Alok Mani Srivastava

Spectroscopic Evaluation of a White Light Phosphor for UV-LEDs— Ca2NaMg2V3O12 : Eu3 +

The luminescence of the vanadate garnet, Ca 2 NaMg 2 V 3 O 1 2 , activated with trivalent europium is evaluated and discussed as a potential ultraviolet light-emitting diode (UV-LED) phosphor. This single phosphor is capable of converting the ultraviolet emission of a UV-LED into white light with good luminosity and color-rendering index. We have evaluated the luminescence of this material at elevated temperatures, because the junction temperatures of typical LEDs can be greater than 100°C. There is significant thermal quenching of this phosphor and emission color shift at T > 100°C. This is attributed to energy migration and transfer to nonradiative traps and Eu 3 + within the host lattice. These implications on the use of this phosphor in UV-LED based light sources is also discussed.

Photon Cascade Luminescence of Pr3 + in LaMgB5 O 10

The observation of photon cascade emission in Pr 3+ activated LaMgB 5 O 10 under 185 nm vacuum ultraviolet excitation is reported and discussed. In this host lattice, the lowest energy state of the Pr 3+ 4f5d excited configuration is located above the 1 S o state. The excitation of the 4f5d state by vacuum ultraviolet photons results in radiative emission originating from the 1 S o level state. The absence of emission from the 3 P o state is attributed to the efficient multiphonon 3 P o → 1 D 2 relaxation in this lattice.

White light with UV LEDs

The advantages of near UV LED chips with phosphors for white light generation are discussed. Recently developed UV LED excitable phosphor blends are presented. Monte Carlo simulations suggest low color point variation (entirely within the first MacAdam oval) for the standard LED chip bin (400-410 nm), compared to high color point variation (outside the fourth MacAdam oval) for the standard bin (460-470 nm) and typical phosphors, modeled as Gaussians of realistic spectral width and targeting the 3000K ANSI color point (x=0.440, y=0.403). A discussion of the full LED package performance is also offered.

On the Vacuum‐Ultraviolet Excited Luminescence of Pr3 + in LaB3 O 6

The observation of luminescence from the Pr 3 1 S D state in LaB 3 0 6 under vacuum ultraviolet excitation is reported and discussed. The photon cascade emission consists of the 1 S 0 → 1 | 6 and 1 D 2 → 3 H 4 transitions. The efficient multiphonon 3 P 0 → 1 D 2 nonradiative relaxation is responsible for the absence of 3 P 0 luminescence in LaB 3 O 6 .

Effects of Localized Heat Generations Due to the Color Conversion in Phosphor Particles and Layers of High Brightness Light Emitting Diodes

The efficiency and reliability of the solid-state lighting devices strongly depend on successful thermal management. Light emitting diodes, LEDs, a strong candidate for the next generation general illumination applications are of interest. Typical white LEDs start with either blue or near UV light generated by the active quantum layers. The light is guided through a transparent encapsulant filled with micron sized phosphor particles. The phosphor particles up-convert the short wavelength light to desired colors, producing white light. Due to low quantum efficiency, during the conversion, localized heating of small particles occurs. Experimental results with high brightness LED packages showed that there is significant light output reduction. Idealized numerical models through Finite element technique were created to evaluate the effects of localized heat generations at particles and layers. Results showed that as small as a 3 mW heat generation on a 20 μm diameter spherical phosphor particle might lead to excessive temperatures which can be a major source of light output degradation and reliability concern for high brightness LEDs.Copyright

Luminescence of Bi3+ in LaGaO3

The luminescence of Bi3+-activated LaGaO3 is evaluated in the temperature range of 15.0 to 400 K. The Stokes shift of the Bi3+ emission is small (0.75 eV). Thermal quenching of Bi3+ luminescence in LaGaO3 is due to energy transfer between the Bi3+ ions, to host lattice quenching sites. It is concluded that the optical properties of Bi3+ are not influenced by the Ga3+ ions (3d10) of the LaGaO3 host lattice. A comparative study of the Bi3+ ion luminescence in the perovskites LaAlO3, LaGaO3, and LaInO3 is presented.

Charge creation, trapping, and long phosphorescence in Sr2MgSi2O7:Eu2+, RE3+

This report describes experiments that elucidate the phosphorescence mechanism in Sr2MgSi2O7:Eu2+, RE3+ phosphors. The first step of phosphorescence, the storage of carriers in traps, is traced to thermally assisted photoionization of Eu2+ ions, supported by thermal quenching studies that give the thermal ionization threshold of Eu2+. The trapping of these delocalized electrons has been detected using the spectroscopic signature of Sm2+ ions created by electron capture by Sm3+ in samples doped with Eu2+ and Sm3+ ions. The final steps for phosphorescence, liberation of trapped carriers and recombination with the luminescence center, have also been studied by monitoring the Eu2+ luminescence induced by optical excitation into the Sm2+ 4f6→4f55d1 absorption band. These results show that the phosphorescence in this system starts with the promotion of electrons into the conduction band and subsequent reduction of RE3+ ions, not from hole formation and RE3+ oxidation, as has been reported for similar systems. F...

Nature of luminescent centers in cerium-activated materials with the CaFe2O4 structure

Abstract The cerium luminescence in SrY 2 O 4 shows two bands emitting in the blue and green regions of the optical spectrum. Based on standard emission and excitation spectroscopy combined with time-resolved emission studies, we can attribute the blue emission to a Ce 3+ 5d–4f transition and tentatively assign the green emission to a “Ce 4+ –O 2− ”charge transfer transition.

The mixing of the 4f2 1S0 state with the 4f5d states in Pr3+ doped SrAl12O19

Abstract The 1 S 0 state of Pr 3+ in SrAl 12 O 19 lies near to the 4f5d band. The admixing of the opposite parity 5d components by the crystal field without a symmetry center can be significant, thus affecting the properties of the 1 S 0 state. In a crystal with D 3h symmetry, the major 4f5d components which can be mixed into 1 S 0 are 2 −1/2 (|fdΓ 1 1 F 3 3〉-|fdΓ 1 1 F 3 -3〉) and 2 −1/2 (|fdΓ 1 1 H 5 3〉-|fdΓ 1 1 H 5 -3〉). A calculation of the 4f5d wavefunctions indicates that the states which may be mixed into 1 S 0 lie mainly at the middle and upper parts of the 4f5d configuration. The mixed 4f 2 1 S 0 wavefunction is then calculated and its spectral properties are discussed.

Optical Spectroscopy and Crystal Field Studies of the Mn4+ Ion (3d3) in the Double Perovskite NaLaMgTeO6

The spectroscopic properties of the Mn4+ ion (3d3) in the double perovskite NaLaMgTeO6 are reported in this work. Evidence is presented for the occupation by the Mn4+ ion of both the six coordinated Mg2+ and Te6+ sites in the host structure. The Mn4+ energy levels are calculated using the exchange charge model of crystal field theory for both occupied sites. The results of our calculations yield the crystal field splitting and Racah parameters of Dq = 2008 cm−1, B = 790 cm−1, C =2881 cm−1, with C/B=3:65 (Mg2+ site) and Dq=2008 cm−1, B=790 cm−1,C =2949 cm−1, with C/B = 3:73 (Te6+ site). A cross-cutting comparative study of the variations in the crystal field splitting and the Racah parameters of the six-coordinated Mn4+ ion in a series of materials with the perovskite structure are presented. Graphical Abstract Optical Spectroscopy and Crystal Field Studies of the Mn4+ Ion (3d3) in the Double Perovskite NaLaMgTeO6