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15 Aug 2010

Volume 108, Issue 4, Articles (04xxxx)

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J. Appl. Phys. 108, 041901 (2010); http://dx.doi.org/10.1063/1.3474648 (2 pages)

Sergei V. Kalinin, Nava Setter, and Andrei L. Kholkin
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back to top Lasers, Optics, and Optoelectronics

Effects of distorted lattice and nonequal-valvence substitution on the long lasting phosphorescence of Eu2+ and Gd3+ doped RMg2(PO4)2 (R = Sr,Ba) phosphors

Liyan Liu, Ran Pang, Chengyu Li, and Qiang Su

J. Appl. Phys. 108, 043101 (2010); http://dx.doi.org/10.1063/1.3475989 (5 pages) | Cited 4 times

Online Publication Date: 16 August 2010

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The afterglow of Eu2+ activated SrMg2(PO4)2 can be greatly enhanced by the codoping of Gd3+, as well as an interesting phenomenon of the improvement of the efficiency of the excitation light. However, Eu2+ activated BaMg2(PO4)2 does not show the phosphorescence until the codoping of Gd3+. It is suggested that the codoping of Gd3+ improve the electron storage ability of material by acting as electron-trapping centers resulted from the nonequal-valence substitution of Gd3+ replacing Sr2+ and Ba3+. And the Gd3+-induced enhancement of the excitation efficiency of Eu and Gd codoped SrMg2(PO4)2 is due to the improvement of the energy transfer efficiency caused by the distortion of lattice of Gd3+ located at the sites of Sr2+.
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78.55.Hx Other solid inorganic materials
78.60.-b Other luminescence and radiative recombination
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Enhancement of photovoltaic cell response due to high-refractive-index encapsulants

Ming Ma, Frank W. Mont, David J. Poxson, Jaehee Cho, E. Fred Schubert, Roger E. Welser, and Ashok K. Sood

J. Appl. Phys. 108, 043102 (2010); http://dx.doi.org/10.1063/1.3466980 (3 pages) | Cited 2 times

Online Publication Date: 18 August 2010

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This study compares the electrical output of photovoltaic (PV) cells encapsulated with silicones having different refractive indices to unencapsulated PV cells. It is demonstrated that the optical concentration ratio of dome-shaped concentrator PV systems can be increased by using a higher refractive-index encapsulant. The short-circuit photocurrent of the PV cell having high-refractive-index encapsulation (n = 1.57) is 71% higher than that of the PV cell having a low-refractive-index encapsulation (n = 1.41), and 316% higher than that of the unencapsulated PV cell. These experimental concentration-ratio enhancements are consistent with the theoretical estimates of concentration ratio dependence on the refractive index of the PV concentrator.
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85.60.-q Optoelectronic devices

ZnO-organic hybrid white light emitting diodes grown on flexible plastic using low temperature aqueous chemical method

N. Bano, S. Zaman, A. Zainelabdin, S. Hussain, I. Hussain, O. Nur, and M. Willander

J. Appl. Phys. 108, 043103 (2010); http://dx.doi.org/10.1063/1.3475473 (5 pages) | Cited 13 times

Online Publication Date: 19 August 2010

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We demonstrate white light luminescence from ZnO-organic hybrid light emitting diodes grown at 90 °C on flexible plastic substrate by aqueous chemical growth. The configuration used for the ZnO-organic hybrid white light emitting diodes (WLEDs) consists of a layer of poly (9, 9-dioctylfluorene) (PFO) on poly (3, 4-ethylenedioxythiophene) poly (styrenesulfonate) coated plastic with top ZnO nanorods. Structural, electrical, and optical properties of these WLEDs were measured and analyzed. Room temperature electroluminescence spectrum reveals a broad emission band covering the range from 420 to 750 nm. In order to distinguish the white light components and contribution of the PFO layer we used a Gaussian function to simulate the experimental data. Color coordinates measurement of the WLED reveals that the emitted light has a white impression. The color rendering index and correlated color temperature of the WLED were calculated to be 68 and 5800 K, respectively.
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85.60.Jb Light-emitting devices
81.05.Dz II-VI semiconductors
81.07.Bc Nanocrystalline materials
78.60.Fi Electroluminescence
68.55.ag Semiconductors
78.66.-w Optical properties of specific thin films

