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27 Oct 2008

Volume 93, Issue 17, Articles (17xxxx)

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Appl. Phys. Lett. 93, 172501 (2008); http://dx.doi.org/10.1063/1.3005561 (3 pages)

Yan Wang, X. F. Han, and X.-G. Zhang
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Gate-field-induced phase transitions in VO2: Monoclinic metal phase separation and switchable infrared reflections

Changhong Chen, Renfan Wang, Lang Shang, and Chongfeng Guo

Appl. Phys. Lett. 93, 171101 (2008); http://dx.doi.org/10.1063/1.3009569 (3 pages) | Cited 12 times

Online Publication Date: 27 October 2008

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In a metal-oxide-semiconductor VO2 active layer under uniaxial stress, gate-field-induced phase transitions are revealed by strongly field-dependent Raman scattering and infrared reflections. A metal-insulator transition (MIT) is demonstrated by a strongly correlated monoclinic metal phase separation that percolates, thereby making the reflections switchable. In addition, the MIT occurs at a gate voltage around 3.36 V, much lower than the threshold of a structural phase transition (SPT). Hence, the MIT is easily controlled by the gate field to avoid the SPT-caused fatigue and breakdown in high-speed operation.
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64.70.K- Solid-solid transitions
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
71.30.+h Metal-insulator transitions and other electronic transitions
78.30.-j Infrared and Raman spectra
64.75.-g Phase equilibria
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder

Optical property of beta barium borate in terahertz region

Jingle Liu, Xiaoyu Guo, Jianming Dai, and X.-C. Zhang

Appl. Phys. Lett. 93, 171102 (2008); http://dx.doi.org/10.1063/1.3009964 (3 pages) | Cited 6 times

Online Publication Date: 27 October 2008

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The optical property of beta barium borate (β−BBO) in the terahertz region (0.2–12 THz) was studied using a broadband air photonic terahertz time-domain spectrometer. The crystal orientation and frequency dependence of β−BBO refractive index and absorption coefficient were experimentally investigated over the temperature range of 10–293 K. Four TO phonon modes were observed below 3.5 THz. The behavior of these phonon modes at different temperatures has been characterized.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
63.20.dd Measurements
78.70.Gq Microwave and radio-frequency interactions
61.66.Fn Inorganic compounds

Autofluorescence insensitive imaging using upconverting nanocrystals in scattering media

Can T. Xu, Niclas Svensson, Johan Axelsson, Pontus Svenmarker, Gabriel Somesfalean, Guanying Chen, Huijuan Liang, Haichun Liu, Zhiguo Zhang, and Stefan Andersson-Engels

Appl. Phys. Lett. 93, 171103 (2008); http://dx.doi.org/10.1063/1.3005588 (3 pages) | Cited 22 times

Online Publication Date: 27 October 2008

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Autofluorescence is a nuisance in the field of fluorescence imaging and tomography of exogenous molecular markers in tissue, degrading the quality of the collected data. In this letter, we report autofluorescence insensitive imaging using highly efficient upconverting nanocrystals (NaYF4:Yb3+/Tm3+) in a tissue phantom illuminated with near-infrared radiation of 85 mW/cm2. It was found that imaging with such nanocrystals leads to an exceptionally high contrast compared to traditional downconverting fluorophores due to the absence of autofluorescence. Upconverting nanocrystals may be envisaged as important biological markers for tissue imaging purposes.
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87.64.km Infrared
87.64.kv Fluorescence

All-optical steering of soliton waveguides in dye-doped liquid crystals

A. Piccardi, G. Assanto, L. Lucchetti, and F. Simoni

Appl. Phys. Lett. 93, 171104 (2008); http://dx.doi.org/10.1063/1.3009658 (3 pages) | Cited 21 times

Online Publication Date: 28 October 2008

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Soliton induced waveguides can be all-optically readdressed in planar cells by modifying the molecular anchoring with an external light source. Using an elliptically shaped control beam we demonstrate refraction and total internal reflection of spatial solitons in dye-doped nematic liquid crystals.
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42.70.Df Liquid crystals
42.60.Fc Modulation, tuning, and mode locking
42.65.Tg Optical solitons; nonlinear guided waves

Stark-tunable electroluminescence from cavity polariton states

Y. Todorov, P. Jouy, A. Vasanelli, L. Sapienza, R. Colombelli, U. Gennser, and C. Sirtori

Appl. Phys. Lett. 93, 171105 (2008); http://dx.doi.org/10.1063/1.3002302 (3 pages) | Cited 2 times

