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24 Jul 2006

Volume 89, Issue 4, Articles (04xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 89, 041105 (2006); http://dx.doi.org/10.1063/1.2234591 (3 pages)

Alireza Khalili, Hopil Bae, and James S. Harris
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Time-resolved broadband analysis of split ring resonators in terahertz region

B. G. Quan, X. L. Xu, H. F. Yang, X. X. Xia, Q. Wang, L. Wang, C. Z. Gu, C. Li, and F. Li

Appl. Phys. Lett. 89, 041101 (2006); http://dx.doi.org/10.1063/1.2220060 (3 pages) | Cited 5 times

Online Publication Date: 24 July 2006

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Split ring resonators (SRRs) in micrometer scale were fabricated by photolithography and lift-off technologies. The transmission and dispersion characteristics of SRR were measured by terahertz spectroscopy in broadband time domain. Significant resonances at about 0.88 and 2.01 THz have been observed, where two stop-band gaps are formed in the dispersion spectrum of SRR. It is suggested that these two minimum dips in the transmission spectrum are induced by dipole and magnetodipole resonances. The results show good agreement with the effective medium theory and the numerical calculation of electromagnetic propagation using finite-difference time-domain method.
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42.70.Qs Photonic bandgap materials
42.79.-e Optical elements, devices, and systems
07.57.Pt Submillimeter wave, microwave and radiowave spectrometers; magnetic resonance spectrometers, auxiliary equipment, and techniques

Temporal and spectral nonspecularities in reflection at surface plasmon resonance

Xiaobo Yin, Lambert Hesselink, Henry Chin, and David A. B. Miller

Appl. Phys. Lett. 89, 041102 (2006); http://dx.doi.org/10.1063/1.2233622 (3 pages) | Cited 1 time

Online Publication Date: 24 July 2006

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Nonspecular reflection of short pulses at surface plasmon resonance is analyzed and experimentally investigated. It is shown that the reflected optical pulse undergoes an extraordinary time delay as well as a central carrier frequency shift as the surface waves excited. The surface plasmon resonance is shown to enhance both nonspecular effects.
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78.68.+m Optical properties of surfaces

Second harmonic generation of continuous wave ultraviolet light and production of β-BaB2O4 optical waveguides

R. Degl’Innocenti, A. Guarino, G. Poberaj, and P. Günter

Appl. Phys. Lett. 89, 041103 (2006); http://dx.doi.org/10.1063/1.2234275 (3 pages) | Cited 14 times

Online Publication Date: 24 July 2006

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We report on the generation of continuous-wave (cw) ultraviolet (UV) laser light at λ = 278 nm by optical frequency doubling of visible light in β-BaB2O4 waveguides. Ridge-type waveguides were produced by He+ implantation, photolithography masking, and plasma etching. The final waveguides have core dimension of a few μm2 and show transmission losses of 5 dB/cm at 532 nm and less than 10 dB/cm at 266 nm. In our first experiments, a second harmonic power of 24 μW has been generated at 278 nm in an 8 mm long waveguide pumped by 153 mW at 556 nm.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.55.Rz Doped-insulator lasers and other solid state lasers
42.60.By Design of specific laser systems

Role of delocalized charges in the pyroelectric effect

D. R. Evans, S. A. Basun, G. Cook, and R. S. Meltzer

Appl. Phys. Lett. 89, 041104 (2006); http://dx.doi.org/10.1063/1.2234565 (3 pages)

Online Publication Date: 24 July 2006

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The temperature dependence of the pyroelectric effect is investigated. An increase in dark conductivity resulting from either trap depopulation or proton migration is shown to suppress the pyroelectric-generated high voltage. Electrically shorting crystal z surfaces coated with a transparent conductive layer is shown to be effective in eliminating the breakdown of the surface charge in air and thus provides a means to study weak thermoluminescence in pyroelectric crystals. In addition, a new method to measure proton migration and dark conductivity is proposed; a direct correlation between the emitted light intensity resulting from ionization of air and the pyroelectric current is observed.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.70.+a Pyroelectric and electrocaloric effects
72.40.+w Photoconduction and photovoltaic effects
78.60.Kn Thermoluminescence
77.22.Jp Dielectric breakdown and space-charge effects
77.80.-e Ferroelectricity and antiferroelectricity