Modeling high power light-emitting diode spectra and their variation with junction temperature

A. Keppens, W. R. Ryckaert, G. Deconinck, and P. Hanselaer

J. Appl. Phys. 108, 043104 (2010); http://dx.doi.org/10.1063/1.3463411 (7 pages) | Cited 5 times

Online Publication Date: 20 August 2010

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Spectral radiant flux is the primary optical characteristic of a light source, determining the luminous flux and color. Much research is dedicated to the modeling of light-emitting diode (LED) spectra and their temperature dependence, allowing for the simulation of optical properties in various applications. Most of the spectral radiant flux models that have been published so far are purely mathematical. For this paper, spectral radiant fluxes of commercial single color LED packages have been measured in a custom made integrating sphere at several junction temperatures by active cooling and heating with a Peltier element. A spectrum model at 300 K is constructed where the Boltzmann free carrier distribution and carrier temperature are included. Subsequently, the model is extended with the carrier temperature variation, the band gap energy shift, and the nonradiative recombination rate decrease with junction temperature. As a result, the skewness variation, peak frequency shift, and peak value change in the spectrum with temperature can be predicted. The model has been validated by comparing flux and color coordinates of measured and simulated spectra at 340 K junction temperature. In practice, only two spectral flux measurements at different junction temperatures are needed to accurately simulate a single color spectrum at any temperature.
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85.60.Jb Light-emitting devices

Intense whole area electroluminescence from low pressure chemical vapor deposition-silicon-rich oxide based light emitting capacitors

A. A. González Fernández, M. Aceves Mijares, A. Morales Sánchez, and K. M. Leyva

J. Appl. Phys. 108, 043105 (2010); http://dx.doi.org/10.1063/1.3465335 (4 pages) | Cited 2 times

Online Publication Date: 24 August 2010

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Light emitting capacitors (LECs) based on silicon-rich oxide (SRO) were fabricated and its electroluminescent (EL) characteristics studied. Thin SRO films with R0 = 30 were deposited by low pressure chemical vapor deposition and submitted to thermal treatment at 1100 °C for 180 min. Photoluminescence was observed in the SRO films and intense visible EL was obtained from fabricated LECs when stimulated with direct current. Strong intensity, naked eye visible, full area EL was obtained after the application of an electrical treatment. The EL was attributed to the presence of Si related defects and full area emission obtained was due to the optimization of carrier injection through the material by the annulations of preferential conductive paths.
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84.32.Tt Capacitors
85.60.Jb Light-emitting devices

Detection of concealed and buried chemicals by using multifrequency excitations

Yaohui Gao, Meng-Ku Chen, Chia-En Yang, Yun-Ching Chang, Jim Yao, Jiping Cheng, Stuart Yin, Rongqing Hui, Paul Ruffin, Christina Brantley, Eugene Edwards, and Claire Luo

J. Appl. Phys. 108, 043106 (2010); http://dx.doi.org/10.1063/1.3474651 (6 pages)

Online Publication Date: 25 August 2010

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In this paper, we present a new type of concealed and buried chemical detection system by stimulating and enhancing spectroscopic signatures with multifrequency excitations, which includes a low frequency gradient dc electric field, a high frequency microwave field, and higher frequency infrared (IR) radiations. Each excitation frequency plays a unique role. The microwave, which can penetrate into the underground and/or pass through the dielectric covers with low attenuation, could effectively transform its energy into the concealed and buried chemicals and increases its evaporation rate from the sample source. Subsequently, a gradient dc electric field, generated by a Van De Graaff generator, not only serves as a vapor accelerator for efficiently expediting the transportation process of the vapor release from the concealed and buried chemicals but also acts as a vapor concentrator for increasing the chemical concentrations in the detection area, which enables the trace level chemical detection. Finally, the stimulated and enhanced vapors on the surface are detected by the IR spectroscopic fingerprints. Our theoretical and experimental results demonstrate that more than sixfold increase in detection signal can be achieved by using this proposed technology. The proposed technology can also be used for standoff detection of concealed and buried chemicals by adding the remote IR and/or thermal spectroscopic and imaging detection systems.
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29.20.Ba Electrostatic accelerators
07.57.Ty Infrared spectrometers, auxiliary equipment, and techniques

