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19 Nov 2007

Volume 91, Issue 21, Articles (21xxxx)

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Appl. Phys. Lett. 91, 212501 (2007); http://dx.doi.org/10.1063/1.2813047 (3 pages)

M. V. Milošević, G. R. Berdiyorov, and F. M. Peeters
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Realistic laser focusing effect on electron acceleration in the presence of a pulsed magnetic field

Devki Nandan Gupta, Hyyong Suk, and Min Sup Hur

Appl. Phys. Lett. 91, 211101 (2007); http://dx.doi.org/10.1063/1.2801392 (3 pages) | Cited 5 times

Online Publication Date: 19 November 2007

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As we know, for a significant electron energy gain, a fast electron should be injected into the highest intensity region of the laser focus. Such intensities may be achieved in the laboratory by tight focusing of a laser. For a tight focused laser beam, it is necessary to consider all field components the arise due to the tight focusing of the laser beam, when the waist of the laser beam is of the order of the laser wavelength. By using the accurate field components of a tightly focused laser beam, we investigate the electron acceleration in the presence of a pulsed magnetic field. Our study shows that the electron energy gain during laser acceleration is found to be considerably higher.
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42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation

Terahertz wave amplification in gases with the excitation of femtosecond laser pulses

Jianming Dai, Xu Xie, and X.-C. Zhang

Appl. Phys. Lett. 91, 211102 (2007); http://dx.doi.org/10.1063/1.2814063 (3 pages) | Cited 17 times

Online Publication Date: 19 November 2007

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We report the observation of terahertz wave amplification in gases excited by femtosecond laser pulses. When a seed terahertz wave is sent into the ∼ 5 mm long plasma created by focusing a 800 nm beam and its second harmonic into nitrogen gas, a peak amplification factor of about 65% is measured with total optical excitation intensity of 8×1014 W/cm2. The amplification effect occurs within a time scale of less than 400 fs of the onset of ionization processes due to the optical excitation pulse duration. Four-wave-mixing parametric processes during the plasma formation are proposed to be responsible for the amplification effect.
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42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
42.55.Lt Gas lasers including excimer and metal-vapor lasers
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation

Dose dependence of x-ray luminescence from CaF2:Eu2+, Mn2+ phosphors

Wei Chen, Sarah L. Westcott, and Jun Zhang

Appl. Phys. Lett. 91, 211103 (2007); http://dx.doi.org/10.1063/1.2816330 (3 pages) | Cited 3 times

Online Publication Date: 20 November 2007

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In CaF2:Mn2+, Eu2+, the emission from Eu2+ decreases while the emission from Mn2+ increases in intensity as the x-ray duration time increases. The decrease of Eu2+ emission is due to the oxidation of Eu2+ to Eu3+ upon x-ray irradiation, while the increase of Mn2+ emission in intensity is likely due to the breakdown of the forbidden transition by the defects. This phenomenon has potential for dose-dependent radiation detection because utilizing the ratio of the two emissions from Mn2+ and Eu2+ for radiation dosimetry is more sensitive and more reliable than using emission intensity change only.
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78.55.Hx Other solid inorganic materials
61.80.Cb X-ray effects
81.65.Mq Oxidation
61.82.Fk Semiconductors
61.72.-y Defects and impurities in crystals; microstructure

Observation of enhanced photoluminescence from silicon photonic crystal nanocavity at room temperature

Satoshi Iwamoto, Yasuhiko Arakawa, and Akiko Gomyo

Appl. Phys. Lett. 91, 211104 (2007); http://dx.doi.org/10.1063/1.2816892 (3 pages) | Cited 16 times

Online Publication Date: 21 November 2007

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We fabricated air-bridge-type silicon photonic crystal (PC) nanocavities and observed significant enhancement of photoluminescence (PL) from crystalline silicon at room temperature. Cavity-resonant peaks shifted toward longer wavelengths when the period of PC was increased and their polarization dependences agree with the calculation. At a cavity mode wavelength, a 310-fold enhancement of PL intensity, compared with that of unpatterned silicon-on-insulator substrate, was demonstrated.
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78.55.Ap Elemental semiconductors
42.70.Qs Photonic bandgap materials
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures

Field effect white and tunable electroluminescence from ion beam synthesized Si- and C-rich SiO2 layers

