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10 Sep 2007

Volume 91, Issue 11, Articles (11xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 91, 112501 (2007); http://dx.doi.org/10.1063/1.2780107 (3 pages)

Y. Liu, S. Gliga, R. Hertel, and C. M. Schneider
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Enhanced tunable Bragg diffraction in large-pore inverse opals using dual-frequency liquid crystal

E. Graugnard, S. N. Dunham, J. S. King, D. Lorang, S. Jain, and C. J. Summers

Appl. Phys. Lett. 91, 111101 (2007); http://dx.doi.org/10.1063/1.2777153 (3 pages) | Cited 8 times

Online Publication Date: 10 September 2007

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Large-pore TiO2 inverse opals were fabricated by atomic layer deposition in sintered polystyrene colloidal crystal templates and infiltrated with dual-frequency liquid crystal. The optical properties of the hybrid organic/inorganic structure were characterized by reflectance measurements of the Bragg peak, the position of which was tuned using a frequency dependent applied electric field. A 6 nm blueshift was observed for frequencies less than 13 kHz and a 13 nm redshift for frequencies above 13 kHz. These results demonstrate enhanced optical tunability in three-dimensional photonic crystals and are important for the development of active photonic devices.
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42.70.Qs Photonic bandgap materials
42.70.Df Liquid crystals
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
81.07.Pr Organic-inorganic hybrid nanostructures

Periodic surface structures on gallium phosphide after irradiation with 150 fs–7 ns laser pulses at 800 nm

E. M. Hsu, T. H. R. Crawford, H. F. Tiedje, and H. K. Haugen

Appl. Phys. Lett. 91, 111102 (2007); http://dx.doi.org/10.1063/1.2779914 (3 pages) | Cited 16 times

Online Publication Date: 10 September 2007

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Gallium phosphide is irradiated with 150 fs–7 ns duration laser pulses at a wavelength of 800 nm. After irradiation with 150 fs pulses, periodic surface structures (ripples) are observed on the GaP surface, exhibiting near-wavelength and substantially subwavelength spatial periods depending on irradiation conditions. As the pulse duration increases, near-wavelength ripples become a more dominant feature, completely replacing subwavelength ripples for pulse lengths beyond 80–130 ps. Overall the results show that subwavelength ripples can be generated for a wide range of pulse durations, and their formation is quite insensitive to pulse intensity.
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68.47.Fg Semiconductor surfaces
68.35.B- Structure of clean surfaces (and surface reconstruction)
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
79.20.Ds Laser-beam impact phenomena
78.47.-p Spectroscopy of solid state dynamics

Dynamical ultrafast all-optical switching of planar GaAs/AlAs photonic microcavities

Philip J. Harding, Tijmen G. Euser, Yoanna-Reine Nowicki-Bringuier, Jean-Michel Gérard, and Willem L. Vos

Appl. Phys. Lett. 91, 111103 (2007); http://dx.doi.org/10.1063/1.2779106 (3 pages) | Cited 16 times

Online Publication Date: 10 September 2007

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The authors study the ultrafast switching-on and -off of planar GaAs/AlAs microcavities. Up to 0.8% refractive index changes are achieved by optically exciting free carriers at λ = 1720 nm and pulse energy Epump = 1.8±0.18 μJ. The cavity resonance is dynamically tracked by measuring reflectivity versus time delay with tunable laser pulses, and is found to shift by as much as 3.3 linewidths within a few picoseconds. The switching-off occurs with a decay time of ∼ 50 ps. They derive the dynamic behavior of the carrier density and of the complex refractive index. They propose that the inferred 10 GHz switching rate may be tenfold improved by optimized sample growth.
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78.47.-p Spectroscopy of solid state dynamics
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Enhanced electroluminescence efficiency of oxidized amorphous silicon nitride light-emitting devices by modulating Si/N ratio

Rui Huang, Kunji Chen, Hengping Dong, Danqing Wang, Honglin Ding, Wei Li, Jun Xu, Zhongyuan Ma, and Ling Xu

Appl. Phys. Lett. 91, 111104 (2007); http://dx.doi.org/10.1063/1.2783271 (3 pages) | Cited 10 times

