Design, fabrication and evaluation of chalcogenide glass Luneburg lenses for LiNbO3 integrated optical devices

Optical waveguide Luneburg lenses of arsenic trisulfide glass are described. The lenses are formed by thermal evaporation of As2S3 through suitably placed masks onto the surface of LiNbO3:Ti indiffused waveguides. The lenses are designed for input apertures up to 1 cm and for speeds of f/5 or better. They are designed to focus the TM sub 0 guided mode of a beam of wavelength, external to the guide, of 633 nm. The refractive index of the As2S3 films and the changes induced in the refractive index by exposure to short wavelength light were measured. Some correlation between film thickness and optical properties was noted. The short wavelength photosensitivity was used to shorten the lens focal length from the as deposited value. Lenses of rectangular shape, as viewed from above the guide, as well as conventional circular Luneburg lenses, were made. Measurements made on the lenses include thickness profile, general optical quality, focal length, quality of focal spot, and effect of ultraviolet irradiation on optical properties

Research and Technology

Langley Research Center is engaged in the basic an applied research necessary for the advancement of aeronautics and space flight, generating advanced concepts for the accomplishment of related national goals, and provding research advice, technological support, and assistance to other NASA installations, other government agencies, and industry. Highlights of major accomplishments and applications are presented

Biological and Bioinspired Photonic Materials for Passive Radiative Cooling and Waveguiding

Animals have evolved diverse strategies to control solar and thermal radiations so that they can better adapt to their natural habitats. Structured materials utilized by these animals to control electromagnetic waves often surpass analogous man-made optical materials in both sophistication and efficiency. Understanding the physical mechanism behind these structured materials of nature inspires one to create novel materials and technologies. Our optical and thermodynamic measurements of insects (Saharan silver ants and cocoons of the Madagascar comet moth) living in harsh thermal environments showed their unique ability to simultaneously enhance solar reflectivity and thermal emissivity, and to maintain a cool body temperature. Saharan silver ants, Cataglyphis bombycina, forage on the desert surface during the middle of the day. The ants’ conspicuous silvery glance is caused by a coating of hairs with unique triangular cross-sections. The hair coating enhances not only the reflectivity of the ant’s body surface in the visible and near-infrared range of the spectrum, where solar radiation culminates, but also the emissivity of the ant in the mid-infrared. The latter effect enables the animals to efficiently dissipate heat back to the surroundings via blackbody radiation under full daylight conditions. The fibers produced by the wild comet moth, Argema mittrei, are populated with a high density of air voids that have a random distribution in the fiber cross-section but are invariant along the fiber. These filamentary air voids strongly back-scatter light in the solar spectrum, which, in combination with the fibers’ intrinsic high emissivity in the mid-infrared, enables the cocoon to function as an efficient radiative-cooling device, preventing the pupa inside from overheating. The reduced dimensionality of the random voids leads to strong optical scattering in the transverse direction of the cocoon fibers. This enables tightly confined optical modes to propagate along the fibers via transverse Anderson localization. We made the first observation of transverse Anderson localization in a natural fiber and further demonstrated light focusing and image transport in the fibers. This discovery opens up the possibility to use wild silk fibers as a biocompatible and bioresorbable material for transporting optical signals and images. Drawing inspirations from these discoveries, we designed and developed high-throughput fabrication processes to create coatings and fibers with passive radiative-cooling properties. The radiative-cooling coatings consist of various nanoparticles imbedded within a silicone thin film. The sizes and materials of the nanoparticles were chosen to provide simultaneously high solar reflectivity and thermal emissivity. The coating has been implemented in two site studies on real roofs and has demonstrated reduced roof temperature by up to 30oC in the summer and associated reduction of electricity usage by up to 30%. We also made biomimetic fibers from regenerated silk fibroin and a thermoplastic using wet spinning. Spectroscopic measurements showed that these man-made fibers exhibit exceptional optical properties for radiative-cooling applications