An evanescent-coupling approach to making stable fiber-coupled semiconductor lasers

Alireza Khalili, Hopil Bae, and James S. Harris

Appl. Phys. Lett. 89, 041105 (2006); http://dx.doi.org/10.1063/1.2234591 (3 pages) | Cited 1 time

Online Publication Date: 24 July 2006

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We present a fiber-coupled GaAs laser, which works on the principle of evanescent coupling between a semiconductor antiresonant reflecting optical waveguide and a side-polished single-mode fiber. Compared to conventional fiber-coupled semiconductor lasers, these devices enjoy more relaxed alignment tolerances and simpler packaging, as well as high immunity to catastrophic mirror damage that limits the maximum power output of most edge-emitting lasers. Moreover, the small coupling window here provides an inherent filter that guarantees wavelength stability without any gratings. The initial data show stable single-frequency operation with more than 27 dB side-mode suppression ratio and a small thermal coefficient of 0.06 nm/°C.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.55.Wd Fiber lasers
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Near infrared absorption and room temperature photovoltaic response in AlN/GaN superlattices grown by metal-organic vapor-phase epitaxy

E. Baumann, F. R. Giorgetta, D. Hofstetter, S. Golka, W. Schrenk, G. Strasser, L. Kirste, S. Nicolay, E. Feltin, J. F. Carlin, and N. Grandjean

Appl. Phys. Lett. 89, 041106 (2006); http://dx.doi.org/10.1063/1.2234847 (3 pages) | Cited 23 times

Online Publication Date: 25 July 2006

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We report on intersubband absorption of near infrared radiation in AlN/GaN superlattice structures grown by metal-organic vapor-phase epitaxy. A good correlation between well thickness and absorption peak energy was obtained. One sample shows a photovoltaic signal which overlaps well with the corresponding absorption curve at around 1.5 μm (830 meV), a common wavelength in optical fiber telecommunication systems. This photovoltaic signal is strongest at temperatures around 75 K and persists up to room temperature. The frequency response of this sample was measured with a modulated 1.5 μm laser diode. The amplitude of the response was highest for a frequency of 36 kHz.
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78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
78.30.Hv Other nonmetallic inorganics
72.40.+w Photoconduction and photovoltaic effects
81.15.Kk Vapor phase epitaxy; growth from vapor phase

Enhanced zero-order transmission of terahertz radiation pulses through very deep metallic gratings with subwavelength slits

Qirong Xing, Shuxin Li, Zhen Tian, Dong Liang, Ning Zhang, Liying Lang, Lu Chai, and Qingyue Wang

Appl. Phys. Lett. 89, 041107 (2006); http://dx.doi.org/10.1063/1.2235844 (3 pages) | Cited 11 times

Online Publication Date: 25 July 2006

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Very deep metallic grating structures with subwavelength slits are processed to study the transmission properties of terahertz radiation pulses. The experiments have been performed with two samples. The delay of the terahertz pulses and the corresponding enhanced unresonant transmission spectra through the samples are observed. A simple interpretation is presented based on the combination of zero-order diffraction of the gratings with the coupling of the surface plasmon polaritons formed on the top and bottom of the gratings. The experimental results are in good agreement with theoretical calculations.
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42.79.Dj Gratings
78.70.Gq Microwave and radio-frequency interactions
78.68.+m Optical properties of surfaces
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)

Ultrafast-laser-induced parallel phase-change nanolithography

Y. Lin, M. H. Hong, T. C. Chong, C. S. Lim, G. X. Chen, L. S. Tan, Z. B. Wang, and L. P. Shi

Appl. Phys. Lett. 89, 041108 (2006); http://dx.doi.org/10.1063/1.2235855 (3 pages) | Cited 22 times

