John D. Joannopoulos

Design and Fabrication of Photonic Crystals for Thermal Energy Conservation

The vision of intelligent and large-area fabrics capable of signal processing, sensing and energy harvesting has made incorporating electronic devices into flexible fibers an active area of research. Fiber-integrated rectifying junctions in the form of photovoltaic cells and light-emitting diodes (LEDs) have been fabricated on optical fiber substrates. However, the length of these fiber devices has been limited by the processing methods and the lack of a sufficiently conductive and transparent electrode. Their cylindrical device geometry is ideal for single device architectures, like photovoltaics and LEDs, but not amenable to building multiple devices into a single fiber. In contrast, the composite preform-to-fiber approach pioneered in our group addresses the key challenges of device density and fiber length simultaneously. It allows one to construct structured fibers composed of metals, insulators and semiconductors and enables the incorporation of many devices into a single fiber capable of performing complex tasks such as of angle of incidence and color detection. However, until now, devices built by the preform-to-fiber approach have demonstrated only ohmic behavior due to the chalcogenide semiconductors amorphous nature and defect density. From a processing standpoint, non-crystallinity is necessary to ensure that the preform viscosity during thermal drawing is large enough tomorexa0» extend the time-scale of breakup driven by surface tension effects in the fluids to times much longer than that of the actual drawing. The structured preform cross-section is maintained into the microscopic fiber only when this requirement is met. Unfortunately, the same disorder that is integral to the fabrication process is detrimental to the semiconductors electronic properties, imparting large resistivities and effectively pinning the Fermi level near mid-gap. Indeed, the defect density within the mobility gap of many chalcogenides has been found to be 1018-1019 cm-3 eV-1, resulting in a narrow depletion width and ohmic behavior at metal-semiconductor junctions. In this work we incorporated phase-changing semiconductors, those that may be easily converted between the amorphous and crystalline states, into composite fibers with a goal towards constructing rectifying junctions in fiber.«xa0less

A TRANSIENT SEMI‐METALLIC LAYER IN DETONATING NITROMETHANE

We present the first ever glimpse behind a detonation shock front in a chemically reactive quantum molecular dynamics simulation of the explosive nitromethane (CH3NO2). We discover that the wide‐bandgap insulator nitromethane undergoes chemical decomposition and a transformation into a semi‐metallic state for a limited distance behind the detonation front. We find this transformation is associated with the production of charged decomposition species.We present the first ever glimpse behind a detonation shock front in a chemically reactive quantum molecular dynamics simulation of the explosive nitromethane (CH3NO2). We discover that the wide‐bandgap insulator nitromethane undergoes chemical decomposition and a transformation into a semi‐metallic state for a limited distance behind the detonation front. We find this transformation is associated with the production of charged decomposition species.