable agreement between them was obtained. Moreover, a deeper practical engineering understanding of how lumped components with tighter tolerances, more accurate transistor models, and integrated circuit-based implementations will lead to more satisfactory performance characteristics of the non-Foster circuit-augmented ESAs was accomplished and is also reported.

A pattern-reconfigurable, low-profile, efficient, electrically small, near-field resonant parasitic (NFRP), Huygens source antenna is presented. The design incorporates both electric and magnetic NFRP elements. The electric ones are made reconfigurable by the inclusion of a set of p-i-n diodes. By arranging these electric and magnetic NFRP elements properly, a set of three Huygens sources are attained, each covering a 120 degrees sector. Pattern reconfigurability is obtained by switching the diodes on or off; it encompasses the entire 360 degrees azimuth range. A prototype was fabricated and tested. The numerical and experimental studies are in good agreement. The experimental results indicate that in each of its instantaneous states at f(0) = 1.564 GHz, the antenna provides uniform peak realized gains, front-toback ratios, and radiation efficiencies, respectively, as high as 3.55 dBi, 17.5 dB, and 84.9%, even though it is electrically small: ka = 0.92, and low profile: 0.05 lambda(0).

Cette thèse présente une étude expérimentale des modifications des propriétés de diffusion résonante de la lumière par des nuages mésoscopiques de rubidium 87 froids dont la densité peut être variée. pour des densités de l’ordre de 1014 atomes/cm3, les distances entre certains atomes deviennent bien plus petites que la longueur d’onde du rayonnement à résonance avec la transition d2 (780 nm) du rubidium 87. l’ampleur des interactions dipôle-dipôle entre ces diffuseurs empêche alors de les considérer comme étant indépendants les uns des autres: on parle de diffusion collective. les méthodes employées pour produire les échantillons étudiés s’appuient sur des techniques de refroidissement (ralentisseur zeeman et piège magnéto-optique) et de piégeage (pinces optiques) d’atomes neutres permettant d’obtenir des ensembles d’atomes froids (environ 100 µK) allant d’exactement un à plusieurs centaines d’entre eux. les observations réalisées portent sur la lumière diffusée par ces atomes, collectée grâce à un système optique de grande ouverture numérique. l’étude consiste dans un premier temps en la caractérisation la diffusion résonante de lumière par un unique atome. elle aboutit notamment à la mesure du retard dans la diffusion par un atome d’un paquet d’ondes lumineuses, appelé délai de wigner, dans des conditions proches d’une expérience de pensée. dans un second temps, cette étude porte sur les propriétés de diffusion collective de lumière par des ensembles denses d’atomes. elle rapporte en particuliers une mesure de la suppression de l’excitation des nano-nuages en fonction de leur densité
This thesis reports an experimental study of the modifications of the near-resonant light scattering properties of mesoscopic cold clouds of rubidium 87 which densities are tunable. for densities of the order of 1014 atoms/cm3, inter-atomic distances can be smaller than the wavelength of the radiation at resonance with the d2 line (780 nm) of rubidium 87. the magnitude of the dipole-dipole interactions between the scatters is such that they can no longer be considered as independent of each other: a phenomenon called collective scattering. the methods used to produce the studied samples are based on cooling (zeeman slower and magneto-optical trap) and trapping (optical tweezers) techniques for neutral atoms. they allow obtaining cold (100 µK) atomic ensembles containing from exactly one to several hundreds of atoms. the observations performed are based on the light scattered by these atoms which is collected thanks to a high numerical aperture optical system. in a first step, this study consists in characterizing the near-resonant light scattering by a single atom. it leads to the measurement of the delay in the scattering by an atom of a wave packet of light, called wigner delay, in conditions close to those of a gedanken experiment. in a second step, this study deals with the collective scattering of light by dense atomic ensembles. it especially reports a measurement of the suppression of the excitation of the nano-clouds versus their densities