Online Publication Date: 25 July 2006

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A phase-change nanolithography technique is developed to fabricate up to millions of two-/three-dimensional nanostructures ( ∼ 50 nm) over a large area at a high speed by combining femtosecond laser, microlens array, and wet etching process. Near-field scanning optical microscopy, electrical force microscopy, and atomic force microscopy were used to characterize optical and electrical properties of crystalline and amorphous states, respectively. Different reactions of both amorphous and crystalline areas in phase-change film to alkaline solution are demonstrated. Multiphoton absorption and ultrashort pulse contribute to nanostructure generation. This method opens up a route for nanodevice fabrication with phase-change material.
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81.16.Nd Micro- and nanolithography
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
73.61.-r Electrical properties of specific thin films

Scaling behavior of ZnO transparent thin-film transistors

Hsing-Hung Hsieh and Chung-Chih Wu

Appl. Phys. Lett. 89, 041109 (2006); http://dx.doi.org/10.1063/1.2235895 (3 pages) | Cited 63 times

Online Publication Date: 25 July 2006

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Scaling behaviors of ZnO transparent thin-film transistors (TTFTs) have been studied by fabricating series of miniaturized ZnO TTFTs having various channel widths and lengths. Mobility of >8 cm2/V s and on/off ratio of up to 107 are achieved with these TTFTs. Results show that these ZnO TTFTs retain rather well-behaved transistor characteristics down to the channel length of ∼ 5 μm, rendering possible high-resolution applications. More apparent short-channel effects (e.g., lowering of threshold voltages, degradation of the subthreshold slope with the decrease of the channel length and the increase of the drain voltage, loss of hard saturation, etc.) are observed when the channel length is reduced below 5 μm.
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85.30.Tv Field effect devices

Twist angle effects on the dynamic response of in-plane-switching liquid crystal displays

Yubao Sun, Hongmei Ma, Zhidong Zhang, Xinyu Zhu, and Shin-Tson Wu

Appl. Phys. Lett. 89, 041110 (2006); http://dx.doi.org/10.1063/1.2236215 (3 pages) | Cited 6 times

Online Publication Date: 25 July 2006

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Twist angle effects on the response time of in-plane-switching liquid crystal displays are analyzed. The authors propose a device configuration whose top and bottom boundary liquid crystal layers are symmetric to each other with respect to the electric field direction. The analytical results of this device configuration indicate that the response time is improved at least four times faster than that of a conventional in-plane-switching twisted-nematic mode and normal in-plane-switching mode.
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42.70.Df Liquid crystals
42.79.Kr Display devices, liquid-crystal devices
85.60.Pg Display systems
61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order

Adiabatic and nonadiabatic nanofocusing of plasmons by tapered gap plasmon waveguides

D. F. P. Pile and D. K. Gramotnev

Appl. Phys. Lett. 89, 041111 (2006); http://dx.doi.org/10.1063/1.2236219 (3 pages) | Cited 54 times

Online Publication Date: 25 July 2006

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Show Abstract
Adiabatic and nonadiabatic nanofocusing of plasmons in tapered gap plasmon waveguides is analyzed using the finite-difference time-domain algorithm. Optimal adaptors between two different subwavelength waveguides and conditions for maximal local field enhancement are determined, investigated, and explained on the basis of dissipative and reflective losses in the taper. Nanofocusing of plasmons into a gap of ∼ 1 nm width with more than 20 times increase in the plasmon energy density is demonstrated in a silver-vacuum taper of ∼ 1 μm long. Comparison with the approximate theory based on the geometrical optics approximation is conducted.
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42.79.Gn Optical waveguides and couplers
78.68.+m Optical properties of surfaces
42.15.-i Geometrical optics

Optical manipulation of single electron spin in doped and undoped quantum dots

Jinshuang Jin, Xin-Qi Li, and YiJing Yan

Appl. Phys. Lett. 89, 041112 (2006); http://dx.doi.org/10.1063/1.2236282 (3 pages)

Online Publication Date: 26 July 2006

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The optical manipulation of electron spins is of great benefit to solid-state quantum information processing. In this letter, we provide a comparative study on the ultrafast optical manipulation of single electron spin in the doped and undoped quantum dots. The study indicates that the experimental breakthrough can be preliminarily made in the undoped quantum dots, because of the relatively less demand.
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78.67.Hc Quantum dots
42.50.-p Quantum optics

Degradation-robust single mode continuous wave operation of 1.46 μm metamorphic quantum dot lasers on GaAs substrate

T. Kettler, L. Ya. Karachinsky, N. N. Ledentsov, V. A. Shchukin, G. Fiol, M. Kuntz, A. Lochmann, O. Schulz, L. Reissmann, K. Posilovic, D. Bimberg, I. I. Novikov, Yu. M. Shernyakov, N. Yu. Gordeev, M. V. Maximov, et al.

