There is an increasing need for high power density light sources, e.g. for the next generation of car headlights, diode laser pumped white light sources and projection devices. However, saturation and droop at high excitation densities limit the light output in high power devices. Excited state absorption and long excited state lifetimes play a role, but the relation between light output and excitation power is a poorly understood and is complex interplay of quenching processes including reabsorption and (transient) color center formation. The development of superior materials is crucial and relies on a better understanding of droop processes and the relation with the nature and processing condition of light conversion materials.
In this presentation a basic (and hopefully insightful) overview of known luminescence quenching processes will be followed by a discussion on how we can increase our understanding of luminescence quenching with a focus on high power applications. A variety of quenching mechanisms will be evaluated and illustrated for known and new luminescent materials. New experimental and theoretical capabilities will be discussed that may help to acquire new insights in what limits the light output in current and future light sources.
We present an optical concept to visualize nerves during surgical interventions. The concept relies on the anisotropic optical properties of the nerves which allows for specific switching of the optical reflection by the nervous tissue. Using a low magnification polarized imaging system we are able to visualize the on and off switching of the optical reflection of the nervous tissue, enabling a non-invasive nerve specific real-time nerve visualization during surgery.
In a Luminescent Solar Concentrator (LSC), short-wavelength light is converted by a luminescent material into longwavelength
light, which is guided towards a photovoltaic cell. In principle, an LSC allows for high concentration, but in
practice this is prevented by loss mechanisms like limited sunlight absorption, limited quantum efficiency and high self
absorption. To tackle these problems, a suitable luminescent material is needed. Another important loss mechanism is the
escape of luminescent radiation into directions that do not stay inside the light guide. To reduce this amount, wavelengthselective
filters can be applied that reflect the luminescent radiation back into the light guide while transmitting the
incident sunlight. In this paper, we discuss experiments and simulations of new luminescent and filter materials. We will
introduce a phosphor with close-to-optimal luminescent properties. A problem for use in an LSC is the large scattering of
this material; we will discuss possible solutions for this. Furthermore, we will discuss the use of broad-band cholesteric
filters in combination with this phosphor.
We review the use of nanometer-sized particles (including quantum dots) in the conversion of parts of the solar spectrum
incident on solar cells to more usable regions. The modification of the solar spectrum ideally would lead to a narrowbanded
incident spectrum at a center wavelength corresponding to an energy that is slightly larger than the band gap of
the semiconductor material employed in the solar cell, which would lead to an enhancement of the overall solar energy
conversion efficiency. Modification of the spectrum requires down and/or up conversion or shifting of the spectrum,
meaning that the energy of photons is modified either to lower (down) or higher (up) energy. Nanostructures such as
quantum dots, luminescent dye molecules, and lanthanide-doped glasses are capable of absorbing photons at a certain
wavelength and emitting photons at a different (shorter or longer) wavelength. We will discuss down and up conversion
and shifting by quantum dots, luminescent dyes, and lanthanide compounds, and assess their potential in contributing to
ultimately lowering the cost per kWh of solar generated power.
Nano structured Lu2O3, both plane and doped with Tb, was prepared utilizing a combustion technique. The best crystallity of the products can be obtained initiating the reaction within 560-700 $DEGC range of temperature. Tb easily enters the nano scaled host lattice both as Tb3+ and Tb4+. The former gives rise to a typical green emission of the ion, while the later introduces a broad-band visible absorption, due to charge transfer transitions. The green emission of Tb3+ from a raw material may be radically increased by after- preparation heat-treatment. Undoped material gives rise to a blue emission, which disappears when Tb content with respect to Lu reaches 0.0001% or higher level.
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