Online Publication Date: 10 September 2007

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The authors had reported green-yellow electroluminescence (EL) from N-rich oxidized amorphous silicon nitride (a-SiN:O) light-emitting devices (LEDs) in a previous work. In this work, a significantly enhanced EL intensity was obtained in the LED by employing Si-rich a-SiN:O instead of N-rich a-SiN:O as luminescent active layer. Moreover, the Si-rich a-SiN:O devices also exhibit lower turn-on voltage and the external quantum efficiency is found to be three times higher than that of the N-rich a-SiN:O devices. The electrical characteristics analyses reveal that the injection barrier for Si-rich a-SiN:O devices is reduced by 30% compared to that of N-rich a-SiN:O devices, which results in a remarkably enhanced carrier-injection efficiency and gives rise to the notable improved performances of the LEDs.
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85.60.Jb Light-emitting devices
85.30.Kk Junction diodes

Nonmagnetic cloak with minimized scattering

Wenshan Cai, Uday K. Chettiar, Alexander V. Kildishev, Vladimir M. Shalaev, and Graeme W. Milton

Appl. Phys. Lett. 91, 111105 (2007); http://dx.doi.org/10.1063/1.2783266 (3 pages) | Cited 122 times

Online Publication Date: 11 September 2007

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In an electromagnetic cloak based on a transformation approach, reduced sets of material properties are generally favored due to their easier implementation in reality, although a seemingly inevitable drawback of undesired scattering exists in such cloaks. Here, the authors suggest the use of high-order transformations to create smooth moduli at the outer boundary of the cloak, therefore completely eliminating the detrimental scattering within the limit of geometric optics. The authors apply this scheme to a nonmagnetic cylindrical cloak and demonstrate that the scattered field is reduced substantially in a cloak with optimal quadratic transformation as compared to its linear counterpart.
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41.20.Jb Electromagnetic wave propagation; radiowave propagation

Via-hole-based vertical GaN light emitting diodes

Hyun-Min Jung, Gi-Yeon Nam, Byung-Kyun Choi, Tae-Hee Lee, Hyun-Suk Kim, Soo-Kun Jeon, Eun-Hyun Park, and Chang-Tae Kim

Appl. Phys. Lett. 91, 111106 (2007); http://dx.doi.org/10.1063/1.2785110 (3 pages) | Cited 10 times

Online Publication Date: 11 September 2007

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A vertical GaN-light emitting diode (LED) has been fabricated on a sapphire substrate with periodic via holes formed by a laser drilling technique. n-contact metal which was deposited on the backside of sapphire substrate was directly connected with an Ohmic metal of n-GaN layer through the via holes. The via-hole-based vertical GaN-LED demonstrated an optical power improvement of up to 12.5% with lower forward operating voltage compared with a conventional GaN-LED. In addition, this vertical LED showed just 0.8% and 1.5% variations of optical power and operation voltage at the 500 h reliability test.
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85.60.Jb Light-emitting devices

Polarization control of 0.85 μm vertical-cavity surface-emitting lasers integrated with gold nanorod arrays

P. Babu Dayal and F. Koyama

Appl. Phys. Lett. 91, 111107 (2007); http://dx.doi.org/10.1063/1.2783281 (3 pages) | Cited 5 times

Online Publication Date: 12 September 2007

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In this letter, the authors unambiguously demonstrate the polarization control of 0.85 μm single transverse mode vertical-cavity surface-emitting lasers (VCSELs) using plasmonic nanorod arrays fabricated on the top surface of VCSELs. The strong direction dependent scattering of light from anisotropic gold nanorods enables stable polarization operations of VCSELs with suppressing the absorption in gold layer and leads to high differential quantum efficiencies. These nanorod arrays integrated VCSELs show negligible penalties in laser performances having the maximum output power of nearly 1 mW with the side mode suppression ratio of 45 dB and the orthogonal polarization suppression ratio of 21 dB.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation
42.82.-m Integrated optics

Demonstration of an ultraviolet ZnO-based optically pumped third order distributed feedback laser

Daniel Hofstetter, Yargo Bonetti, Fabrizio R. Giorgetta, Abdel-Hamid El-Shaer, Andrey Bakin, Andreas Waag, Rüdiger Schmidt-Grund, Mathias Schubert, and Marius Grundmann

Appl. Phys. Lett. 91, 111108 (2007); http://dx.doi.org/10.1063/1.2783965 (3 pages) | Cited 9 times

