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25 Jul 2011

Volume 99, Issue 4, Articles (04xxxx)

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Appl. Phys. Lett. 99, 041102 (2011); http://dx.doi.org/10.1063/1.3615051 (3 pages)

M. Davanço, M. T. Rakher, D. Schuh, A. Badolato, and K. Srinivasan
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High Q microcavity light emitting diodes with buried AlN current apertures

Bo-Siao Cheng, Yun-Lin Wu, Tien-Chang Lu, Ching-Hsueh Chiu, Cheng-Hung Chen, Po-Min Tu, Hao-Chung Kuo, Shing-Chung Wang, and Chun-Yen Chang

Appl. Phys. Lett. 99, 041101 (2011); http://dx.doi.org/10.1063/1.3617418 (3 pages) | Cited 1 time

Online Publication Date: 25 July 2011

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We demonstrate a GaN-based high-Q microcavity light emitting diode (MCLED) with a buried AlN current aperture. A thin AlN layer is inserted on the InGaN/GaN multiple quantum wells as a current blocking layer and an optical confinement layer. The GaN-based MCLED is composed of a 29-pair GaN/AlN distributed Bragg reflector (DBR), an eight-pair of SiO2/Ta2O5 dielectric DBR, and a three-λ optical thickness InGaN/GaN active region. The current can be injected more effectively in the MCLED with a buried AlN current aperture. The output emission has a dominant emission peak wavelength at 440 nm with a very narrow linewidth of 0.52 nm, corresponding to a cavity Q-value of 846 at a driving current of 5 mA.
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85.60.Jb Light-emitting devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
42.82.-m Integrated optics

A circular dielectric grating for vertical extraction of single quantum dot emission

M. Davanço, M. T. Rakher, D. Schuh, A. Badolato, and K. Srinivasan

Appl. Phys. Lett. 99, 041102 (2011); http://dx.doi.org/10.1063/1.3615051 (3 pages) | Cited 4 times

Online Publication Date: 25 July 2011

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We demonstrate a nanostructure composed of partially etched annular trenches in a suspended GaAs membrane, designed for efficient and moderately broadband (≈5 nm) emission extraction from single InAs quantum dots. Simulations indicate that a dipole embedded in the nanostructure center radiates upward into free space with a nearly Gaussian far field, allowing a collection efficiency >80% with a high numerical aperture (NA = 0.7) optic and with ≈12× Purcell radiative rate enhancement. Fabricated devices exhibit a ≈10% photon collection efficiency with a NA = 0.42 objective, a 20× improvement over quantum dots in unpatterned GaAs. A fourfold exciton lifetime reduction indicates moderate Purcell enhancement.
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78.67.Hc Quantum dots
81.07.Ta Quantum dots
71.35.-y Excitons and related phenomena
78.55.Cr III-V semiconductors
81.05.Ea III-V semiconductors

Photoluminescence dynamics in GaAs/AlGaAs quantum wells under pulsed intersubband excitation

S. Zybell, H. Schneider, S. Winnerl, M. Wagner, K. Köhler, and M. Helm

Appl. Phys. Lett. 99, 041103 (2011); http://dx.doi.org/10.1063/1.3615298 (3 pages) | Cited 4 times

Online Publication Date: 25 July 2011

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We investigate the time-resolved photoluminescence (PL) dynamics of an undoped GaAs/AlGaAs multiple quantum well under mid-infrared (MIR) irradiation. A time-delayed MIR laser pulse from a free-electron laser, tuned to the intersubband transition energy of the quantum well, induces temporal quenching of the PL intensity with subsequent recovery. The experimental data can be accurately described by a simple rate-equation model, which accounts for the cooling of the non-radiative states to radiative states. By performing polarization sensitive measurements, we are able to discriminate the contributions of free-carrier absorption from that of intersubband absorption, where the latter is about 50 times more efficient.
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78.67.De Quantum wells
81.05.Ea III-V semiconductors
81.07.St Quantum wells
78.47.jd Time resolved luminescence
78.55.Cr III-V semiconductors