Appl. Phys. Lett. 89, 041113 (2006); http://dx.doi.org/10.1063/1.2236291 (3 pages) | Cited 8 times

Online Publication Date: 26 July 2006

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Narrow ridge lasers of 1.5 μm range based on InAs/InGaAs quantum dots grown on metamorphic (In,Ga,Al)As layers deposited on GaAs substrates using defect reduction technique are studied. It is shown that the lasers operate continuous wave (cw) in a single transverse mode. Single-mode 800 mW output power in the pulsed regime is obtained for a 6 μm ridge width. The dynamic studies of the lasers show a modulation bandwidth of ∼ 3 GHz. Aging tests demonstrate >800 h of cw operation at ∼ 50 mW at 10 °C (60 °C) and >200 h at 20 °C (70 °C) heat sink (junction) temperature without noticeable degradation.
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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

THz spectroscopic characterization of biomolecule/water systems by compact sensor chips

J. Kitagawa, T. Ohkubo, M. Onuma, and Y. Kadoya

Appl. Phys. Lett. 89, 041114 (2006); http://dx.doi.org/10.1063/1.2236295 (3 pages) | Cited 10 times

Online Publication Date: 26 July 2006

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We demonstrate the terahertz (THz) spectroscopic performance of highly integrated sensor chips based on microstrip lines by measuring biomolecule/water systems. The concentration resolution of the present chips reaches down to 0.05 g/ml. We have confirmed that the number of bound water molecules per biomolecule can be obtained with precision using solid state transmission lines. The chips are highly suitable for the inspection of small amounts of specimen and for the application to a wide range of water rich materials. Our method may therefore be a good candidate for a simple liquid sensor working in the THz regime.
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87.80.-y Biophysical techniques (research methods)
07.57.Pt Submillimeter wave, microwave and radiowave spectrometers; magnetic resonance spectrometers, auxiliary equipment, and techniques
87.14.-g Biomolecules: types
82.80.-d Chemical analysis and related physical methods of analysis
84.40.Az Waveguides, transmission lines, striplines

Mode tuning of quantum cascade lasers through optical processing of chalcogenide glass claddings

Shanshan Song, Scott S. Howard, Zhijun Liu, Afusat O. Dirisu, Claire F. Gmachl, and Craig B. Arnold

Appl. Phys. Lett. 89, 041115 (2006); http://dx.doi.org/10.1063/1.2236296 (3 pages) | Cited 17 times

Online Publication Date: 26 July 2006

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In this letter, we demonstrate a method of tuning quantum cascade (QC) lasers by modifying the optical properties of an overlying cladding material. An amorphous chalcogenide cladding layer is deposited through a low temperature, solvent-casting technique that is compatible with current QC laser fabrication and operation. Above band gap illumination (λ<530 nm) of this cladding causes a permanent change in its index of refraction leading to a change of the modal refractive index and a corresponding modal shift in the laser. Combined with deep-etched distributed Bragg gratings, a tuning of over 30 nm is obtained at an operating wavelength of 7.9 μm for constant current and temperature.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.60.Fc Modulation, tuning, and mode locking
42.79.Dj Gratings
42.70.Ce Glasses, quartz

Improved luminance intensity of InGaN–GaN light-emitting diode by roughening both the p-GaN surface and the undoped-GaN surface

Wei Chih Peng and Yew Chung Sermon Wu

Appl. Phys. Lett. 89, 041116 (2006); http://dx.doi.org/10.1063/1.2236462 (3 pages) | Cited 31 times