Online Publication Date: 12 September 2007

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The authors demonstrate an optically pumped ZnO distributed feedback laser operating at 383 nm. For a large temperature range between 10 and 270 K, the device lased in a single longitudinal mode. Mode selection was accomplished via a third order diffraction grating, which was dry etched into a 120 nm thick Si3N4 layer deposited on the ZnO active region. They observed a spectral linewidth of 0.4 nm, a pump threshold intensity of 0.12 MW/cm2, and a peak output power of 14 mW. From wavelength versus temperature measurements, they deduced a temperature tuning coefficient of the ZnO refractive index of 9×10−5 K−1.
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42.55.Px Semiconductor lasers; laser diodes

Strong enhancement of second-harmonic generation in all-dielectric resonant waveguide grating

Mikael Siltanen, Samuli Leivo, Pauliina Voima, Martti Kauranen, Petri Karvinen, Pasi Vahimaa, and Markku Kuittinen

Appl. Phys. Lett. 91, 111109 (2007); http://dx.doi.org/10.1063/1.2783969 (3 pages) | Cited 12 times

Online Publication Date: 12 September 2007

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The authors demonstrate that a dielectric resonant waveguide grating can enhance optical second-harmonic generation by a factor of 550 compared to a similar flat surface. Their structure, which consists of purely dielectric and thereby transparent materials, has a low index silicon dioxide grating covered by a high index titanium dioxide layer and it is optimized for the fundamental wavelength of 1064 nm. Polarization dependence of the second-harmonic response suggests that the enhancement arises from the favorable interaction of the resonant waveguide mode and its strong local field with the inherent surface nonlinearity of the dielectric materials.
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42.79.Dj Gratings
42.79.Gn Optical waveguides and couplers
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation

Single photon emission based on coherent state preparation

P. Ester, L. Lackmann, S. Michaelis de Vasconcellos, M. C. Hübner, A. Zrenner, and M. Bichler

Appl. Phys. Lett. 91, 111110 (2007); http://dx.doi.org/10.1063/1.2784173 (3 pages) | Cited 9 times

Online Publication Date: 12 September 2007

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The authors report here on deterministic single photon emission after coherent optical state preparation in the p-shell of a single InGaAs/GaAs quantum dot. In the approach, they use p-shell Rabi flopping followed by relaxation to the s-shell ground state with subsequent spontaneous single photon emission. Pulsed photon correlation experiments show complete suppression of the correlation peak at zero time delay and hence demonstrate clean single photon emission.
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78.67.Hc Quantum dots
78.55.-m Photoluminescence, properties and materials

Controllable electromagnetic transmission based on dual-metallic grating structures composed of subwavelength slits

Chen Cheng, Jing Chen, Qi-Yang Wu, Fang-Fang Ren, Ji Xu, Ya-Xian Fan, and Hui-Tian Wang

Appl. Phys. Lett. 91, 111111 (2007); http://dx.doi.org/10.1063/1.2784175 (3 pages) | Cited 24 times

Online Publication Date: 12 September 2007

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Electromagnetic (EM) transmission through dual-metallic grating (DMG) structures composed of slits arrays, with the longitudinal interval G and the lateral displacement L, is investigated by using the finite-difference time-domain method. The results show that the EM transmission property can be controlled by changing G or/and L, such as tunable wavelength of high transmission, changeable transmittance for a special wavelength, and suppressed EM transmission over a broad spectral range. The DMG structures have potential applications in the future photonics. In addition, some advantages of DMGs with respect to the single-metallic gratings are also discussed.
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42.79.Dj Gratings

Vertical dielectric-sandwiched thin metal layer for compact, low-loss long range surface plasmon waveguiding

Zhijun Sun

Appl. Phys. Lett. 91, 111112 (2007); http://dx.doi.org/10.1063/1.2784177 (3 pages) | Cited 4 times

Online Publication Date: 12 September 2007

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In this letter, the author proposes a long range surface plasmon waveguide structure of a vertically standing thin metal layer sandwiched between finite-width dielectric layers to achieve both compact and low-loss photonic waveguiding. Rounded 90° bends in such waveguides, with radii of curvature on the scale of the incident wavelength, are studied. The author analyzes waveguiding modes of such waveguides at the straight and bend parts, and also the coupling of surface plasmon waves between them. The analyses are verified with numerical simulations. High transmission of surface plasmon waves through the bends is demonstrated.
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78.66.Bz Metals and metallic alloys
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
42.82.Et Waveguides, couplers, and arrays

Observation of two-dimensional holographic photovoltaic bright solitons in a photorefractive-photovoltaic crystal