Photonic THz generation in GaAs via resonantly enhanced intracavity multispectral mixing

K. L. Vodopyanov, W. C. Hurlbut, and V. G. Kozlov

Appl. Phys. Lett. 99, 041104 (2011); http://dx.doi.org/10.1063/1.3617435 (3 pages) | Cited 4 times

Online Publication Date: 26 July 2011

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We generate tunable (1.5–2 THz) terahertz output with up to 200 μW average power in periodically inverted GaAs using resonantly enhanced multispectral frequency mixing inside the cavity of a type-0 optical parametric oscillator operating at degeneracy. The optical parametric oscillator was synchronously pumped by a 1064-nm picosecond Yb-fiber laser and produced, due to the presence of an intracavity Fabry-Pérot etalon, a set of optical frequency peaks spaced at the desired THz interval that allows efficient THz wave production via difference frequency generation. The proposed method is well adapted for cascaded THz generation, where the quantum conversion limit can be significantly surpassed.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.55.Wd Fiber lasers
42.60.By Design of specific laser systems
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation

Effects of polarization-field tuning in GaInN light-emitting diodes

Jiuru Xu, Martin F. Schubert, Di Zhu, Jaehee Cho, E. Fred Schubert, Hyunwook Shim, and Cheolsoo Sone

Appl. Phys. Lett. 99, 041105 (2011); http://dx.doi.org/10.1063/1.3609783 (3 pages) | Cited 3 times

Online Publication Date: 26 July 2011

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III-V nitrides form the backbone of light-emitting diode (LED) technology. However, the relevance of the very strong polarization fields in III-V nitride LEDs remains unclear. Here, we demonstrate the tuning of polarization fields by mechanical force. For compressive strain in a GaInN LED epitaxial layer, we find: (i) redistribution of intensity within the electroluminescence spectrum; (ii) a decrease in the peak efficiency at low current densities; and (iii) an increase in light-output power at high current densities. These findings show the relevance of transport effects in the efficiency droop.
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85.60.Jb Light-emitting devices

Photoelectron emission control with polarized light in plasmonic metal random structures

R. C. Word, J. Fitzgerald, and R. Könenkamp

Appl. Phys. Lett. 99, 041106 (2011); http://dx.doi.org/10.1063/1.3615783 (3 pages) | Cited 5 times

Online Publication Date: 26 July 2011

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We report on the possibility of switching the emission rate of photoelectrons by polarization changes in the plasmon excitation light. Photoelectron emission is strongly enhanced in the near-field of localized surface plasmons and occurs from areas with typical diameters of 20-70 nm. The underlying physical process involves excitation of a localized surface plasmon polariton with a femtosecond laser pulse, and a subsequent multi-photon photoemission process. The non-linearity of this process leads to a sharp polarization dependence that allows efficient switching of the emission. We demonstrate that a 90° polarization change can result in on/off ratios of ∼100 for electron emission.
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81.05.Bx Metals, semimetals, and alloys
81.07.-b Nanoscale materials and structures: fabrication and characterization
79.60.Jv Interfaces; heterostructures; nanostructures
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
73.22.Lp Collective excitations
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)

Surface plasmon-enhanced light-emitting diodes with silver nanoparticles and SiO2 nano-disks embedded in p-GaN

Chu-Young Cho, Ki Seok Kim, Sang-Jun Lee, Min-Ki Kwon, Hyungduk Ko, Sung-Tae Kim, Gun-Young Jung, and Seong-Ju Park

Appl. Phys. Lett. 99, 041107 (2011); http://dx.doi.org/10.1063/1.3616149 (3 pages) | Cited 13 times

