Nanoparticles in Solution-Derived Chalcogenide Glass Films

The results in this thesis are from our efforts to modify the optical properties of solution-derived chalcogenide glass films by the incorporation of nanomaterials. First, the composition Ge23Sb7S70 was selected as the appropriate glass matrix for testing because solution-derived films of this composition have been well-studied in our group. Additionally, this composition was found to be less sensitive to certain processing parameters than As2S3, another well-studied, candidate chalcogenide glass composition, making Ge23Sb7S70 more suitable for the addition of nanomaterials. Optimization of film process parameters was performed to obtain high-quality films appropriate for doping with nanomaterials. This consisted of determining the maximum solubility of glass in propylamine solvent to obtain films of adequate thickness, as well as optimizing the water content in the propylamine to minimize surface roughness and cracking. Two classes of nanomaterials were used to investigate the principles of doped films, spherical metallic nanoparticles (MNPs), and spherical semiconductor nanoparticles, also known as quantum dots (QDs). Gold was the particular type of MNP used, and is characterized by its surface plasmon resonance (SPR) absorption band, which is tunable and environment sensitive, and leads to interesting properties such as magneto-optic effects. Two types of QDs were used, CdSe and PbS. QDs are widely known for their high photostability and luminescence, which is tunable by varying the size of the QD. CdSe exhibits luminescence in the visible spectral region, while PbS emits in the near-infrared (NIR). With Au nanoparticles, experiments were performed to determine the maximum nanoparticle concentration in the glass solution by utilizing UV-vis-NIR spectroscopy. Films were then deposited and characterized by their absorption spectra. In the case of QDs, solutions were not stable for long enough periods of time, so only the films deposited from the solutions could be analyzed. UV-vis-NIR spectroscopy and photoluminescence measurements were used to observe the intensity and location of the characteristic absorption and luminescence bands of the QDs. Quantum yield and luminescence lifetime were used to quantitatively characterize the behavior of the QDs in different environments when possible. Different organic ligands on the surface of the QDs were tested and compared to evaluate their effect on the behavior of the QD. The results show that small amounts of Au MNPs can be dispersed in a chalcogenide glass solution with minimal aggregation, as quantified by the absorption spectra. Comparison of the optical behavior of the films to that of the solutions showed that the concentration of Au MNPs was too low to observe the characteristic SPR band. The results of the QD testing show that luminescence can be observed from a deposited film, and that the behavior of the QDs, characterized by quantum yield and lifetime, varies greatly in different environments. Furthermore, it was found that different capping agents led to different behavior of the QDs in the glass solution, affecting the properties of the deposited film

Electrospray deposition of chalcogenide glass films for gradient refractive index and quantum dot incorporation

Chalcogenide glasses (ChGs) are well-known for their optical properties, making them ideal candidates for emerging applications of mid-infrared microphotonic devices, such as lab-on-a-chip chemical sensing devices, which currently demand additional flexibility in processing and materials available to realize new device designs. Solution-derived processing of ChG films, initially developed in the 1980s by Chern and Lauks, has consisted mainly of spin-coating and offers unique advantages over the more traditional physical vapor deposition techniques. In the present effort, the nanoparticles of interest are luminescent quantum dots (QDs), which can be used as an on-chip source of light for a planar chemical sensing device. Prior efforts of QD incorporation have exposed limitations of spin-coating of ChG solutions, namely QD aggregation and material waste, along with incompatibility with larger scale manufacturing methods such roll-to-roll processing. This dissertation has evaluated electrospray (ES) as an alternative method of solution-derived chalcogenide glass film deposition. While employed in other materials systems, deposition of optical quality ChG films via electrospray has not been previously attempted, nor have parameters until now, been defined. This study has defined pre-cursor solution chemistry, electrospray jet process parameters required for formation of stable films, annealing protocols and resulting film attributes, yielding important correlations needed to realize high optical quality films. Electrosprayed films attributes were compared to those seen for spin coating and trade-offs in processing route and resulting quality, were identified. Optical properties of importance to device applications were defined, including surface roughness, refractive index, and infrared transmission. The use of a serpentine path of the spray over the substrate was demonstrated to obtain uniform thickness, blanket films, and demonstrates process compatibility with roll-to-roll processing whereby (theoretically) 100% of starting solution can be utilized in a continuous process. The present effort has shown that electrospray offers the advantage of spatially defined, localized deposition, which enables direct 2-D and 3-D printing, though with limited (unoptimized) spatial resolution on the order of millimeters. Knowledge of processing protocols were exploited to fabricate multi-layer films from two different glass compositions to yield an effective refractive index gradient (GRIN). GRIN coatings were fabricated and refractive index variations were predicted. The advantages of electrospray deposition were also explored for the enhancement of quantum dot doping in ChG films. A hypothesis whereby electrospray would enable deposition of films based on consolidation of many, single QD doped aerosol droplets was developed and evaluated. Experimental validation of this premise in CdSe and PbS doped ChG films was shown, indicating that electrospray offers a kinetic barrier to QD movement preventing aggregation from occurring, not seen in spin-coating. Two types of organic ligands were found to enhance dispersion of QDs in amine solvents, octadecylamine and mercaptopropionic acid. Utilizing TEM characterization, evidence that electrospray may be more suitable than spin-coating for the dispersion of QDs in solution-derived ChG films was confirmed. However, the ultimate effectiveness of this approach was limited due to the ability to quantify direct loading levels of the QD and surrounding glass matrix. This work demonstrates that electrospray offers additional flexibility over spin-coating and other evaporation methods for the deposition of ChG coatings. Electrospray processing of doped and undoped ChG solutions for microphotonic applications has been shown as a viable alternative in the processing and material toolbox where spatially defined index and dopant control is required