Bandwidth enhancement of low-profile omnidirectional, electrically-small antennas has evolved from the design and construction of AM transmitter towers eighty years ago to current market demand for battery-powered personal communication devices. Electrically-small antenna theory developed with well-known approximations for characterizing radiation properties of antenna structures that are fractions of the radiansphere. Current state-of-the-art wideband small antennas near kaH1 have achieved multiple-octave impedance bandwidths when utilizing volume-efficient designs.
Significant advances in both the power and miniaturization of microelectronics have created a second possible approach to enhance bandwidth. Frequency agility, via switch tuning of reconfigurable structures, offers the possibility of the direct integration of high-speed electronics to the antenna structure. The potential result would provide a means to translate a narrow instantaneous bandwidth across a wider operating bandwidth.
One objective of the research was to create a direct comparison of the passive- multi-resonant and active-reconfigurable approaches to enhance bandwidth. Typically, volume-efficient, wideband antennas are unattractive candidates for low-profile applications and conversely, active electronics integrated directly antenna elements continue to introduce problematic loss mechanisms at the proof-of-concept level.
The dissertation presents an analysis method for wide bandwidth self-resonant antennas that exist in the 0.5dkad1.0 range. The combined approach utilizes the quality factor extracted directly from impedance response data in addition to near-and-far field modal analyses. Examples from several classes of antennas investigated are presented with practical boundary conditions. The resultant radiation properties of these antenna-finite ground plane systems are characterized by an appreciable percentage of radiated power outside the lowest-order mode.
Volume-efficient structures and non-omnidirectional radiation characteristics are generally not viable for portable devices. Several examples of passive structures, representing different antenna classes are investigated. A PIN diode, switch-tuned low-profile antenna prototype was also developed for the comparison which demonstrated excessive loss in the physical prototype.
Lastly, a passive, low-profile multi-resonant antenna element with monopole radiation is introduced. The structure is an extension of the planar inverted-F antenna with the addition of a capacitance-coupled parasitic to enhance reliable operation in unknown environments.


Ph. D.

A rise in the need for dynamic energy allocation has been associated with the saturation of available portable wireless electronic devices. Currently, the methods for transmitting this energy efficiently have been limited to a number of options, including near field resonant magnetic coupling. Previous research with mid-range (dâ 4r) wireless power transfer has resulted in coupling efficiencies of close to 40%. In order to increase efficiency in transfer a more directive transmission system was developed using a phased array. Coupling networks were used to shift the resonance of the coupling device, leading to a tightly coupled network by array phasing. Coupling networks for the phased array were optimized using a hybrid combination of a full wave Method of Moments simulation with circuit simulation. Results were validated in a full wave simulator, and field results were shown during resonance. S-parameter results show simulated transfer efficiencies of 70% (-1.5dB) for a phased array structure and 62.3% (-2.4dB) for a single feed structure. Single feed prototyping S-parameter results show coupling efficiencies of 25% (-5.9dB). All coupling measurements are at a distance 4r with reference to the largest transmitting coupler.
Master of Science

Atoms, molecules and clusters all constitute building blocks of macroscopic matter. Therefore, understanding the electronic and geometrical properties of such systems is the key to understanding the properties of solid state objects.

In this thesis, some atomic, molecular and cluster systems (clusters of O₂, CH₃Br, Ar/O₂, Ar/Xe and Ar/Kr; dimers of Na; Na and K atoms) have been investigated using synchrotron radiation, and in the two last instances, laser light. We have performed x-ray photoelectron spectroscopy (XPS) on all of these systems. We have also applied ultraviolet photoelectron spectroscopy (UPS), resonant Auger spectroscopy (RAS) and near-edge x-ray absorption spectroscopy (NEXAFS) to study many of the systems. Calculations using ab initio methods, namely density functional theory (DFT) and Møller-Plesset perturbation theory (MP), were employed for electronic structure calculations. The geometrical structure was studied using a combination of ab initio and molecular dynamics (MD) methods.

Results on the dissociation behavior of CH₃Br and O₂ molecules in clusters are presented. The dissociation of the Na₂ molecule has been characterized and the molecular field splitting of the Na 2p level in the dimer has been measured. The molecular field splitting of the CH₃Br 3d level has been measured and the structure of CH₃Br clusters has been determined to be similar to the structure of the bulk solid. The diffusion behavior of O₂, Kr and Xe on large Ar clusters, as a function of doping rate, has been investigated. The shake-down process has been observed from excited states of Na and K. Laser excited Na atoms have been shown to be magnetically aligned. The shake-down process was used to characterize the origin of various final states that can be observed in the spectrum of ground-state K.