Online Publication Date: 26 July 2011

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We report the fabrication and characterization of surface plasmon (SP)-enhanced blue light-emitting diodes (LEDs) with Ag nanoparticles and SiO2 nano-disks embedded in the p-GaN layer. Compared with LEDs without Ag nanoparticles, the optical output power increases for the SP-enhanced LEDs with and without SiO2 nano-disks are 72% and 49%, respectively. The greater increase in optical output power for the SP-enhanced LEDs with SiO2 nano-disks compared with SP-enhanced LEDs without SiO2 nano-disks can be attributed to the SiO2 nano-disks in the p-GaN layer, which reduce the Ag nanoparticle-induced defects and enhance the light extraction efficiency of the LEDs.
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85.60.Jb Light-emitting devices

A stable and switchable uniform lying helix structure in cholesteric liquid crystals

Chun-Ta Wang, Wei-Yuan Wang, and Tsung-Hsien Lin

Appl. Phys. Lett. 99, 041108 (2011); http://dx.doi.org/10.1063/1.3615293 (3 pages) | Cited 4 times

Online Publication Date: 27 July 2011

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This investigation demonstrates an electrically switchable uniformly lying helix (ULH) stable state in cholesteric liquid crystal. A stable ULH state can be achieved by applying the low-frequency (30 Hz) pulse electrical field via an electro-hydrodynamatic effect. The ULH state can be stably maintained with a helical pitch in the visible range (450 nm–630 nm) and exhibit a tunable uniaxial crystal wave plate property under 1 kHz electrical field. The study examines the electro-optical property of ULH state and driving scheme for switching among the three stable states. A multi-stable and electrically switchable cholesteric liquid crystal can provide various optical properties and has extensive potential applications.
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61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
77.84.Nh Liquids, emulsions, and suspensions; liquid crystals
78.20.Jq Electro-optical effects
61.30.Eb Experimental determinations of smectic, nematic, cholesteric, and other structures

An optical magnetic metamaterial working at multiple frequencies simultaneously

Qiuze Li, Weihua Lin, and Guo Ping Wang

Appl. Phys. Lett. 99, 041109 (2011); http://dx.doi.org/10.1063/1.3617465 (3 pages) | Cited 1 time

Online Publication Date: 28 July 2011

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We theoretically design and experimentally demonstrate a multilayered optical magnetic metamaterial working at multiple frequencies simultaneously. The metamaterial is constructed with tapered ellipsoidal metal-dielectric-metal particles (EMDMPs), which is obtained through alternately depositing multiple layers of Ag and SiO2 films on a dielectric template of ellipsoidal nanohole arrays created by holographic lithography and lift-off procedure. Transmission and reflection spectra measurements reveal that the metamaterial exhibits multiband magnetic resonances at near infrared due to the size variation of EMDMPs in depositing and lift-off procedures. Our results may prove a step towards the construction of realistic metamaterials for various photonics applications.
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42.70.Ln Holographic recording materials; optical storage media
68.65.Ac Multilayers
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
42.40.Kw Holographic interferometry; other holographic techniques

Concurrent field enhancement and high transmission of THz radiation in nanoslit arrays

Mostafa Shalaby, Hannes Merbold, Marco Peccianti, Luca Razzari, Gargi Sharma, Tsuneyuki Ozaki, Roberto Morandotti, Thomas Feurer, Anja Weber, Laura Heyderman, Bruce Patterson, and Hans Sigg

Appl. Phys. Lett. 99, 041110 (2011); http://dx.doi.org/10.1063/1.3617476 (3 pages) | Cited 6 times

Online Publication Date: 28 July 2011

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multimedia

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We experimentally and numerically investigate the transmission of THz radiation through uniform nanoslit arrays. These structures are capable of inducing plasmon-mediated field enhancement while concurrently providing high field transmission. Combined with intense THz radiation, estimated field strengths as high as 26 MV/cm are obtained in the near-field regime which will facilitate nonlinear THz experiments.
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78.66.Bz Metals and metallic alloys

High resolution, large field of view x-ray differential phase contrast imaging on a compact setup

T. Thuering, P. Modregger, T. Grund, J. Kenntner, C. David, and M. Stampanoni

Appl. Phys. Lett. 99, 041111 (2011); http://dx.doi.org/10.1063/1.3618672 (3 pages) | Cited 8 times