Short-term Effects of 308-nm Xenon-chloride Excimer Laser and Narrow-band Ultraviolet B in the Treatment of Vitiligo: A Comparative Study

We compared the clinical efficacy of a short-term intervention of 308-nm excimer laser with that of narrow-band UVB (NBUVB) phototherapy for vitiligo patients to see the early response. Twenty-three symmetrically patterned patches of vitiligo on 8 patients were selected. Vitiligo patches on one side of the body were treated 2 times per week for a maximum of 20 treatments with the excimer laser, and NBUVB phototherapy was used on patches on the other side. Improvement (repigmentation) was assessed on a visual scale via serial photographs taken every five treatments and scored as follows: 0, ≤1% improvement; 1, ≤25% improvement; 2, 26-50% improvement; 3, 51-75% improvement; and 4, ≥75% improvement. At five treatments, the excimer laser-treated patches had an average score of 0.26, compared with 0.04 for patches treated with NBUVB phototherapy. A slightly higher repigmentation (p>0.05) in the excimer treated area was thus observed. At 10, 15, or 20 treatments, the differences between the average scores were significant: 0.83, 1.17, and 1.39 for the excimer-treated patches, and 0.17, 0.30, and 0.74 for the NBUVB phototherapy-treated areas (p<0.05). In conclusion, the 308-nm excimer laser appears to be more effective than NBUVB phototherapy, as it produces more rapid and profound repigmentation

Low-loss, submicron chalcogenide integrated photonics with chlorine plasma etching

A chlorine plasma etching-based method for the fabrication of high-performance chalcogenide-based integrated photonics on silicon substrates is presented. By optimizing the etching conditions, chlorine plasma is employed to produce extremely low-roughness etched sidewalls on waveguides with minimal penalty to propagation loss. Using this fabrication method, microring resonators with record-high intrinsic Q-factors as high as 450 000 and a corresponding propagation loss as low as 0.42 dB/cm are demonstrated in submicron chalcogenide waveguides. Furthermore, the developed chlorine plasma etching process is utilized to demonstrate fiber-to-waveguide grating couplers in chalcogenide photonics with high power coupling efficiency of 37% for transverse-electric polarized modes

Invasion of Europe by the western corn rootworm, Diabrotica virgifera virgifera: multiple transatlantic introductions with various reductions of genetic diversity

The early stages of invasion involve demographic bottlenecks that may result in lower genetic variation in introduced populations as compared to source population/s. Low genetic variability may decrease the adaptive potential of such populations in their new environments. Previous population genetic studies of invasive species have reported varying levels of losses of genetic variability in comparisons of source and invasive populations. However, intraspecific comparisons are required to assess more thoroughly the repeatability of genetic consequences of colonization events. Descriptions of invasive species for which multiple introductions from a single source population have been demonstrated may be particularly informative. The western corn rootworm (WCR), Diabrotica virgifera virgifera, native to North America and invasive in Europe, offers us an opportunity to analyse multiple introduction events within a single species. We investigated within- and between-population variation at eight microsatellite markers in WCR in North America and Europe to investigate the routes by which WCR was introduced into Europe, and to assess the effect of introduction events on genetic variation. We detected five independent introduction events from the northern USA into Europe. The diversity loss following these introductions differed considerably between events, suggesting substantial variation in introduction, foundation and/or establishment conditions. Genetic variability at evolutionarily neutral loci does not seem to underlie the invasive success of WCR in Europe. We also showed that the introduction of WCR into Europe resulted in the redistribution of genetic variance from the intra- to the interpopulational level contrary to most examples of multiple introductions

