Ultra-high, broadband transmittance through coated glass windows is demonstrated over a wide range of incident angles. Near perfect 100% transmittance through a glass substrate has been achieved over select
spectral bands, and the average transmittance increased to over 97% for photons incident between 0° and 75° with wavelengths between 400 nm and 1600 nm. The measured improvements in transmittance result from coating the windows with a new class of materials consisting of porous SiO2 nanorods.
There exists a fundamental trade-off relation between color rendering index (CRI) and luminous efficacy; in other words,
improvements in one are generally detrimental to the other. We analyze and demonstrate through simulation that
phosphor-converted white LEDs with dual-blue emitting active regions, as opposed to single-blue emitting active
regions, significantly enhance color rendering ability while maximizing the output luminous flux. The improvements are
achieved over a broad range of correlated color temperatures.
Oblique angle deposition allows the fabrication of nano-structured porous thin films of high optical quality. By selecting
the incident angle, the porosity - and thereby, the refractive index - of the deposited film can be tuned to a specific
desired value. This makes it possible to fabricate multi-layer optical thin film components consisting entirely of a single
material which is chosen for its properties other than refractive index, such as optical absorption or conductivity. As an
application for this technique we demonstrate a conductive distributed Bragg reflector (DBR) designed for 460 nm.
Common material choices in this wavelength range are SiO2 and TiO2; however, both materials are insulating.
Conductive DBRs are limited to epitaxially grown doped semiconductors, which generally have low index contrast. The
DBR reported here is composed entirely of indium tin oxide (ITO), chosen for its conductivity and low absorption. By
varying the deposition angle a refractive index contrast of Δn = 0.4 is achieved, which yields a measured reflectivity of
72.7% for a three-period low-porosity-ITO/high-porosity-ITO DBR. The reflectivity is in excellent agreement with
The junction temperature of red (AlGaInP), green (GaInN), blue (GaInN), and ultraviolet (GaInN) light-emitting diodes (LEDs) is measured using the temperature coefficients of the diode forward voltage and of the emission-peak energy. The junction temperature increases linearly with DC current as the current is increased from 10 mA to 100 mA. For comparison, the emission-peak-shift method is also used to measure the junction temperature. The emission-peak-shift method is in good agreement with the forward-voltage method. The carrier temperature is measured by the high-energy-slope method, which is found to be much higher than the lattice temperature at the junction. Analysis of the experimental methods reveals that the forward-voltage method is the most sensitive and its accuracy is estimated to be ± 3°C. The peak position of the spectra is influenced by alloy broadening, polarization, and quantum confined Stark effect thereby limiting the accuracy of the emission-peak-shift method to ±15°C. A detailed analysis of the temperature dependence of a tri-chromatic white LED source (consisting of three types of LEDs) is performed. The analysis reveals that the chromaticity point shifts towards the blue, the color-rendering index (CRI) decreases, the color temperature increases, and the luminous efficacy decreases as the junction temperature increases. A high CRI > 80 can be maintained, by adjusting the LED power so that the chromaticity point is conserved.