Multiphoton-absorption induced ultraviolet luminescence of ZnO nanorods using low-energy femtosecond pulses

Susanta Kumar Das, Mahua Biswas, Daragh Byrne, Martin Bock, Enda McGlynn, Markus Breusing, and Ruediger Grunwald

J. Appl. Phys. 108, 043107 (2010); http://dx.doi.org/10.1063/1.3468632 (6 pages) | Cited 4 times

Online Publication Date: 25 August 2010

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Multiphoton-absorption (MPA) induced ultraviolet (UV) luminescence of ZnO nanorods grown by vapor phase transport was demonstrated using ultrafast excitation at pulse energies in the few nanojoules range, directly generated by a Ti:sapphire laser oscillator at wavelengths around 800 nm. The dependence of the UV luminescence on the excitation density reveals a two-photon absorption process as the responsible excitation mechanism. The broad spectral bandwidth of the excitation pulses obviously promotes the feasibility of the observed two-photon channel. Theoretical estimates concerning the contribution of nonlinear absorbance strongly support the experimental findings. The essential conditions for proper utilization of this process are discussed.
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78.55.Et II-VI semiconductors
78.67.Qa Nanorods

The measured dependence of the lateral ambipolar diffusion length on carrier injection-level in Stranski-Krastanov quantum dot devices

D. Naidu, P. M. Smowton, and H. D. Summers

J. Appl. Phys. 108, 043108 (2010); http://dx.doi.org/10.1063/1.3471812 (10 pages)

Online Publication Date: 26 August 2010

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Using the segmented contact method we separate and numerically evaluate the components making up the threshold current density dependence of quantum dot ridge waveguide lasers. An increasing internal optical mode loss and an increasing lateral out-diffusion current are the significant processes in ridges of widths between 4 and 10 μm, with no significant contribution from a deteriorating gain-mode overlap. By fitting a diffusion length model to the lateral out-diffusion process, we extract the ambipolar diffusion length, Ld, as a function of intrinsic carrier injection-level which covers carrier densities appropriate for functioning light-emitting diode and laser devices. The measured dependence fits a diffusion mechanism involving the thermal redistribution of carriers via the wetting-layer and most significantly leads to two regimes where Ld can be reduced in self-assembled quantum-dot systems. Only one of these is shown to be beneficial to the overall efficiency of the device, while the other is at the expense of undesired high-order nonradiative recombination processes at high injection-levels. Covering a peak modal gain range of approximately 5 to 11 cm−1 over injection-levels of 65 to 122 meV at 350 K, this dependence caused Ld to change from 0.75 to 1.50 μm, with the maximum occurring at 84 meV where the peak modal gain is 6 cm−1. Decreasing the temperature to 300 K reduced Ld to <0.75 μm over approximately the same injection-level range.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.79.Gn Optical waveguides and couplers

Influence of period and amplitude of microwaviness on KH2PO4 crystal’s laser damage threshold

Mingjun Chen, Mingquan Li, Wei Jiang, and Qiao Xu

J. Appl. Phys. 108, 043109 (2010); http://dx.doi.org/10.1063/1.3462430 (6 pages) | Cited 3 times

Online Publication Date: 31 August 2010

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The laser induced damage threshold (LIDT) has become a technical bottleneck which restricts the application and development of the inertial confinement fusion. Using single point diamond turning method to process KH2PO4 (KDP) crystals, the periodic microwaviness will be left on the machined surface which has a significant impact on the LIDT. In this paper, after acquiring the frequency information of machined surface of KDP crystals with the power spectral density method, we analyze quantitatively the influence of microwaviness on the LIDT of KDP crystal with the Fourier modal theory. Research results indicate that: the surface morphology of KDP crystal is consisted of the subwaviness with different characteristic spatial frequencies; and the reduced amount of the LIDT of KDP crystal leaded by such subwaviness is different; the experimental results of the LIDT are consistent with the theoretical calculations basically; for the machine tool and the processing technology we used, the leading frequency of microwaviness which caused the LIDT decreasing is between (350 μm)−1 and (30 μm)−1, especially between (90 μm)−1 and (180 μm)−1.
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61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.82.Ms Insulators
68.35.bt Other materials
79.20.Ds Laser-beam impact phenomena
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