碩士
建國科技大學
電機工程系暨研究所
99
This study proposed a new LLC resonant converter with near constant-frequency. With the characteristic of soft switching, LLC resonant converters presents the advantages of high conversion efficiency and low electromagnetic interference (EMI) that they are suitable for high-frequency power converters. Nonetheless, LLC resonant converters are operated with variable-frequency control that the designs of EMI filters become more complicated and difficult. Besides, when working in light load, the efficiency conversion is lower and the output voltage ripples are larger. In this case, the LLC resonant converter, which could work in Near Constant-Frequency, is proposed in this study. To achieve the function of Near Constant-Frequency, a frequency-converted Resonant Tank is designed to change the resonant frequency with the change of load. Furthermore, the LLC resonant converter with Near Constant-Frequency remains the characteristic of high efficiency conversion and largely improves the conversion efficiency with light load. The LLC resonant converter with near constant-frequency control is first analyzed; then, chaotic ganetic algorithm is utilized to design the key circuit element parameters of the converter; and finally, the near constant-frequency LLC resonant converter is completed. The experimental results show that the efficiency of the ultra-light load 6W is 78.2%, the efficiency of 18W reaches 89.3%, the maximum efficiency of the circuit presents 94.5%, and the change of the operation frequency 24W~240W appears 1.1kHz.

碩士
建國科技大學
電機工程系暨研究所
103
LLC resonant converter functions in accordance load variation via frequency switching, thus making it difficult to control the high frequency harmonic interference issue resulted from frequency switching. In order to achieve the objective of narrowing the variation range of switching fre-quency, senior fellows and scholars have tried to grind iron cores of various shapes to change the resonant inductor value by the feature of saturation of magnetic device. The molding of various forms of iron cores will be rather time consuming, thus the coil is moved to a main transformer with high leak inductance for the resonance between the leak inductance and a reso-nant inductor with lower inductance, such that the saturation of the addi-tional resonant inductor along with the load variation can change the in-ductance and achieve the narrowing of frequency band. In this article the design of traditional and nearly constant frequency LLC resonant converter with saturable resonant inductor have been completed. The experimental results reveal that the variation range of traditional switching frequency for the load changed from 30W to the full load of 240W is 24.9kHz, yet the range of switching frequency of the nearly constant frequency LLC reso-nant converter with saturable resonant inductor designed in this article is only 5.9kHz. In addition, the highest efficiency of the nearly constant fre-quency LLC resonant converter with saturable resonant inductor has reached 91.5%, and 91.1% when it is fully loaded. Even though the effi-ciency is not as high as the traditional type, in this article the LLC resonant converter with ultra-narrow frequency variation and the efficiency above 90% has been developed to overcome the EMI issue, thus facilitating the design of EMI rectifier. Keywords : Saturable; Near-Constant Frequency; Rectifier

Metal nanostructures, with their extraordinary optical properties, have attracted great attention in recent years. Subwavelength-scaled metal elements, without involving array effects, have the unique ability to confine or route light at the nano-scale. In this thesis, we provide three topics relating to the manipulation of light using metal nanostructures. We first present a theory to solve the end-face reflection of a subwavelength metal stripe, which is beneficial to the design of optical resonator antennas. Subsequently, we take the advantage of the destructive interference among triple nano-slits to sharpen the focus beam in the near-field at near-infrared wavelengths, which is of interest to the study of near-field optical phase imaging and lithography. Lastly, we demonstrate a rectangular subwavelength aperture quad to convert linearly polarized radiation to a radially polarized beam, which is useful to create a deep-subwavelength focus and for optical trapping.
Graduate