Jinsong Liu, Shixong Liu, Guangyong Zhang, and Cheng Wang

Appl. Phys. Lett. 91, 111113 (2007); http://dx.doi.org/10.1063/1.2778469 (3 pages) | Cited 4 times

Online Publication Date: 13 September 2007

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Two-dimensional holographic photovoltaic bright spatial solitons are observed in a Cu:K0.25Na0.75Sr1.5Ba0.5Nb5O15 crystal in which two coherent laser beams, a signal beam, as well as a strong and uniform pump beam at 532 nm are coupled to each other via two-wave mixing. Such solitons originate from the self-trapping of the signal beam due to both photovoltaic effects and holographic focusing effects.
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42.65.Tg Optical solitons; nonlinear guided waves
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
42.40.-i Holography
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
72.40.+w Photoconduction and photovoltaic effects

Convolution theorem for nonlinear optics

Hernando Garcia and Ramki Kalyanaraman

Appl. Phys. Lett. 91, 111114 (2007); http://dx.doi.org/10.1063/1.2780082 (3 pages) | Cited 3 times

Online Publication Date: 13 September 2007

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The authors have expressed the nonlinear optical absorption of a semiconductor in terms of its linear spectrum. They determined that the two-photon absorption coefficient in a strong dc electric field of a direct gap semiconductor can be expressed as the product of a differential operator times the convolution integral of the linear absorption without a dc electric field and an Airy function. They have applied this formalism to calculate the two-photon absorption coefficient and nonlinear refraction for GaAs and ZnSe using their linear absorption and have found excellent agreement with available experimental data.
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42.65.-k Nonlinear optics
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

InGaAs/AlAsSb quantum cascade detectors operating in the near infrared

F. R. Giorgetta, E. Baumann, D. Hofstetter, C. Manz, Q. Yang, K. Köhler, and M. Graf

Appl. Phys. Lett. 91, 111115 (2007); http://dx.doi.org/10.1063/1.2784289 (3 pages) | Cited 20 times

Online Publication Date: 13 September 2007

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The authors report on short-wavelength In0.53Ga0.47As/AlAs0.56Sb0.44 quantum cascade detectors (QCDs). At room temperature, one device detects at 505 meV (2.46 μm) with a responsivity of 2.57 mA/W, while a second QCD is sensitive at 580 meV (2.14 μm) with a responsivity of 0.32 mA/W.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Ultrabroad stimulated emission from quantum-dash laser

H. S. Djie, C. L. Tan, B. S. Ooi, J. C. M. Hwang, X.-M. Fang, Y. Wu, J. M. Fastenau, W. K. Liu, G. T. Dang, and W. H. Chang

Appl. Phys. Lett. 91, 111116 (2007); http://dx.doi.org/10.1063/1.2784969 (3 pages) | Cited 15 times

Online Publication Date: 13 September 2007

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The authors demonstrate the generation of ultrabroadband stimulated emission in the quasi-zero-dimensional InAs/InAlGaAs quantum-dash laser grown on InP substrate. The laser exhibits lasing wavelength coverage of up to 76 nm at ∼ 1.64 μm from simultaneous multiple confined states lasing at room temperature. Unlike the conventional interband diode laser, the rule changing broadband lasing signature is achieved from the quasicontinuous interband transition formed by the inhomogeneous quantum-dash nanostructure.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems

Ultrafast carrier dynamics on conjugated poly(3-hexylthiophene)/[6,6]-phenylC61-butyric acid methyl ester composites

Emmanouil Lioudakis, Andreas Othonos, Ioannis Alexandrou, and Yasuhiko Hayashi

Appl. Phys. Lett. 91, 111117 (2007); http://dx.doi.org/10.1063/1.2785120 (3 pages) | Cited 12 times

Online Publication Date: 14 September 2007

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The authors have studied ultrafast carrier dynamics on poly(3-hexylthiophene)/[6,6]-phenylC61-butyric acid methyl ester (PCBM) composites up to 50% PCBM concentration. They have resolved the exciton intraband relaxation of composites as well as the subsequent electron/polaron relaxation of dissociated excitons. The observed fast time constant of intraband relaxation is found to be 0.5–1.5 ps, whereas the second long-live relaxation (0.5–1 ns) is strongly fullerene related. A wavelength dependent ultrafast study is performed giving fundamental information on the nonradiative exciton relaxation, exciton dissociation, and electron relaxation of PCBM-related states.
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81.05.Qk Reinforced polymers and polymer-based composites
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
82.35.Cd Conducting polymers
71.38.-k Polarons and electron-phonon interactions
71.35.Lk Collective effects (Bose effects, phase space filling, and excitonic phase transitions)
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
72.80.Tm Composite materials
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Study of radial growth rate and size control of silicon nanocrystals in square-wave-modulated silane plasmas