Online Publication Date: 28 October 2008

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Electric-field tunable electroluminescence from intersubband transitions in a quantum well has been demonstrated via the strong coupling of the electronic transitions with an optical cavity mode. The device consists of a quantum cascade structure embedded in a planar metal-dielectric microcavity where electrons can be resonantly injected at different energies, thanks to the polariton dispersion curve. The electroluminescence tuning shows a strong far field angular dependence in accordance with the conservation of the in-plane momentum. Our experiment illustrates that it is possible to connect quantum optics and electronic transport in semiconductor heterostructures.
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78.67.De Quantum wells
78.60.Fi Electroluminescence
78.20.Jq Electro-optical effects
73.21.Fg Quantum wells
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)

A physical model for the rapid degradation of semiconductor laser diodes

A. Martín-Martín, M. Avella, M. P. Iñiguez, J. Jiménez, M. Oudart, and J. Nagle

Appl. Phys. Lett. 93, 171106 (2008); http://dx.doi.org/10.1063/1.3009290 (3 pages) | Cited 4 times

Online Publication Date: 28 October 2008

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The degradation of AlGaAs based high power laser bars (808 nm) is modeled in terms of the thermal stress gradient induced by the overheating produced at a facet defect by self-absorption and nonradiative recombination. Using a thermomechanical model, the local heating at the defect is shown to induce local stress above the yield strength necessary for plastic deformation. Cathodoluminescence images of the facets show the formation of large facet defects. The role of the packaging stress is also elucidated. The power density dissipation and the local temperature necessary to achieve the plastic deformation are in good agreement with the experimental values reported for laser degradation.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems

Simultaneous observation of single- and two-photon excitation photoluminescence on optically quenched wide-gap semiconductor crystals

A. S. M. Noor, M. Torizawa, A. Miyakawa, and Y. Kawata

Appl. Phys. Lett. 93, 171107 (2008); http://dx.doi.org/10.1063/1.3009300 (3 pages)

Online Publication Date: 28 October 2008

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Simultaneous detection of a quenched wide-gap semiconductor crystal is observed using single- and two-photon excitation photoluminescence. The quenching process involved two-photon excitation method induced defects to the crystal. It was found that the quenched area, photoluminescence at the bandgap wavelength is detected using single-photon excitation while no photoluminescence is observed from two-photon excitation. This is due to the damage occurred to the transition states of the two-photon excitation. The dependency of respective excitation methods with photoluminescence was elaborated to confirm the single- and two-photon excitation photoluminescence methods.
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78.55.-m Photoluminescence, properties and materials
71.20.Nr Semiconductor compounds

The focusing effect of graded index photonic crystals

H. Kurt, E. Colak, O. Cakmak, H. Caglayan, and E. Ozbay

Appl. Phys. Lett. 93, 171108 (2008); http://dx.doi.org/10.1063/1.3009965 (3 pages) | Cited 20 times

Online Publication Date: 29 October 2008

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We describe an approach to implement graded index (GRIN) structures using two-dimensional photonic crystals (PCs). The lattice spacing along the transverse direction to propagation is altered and we show, both theoretically and experimentally, that such a spatial perturbation is an effective way to obtain GRIN PC. The response of the structure to spatially wide incident beams is investigated and strong focusing behavior is observed. The large spot size conversion ratio can be attainable and is mainly limited by the finite size of the structure. The designed GRIN PC shows promise for use in optical systems that require compact and powerful focusing elements compared to the traditional bulky lenses.
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42.70.Qs Photonic bandgap materials
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
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Ultrashort-pulse laser calligraphy

Weijia Yang, Peter G. Kazansky, Yasuhiko Shimotsuma, Masaaki Sakakura, Kiyotaka Miura, and Kazuyuki Hirao

Appl. Phys. Lett. 93, 171109 (2008); http://dx.doi.org/10.1063/1.3010375 (3 pages) | Cited 12 times

Online Publication Date: 29 October 2008

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Control of structural modifications inside silica glass by changing the front tilt of an ultrashort pulse is demonstrated, achieving a calligraphic style of laser writing. The phenomena of anisotropic bubble formation at the boundary of an irradiated region and modification transition from microscopic bubbles formation to self-assembled form birefringence are observed, and the physical mechanisms are discussed. The results provide the comprehensive evidence that the light beam with centrosymmetric intensity distribution can produce noncentrosymmetric material modifications.
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42.62.Cf Industrial applications
81.65.-b Surface treatments
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
61.43.Fs Glasses

Plasmonic antenna array at optical frequency made by nanoapertures

Z. J. Zhang, R. W. Peng, Z. Wang, F. Gao, X. R. Huang, W. H. Sun, Q. J. Wang, and Mu Wang

Appl. Phys. Lett. 93, 171110 (2008); http://dx.doi.org/10.1063/1.3010741 (3 pages) | Cited 10 times