碩士
建國科技大學
電機工程系暨研究所
102
In this thesis the finite element simulation software COMSOL Mul-tiphysics is used for the aided design of resonant inductor in order to real-ize a near-constant frequency LLC resonant converter. LLC resonant converter functions on variable frequency switching in accordance with load variation such that the high frequency harmonic in-terference problem caused by switching frequency becomes more difficult to be handled. For the purpose of narrow variable range of switching fre-quency, it has been proposed by numerous studies that the load variation can be used to change the saturation characteristics of magnetic core and thus changing the resonant inductance to achieve the narrow variable range of switching frequency of LLC resonant converter. However, the saturation design of magnetic core can be difficult to control, thus it is necessary to further investigate and study this subject. In light of this, in this thesis the finite element simulation software COMSOL Multiphysics is used for the aided design of variable resonant inductor, and the designs of conventional frequency-variable and near-constant frequency LLC resonant converters were completed. The experimental results indicate that, when the load is changed from 12W to the full load of 240W, the variable range of switching frequency of the conventional type is 12.8kHz, while the variable range of switching frequency of the proposed converter is only 4.5kHz. In addition, the highest efficiency of the nearly constant frequency LLC resonant converter can reach 94.5% with full-load effi-ciency as high as 92.1%.

Near-resonant triad interactions and wave generation theory are investigated for continuously stratified fluids. Interaction equations are derived for spatially-varying wave trains under the inviscid Boussinesq approximation. Rotational effects are included, and properties of the underlying eigenvalue problem are explored. To facilitate a numerical study of the near-resonant interactions, numerical methods are developed and an analysis of wave generation on a periodic domain is performed. Numerical experiments using laboratory and ocean-scale parameters are conducted, and the simulations confirm the validity of the wave forcing theory. Interaction experiments demonstrate a strong tendency for waves to exhibit nonlinear behaviour. While resonant interactions are observed in the laboratory scale simulations, nonlinear steepening effects and the formation of solitary-like waves dominate the ocean-scale experiments. The results suggest that the weakly-nonlinear interaction theory is only appropriate in a limited parameter regime. The problem of analyzing forced wave equations on an infinite domain is also considered. Motivated by the results obtained on a periodic domain, asymptotic analysis is applied to three important wave equations. The method of steepest descents is used to determine the large-time behaviour for the linearized Korteweg-de Vries, Benjamin-Bona-Mahony, and internal gravity wave equations. The asymptotic results are compared with numerical experiments and found to agree to high precision.

碩士
國立交通大學
光電工程系所
96
Near resonant high-order optical nonlinearities of ZnO thin film made by the laser MBE were investigated by the Z-scan method using the high repetition rate femtosecond Ti:sapphire laser with doubled-frequency wavelength operated between 390 nm to 420 nm. We observed an enormous enhancement of two photon absorption (TPA) coefficient due to near-exciton resonance. When photoexcitation energy is close to exciton transition energy, the TPA coefficient is about 3000 cm/GW, which is 710 times larger than that of ZnO bulk measured at 532 nm. The order of enhancement is even greater than that in near-IR regime. Besides, the higher order nonlinearity, free-carrier absorption, becomes more evident in the near band edge region than that in the off-resonant regime due to the enhanced TPA coefficients. A modified method, which considered higher nonlinearity, was then used and the calculated values were sensible. After considering the contributions of every possible mechanism, we found nonlinear absorption mainly resulted from TPA. Furthermore, the effects of band tailing states on nonlinear absorption were also observed under low input intensity through z-scan method.

The possibilities to use methods and tools of the nearfield microwave microscopy for studying small-dimension biological objects are analyzed. Methods for optimization of the resonant sensor structure with regard to the objects features are developed, the system of the information signals formation is proposed.

This thesis investigates the effects of the earth's rotation on internal waves from two perspectives of nonlinear internal wave theory: near-resonant triads and weakly nonlinear models. We apply perturbation theory (multiple scale analysis) to the governing equations of internal waves and develop a near-resonant internal wave triad theory. This theory explains a resonant-like phenomenon in the numerical results obtained from simulating internal waves generated by tide topography interaction. Furthermore, we find that the inclusion of the earth's rotation (nonzero $f$) in the numerical runs leads to a very special type of resonance: parametric subharmonic instability. Through using perturbation expansion to solve separable solutions to the governing equations of internal waves, we derive a new rotation modified KdV equation (RMKdV). Of particular interest, the dispersion relation of the new equation obeys the exact dispersion relation for internal waves for both small and moderate wavenumbers ($k$). Thus this new RMKdV is able to model wea kly nonlinear internal waves with various wavenumbers ($k$), better than the Ostrovsky equation which fails at describing waves of small $k$.