Online Publication Date: 26 July 2006

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The InGaN–GaN epitaxial films were grown by low-pressure metal-organic chemical vapor deposition on a sapphire substrate, and then the light-emitting diode (LED) with double roughened (p-GaN and undoped-GaN) surfaces was fabricated by surface-roughening, wafer-bonding, and laser lift-off technologies. It was found that the front side luminance intensity of double roughened LED was 2.77 times higher than that of the conventional LED at an injection current of 20 mA. The backside luminance intensity was 2.37 times higher than that of the conventional LED. This is because the double roughened surfaces can provide photons multiple chances to escape from the LED surface, and redirect photons, which were originally emitted out of the escape cone, back into the escape cone.
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85.60.Jb Light-emitting devices
85.30.-z Semiconductor devices

Propagation of a partially coherent twisted anisotropic Gaussian Schell-model beam in a turbulent atmosphere

Yangjian Cai and Sailing He

Appl. Phys. Lett. 89, 041117 (2006); http://dx.doi.org/10.1063/1.2236463 (3 pages) | Cited 83 times

Online Publication Date: 26 July 2006

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Analytical formulas are derived for the propagation of a partially coherent twisted anisotropic Gaussian Schell-model (TAGSM) beam in a turbulent atmosphere. Propagation properties of a TAGSM beam in a turbulent atmosphere are investigated in detail. It is found that a TAGSM beam will become a circular beam in a turbulent atmosphere, and low coherence and larger twist have an effect of anticircularization of the beam spot. The beam spot spreads more rapidly for lower coherence, larger twist, or stronger turbulence.
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42.68.Ay Propagation, transmission, attenuation, and radiative transfer
42.68.Bz Atmospheric turbulence effects
92.60.hk Convection, turbulence, and diffusion

Tuning photonic nanocavities by atomic force microscope nano-oxidation

K. Hennessy, C. Högerle, E. Hu, A. Badolato, and A. Imamoğlu

Appl. Phys. Lett. 89, 041118 (2006); http://dx.doi.org/10.1063/1.2236954 (3 pages) | Cited 62 times

Online Publication Date: 26 July 2006

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The authors demonstrate a technique to achieve high-precision tuning of photonic crystal nanocavities by atomic force microscope nano-oxidation of the cavity surface. Relative tuning between two nanocavity modes is achieved though careful choice of the oxide pattern, allowing them to restore the spectral degeneracy conditions necessary to create polarization-entangled quantum states. Tuning steps less than the linewidth (1 Å) of the high quality factor modes are obtained, allowing for virtually continuous tuning ability.
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42.70.Qs Photonic bandgap materials
81.16.Pr Micro- and nano-oxidation
81.65.Mq Oxidation

Spectral and temporal evolutions of ultrashort pulses diffracted through a slit near phase singularities

Suhas Poyyil Veetil, Nirmal K. Viswanathan, C. Vijayan, and Frank Wyrowski

Appl. Phys. Lett. 89, 041119 (2006); http://dx.doi.org/10.1063/1.2236979 (3 pages) | Cited 11 times

Online Publication Date: 26 July 2006

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The authors present the evolution of ultrashort pulses diffracted through a slit in the complementary space-frequency and space-time domains. Spectral anomalies near phase singularities are found to influence the pulse evolution in the space-time domain. The results of this work have important implications in free-space information transmission.
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42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.25.Fx Diffraction and scattering

On the photoluminescence from Si nanocrystals in Er-doped silica by a double-pulse technique

E. Borsella, M. Falconieri, F. Gourbilleau, and R. Rizk

Appl. Phys. Lett. 89, 041120 (2006); http://dx.doi.org/10.1063/1.2240306 (3 pages)

Online Publication Date: 26 July 2006

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The mechanism of photoluminescence (PL) from Si nanocrystals (Si-nc’s) in Er-doped silica is investigated by a double-pulse technique. It is shown that the decay time and the spectral intensity of PL emission from Si-nc’s do not change when Si-nc’s are reexcited by a delayed pulse in the presence of still excited Er ions. Results are compatible with a strong quenching of photoexcited Si-nc’s through trap states and/or Auger-like interaction between Si-nc’s and Er ions, both in the excited states, while PL emission originates from excitonic recombination in Si-nc’s that do not couple to Er3+.
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78.66.Db Elemental semiconductors and insulators
78.55.Ap Elemental semiconductors
71.35.-y Excitons and related phenomena

Bright semipolar GaInN/GaN blue light emitting diode on side facets of selectively grown GaN stripes