Plasmon-Enhanced Optical Sensing by Engineering Metallic Nanostructures

The world’s booming population projected to reach 10 billion by 2050 causes enormous stresses on environmental safety, food supply, and healthcare, which in return threatens human civilizations. One of the most promising solutions lies at innovating point-of-care (POC) sensing technologies to conduct detection of environmental hazards, monitoring of food safety, and early diagnosis of diseases in a timely and accurate manner. The discovery of surface-enhanced spectroscopy in the 1970s has significantly stimulated research on light-matter interaction which gives rise to enhanced optical phenomena such as surface-enhanced Raman scattering (SERS), plasmon-enhanced fluorescence (PEF), and particularly, they have found enormous applications in optical sensing. To fully exploit surface-enhanced spectroscopy to advance sensing technologies, it requires innovations in the sensor design as well as the plasmonic metallic nanostructures, which is exactly the focus of this dissertation. Owing to their strong capabilities of revealing molecular fingerprints and conducting single molecule analysis, both SERS and PEF have received extensive research interests. Since SERS directly correlates with the local electromagnetic (EM) field enhancement, it is featured by the simplicity in signal amplification. However, high SERS spectral resolution cannot be achieved without a tightly focused laser beam, which compromises the design of SERS-based POC sensing platforms. In contrast, the emission nature of fluorescence makes PEF easily coupled with POC readers, but optimal PEF requires a delicate control of the separation distance between the fluorophore and the nanostructure to minimize fluorescence quenching. SERS and PEF are essentially two complementary techniques and both hold great promise for POC sensing technologies. In the dissertation, in the first place, two label-free SERS sensors have been developed aiming to reduce the number of elements used in a sensor, which could potentially minimize interference, reduce the cost, and enhance the performance. In this regard, a label-free SERS sensor for mercury ions (Hg2+) detection has been developed based on functionalized gold nanoparticles, which employs a small molecule 4-mercaptobenzoic acid to capture mercury ions. A coordination bond formed only in the presence of mercury ions produces a new SERS peak at 374 cm-1, allowing unique detection of mercury ions. The other label-free SERS sensor has been developed for nitrite (NO2-) detection following the mechanism of Griess reaction based on the plasmonic coupling between gold nanostars and silver nanopyramid arrays. A newly formed azo compound produces at least three characteristic SERS peaks at 1140 cm-1, 1389 cm-1, and 1434 cm-1, which allow a highly specific detection of nitrite. While label-free SERS sensing has proved effective to enhance the performance, the need for a tightly focused laser beam hinders SERS from being easily coupled with POC readers for rapid signal readout. To address this limitation of SERS, on-chip PEF sensors have been developed, which can be inserted into POC readers for rapid signal readout. Optimizing PEF usually requires a delicate control of the separation distance between the fluorophore and the nanostructure to balance the excitation and emission enhancement which have different distance dependence. In addition to the separation distance, scattering has been found to be strongly correlated with quantum efficiency enhancement, which has been established as another tuning parameter in optimizing PEF. By making PEF work in the near-infrared (NIR) biological transparency window, the strength of PEF is further manifested by its compatibility with biological matrix featured as low background interference and high penetration depth. As a proof of concept, a NIR fluorescent biosensor has been developed for detection of traumatic brain injury biomarker in the blood plasma. The selection of a gold nanopyramid array pattern as the sensing platform not only generates intense localized EM field for the excitation enhancement, but also allows all the tests to be conducted using a POC fluorescence reader. While noble metals such as gold and silver are often used in developing sensing technologies as they support strong localized surface plasmon resonance (LSPR), it remains an open question as whether they could be replaced by alternative inexpensive metals such as copper and aluminum without compromising the performance. The discovery of a strong and sharp LSPR on copper nanoparticles when the shape is made cubic strongly suggests this possibility. By means of a numeric and theoretical study, it is found that the observed LSPR on copper nanocubes originates from the corner mode which survives damping as it is spectrally separated from the interband transitions. Compared to the dipole mode of a gold nanosphere of the same volume, a copper nanocube displays a comparable extinction coefficient but a local EM field enhancement 7.2 times larger. Furthermore, a film-coupled copper nanocube system has been designed for plasmon-enhanced NIR fluorescence. Because of the coupling between the copper nanocube and the underlying film, a plasmonic cavity mode is generated and featured as a spectrally tunable LSPR and an intense local EM field. By tailoring the resonance to the NIR wavelength region, the film-coupled copper nanocube system has been demonstrated to support a large NIR fluorescence enhancement owing to the strong excitation enhancement and the quantum efficiency enhancement