Online Publication Date: 31 October 2008

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We show here that the plasmonic array based on nanoapertures in ultrathin silver film radiates at optical frequency and behaves as an optical antenna array (OAA). The far-field radiation originates from the coherent superposition of plasmonic emissions on each bank of the aperture. The radiation of OAA presents a strong directivity, which depends on the in-plane rotation of aperture array, and on the polarization and incidence angle of the excitation light as well. We suggest that these features have potential applications in photovoltaics, light-emitting devices, and optical sensors.
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42.79.Wc Optical coatings
42.79.Ag Apertures, collimators

All-angle zero reflection at metamaterial surfaces

Xin Li, Zixian Liang, Xiaohan Liu, Xunya Jiang, and Jian Zi

Appl. Phys. Lett. 93, 171111 (2008); http://dx.doi.org/10.1063/1.3012375 (3 pages) | Cited 7 times

Online Publication Date: 31 October 2008

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The authors study theoretically reflection on the surface of a metamaterial with a hyperbolic dispersion. It is found that reflection is strongly dependent on how the surface is terminated with respect to the asymptote of the hyperbolic dispersion. For a surface terminated normally to the asymptote, zero reflection occurs for all incident angles. It is exemplified by a metamaterial made of a periodic metal-dielectric layered structure with its surface properly cut through numerical simulations.
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42.25.Gy Edge and boundary effects; reflection and refraction
42.70.-a Optical materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Surface-emitting photonic crystal terahertz quantum cascade lasers

Owen P. Marshall, Vasilis Apostolopoulos, Joshua R. Freeman, Rakchanok Rungsawang, Harvey E. Beere, and David A. Ritchie

Appl. Phys. Lett. 93, 171112 (2008); http://dx.doi.org/10.1063/1.3012385 (3 pages) | Cited 11 times

Online Publication Date: 31 October 2008

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Surface-emitting terahertz quantum cascade lasers based on double-metal waveguides incorporating photonic crystal structures have been demonstrated. Far-field emission patterns are dominated by lobes close to the surface normal. In addition to modified emission profiles, enhanced output powers are also displayed in comparison to standard ridge waveguides, with over 2 mW peak power observed at a heat sink temperature of 10 K.
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42.55.Px Semiconductor lasers; laser diodes
42.70.Qs Photonic bandgap materials
42.55.Tv Photonic crystal lasers and coherent effects
42.79.Gn Optical waveguides and couplers
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Reduction of efficiency droop in InGaN light emitting diodes by coupled quantum wells

Xianfeng Ni, Qian Fan, Ryoko Shimada, Ümit Özgür, and Hadis Morkoç

Appl. Phys. Lett. 93, 171113 (2008); http://dx.doi.org/10.1063/1.3012388 (3 pages) | Cited 66 times

Online Publication Date: 31 October 2008

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Light emitting diodes (LEDs) based on InGaN suffer from efficiency droop at current injection levels as low as 50 A cm−2. We investigated multiple quantum well InGaN LEDs with varying InGaN barrier thicknesses (3–12 nm) emitting at ∼ 400–410 nm to investigate the effect of hole mass and also to find out possible solutions to prevent the efficiency droop. In LEDs with electron blocking layers, when we reduced the InGaN barriers from 12 to 3 nm, the current density for the peak or saturation of external quantum efficiency increased from 200 to 1100 A cm−2 under pulsed injection conditions, which eliminates the heating effects to a large extent. Our calculations show that such reduction in the barrier thickness makes the hole distribution more uniform among the wells. These results suggest that the inferior low hole transport through the barriers exacerbated by large hole effective mass and low hole injection due to relatively low hole concentration and the consequent electron leakage are responsible for the efficiency droop at high current injection levels.
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85.60.Jb Light-emitting devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Spontaneous luminescence polarizations of wurtzite InGaN/GaN quantum wells

Chuanyu Jia, Tongjun Yu, Renchun Tao, Xiaodong Hu, Zhijian Yang, Zhixin Qin, Zhizhong Chen, and Guoyi Zhang

Appl. Phys. Lett. 93, 171114 (2008); http://dx.doi.org/10.1063/1.2999540 (3 pages) | Cited 2 times

Online Publication Date: 31 October 2008

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In this paper, we analyze the polarization selection rules of wurtzite InGaN/GaN quantum wells using the k⋅p perturbation method. It was found that the symmetry properties of the valence subbands’ wavefunctions at kt ≠ 0 are quite different with those at kt = 0. These symmetry properties of valence subbands’ wavefunctions influence the momentum matrix element for TE and TM modes, leading to a different polarization selection rules from the ones at kt = 0 and the absence of spectra peak shift between TE and TM modes. It is suggested that the polarization selection rule at kt ≠ 0 should be considered in the main transition process for wurtzite III-V semiconductors.
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78.67.De Quantum wells
78.55.Cr III-V semiconductors
78.60.Fi Electroluminescence
73.21.Fg Quantum wells
73.20.At Surface states, band structure, electron density of states
71.15.-m Methods of electronic structure calculations
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