Th. Nguyen-Tran, P. Roca i Cabarrocas, and G. Patriarche

Appl. Phys. Lett. 91, 111501 (2007); http://dx.doi.org/10.1063/1.2784294 (3 pages) | Cited 3 times

Online Publication Date: 14 September 2007

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The growth of silicon nanocrystals in high pressure and high dilution silane plasmas is investigated by using the temporal evolution of the self-bias on the radio frequency electrode and transmission electron microscopy. A square-wave-modulated plasma was used in order to control the growth of monodispersed nanoparticles with sizes smaller than 12 nm. To this end, the plasma on time was kept below 1 s. The radial growth rate of nanoparticles was varied in the range from 7.5 to 75 nm/s by changing silane partial pressure. Nanoparticles grown in silane-helium discharges have been found amorphous while they are crystalline in silane-hydrogen-argon discharges. Surprisingly, the crystallization in the gaseous phase does not depend on how slow or fast the particles grow but on the presence of atomic hydrogen.
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81.05.Cy Elemental semiconductors
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
68.47.Fg Semiconductor surfaces
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
52.77.Dq Plasma-based ion implantation and deposition
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Local structure characterization in quasicrystal-forming Zr80Pt20 binary amorphous alloy

Junji Saida, Takashi Sanada, Shigeo Sato, Muneyuki Imafuku, and Akihisa Inoue

Appl. Phys. Lett. 91, 111901 (2007); http://dx.doi.org/10.1063/1.2778750 (3 pages) | Cited 4 times

Online Publication Date: 11 September 2007

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The local structure of melt-spun Zr80Pt20 alloy was investigated in the amorphous and icosahedral quasicrystal (QC)-formed states by x-ray diffraction and extended x-ray absorption fine structure measurements. While the local environment around the Zr atom in the amorphous state is considerably different from that in the QC-formed state, it remains during the quasicrystallization around the Pt atom. It is suggested that the stable icosahedral local structure is mainly formed in the center of the Pt atom in the amorphous state.
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61.44.Br Quasicrystals
78.70.Dm X-ray absorption spectra

Thermodynamic formalism of minimum heat source temperature for driving advanced adsorption cooling device

Bidyut Baran Saha, Anutosh Chakraborty, Shigeru Koyama, Kandadai Srinivasan, Kim Choon Ng, Takao Kashiwagi, and Pradip Dutta

Appl. Phys. Lett. 91, 111902 (2007); http://dx.doi.org/10.1063/1.2780117 (3 pages) | Cited 10 times

Online Publication Date: 11 September 2007

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This letter presents a thermodynamic formulation to calculate the minimum driving heat source temperature of an advanced solid sorption cooling device, and it is validated with experimental data. This formalism has been developed from the rigor of the Boltzmann distribution function and the condensation approximation of adsorptive molecules. An interesting and useful finding has been established from this formalism that it is possible to construct a solid sorption refrigeration device that operates in a cycle transferring heat from a low temperature source to a heat sink with a driving heat source at a temperature close to but above ambient.
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44.10.+i Heat conduction
07.20.Mc Cryogenics; refrigerators, low-temperature detectors, and other low-temperature equipment

Resonant Raman scattering in hydrogen and nitrogen doped ZnO

F. Friedrich and N. H. Nickel

Appl. Phys. Lett. 91, 111903 (2007); http://dx.doi.org/10.1063/1.2783222 (3 pages) | Cited 24 times

Online Publication Date: 11 September 2007

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Raman backscattering measurements were performed on single crystal ZnO for different excitation wavelengths before and after ion implantation with hydrogen and nitrogen. In addition to the formation of H- and N-related defects due to implantation, anomalous Raman modes were observed. Recently, the anomalous Raman modes have been attributed to the disorder-induced activation of silent modes. However, we will show that part of the observed modes are due to the resonantly enhanced longitudinal optical phonons.
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78.30.Fs III-V and II-VI semiconductors
63.20.D- Phonon states and bands, normal modes, and phonon dispersion