碩士
國立中正大學
物理系研究所
104
The enhanced upconversion luminescence of lanthanide-ion(Ln3+) doped upconversion nanoparticles (UCNPs) is particularly important and highly required for their innumerable applications in fluorescence microscope, bio-imaging, sensing, solid-state lasers, 3D displays, and solar cells.In this study, we investigate the enhancement effect of upconversion fluorescence emission of UNCP by using low refractive index resonant waveguide grating(RWG) when both excitation and extraction resonance occur in the waveguide structure.The method to fabricate one-dimensional grating patterns with nanoporous SiO2, a low refractive index material (n=1.22), using nanoimprint lithography is reported. A guided mode resonant structure is obtained by subsequent sputtering of thin film of high refractive index titanium dioxide (TiO2) on the sample surface, and then NaYF4:Yb3+,Tm3+ nanoparticles are deposited onto the top cladding layer of a RWG structure by dip-coating method.We demonstrate the up-conversion photoluminescence signal generated from the rare-earth nanocrystal coated on top of such RWG structure can be enhanced up to 10^4 times when both excitation and extraction resonance are simultaneously achieved, compared with UCNPs on an non-patterned surface. Keywords : Low-refractive index , Up conversion, Rare earth, Fluorescence, Guided-mode resonance

Indiana University-Purdue University of Indianapolis (IUPUI)
A Near-field Scanning Optical Microscope (NSOM) is used to study the resonant energy transfer between different size CdSe/ZnS quantum dots (QDs). The NSOM system is used to bring the small QDs which are 6 nm in diameter close to 8 nm diameter QDs which are embed with PMMA on a cover glass. The PMMA is used to prevent the 8 nm QDs from aggregation, which allows us to locate one dot on the cover slide and have the potential to get the interaction of two individual dots. A systematic methodology is used to localize a single QD on the cover glass and align the small and large QDs. Since the ground energy state of the small QDs match the excitation energy level of the large QDs. When the small dots get excited, part of the energy transfers to the large QDs. As the separation between small and large QDs is changed in near-field range (20-50nm), the transition probability is observed, indicating that the FRET level changes as a function of separation between small and large QDs. Possible future improvements are also discussed.

