René Theodorus Wegh

Visible quantum cutting via downconversion in LiGdF4 : Er3+, Tb3+ upon Er3+ 4f11→4f105d excitation

Abstract Visible quantum cutting (the emission of two photons of visible light per absorbed photon) is reported for LiGdF4 codoped with Er3+ and Tb3+. In this compound visible quantum cutting is achieved through downconversion, i.e., by making use of energy transfer between different lanthanide ions. Upon VUV excitation in the 4f105d levels of Er3+, part of the excitation energy can be transferred to Gd3+ by cross-relaxation, yielding both Er3+ and Gd3+ in an excited state. This is followed by the emission of two visible photons: one from Er3+ due to the 4 S 3/2 → 4 I 15/2 transition, and, after energy transfer from Gd3+ to Tb3+, one from Tb3+ due to the 5 D J → 7 F J transition. It is shown that this process occurs, next to other energy transfer processes from the Er3+ 4f105d states to Gd3+ yielding only one visible photon.

Downconversion: a new route to visible quantum cutting

Abstract To obtain vacuum ultraviolet (VUV) phosphors with quantum efficiencies higher than 100%, the concept of downconversion is used. In a downconversion process a VUV photon is split into two visible photons by making use of energy transfer between different rare earth ions. Two examples of downconversion couples are discussed, viz. the Gd3+–Eu3+ couple and the Er3+–Gd3+–Tb3+ system.

Emission spectra and trends for 4fn−15d↔4fn transitions of lanthanide ions: Experiment and theory

Luminescence spectra of 4fn↔4fn−15d transitions of lanthanide ions in LiYF4 are reported. Some interesting trends are observed. Lifetime measurements for the spin allowed fd emission show that the electric dipole matrix element for the fd transition decreases through the lanthanide series. Also the splitting between the high spin and low spin fd states decreases through the lanthanide series. Both effects are reproduced by calculations using a theoretical model that extends established models for the 4fn configuration by including crystal field and spin–orbit interactions for the 5d electron and the Coulomb interaction between the 4f and 5d electrons.

Remote phosphor LED modules for general illumination: toward 200 lm/W general lighting LED light sources

Phosphor converted white LEDs are becoming more and more attractive for general lighting applications because of the steadily increasing luminous efficacy numbers reported by LED-suppliers. Despite these high numbers, a further significant improvement step can be made when a low-to-medium brightness (<500 kCd/m2) source is acceptable. The wall plug efficiency of a blue LED is generally better than that of a conventional white LED made from the same die. To take full advantage of this, we have developed medium-brightness LED-modules (~150 kCd/m2) for general lighting in which the phosphor is applied remote from the blue LEDs. By direct comparison with modules in which conventional high power white LEDs with almost identical dies are applied, we have shown that on system level the remote phosphor modules can have up to 50% better efficacy. Using a downlight module as a carrier, we have shown that in the relevant color temperature range of 2700 to 4000K a high CRI (>80) can be obtained in combination with a high luminous efficacy, while the optical efficiency of the module can be over 85%. A module efficacy of over 100 lm/W at 4000K with CRI 80 seems to be within reach, with a long-term expectation of over 180 lm/W. The remote phosphor LED modules deliver well homogenized white light with a Lambertian radiation profile. They are ideal for general illumination, as they combine glare reduction with high system efficacy and enable high optical efficiencies of the luminaries. The RP modules enable forward compatibility by well defined interfaces and optical properties that are decoupled from the actual performance, form factor and number of LEDs in the module. The Philips Fortimo downlight system is based on this remote phosphor concept, featuring forward compatibility and a total system efficacy (including driver) of over 60 lm/W under operating conditions using currently available Luxeon Rebel emitters.

Luminescence spectroscopy of high-energy 4f11 levels of Er3+ in fluorides

The 4f11 energy levels of Er3+ in LiYF4 in the spectral region 39000–65000 cm−1 have been studied. The agreement between experimental energy levels, obtained from luminescence excitation spectra, and calculated energy levels is good. Luminescence originating from high-lying energy levels has been investigated. Emission from the states 4D1/2 (∼47 200cm−1), 2F(2)7/2 (∼54700cm−1) and 2F(2)5/2 (∼63 300cm−1) is observed. 2F(2)5/2 emission occurs for Er3+ in LaF3, where the 2F(2)5/2 level is situated just below the lowest 4f105d state, but also for Er3+ in LiYF4, where it lies in between the two lowest 4f105d states.

4fn↔4fn−15d transitions of the trivalent lanthanides: experiment and theory

Abstract Emission and excitation spectra of 4fn↔4fn−15d transitions of lanthanide ions in LiYF4 are reported and are succesfully reproduced by energy level calculations using a theoretical model that extends established models for the 4fn configuration to include fd states. Some interesting trends are observed. Lifetime measurements for the spin-allowed fd emission show that the electric dipole matrix element for the fd transition decreases through the lanthanide series. Also the splitting between the high spin and low spin fd states decreases through the lanthanide series. Both effects are reproduced by our model.

27.3: New Cholesteric Colour Filters for Reflective LCDs

In this paper, we describe a new method to manufacture cholesteric colour filters. The cholesteric colour filter is made by a simple method, comprising one coating step and two UV exposure steps. The optical performance of the cholesteric colour filter and a reflective LCD with a cholesteric colour filter inside the cell will be discussed. The cholesteric colour filter has a high colour purity and the display shows a high contrast ratio of 35:1.

The variation of the electron-phonon coupling strength through the trivalent lanthanide ion series

Abstract The electron-phonon coupling for trivalent lanthanides is studied by temperature-dependent linewidth measurements. The results indicate that the coupling is strong at the beginning and at the end of the lanthanide series and weak in the middle.

New concepts for VUV phosphors with high quantum efficiency (q > 100%)

The first overview of the 4f levels of Gd 3+ in the VUV is reported. It is shown that quantum cutting is a very promising phenomenon to make VUV phosphors with a quantum efficiency greater than 100% and that the efficiency of this process can be improved by cross-relaxation. The visible quantum efficiency of LiGdF 4 :Eu 3+ 0.5 mol% upon excitation in the VUV is shown to be 195%.

26.5: Requirements on Backlights for LCD's with Transmissive Cholesteric Color Filters

Transmission spectra of interference color filters are calculated for various light sources and angular distributions of incident light. Using conventional lamps, the achievable color gamut is poor for all angular distributions. To improve performance, a different light source and pre-shifted plus broadened reflection windows of the interference filters should be used.