O. Jambois, Josep Carreras, A. Pérez-Rodríguez, B. Garrido, C. Bonafos, S. Schamm, and G. Ben Assayag

Appl. Phys. Lett. 91, 211105 (2007); http://dx.doi.org/10.1063/1.2807281 (3 pages) | Cited 7 times

Online Publication Date: 21 November 2007

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White and tunable electroluminescence has been obtained by field effect injection in 40 nm thick Si- and C-rich SiO2 layers. The films, synthesized by ion implantation, contain Si and C-rich nanoparticles embedded in SiO2 which were formed by annealing at 1100 °C. Shifting of the distribution of C-related centers toward the interface region with the substrate allows us to obtain a characteristic white electroluminescence emission under pulsed excitation conditions. Moreover, an evolution of the emission spectrum from white to red is observed by changing the frequency of the pulse. A power efficiency higher than 10−3% is estimated. This work opens interesting perspectives on color tunability of field effect electroluminescent devices.
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78.60.Fi Electroluminescence
61.72.Cc Kinetics of defect formation and annealing

Highly efficient red electrophosphorescence from a solution-processed zwitterionic cyclometalated iridium(III) complex

Younghun Byun, Yi-Yeol Lyu, Rupasree Ragini Das, Ohyun Kwon, Tae-Woo Lee, and Young Ja Park

Appl. Phys. Lett. 91, 211106 (2007); http://dx.doi.org/10.1063/1.2809375 (3 pages) | Cited 8 times

Online Publication Date: 21 November 2007

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A mononuclear red-emitting zwitterionic iridium(III) complex, bis(1-(phenyl)isoquinolinato) iridium(III) (2,2′-bipyridine-3-ol-3′-olate), was synthesized and used as a triplet emitter doped in a polymeric host in solution-processed electrophosphorescent light-emitting diodes. We achieved a high luminous effieciency (LE) of 12.62 cd/A with fast response time in the device using the dopant unlike in the device using cationic irridium(III) phosphorescent materials. A hole transporting polymeric interlayer between the anode and the emitting layer improved the LE and decreased the exciton quenching at high electric fields.
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78.55.Kz Solid organic materials
71.35.-y Excitons and related phenomena
81.20.-n Methods of materials synthesis and materials processing

Enhanced forward emission from ZnO via surface plasmons

D. Y. Lei and H. C. Ong

Appl. Phys. Lett. 91, 211107 (2007); http://dx.doi.org/10.1063/1.2805022 (3 pages) | Cited 16 times

Online Publication Date: 21 November 2007

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By using metal-insulator-metal (MIM) as the cap layer, we have enhanced the photoluminescent emission from ZnO in the forward direction via coupling of surface plasmons. The basic principle behind the enhancement involves two steps. First, the overall light emission efficiency of ZnO is increased significantly due to the coupling and scattering of nonradiative surface plasmons at the metal/ZnO interface. The forward emission is then increased by enhanced transmission due to the excitation of radiative surface plasmons in the MIM structure. As a result, the forward emission from MIM/ZnO is found to be 7 and 3.5 times stronger than that from uncapped ZnO and M/ZnO. We expect that the MIM structure could evolve as an effective means in fabricating high brightness light emitting diodes from semiconductors.
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71.20.Nr Semiconductor compounds
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.40.Rw Metal-insulator-metal structures
85.60.Jb Light-emitting devices

Gallium-nitride-based microcavity light-emitting diodes with air-gap distributed Bragg reflectors

Rajat Sharma, Yong-Seok Choi, Chiou-Fu Wang, Aurélien David, Claude Weisbuch, Shuji Nakamura, and Evelyn L. Hu

Appl. Phys. Lett. 91, 211108 (2007); http://dx.doi.org/10.1063/1.2805028 (3 pages) | Cited 11 times

Online Publication Date: 21 November 2007

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We report on the realization of highly efficient InGaN microcavity light-emitting diodes incorporating a high index contrast air-gap distributed Bragg reflector (DBR). Detailed analysis deduces an effective cavity length of ∼ 500 nm and cavity mode orders of 5 and 6 for measured Fabry-Pérot fringes. A value reflectivity of ∼ 70% was determined for the 4.5 period air∕Al0.08Ga0.92N DBR through the analysis of cavity finesse based on the angle-resolved photoluminescence (PL) data. A fivefold improvement in light extraction efficiency was verified by electrical probing as well as angle-resolved PL measurements.
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85.60.Jb Light-emitting devices
85.60.Bt Optoelectronic device characterization, design, and modeling
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
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