Doctor of Philosophy

dissertationThe synthesis, characterization, and nonclassical optical properties of photonic crystals (PCs) created from naturally occurring biological templates was studied. Biotemplated PCs were created from several different natural structures using sol-gel chemistry methods. PCs were characterized using a combination of reflection spectroscopy, SEM image analysis, three-dimensional structure modeling, photonic band structure calculations, and density of optical states calculations. The effect our PCs had on the density of optical states (DOS) was probed using time correlated single photon counting spectroscopy. By carefully controlling the sol-gel chemistry used in the templating process, it is possible to synthesize hollow silica inverse, solid silica inverse, hollow titania inverse, solid titania inverse, and solid titania replicate structures. The inverse-type structures have the advantage of being accessible through a single templating step, while the titania replica is capable of a predicted full photonic band gap. Each structure was investigated using methods mentioned above. The reliability of reflectance spectroscopy was investigated. It was found that in certain cases, a continuum of structural parameters yield reflections that match photonic band structure calculations. Methods to improve this situation are discussed. When applied to titania inverse opals, it was found that the refractive index could be determined to ±0.05 and the volume fraction to ±0.5%. Accurately determining the refractive index of inverse opals is useful in estimating the refractive index of other PCs made from the same sol-gel. Calculation of the DOS using a combination of MIT's photonic bands package and house-written software was applied to biotemplated photonic crystals. It was found that even partial band gap photonic crystals can greatly modify the DOS. Finally, the rate of spontaneous emission of quantum dots embedded in photonic crystals was measured to indirectly probe the DOS. Three different models were used to extract the lifetime from radiative decay curves. It was found that a log-normal distribution of lifetimes was the most meaningful model. The radiative lifetime of quantum dots embedded in titania photonic crystals replicated from Lamprocyphus augustus was modified by up to a factor of ten, an amount unprecedented in the photonic crystal literature

An experimental and numerical study of evaporation enhancement and combustion in porous media.

The results showed that the pressure drop across the porous media increased as the coflow air velocity, temperature, and linear pore density of the medium were increased. The measured and predicted surface temperatures of evaporation and combustion porous media showed that the temperature distribution was uniform within +/- 25 K and 50 K, respectively. The droplet Sauter mean diameter data revealed that the spray core region contained droplets with lower diameter, and the droplet diameter increased radially outward. A heat feedback rate to the evaporation porous medium section of about 1% of the average heat release in the combustion section was needed to completely vaporize the kerosene fuel. The vapor concentration level downstream of evaporation porous medium with 1% combustion heat release feedback was 63% higher than that with no heat feedback.Our results also suggest that the use of porous media in combustors allows operation at a lower coflow air temperature or with a shorter evaporation section. The porous-medium-burner concepts developed in this dissertation can be employed in many practical liquid combustion systems such as gas turbine combustors, air-heating systems, industrial burners, porous chemical reactors, heat recovery systems, and hybrid burners for bio-fuels.Stable spray flames were established both inside (referred to as interior flames) and on the downstream exit surface (surface flames) of the combustion porous medium. The equivalence ratio at flame extinction in each mode was determined. The extinction equivalence ratio decreased with a decrease in coflow air velocity. A nominal value of Damkohler number of 5.0 was required to initiate the interior combustion mode. As Damkohler number was increased, the extinction equivalence ratio decreased (i.e., extending the fuel lean operation). The axial temperature profiles in evaporation and combustion porous media were measured. Also measured were the radiative heat release from porous medium downstream exit surface, and pollutant emissions of carbon monoxide and nitric oxide. The results demonstrate the benefits of porous medium in making NO emission somewhat insensitive to operating parameters such as equivalence ratio and location of injector.Blocks of open-cell, silicon carbide coated, carbon-carbon ceramic foam of bulk cross section 4 x 4 cm and thickness of 2.5 cm were used as porous medium sections for liquid evaporation and subsequent combustion. Liquid fuel (kerosene, n-heptane, and methanol) was sprayed into a co-flowing, preheated (350--490 K) air environment using an air-blast atomizer, and the spray subsequently entered the porous medium. In controlled evaporation studies, combustion heat feedback to evaporation porous medium was simulated with a resistive heating mechanism. The minimum heat feedback rate required for complete vaporization of liquid and the vapor concentration profiles downstream of evaporation porous medium were measured. The stable operating regimes of spray flames in the combustion porous medium were determined and a general understanding of flame extinction in porous media was developed using a Damkohler number analysis.A two-energy equation model was developed to study the evaporation enhancement of liquid spray in the porous media. Combustion in the porous media was simulated by using a uniform volumetric heat source in the porous region. The solid and gas phase equations were coupled using a volumetric heat transfer coefficient. The computer simulations were performed with a commercial code, Fluent(TM) 6.0.Combustion of gaseous fuels in porous media improves combustion performance and reduces pollutant emissions by transferring combustion heat upstream via conduction and radiation to preheat reactants. Such heat feedback may be beneficially exploited to enhance vaporization of a liquid sprayed upstream of the porous medium, in addition to improving combustion performance. This dissertation presents an experimental and computational study of evaporation enhancement and combustion of liquid spray aided by porous media