Dissertação de mestrado em Physics
Förster resonance energy transfer (FRET) is a non-radiative energy transfer mechanism between two light-emitting systems, such as two quantum dots (QDots) or molecules. This mechanism involves an excited donor fluorophore (e.g., a QDot or a dye molecule) which transfers its energy of excitation to an acceptor (another QDot or molecule which is in resonance with the donor), via dipole-dipole coupling. FRET is the dominant type of energy transfer between emitters at a nanometre proximity. Other factors that influence the efficiency of this energy transfer mechanism include the spectral overlap of the donor emission spectrum and the acceptor absorption spectrum and the relative orientation of the dipole moment of both particles. In nature, for instance, FRET plays a dominant role in the energy transfer in photosynthetic apparatus of plants and bacteria. Some interesting applications of FRET can be found in photovoltaics, probing of molecular distances and molecular interactions, and storage and transfer of quantum information. The main goal of this master thesis lies in detecting the presence of the FRET mechanism when two different colloidal QDot samples of PbS (short for Lead Sulfide), with different QDot size, are linked together via surface chemistry. This chemical procedure activates carboxyl or phosphate groups, which promote the binding of primary amines of organic glutathione QDot shell molecules. In other words, it promotes a cross-linkage between the organic shells of two, or more, quantum dots at a distance at which FRET is present. Using PbS quantum dots, which emit in the near-infrared (NIR) region of the light spectrum, these experiments can be reported as one of the first attempts to find The FRET mechanism in a near-infrared system of QDots. Most previous reports of FRET mechanisms were concerned with QDots which emit in the visible range, such as CdTe and CdSe QDots. The NIR spectral range, for instance, promotes interesting applications in photonic crystals, where FRET can be enhanced by spontaneous emission inhibition, in photovoltaics, in order to greatly absorb infrared light, and in the production of near-infrared QDot lasers. In order to find evidences of the presence of the FRET mechanism, emission spectra and time-resolved measurements, using the time correlated single photon counting technique (TCSPC), of cross-linked colloidal PbS QDot solutions have been performed and will be shown in this master thesis. Along with the experimental results, a study of statistical moments of the PbS quantum dot photoluminescence kinetics will be presented in this thesis, which experimental kinetics were acquired with TCSPC. With this statistical analysis, it is possible to evaluate and compare various decay properties, such as the average decay time, the mean-squared value of the decay and the measure of asymmetry of the time-resolved distribution. In order to understand the obtained results, some donor decay models were developed and studied, alongside with other decay functions found in the literature. A theoretical description of the FRET mechanism will be presented in order to understand the proposed decay models.
A transferência ressonante de energia de Förster (em inglês, Förster resonance energy transfer, ou, simplesmente, FRET) é um mecanismo de transferência de energia não radiativo presente entre duas unidades fluorescentes, como dois pontos quânticos ou duas moléculas. Neste mecanismo, um dador excitado (por exemplo, um ponto quântico ou uma molécula) transfere a sua energia de excitação para um aceitador (outro ponto quântico ou molécula) com o qual se encontre em ressonância, por acoplamento dipolo-dipolo. O FRET é um mecanismo dominante entre emissores distanciados entre si a uma ordem dos nanómetros. Outros fatores dominantes que influenciam a eficiência deste mecanismo de transferência são a sobreposição do espetro de emissão do dador com o espetro de absorção do aceitador e a orientação relativa do momento dipolar de ambas as partículas. Este mecanismo tem também um papel fundamental em processos biológicos como a fotossíntese em plantas e bactérias. Algumas aplicações de FRET podem ser encontradas em sistemas fotovoltaicos, na análise de distâncias e interações moleculares, e no armazenamento de informação quântica. O objetivo principal desta tese de mestrado é a deteção do mecanismo de FRET numa mistura coloidal de duas amostras pontos quânticos de PbS (símbolo químico para Galena, ou sulfeto de chumbo) com diferentes tamanhos, que são unidas por processos de química de superfícies. Estes processos químicos promovem, neste caso específico, ligações cruzadas físicas entre dois, ou mais, pontos quânticos, a uma distância da ordem dos nanómetros, que promove a presença de FRET. A deteção de FRET em sistemas de pontos quânticos que emitem no espetro infravermelho próximo (isto é, com comprimentos de onda entre 0.7 􀀀 1.4mm), como os pontos quânticos de PbS, não é amplamente encontrada na literatura, a qual foca na deteção de mecanismos de FRET em pontos quânticos que emitem no espetro visível, como pontos quânticos de CdTe ou CdSe. O espetro infravermelho próximo permite, como exemplo, aplicações em cristais fotónicos, aonde o mecanismo de FRET é predominante pela inibição da emissão radiativa espontânea, assim como a aplicação em lasers que emitem no infravermelho e absorção de luz no infravermelho em sistemas fotovoltaicos. De forma a detetar a presença de FRET nas amostras de pontos quânticos, foram medidos espetros de emissão e cinéticas fotoluminescentes, estas obtidas por técnicas de resolução temporal de fotoluminescência como TCSPC (em inglês, Time Correlated Single Photon Counting). Em conjunto com os resultados experimentais obtidos, é apresentado nesta tese o estudo estatístico das cinéticas fotoluminescentes das amostras de pontos quânticos PbS, cujas cinéticas foram obtidas pela técnica de TCSPC. A partir desta análise estatística, é possível de avaliar e comparar várias propriedades das cinéticas, tais como o tempo médio de decaimento, o erro quadrático médio e a medida da assimetria da cinética obtida. De forma a compreender os resultados obtidos, alguns modelos de decaimentos do dador foram desenvolvidos e estudados, em conjunto com outras funções de decaimento que são encontradas na literatura. De forma a sustentar os modelos teóricos de decaimento, é também apresentado um tratamento teórico do mecanismo de FRET.