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4 Apr 2011

Volume 98, Issue 14, Articles (14xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 98, 141903 (2011); http://dx.doi.org/10.1063/1.3548546 (3 pages)

H. Hattab, A. T. N’Diaye, D. Wall, G. Jnawali, J. Coraux, C. Busse, R. van Gastel, B. Poelsema, T. Michely, F.-J. Meyer zu Heringdorf, and M. Horn-von Hoegen
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Two-dimensional broadband distributed-feedback quantum cascade laser arrays

Elvis Mujagić, Clemens Schwarzer, Yu Yao, Jianxin Chen, Claire Gmachl, and Gottfried Strasser

Appl. Phys. Lett. 98, 141101 (2011); http://dx.doi.org/10.1063/1.3574555 (3 pages) | Cited 6 times

Online Publication Date: 4 April 2011

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We present two-dimensional broadband quantum cascade laser arrays based on distributed-feedback (DFB) ring cavity surface emitting lasers. The 16-element arrays exhibit a linear tuning range of 180 cm−1 centered at a wavelength of 8.2 μm when operated in pulsed mode at room temperature. The devices show single-mode emission with a side mode suppression ratio of 30 dB. Given by the facetless nature of the single emitters, the spectral dependent threshold current densities and optical power reflect the gain profile of the incorporated material and are not impaired by the diversity of underlying DFB designs.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.60.Fc Modulation, tuning, and mode locking

Band-edge and random lasing in paintable liquid crystal emulsions

P. J. W. Hands, D. J. Gardiner, S. M. Morris, C. Mowatt, T. D. Wilkinson, and H. J. Coles

Appl. Phys. Lett. 98, 141102 (2011); http://dx.doi.org/10.1063/1.3574915 (3 pages) | Cited 8 times

Online Publication Date: 4 April 2011

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Lasing mechanisms within paintable dye-doped chiral liquid crystal emulsions are investigated. Evidence shows that by variation in liquid crystal droplet size, by simple control of mechanical mixing speeds, a change in the lasing mechanism from band-edge lasing (large droplets) to diffuse nonresonant random lasing (small droplets) can be facilitated. This approach represents a facile technique for the variation in lasing mechanism, within a self-organizing, flexible, and conformable system, and offers the opportunity of developing controllable linewidth laser sources.
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42.55.Mv Dye lasers
42.55.Zz Random lasers
42.60.By Design of specific laser systems
42.70.Hj Laser materials
42.70.Df Liquid crystals

Electrochemically tunable ultrafast optical response of graphene oxide

Ulaş Kürüm, Okan Öner Ekiz, H. Gul Yaglioglu, Ayhan Elmali, Mustafa Ürel, Hasan Güner, Alpay Koray Mızrak, Bülend Ortaç, and Aykutlu Dâna

Appl. Phys. Lett. 98, 141103 (2011); http://dx.doi.org/10.1063/1.3573797 (3 pages) | Cited 4 times

Online Publication Date: 5 April 2011

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We demonstrate reversible and irreversible changes in the ultrafast optical response of multilayer graphene oxide thin films upon electrical and optical stimulus. The reversible effects are due to electrochemical modification of graphene oxide, which allows tuning of the optical response by externally applied bias. Increasing the degree of reduction in graphene oxide causes excited state absorption to gradually switch to saturable absorption for shorter probe wavelengths. Spectral and temporal properties as well as the sign of the ultrafast response can be tuned either by changing the applied bias or exposing to high intensity femtosecond pulses.
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78.47.J- Ultrafast spectroscopy (<1 psec)
78.67.Wj Optical properties of graphene
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
78.40.Ri Fullerenes and related materials
82.45.-h Electrochemistry and electrophoresis

Extreme ultraviolet detection using AlGaN-on-Si inverted Schottky photodiodes

Pawel E. Malinowski, Jean-Yves Duboz, Piet De Moor, Kyriaki Minoglou, Joachim John, Sara Martin Horcajo, Fabrice Semond, Eric Frayssinet, Peter Verhoeve, Marco Esposito, Boris Giordanengo, Ali BenMoussa, Robert Mertens, and Chris Van Hoof

Appl. Phys. Lett. 98, 141104 (2011); http://dx.doi.org/10.1063/1.3576914 (3 pages) | Cited 4 times