Selection on stability across ecological scales

Much of the focus in evolutionary biology has been on the adaptive differentiation among organisms. It is equally important to understand the processes that result in similarities of structure among systems. Here, we discuss examples of similarities occurring at different ecological scales, from predator–prey relations (attack rates and handling times) through communities (food-web structures) to ecosystem properties. Selection among systemic configurations or patterns that differ in their intrinsic stability should lead generally to increased representation of relatively stable structures. Such nonadaptive, but selective processes that shape ecological communities offer an enticing mechanism for generating widely observed similarities, and have sparked new interest in stability properties. This nonadaptive systemic selection operates not in opposition to, but in parallel with, adaptive evolution

Chalcogenide Glass-on-Graphene Photonics

Two-dimensional (2-D) materials are of tremendous interest to integrated photonics given their singular optical characteristics spanning light emission, modulation, saturable absorption, and nonlinear optics. To harness their optical properties, these atomically thin materials are usually attached onto prefabricated devices via a transfer process. In this paper, we present a new route for 2-D material integration with planar photonics. Central to this approach is the use of chalcogenide glass, a multifunctional material which can be directly deposited and patterned on a wide variety of 2-D materials and can simultaneously function as the light guiding medium, a gate dielectric, and a passivation layer for 2-D materials. Besides claiming improved fabrication yield and throughput compared to the traditional transfer process, our technique also enables unconventional multilayer device geometries optimally designed for enhancing light-matter interactions in the 2-D layers. Capitalizing on this facile integration method, we demonstrate a series of high-performance glass-on-graphene devices including ultra-broadband on-chip polarizers, energy-efficient thermo-optic switches, as well as graphene-based mid-infrared (mid-IR) waveguide-integrated photodetectors and modulators

Glass-on-2-D-material photonics

Due to their extraordinary optoelectronic properties, 2-D materials have been identified as promising materials for integrated photonics. However, most 2-D material-integrated photonic devices demonstrated to date are fabricated by transferring a layer of 2-D material on top of already fabricated photonic structures, which limits full utilization of their capability. Here we introduce a new photonic integration approach via direct deposition and fabrication of chalcogenide glass photonic devices on 2-D materials.We have applied the new process to fabricate high-performance, broadband on-chip graphene-based optical polarizers with a high contrast ratio of 740 dB/cm leveraging the remarkable optical anisotropy of graphene, and thermo-optic switches with a record heating efficiency of 10 nm/mW using in-waveguide low-loss (20 dB/cm) graphene transparent electrodes. The low processing temperatures of chalcogenide glasses further enables monolithic integration on plastics and the first waveguide-integrated graphene photodetector on flexible substrates. Last but not least, we have also demonstrated monolithic integration of chalcogenide photonic components on several other 2-D materials including WSe2, WS2, and MoTe2. The glass-on-2-D-material approach therefore provides a facile universal route for photonic integration based on 2-D materials

Empirical modelling of the BLASTPol achromatic half-wave plate for precision submillimetre polarimetry

A cryogenic achromatic half-wave plate (HWP) for submillimetre astronomical polarimetry has been designed, manufactured, tested and deployed in the Balloon-borne Large-Aperture Submillimeter Telescope for Polarimetry (BLASTPol). The design is based on the five-slab Pancharatnam recipe and itworks in thewavelength range 200–600 μm, making it the broadestband HWP built to date at (sub)millimetre wavelengths. The frequency behaviour of the HWP has been fully characterized at room and cryogenic temperatures with incoherent radiation from a polarizing Fourier transform spectrometer. We develop a novel empirical model, complementary to the physical and analytical ones available in the literature, that allows us to recover the HWP Mueller matrix and phase shift as a function of frequency and extrapolated to 4 K. We show that most of the HWP non-idealities can be modelled by quantifying one wavelength-dependent parameter, the position of the HWP equivalent axes, which is then readily implemented in a map-making algorithm. We derive this parameter for a range of spectral signatures of input astronomical sources relevant to BLASTPol, and provide a benchmark example of how our method can yield improved accuracy on measurements of the polarization angle on the sky at submillimetre wavelengths

Is administration of gadolinium‐based contrast media to pregnant women and small children justified?

The use of gadolinium‐based contrast media in pregnant or lactating women has been discouraged at many radiology departments due to the lack of knowledge of the risks for the fetus and the unwillingness to expose neonates to unnecessary drugs. In the present review the current literature and present guidelines regarding the use of gadolinium‐based contrast media have been reviewed to validate the justification for their administration to pregnant or lactating women and small children. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/86793/1/22413_ftp.pd