Dr. Ross S. Fontenot Profile

Dr. Ross S. Fontenot

Lead Technical Analyst at DHS

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Proceedings Article | 17 May 2016 Paper

Measuring the anti-Stokes luminescence of CdSe/ZnS quantum dots for laser cooling applications

Ross Fontenot, Veerendra Mathur, John Barkyoumb, Carl Mungan, Thanh Tran

Proceedings Volume 9821, 982103 (2016) https://doi.org/10.1117/12.2222826

KEYWORDS: Cooling systems, Solids, Luminescence, Quantum dots, Laser applications, Nanomaterials, Cadmium sulfide, Optical cooling, Semiconductor lasers, Phonons, Polymers, Photons, Semiconductors, Quantum efficiency

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The first demonstration of laser cooling of solids was of an ytterbium doped fluorozirconate glass. While this groundbreaking work successfully showed that it is possible to cool solids using laser cooling, rare-earth materials are governed by Boltzmann statistics limiting their cooling ability to about 100 K. Direct-bandgap semiconductors, on the other hand, are governed by Fermi-Dirac statistics, which allows for a theoretical cooling limit of 10 K as well as higher cooling efficiencies. Recently, it was demonstrated that it is possible to cool CdS nanoribbons by 40 K. That success was attributed to CdS strong electron-phonon coupling, which makes it possible to resonantly annihilate more than one longitudinal optical phonon during each up conversion cycle. To further increase the cooling power, large external quantum efficiency is required. A nanostructure is preferred because it creates confined excitons of tunable wavelength and reduces the self-absorption of the anti-Stokes fluorescence owing to the shorter path length for photons to escape the crystal. However, organically passivated quantum dots have a low quantum yield due to surface related trap states. A core-shell nanostructure alleviates this problem by passivating the surface trap states and protecting against environmental changes and photo-oxidative degradation. As such, we chose to investigate CdSe/ZnS core shell structure for laser cooling applications. This article highlights the measurement of the anti-Stokes luminescence, the dependence of the laser wavelength on the anti-Stokes emission of colloidal quantum dots, and the successful incorporation of CdSe/ZnS into polymers.

Proceedings Article | 13 May 2016 Paper

Using luminescent materials as the active element for radiation sensors

William Hollerman, Ross Fontenot, Stephen Williams, John Miller

Proceedings Volume 9838, 98380Z (2016) https://doi.org/10.1117/12.2228934

KEYWORDS: Sensors, Luminescence, Ionizing radiation, Lead, Europium, Glasses, Ultraviolet radiation, Crystals, Particles, Uranium

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Ionizing radiation poses a significant challenge for Earth-based defense applications as well as human and/or robotic space missions. Practical sensors based on luminescence will depend heavily upon research investigating the resistance of these materials to ionizing radiation and the ability to anneal or self-heal from damage caused by such radiation. In 1951, Birks and Black showed experimentally that the luminescent efficiency of anthracene bombarded by alphas varies with total fluence (N) as (I/I₀) = 1/(1 + AN), where I is the luminescence yield, I₀ is the initial yield, and A is a constant. The half brightness (N_1/2) is defined as the fluence that reduce the emission light yield to half and is equal to is the inverse of A. Broser and Kallmann developed a similar relationship to the Birks and Black equation for inorganic phosphors irradiated using alpha particles. From 1990 to the present, we found that the Birks and Black relation describes the reduction in light emission yield for every tested luminescent material except lead phosphate glass due to proton irradiation. These results indicate that radiation produced quenching centers compete with emission for absorbed energy. The purpose of this paper is to present results from research completed in this area over the last few years. Particular emphasis will be placed on recent measurements made on new materials such as europium tetrakis dibenzoylmethide triethylammonium (EuD₄TEA). Results have shown that EuD₄TEA with its relatively small N_1/2 might be a good candidate for use as a personal proton fluence sensor.

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