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1 Mar 2010

Volume 96, Issue 9, Articles (09xxxx)

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Appl. Phys. Lett. 96, 091102 (2010); http://dx.doi.org/10.1063/1.3332591 (3 pages)

A. Schropp, P. Boye, J. M. Feldkamp, R. Hoppe, J. Patommel, D. Samberg, S. Stephan, K. Giewekemeyer, R. N. Wilke, T. Salditt, J. Gulden, A. P. Mancuso, I. A. Vartanyants, E. Weckert, S. Schöder, et al.
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Enhanced terahertz emission from a multilayered low temperature grown GaAs structure

Samir Rihani, Richard Faulks, Harvey E. Beere, Ian Farrer, Michael Evans, David A. Ritchie, and Michael Pepper

Appl. Phys. Lett. 96, 091101 (2010); http://dx.doi.org/10.1063/1.3332587 (3 pages) | Cited 2 times

Online Publication Date: 1 March 2010

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We report the use of a multilayered structure comprising of alternating layers of low temperature grown GaAs and high temperature grown AlAs, as a terahertz (THz) photoconductive antenna emitter and receiver. Devices based on 10×10 μm2 mesa defined photoconductive gaps were fabricated on the multilayered structure, and a comparison made to conventional planar devices. The mesa defined photoconductive antennas allowed successive contact through the multilayered structure, which resulted in an increase in THz emission power and detection responsivity with increasing number of layers in contact with the antenna electrodes. A comparison with a conventional single layered device, processed in an identical mesa geometry, confirmed that the enhancement in THz emission is solely due to the multilayered nature of the device, whereas the improved receiver performance can be partially attributed to the mesa geometry.
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85.60.-q Optoelectronic devices
84.40.Ba Antennas: theory, components and accessories

Hard x-ray nanobeam characterization by coherent diffraction microscopy

A. Schropp, P. Boye, J. M. Feldkamp, R. Hoppe, J. Patommel, D. Samberg, S. Stephan, K. Giewekemeyer, R. N. Wilke, T. Salditt, J. Gulden, A. P. Mancuso, I. A. Vartanyants, E. Weckert, S. Schöder, et al.

Appl. Phys. Lett. 96, 091102 (2010); http://dx.doi.org/10.1063/1.3332591 (3 pages) | Cited 34 times

Online Publication Date: 1 March 2010

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We have carried out a ptychographic scanning coherent diffraction imaging experiment on a test object in order to characterize the hard x-ray nanobeam in a scanning x-ray microscope. In addition to a high resolution image of the test object, a detailed quantitative picture of the complex wave field in the nanofocus is obtained with high spatial resolution and dynamic range. Both are the result of high statistics due to the large number of diffraction patterns. The method yields a complete description of the focus, is robust against inaccuracies in sample positioning, and requires no particular shape or prior knowledge of the test object.
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07.85.Tt X-ray microscopes

Transient thermoreflectance imaging of active photonic crystals

Virginie Moreau, Gilles Tessier, Fabrice Raineri, Maia Brunstein, Alejandro Yacomotti, Rama Raj, Isabelle Sagnes, Ariel Levenson, and Yannick De Wilde

Appl. Phys. Lett. 96, 091103 (2010); http://dx.doi.org/10.1063/1.3323100 (3 pages) | Cited 3 times

Online Publication Date: 3 March 2010

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Transient thermoreflectance imaging is used to study the dynamics of the temperature inside active two-dimensional photonic crystals (PhCs). We developed a pump-probe setup suited for optically pumped devices that presents submicrosecond time resolution and submicrometer spatial resolution. Characteristic thermal dissipation times of 429 ns in a PhC Bloch mode cavity and of 999 ns in a PhC membrane are measured. This technique gives also access to the diffusivity of the suspended PhC.
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78.20.nd Thermophotonic effects
42.70.Qs Photonic bandgap materials
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials

Stable temperature characteristics of InGaN blue light emitting diodes using AlGaN/GaN/InGaN superlattices as electron blocking layer

Kyu Sang Kim, Jin Ha Kim, Su Jin Jung, Yong Jo Park, and S. N. Cho

Appl. Phys. Lett. 96, 091104 (2010); http://dx.doi.org/10.1063/1.3340939 (3 pages) | Cited 7 times