Thickness and stoichiometry dependence of the thermal conductivity of GeSbTe films

John P. Reifenberg, Matthew A. Panzer, SangBum Kim, Aaron M. Gibby, Yuan Zhang, Simon Wong, H.-S. Philip Wong, Eric Pop, and Kenneth E. Goodson

Appl. Phys. Lett. 91, 111904 (2007); http://dx.doi.org/10.1063/1.2784169 (3 pages) | Cited 27 times

Online Publication Date: 11 September 2007

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Thermal conduction in GeSbTe films strongly influences the writing energy and time for phase change memory (PCM) technology. This study measures the thermal conductivity of Ge2Sb2Te5 between 25 and 340 °C for layers with thicknesses near 60, 120, and 350 nm. A strong thickness dependence of the thermal conductivity is attributed to a combination of thermal boundary resistance (TBR) and microstructural imperfections. Stoichiometric variations significantly alter the phase transition temperatures but do not strongly impact the thermal conductivity at a given temperature. This work makes progress on extracting the TBR for Ge2Sb2Te5 films, which is a critical unknown parameter for PCM simulations.
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66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
68.55.-a Thin film structure and morphology
61.43.Dq Amorphous semiconductors, metals, and alloys
61.66.Bi Elemental solids
61.66.Dk Alloys
64.70.K- Solid-solid transitions
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder

Effect of measured distance and angle tuning on transmission properties of split-ring resonators

Chuang Li, Huiqing Fan, and Nanjing Li

Appl. Phys. Lett. 91, 111905 (2007); http://dx.doi.org/10.1063/1.2785117 (3 pages)

Online Publication Date: 11 September 2007

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Transmission properties of a layer of split-ring resonators (SRRs) are investigated experimentally in free space with the propagation direction of the electromagnetic field perpendicular to the SRR-patterned substrate. It is shown that the response frequency of SRRs is independent of the measured distance. In addition, the qualitative change from electric resonance of SRRs to magnetic resonance with the variation of angle between electric field and the symmetric axis of SRR is presented. The results suggest a simple way of obtaining magnetic resonance of SRRs, even left-handed behavior.
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81.05.-t Specific materials: fabrication, treatment, testing, and analysis
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
42.70.-a Optical materials

Low temperature LO-phonon dynamics of MgZnO nanoalloys

Jesse Huso, John L. Morrison, Heather Hoeck, Erin Casey, Leah Bergman, T. D. Pounds, and M. G. Norton

Appl. Phys. Lett. 91, 111906 (2007); http://dx.doi.org/10.1063/1.2784189 (3 pages) | Cited 10 times

Online Publication Date: 12 September 2007

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In this paper, the authors present research on the low temperature resonant Raman scattering of the LO modes of Mg0.2Zn0.8O nanocrystals. They found that in the LO spectral range, two coupled modes appear: the main LO mode of the alloy and a low frequency mode redshifted by ∼ 27 cm−1 relative to the main one. The low frequency mode was analyzed in terms of three mechanisms applicable to Raman scattering of an alloy system that might result in its activation: phase segregation due to the low solubility limit of the MgO–ZnO alloy system, a convolution of the A1(LO) with the E1(LO) mode, and an order-disorder state. Their analyses indicate that the principle mechanism that activates the low frequency mode is scattering due to the disordered state of the nanoalloys.
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63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.30.Fs III-V and II-VI semiconductors
64.75.-g Phase equilibria

Acoustic and optical phonon assisted formation of biexcitons

Ching-Ju Pan, Kuo-Feng Lin, and Wen-Feng Hsieh

Appl. Phys. Lett. 91, 111907 (2007); http://dx.doi.org/10.1063/1.2784180 (3 pages) | Cited 10 times

Online Publication Date: 13 September 2007

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Efficient exciton relaxation is required for bounding two cooled excitons to form biexciton. Acoustic and optical phonon scatterings playing key roles in exciton relaxation are responsible for formation of biexcitons at various temperatures. Using ZnO powders, the authors observed a sublinear dependence on excitation power at low temperature, in which the relaxation process involves only emission of acoustic phonons due to the excitons having kinetic energy lower than those of the optical phonons. However, the exponent comes near theoretical value of 2 for participation of optical phonons when the exciton kinetic energy approaches to the energy of the lowest optical phonon.
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71.35.-y Excitons and related phenomena
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
78.55.Et II-VI semiconductors
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