Active tuning has long been a goal for photonic metamaterial devices. Several approaches have succeeded in providing active tuning. These include mechanical deformation, the incorporation of an active liquid crystal layer, electrically induced permittivity modulation, and the use of phase change materials. In this presentation, we describe a novel method of tuning the resonance of a metamaterial in which an optically transparent thin film, referred to here as a “shifter,” is placed in proximity to a dielectric metasurface. The spacing between the metasurface and the shifter is carefully controlled by a piezoelectric transducer. Device designs for the midwave infrared (MWIR) based on chalcogenide glass films are presented. Modelling shows a tuning range of approximately 500 nm in the MWIR for a change in shifter spacing of approximately 700 nm. It is shown that the size and shape of the field at an individual resonator changes significantly based on shifter spacing, resulting in a large tuning range. The piezo shifting method described here represents a new technique for tuning the resonance of a metasurface over a large-area with a large tuning range.
Degraded visual environments (DVEs) are the result of a high concentration of obscurants in the air, and occur often during rotary wing aircraft landings. When the obscurants are comprised of sand this leads to a condition called “brownout.” Current DVE penetrating technology is severely limited by a weak return signal in severe brownout conditions. One method of overcoming this weak return signal is to use light tuned to the Christiansen wavelength, the wavelength at which the refractive index of the scatterer matches that of the surrounding medium, eliminating the scattering effect associated with the obscurant particles. We have previously shown a novel method for determining the optical constants of particulate samples using spectroscopic ellipsometry and determined that, for several different sand samples, the Christiansen wavelength is approximately 8 microns. We present data from falling sand of several different types using Fourier transform infrared (FTIR) spectroscopy showing a higher transmission percentage at the Christiansen wavelength. We explain features in this data using previous measurements of the optical properties of these sands.
All organisms carry out metabolic processes to produce chemical energy, but these biochemical pathways are not perfectly efficient and energetic waste is lost as heat. The interplay of heat production/retention/loss in endotherms has been well studied via thermal imaging. However, there is a striking absence of literature on the thermal output of ectotherms, especially invertebrate animals. We have developed a new thermal imaging technique to investigate waste heat production in the nematode worm C. elegans. No direct measurement of this metabolic waste heat has been made in C. elegans or any other mesoinvertebrate using IR imaging techniques.
In this study, thermal IR imaging was used to examine the difference in heat output between living and dead C. elegans. Living and dead C. elegans were imaged simultaneously providing a way to directly compare the temperatures of the worms. Temperature difference was used as a marker of difference in waste heat production, not absolute temperature. A cold object was used in reflectance mode to suppress the thermal background of the imaging substrate. Several different substrates with differing thermal properties were tested to minimize thermal background. The tendency for C. elegans to desiccate necessitated the development of sample preparation techniques that ensured the survival of the animals during imaging. Imaging revealed that there is a clear, repeatable difference in the thermal output of living C. elegans compared to dead animals (whose metaobolic processes have ceased). This is exciting as it points to IR imaging as being a novel investigative tool to be applied to the study of metabolism in C. elegans. In the future this opens the door to screening genetic mutants with known metabolic defects, thus providing useful data for the study of genes that impact metabolism.
Inadequate tissue perfusion is a fundamental cause of early complications following a range of procedures including the creations of skin flaps/grafts during reconstructive microsurgery and complex closures during amputation. Clinical examination remains the primary means of evaluating tissue perfusion intraoperatively. Recently, indocyanine green (ICG) angiography has been used as an adjunt to physical examination. However, ICG angiography is an invasive procedure that requires the intravenous application of a fluorescent dye.
Enhanced thermal imaging (ETI) is a non-invasive, real-time infrared imaging technique that can detect blood vessels embedded in soft tissue. ETI uses selective heating of blood via illumination with a green (532 nm) LED to produce a thermal contrast (≥ 0.5 ◦C) between blood vessels and surrounding water-rich tissue. Vessel-rich regions appear brighter in the thermal image. ETI does not require the use of dyes and recent improvements to the acquisition software have enabled real-time imaging. The compact footprint of the system could allow for use both intraoperatively and at the bedside.
In this study we evaluate the ability of ETI to assess tissue perfusion of skin flaps in a murine model. The healing and perfusion of these flaps was monitored via the density of capillary beds and vascular networks using visual inspection, fluorescent imaging, and ETI over a 12-day study period. We compare the ability of these techniques to detect early indications of necrosis and re-vascularization in grafts.
Enhanced Thermal Imaging (ETI) is a new thermal infrared (8-10 μm) imaging technique that delineates blood vessels embedded in water-rich tissue in real time. ETI uses selective heating of blood via illumination with a green (532 nm) LED to produce a thermal contrast (∼ 0.5°C) between blood vessels and surrounding water-rich tissue. The warmer blood vessels appear brighter in the thermal image. In a previous study, the growth of breast cancer tumors in an 4T1 murine orthotopic model was successfully monitored in vivo using ETI. The images highlighted regions that are routinely targeted for surgical excision around solid mass tumors. Recently, improvements to the acquisition software have enabled real-time imaging with this technique, highlighting ETI’s potential use as an intraoperative imaging tool. In this study, simulations of direct illumination and heating of the blood vessels embedded in tissue were conducted to understand the effects of LED power and vessel depth on the ability of ETI to detect vascular structures. The simulations were performed with an open-source MATLAB integrated solver, MCmatlab.
We are developing a new IR imaging technique, enhanced thermal imaging, that can delineate blood vessels embedded in water-rich tissue in real time. The technique is a combination of thermal IR imaging (8-10 μm) and selective heating of blood (∼0.5°C) relative to surrounding water-rich tissue using LED sources (530 nm) at low powers (irradiance <500 mW/cm2 ). Blood absorbs light strongly at these wavelengths, while absorption by soft tissues is lower. Illumination of tissue containing vessels with the LED heats the blood compared to surrounding tissue so that the vessels look brighter in the thermal image. In previous work, we successfully highlighted vessels embedded in porcine heart tissue and the blood-rich vascular region that is routinely targeted for surgical excision around mammary tumor masses in a murine model. However, in this earlier work the IR images were recorded, and post-acquisition processing was necessary to highlight the presence of the vascular structures. In this work, we describe recent progress in making enhanced thermal imaging a real-time technique. A porcine tissue model with simulated blood vessels and the wrist/forearms of two human subjects were imaged with a FLIR thermal camera. The frame-to-frame temporal and spatial derivatives were calculated in real-time and then used to identify regions exhibiting rapid changes in temperature, thus highlighting the structure of blood-rich regions.
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