Online Publication Date: 28 July 2011

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X-ray grating interferometry is a well established technique to perform differential phase contrast imaging on conventional x-ray tubes. So far, the application of this technique in commercial micro computed tomography scanners has remained a major challenge due to the compact setup geometry. In this letter, we report on the design of a compact imaging setup using a microfocus source. Due to the extreme wave front curvature, the gratings are fabricated on a flexible substrate, enabling precise cylindrical shaping. A laboratory setup and a modified SCANCO μCT100 scanner have been built, allowing high resolution and large field of view imaging.
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07.85.-m X- and γ-ray instruments
42.79.Dj Gratings
07.60.Ly Interferometers

On the temperature dependence of electron leakage from the active region of GaInN/GaN light-emitting diodes

David S. Meyaard, Qifeng Shan, Qi Dai, Jaehee Cho, E. Fred Schubert, Min-Ho Kim, and Cheolsoo Sone

Appl. Phys. Lett. 99, 041112 (2011); http://dx.doi.org/10.1063/1.3618673 (3 pages) | Cited 9 times

Online Publication Date: 28 July 2011

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Reduction in the light-output power in GaN-based light-emitting diodes (LEDs) with increasing temperature is a well-known phenomenon. In this work, temperature dependent external-quantum-efficiency versus current curves are measured, and the mechanisms of recombination are discussed. Shockley-Read-Hall recombination increases with temperature and is found to greatly reduce the light output at low current densities. However, this fails to explain the drop in light-output power at high current densities. At typical current density (35 A/cm2), as temperature increases, our results are consistent with increased Shockley-Read-Hall recombination and increased electron leakage from the active region. Both of these effects contribute to the reduction in light-output power in GaInN/GaN LEDs at high temperatures.
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85.60.Jb Light-emitting devices

Room temperature and narrow intersubband electroluminescence from ZnCdSe/ZnCdMgSe quantum cascade laser structures

Yu Yao, Adrian Alfaro-Martinez, Kale J. Franz, William O. Charles, Aidong Shen, Maria C. Tamargo, and Claire F. Gmachl

Appl. Phys. Lett. 99, 041113 (2011); http://dx.doi.org/10.1063/1.3614561 (3 pages) | Cited 5 times

Online Publication Date: 29 July 2011

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We report ZnCdSe/ZnCdMgSe Quantum Cascade structures with “two-phonon” and “bound-to-continuum” active region designs. The electroluminescence shows more than 3 times higher luminescence efficiency and 40% narrower linewidth (<30 meV) than previous reports. The measured turn-on voltage matches closely the calculated value, indicating the improved electron transport characteristics in these structures. A waveguide design suitable for mode confinement in this material system is also presented, which resulted in a structure with a single narrow electroluminescence peak for all temperatures from 80 to 300 K.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems

Random lasing in low molecular weight organic thin films

S. Kéna-Cohen, P. N. Stavrinou, D. D. C. Bradley, and S. A. Maier

Appl. Phys. Lett. 99, 041114 (2011); http://dx.doi.org/10.1063/1.3619169 (3 pages) | Cited 5 times

Online Publication Date: 29 July 2011

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We demonstrate that coherent random lasing can occur in solid-state dispersions of laser dyes 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran and 4-dicyanomethylene-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran within a tris(8-hydroxy-quinolinato)aluminum matrix without the need for infiltrated scatterers. Random lasing thresholds as low as 90 μJ/cm2 are observed in 250 nm-thick films with 2-2.5% dye concentration. We find that the lasing threshold exhibits a power law decay with sample area, providing a simple test for the occurrence of random lasing. We discuss the implications of this result in the context of previous reports of lasing in these materials where feedback was provided by conventional optical resonators.
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42.55.Mv Dye lasers
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
78.66.Qn Polymers; organic compounds

Metal/graphene sheets as p-type transparent conducting electrodes in GaN light emitting diodes