Online Publication Date: 3 March 2010

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P-type AlGaN/GaN/InGaN superlattices were incorporated in a InGaN based blue light emitting diode as electron blocking layer to minimize the temperature dependence on optical output power. For the characteristic temperatures in range of 10 to 100 °C and at operation current of 350 mA, the external quantum efficiency varied by less than 0.5%. For the presented device, the negative characteristic temperature was shown to occur below temperature of 50 °C. The improved temperature stability in optical output power is thought to be attributed to (1) the efficiency of hole carrier transport in AlGaN/GaN/InGaN superlattices and (2) the enhanced blocking of electron overflow between multiple quantum wells and AlGaN/GaN/InGaN superlattices.
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85.60.Jb Light-emitting devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Competitiveness between direct and indirect radiative transitions of Ge

T.-H. Cheng, C.-Y. Ko, C.-Y. Chen, K.-L. Peng, G.-L. Luo, C. W. Liu, and H.-H. Tseng

Appl. Phys. Lett. 96, 091105 (2010); http://dx.doi.org/10.1063/1.3352048 (3 pages) | Cited 17 times

Online Publication Date: 4 March 2010

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Both direct and indirect transitions of photoluminescence and electroluminescence are observed in a Ge n+p diode. The relative intensity of direct radiative recombination with respect to indirect radiative recombination increases with the increase in the optical pumping power, injection current density, and temperature. The increase in electron population in the direct valley is responsible for the enhancement. The spectra can be fitted by the combination of direct and indirect transition models. The direct radiative transition rate is ∼ 1600 times of the indirect transition, estimated by electroluminescence and photoluminescence spectra near room temperature.
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78.55.Ap Elemental semiconductors
78.60.Fi Electroluminescence
85.30.Kk Junction diodes
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions

Ultrathin Cu-Ti bilayer transparent conductors with enhanced figure-of-merit and stability

D. S. Ghosh, T. L. Chen, and V. Pruneri

Appl. Phys. Lett. 96, 091106 (2010); http://dx.doi.org/10.1063/1.3341201 (3 pages) | Cited 11 times

Online Publication Date: 5 March 2010

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We propose a transparent conductor (TC) structure based on the combination of ultrathin Cu and Ti films. The conductive Cu layer becomes continuous, i.e., it reaches the percolation threshold, for thicknesses in the range of 5.5–6.5 nm. However, without any adequate countermeasure, such an ultrathin layer would unavoidably oxidize even at ambient conditions. In this letter, we show that Ti capping layer in situ treated by oxygen plasma can protect the underlying Cu ultrathin layer and enhances its figure-of-merit. The improved optical properties can be attributed to multiple reflection and refraction effects in Cu-Ti bilayer system. The obtained Cu-Ti bilayer ultrathin films show transparency of 86% at wavelengths around 630 nm and sheet resistance of 16 Ω/sq, and exhibit excellent stability, as demonstrated by the fact that their properties do not significantly change after thermal treatment up to 120 °C for a dwell time of 45 min in ambient atmosphere.
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78.66.Bz Metals and metallic alloys
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
68.65.Ac Multilayers

Luminescence-induced photorefractive spatial solitons

E. Fazio, M. Alonzo, F. Devaux, A. Toncelli, N. Argiolas, M. Bazzan, C. Sada, and M. Chauvet

Appl. Phys. Lett. 96, 091107 (2010); http://dx.doi.org/10.1063/1.3313950 (3 pages) | Cited 2 times

Online Publication Date: 5 March 2010

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We report the observation of spatial confinement of a pump beam into a photorefractive solitonic channel induced by luminescence [luminescence induced spatial soliton (LISS)]. Trapped beams have been obtained in erbium doped lithium niobate crystals at concentrations as high as 0.7 mol % of erbium. By pumping at 980 nm, erbium ions emit photons at 550 nm by two-step absorption, wavelength which can be absorbed by lithium niobate and originates the photorefractive effect. The luminescence at 550 nm generates at the same time the solitonic channel and the background illumination reaching a steady-state soliton regime.
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78.20.Mg Photorefractive effects
42.70.Gi Light-sensitive materials
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
78.55.Hx Other solid inorganic materials
42.65.Tg Optical solitons; nonlinear guided waves
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
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