Online Publication Date: 5 April 2011

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We report on the fabrication of aluminum gallium nitride (AlGaN) Schottky diodes for extreme ultraviolet (EUV) detection. AlGaN layers were grown on silicon wafers by molecular beam epitaxy with the conventional and inverted Schottky structure, where the undoped, active layer was grown before or after the n-doped layer, respectively. Different current mechanisms were observed in the two structures. The inverted Schottky diode was designed for the optimized backside sensitivity in the hybrid imagers. A cut-off wavelength of 280 nm was observed with three orders of magnitude intrinsic rejection ratio of the visible radiation. Furthermore, the inverted structure was characterized using a EUV source based on helium discharge and an open electrode design was used to improve the sensitivity. The characteristic He I and He II emission lines were observed at the wavelengths of 58.4 nm and 30.4 nm, respectively, proving the feasibility of using the inverted layer stack for EUV detection.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
85.30.Kk Junction diodes
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Influence of defects and interface on radiative transition of Ge

S.-R. Jan, C.-Y. Chen, C.-H. Lee, S.-T. Chan, K.-L. Peng, C. W. Liu, Y. Yamamoto, and B. Tillack

Appl. Phys. Lett. 98, 141105 (2011); http://dx.doi.org/10.1063/1.3571439 (3 pages) | Cited 4 times

Online Publication Date: 5 April 2011

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The influences of defects and surface roughness on the indirect bandgap radiative transition of Ge were studied. Bulk Ge has 15 times the integrated intensity of photoluminescence of Ge-on-Si. However, for Ge-on-Si sample, the direct transition related photoluminescence intensity is higher than the indirect transition related one. We affirm that the defects in the Ge-on-Si are responsible for the weak indirect transition and relatively strong direct transition. The scattering of electrons by roughness at Ge/oxide interface can provide extra momentum of the indirect band transition of Ge, and thus enhance the indirect radiative transition.
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68.35.bg Semiconductors
71.20.Mq Elemental semiconductors
78.55.Ap Elemental semiconductors
68.35.Ct Interface structure and roughness
68.47.Fg Semiconductor surfaces

Observation and characterization of mode splitting in microsphere resonators in aquatic environment

Woosung Kim, Şahin Kaya Özdemir, Jiangang Zhu, and Lan Yang

Appl. Phys. Lett. 98, 141106 (2011); http://dx.doi.org/10.1063/1.3571555 (3 pages) | Cited 6 times

Online Publication Date: 5 April 2011

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Whispering gallery mode (WGM) optical resonators utilizing resonance shift (RS) and mode splitting (MS) techniques have emerged as highly sensitive platforms for label-free detection of nanoscale objects. RS method has been demonstrated in various resonators in air and liquid. MS in microsphere resonators has not been achieved in aqueous environment up to date, despite its demonstration in microtoroid resonators. Here, we demonstrate scatterer-induced MS of WGMs in microsphere resonators in water. We determine the size range of particles that induces MS in a microsphere in water as a function of resonator mode volume and quality factor. The results are confirmed by the experimental observations.
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42.79.-e Optical elements, devices, and systems

Real time achromatic measurement of space-variant polarizations

Moti Fridman, Eran Grinvald, Amit Godel, Micha Nixon, Asher A. Friesem, and Nir Davidson

Appl. Phys. Lett. 98, 141107 (2011); http://dx.doi.org/10.1063/1.3575567 (3 pages)

Online Publication Date: 7 April 2011

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A compact configuration for real-time achromatic measurements of space-variant light polarization is presented. The experimental results reveal that the full state of polarization at each location within a light beam or at each wavelength can be obtained with accuracy of over π/18.
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42.25.Ja Polarization
07.60.-j Optical instruments and equipment

Microspectroscopy of ultrafast laser inscribed channel waveguides in Yb:tungstate crystals

F. M. Bain, W. F. Silva, A. A. Lagatsky, R. R. Thomson, N. D. Psaila, A. K. Kar, W. Sibbett, D. Jaque, and C. T. A. Brown

Appl. Phys. Lett. 98, 141108 (2011); http://dx.doi.org/10.1063/1.3573999 (3 pages) | Cited 1 time