Jung Min Lee, Hae Yong Jeong, Kyoung Jin Choi, and Won Il Park

Appl. Phys. Lett. 99, 041115 (2011); http://dx.doi.org/10.1063/1.3595941 (3 pages) | Cited 15 times

Online Publication Date: 29 July 2011

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We demonstrate the use of graphene based transparent sheets as a p-type current spreading layer in GaN light emitting diodes (LEDs). Very thin Ni/Au was inserted between graphene and p-type GaN to reduce contact resistance, which reduced contact resistance from ∼ 5.5 to ∼ 0.6Ω/cm2, with no critical optical loss. As a result, LEDs with metal-graphene provided current spreading and injection into the p-type GaN layer, enabling three times enhanced electroluminescent intensity compared with those with graphene alone. We confirmed very strong blue light emission in a large area of the metal-graphene layer by analyzing image brightness.
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85.60.Jb Light-emitting devices

Morphology-induced plasmonic resonances in silver-aluminum alloy thin films

Sabine Auer, Wenjie Wan, Xu Huang, Ainissa G. Ramirez, and Hui Cao

Appl. Phys. Lett. 99, 041116 (2011); http://dx.doi.org/10.1063/1.3619840 (3 pages) | Cited 4 times

Online Publication Date: 29 July 2011

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We have investigated the optical properties of sputter-deposited silver-aluminum alloy thin films on silicon substrates at room temperature. In addition to the primary feature that corresponds to the bulk plasma resonance, a secondary dip appears in the optical reflectance spectra, which shifts and diminishes with thermal annealing. Careful structural characterization of both the as-deposited and annealed films suggests that the resonant feature originates from the surface plasmon resonances, which are localized in the dielectric gap between grains. This result indicates that the morphology of metal alloys could have a significant effect on their optical properties.
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78.66.Bz Metals and metallic alloys
78.68.+m Optical properties of surfaces
81.15.Cd Deposition by sputtering
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.30.Er Solid metals and alloys
78.40.Kc Metals, semimetals, and alloys

True-green (11-22) plane optically pumped laser with cleaved m-plane facets

Dmitry Sizov, Rajaram Bhat, Albert Heberle, Nick Visovsky, and Chung-en Zah

Appl. Phys. Lett. 99, 041117 (2011); http://dx.doi.org/10.1063/1.3614436 (3 pages) | Cited 4 times

Online Publication Date: 29 July 2011

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We present a detailed experimental study of optical property of green InGaN quantum wells and optically pumped lasers, with cleaved m-plane facets and a lasing wavelength in the range of 520-530 nm, grown on semipolar (11-22) planes. Taking advantage of low transparency carrier density of the lowest-energy valence band and overcoming low differential gain by minimizing the optical loss of the laser structure, we demonstrated a low threshold pumping power of 120 mW.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

A thin metal sheath lifts the EH to HE degeneracy in the cladding mode refractometric sensitivity of optical fiber sensors

C. Caucheteur, C. Chen, V. Voisin, P. Berini, and J. Albert

Appl. Phys. Lett. 99, 041118 (2011); http://dx.doi.org/10.1063/1.3615712 (3 pages) | Cited 6 times

Online Publication Date: 29 July 2011

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The effective indices of the cladding modes of optical fibers depend on the refractive index of the medium surrounding the fiber. We show experimentally and theoretically that while cladding modes with similar effective indices normally have similar refractometric sensitivities, the addition of a 50 nm thick gold sheath enhances the sensitivity of some EH modes by more than one order of magnitude while nearly completely suppressing the sensitivity of neighbouring HE modes (by three orders of magnitude, down to insignificant levels). A differential sensitivity of ∼1000 nm/(refractive index unit) is experimentally reported between adjacent EH and HE grating resonances.
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42.81.Pa Sensors, gyros
42.81.Bm Fabrication, cladding, and splicing
42.79.Dj Gratings
07.60.Hv Refractometers and reflectometers
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