Online Publication Date: 7 April 2011

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We report microspectroscopy measurements of crystalline channel waveguides fabricated in Yb:KGd(WO4)2 and Yb:KY(WO4)2 using the ultrafast laser inscription technique. From these measurements we find that densification of the WO2W bridge in the double tungstate crystal lattice is responsible for the refractive index increase, which creates the waveguide confinement. We identified that shifts toward lower energies in the ∼ 760 cm−1 Raman mode indicate regions, which guide light polarized along the crystallographic b axis, while higher energy shifts in the 682 and 898 cm−1 Raman lines correspond to guiding regions for light polarized along the crystallographic a axis.
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42.79.Gn Optical waveguides and couplers
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
42.62.-b Laser applications
78.30.Hv Other nonmetallic inorganics

Laser-excited acoustical phonons probed by ultrashort pulses from a laser-driven x-ray diode

K. S. Schulze, T. Kämpfer, I. Uschmann, S. Höfer, R. Loetzsch, and E. Förster

Appl. Phys. Lett. 98, 141109 (2011); http://dx.doi.org/10.1063/1.3577605 (3 pages) | Cited 1 time

Online Publication Date: 7 April 2011

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We demonstrate that an ultrashort-pulse laser-driven x-ray diode can be used for time-resolved experiments on a picosecond timescale. Hence, acoustical phonons in germanium are observed after ultrashort laser-excitation and the results are compared with calculations according to a microphysical model. We also show the advantages of this kind of picosecond x-ray source compared to other sources on the basis of its properties.
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63.20.-e Phonons in crystal lattices
42.62.-b Laser applications

Plasma structures for quasiphase matched high harmonic generation

A. H. Sheinfux, Z. Henis, M. Levin, and A. Zigler

Appl. Phys. Lett. 98, 141110 (2011); http://dx.doi.org/10.1063/1.3578407 (3 pages) | Cited 2 times

Online Publication Date: 7 April 2011

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A scheme for creation of periodic plasma structures by ablating a lithographic pattern is demonstrated. A proof of principle experiment was conducted, and plasma parameters were measured as a function of time with spatial resolution <10 and 100 μm periodicity. Several possible applications, in particular, quasiphase matching for high harmonic generation in plasma are considered.
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52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.38.Mf Laser ablation
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.75.-d Plasma devices

Controlling fluorescence from N2 inside femtosecond laser filaments in air by two-color laser pulses

H. L. Xu, A. Azarm, and S. L. Chin

Appl. Phys. Lett. 98, 141111 (2011); http://dx.doi.org/10.1063/1.3579246 (3 pages) | Cited 2 times

Online Publication Date: 8 April 2011

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We experimentally demonstrate the feasibility of controlling the fluorescence emission of nitrogen molecules in air induced by femtosecond laser filamentation by using a pump-probe method. An obvious enhancement or reduction in the filament-induced fluorescence signals of nitrogen molecules can be realized when a blue (400 nm) or an infrared (1338 nm) laser pulse is used as the probe. The completely opposite effect is ascribed to the excitation enhancement of ionization and population trapping of some highly excited states including Rydberg states of nitrogen molecules.
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33.50.Dq Fluorescence and phosphorescence spectra
33.80.Be Level crossing and optical pumping
37.10.Vz Mechanical effects of light on atoms, molecules, and ions

Directed self-assembly of InAs quantum dots on nano-oxide templates

N. L. Dias, A. Garg, U. Reddy, J. D. Young, V. B. Verma, R. P. Mirin, and J. J. Coleman

Appl. Phys. Lett. 98, 141112 (2011); http://dx.doi.org/10.1063/1.3579253 (3 pages) | Cited 2 times

Online Publication Date: 8 April 2011

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We describe the growth and characterization of InAs quantum dots on SiO2 patterned GaAs by metal organic chemical vapor deposition. Arrays of quantum dots with densities as high as 1.8×1010 cm−2 fabricated by electron beam lithography are demonstrated. A process consisting of dry and wet etching to minimize etch damage is developed. As the mask diameter increases, the nucleation transitions from single dots to multidot clusters. We achieve more uniform size and shape distributions of dots on patterned regions relative to unpatterned dots as revealed by structural characterization and room temperature photoluminescence emission spectra.
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81.07.Ta Quantum dots
81.16.Dn Self-assembly
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
68.65.Hb Quantum dots (patterned in quantum wells)
78.55.Cr III-V semiconductors
81.05.Ea III-V semiconductors
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