Thomas Wunderer, Peter Brückner, Barbara Neubert, Ferdinand Scholz, Martin Feneberg, Frank Lipski, Martin Schirra, and Klaus Thonke

Appl. Phys. Lett. 89, 041121 (2006); http://dx.doi.org/10.1063/1.2240307 (3 pages) | Cited 33 times

Online Publication Date: 26 July 2006

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The authors demonstrate the fabrication and evaluation of bright semipolar GaInN/GaN blue light emitting diodes (LEDs). The structures are realized by growing five GaInN/GaN quantum wells on the {1math01} side facets of selectively grown n-GaN stripes with triangular shape running along the 〈11math0〉 direction covered with a Mg-doped GaN top layer. The growth was done by metal organic vapor phase epitaxy using a conventional [0001] sapphire substrate. The devices have circular mesa structures with diameters between 70 and 140 μm. Continuous wave on-wafer optical output powers as high as 700 μW and 3 mW could be achieved under dc conditions for 20 and 110 mA, respectively. The current dependent blueshift of the peak emission wavelength caused by screening effects of the piezoelectric field was only 1.5 nm for currents between 1 and 50 mA. This is less than half the value measured on c-plane LEDs and confirms the reduced piezoelectric field in our LED structures.
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85.60.Jb Light-emitting devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase

Coupled resonator optical waveguides based on silicon-on-insulator photonic wires

Fengnian Xia, Lidija Sekaric, Martin O’Boyle, and Yurii Vlasov

Appl. Phys. Lett. 89, 041122 (2006); http://dx.doi.org/10.1063/1.2234721 (3 pages) | Cited 34 times

Online Publication Date: 26 July 2006

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Coupled resonator optical waveguides (CROWs) comprised of up to 16 racetrack resonators based on silicon-on-insulator (SOI) photonic wires were fabricated and characterized. The optical properties of the CROWs were simulated using measured single resonator parameters based on a matrix approach. The group delay property of CROWs was also analyzed. The SOI based CROWs consisting of multiple resonators have extremely small footprints and can find applications in optical filtering, dispersion compensation, and optical buffering. Moreover, such CROW structure is a promising candidate for exploration of low light level nonlinear optics due to its resonant nature and compact mode size ( ∼ 0.1 μm2) in photonic wire.
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42.82.Et Waveguides, couplers, and arrays
42.79.Gn Optical waveguides and couplers
42.65.Wi Nonlinear waveguides
42.79.Ci Filters, zone plates, and polarizers

Excitation of a surface plasmon with an elastomeric grating

A. Kocabas, A. Dâna, and A. Aydinli

Appl. Phys. Lett. 89, 041123 (2006); http://dx.doi.org/10.1063/1.2222344 (3 pages) | Cited 8 times

Online Publication Date: 26 July 2006

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We report on a new method to excite surface plasmon polaritons on a thin metal slab surface using an elastomeric grating which is fabricated by replica molding technique. The grating is placed on the metal surface which creates a periodic perturbation on the surface matching the momentum of the incident light to that of the surface plasmon. The conformal contact between the metal surface and the elastomeric grating changes the dielectric medium periodically and allows the observation of an effective surface plasmon polariton at the metal-air and metal-polymer interfaces of the grating. To clarify the nature of the observed plasmon, comparison of the elastomeric grating with elastomeric slabs was performed with the attenuated total reflection method.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)
78.68.+m Optical properties of surfaces
42.79.Dj Gratings

Direct computed tomographic reconstruction for directional-derivative projections of computed tomography of diffraction enhanced imaging

Zhi-Feng Huang, Ke-Jun Kang, Zheng Li, Pei-Ping Zhu, Qing-Xi Yuan, Wan-Xia Huang, Jun-Yue Wang, Di Zhang, and Ai-Min Yu

Appl. Phys. Lett. 89, 041124 (2006); http://dx.doi.org/10.1063/1.2219405 (3 pages) | Cited 27 times

Online Publication Date: 26 July 2006

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X-ray diffraction enhanced imaging based on synchrotron radiation has extremely high sensitivity of weakly absorbing low-Z samples in medical and biological fields. This letter is dedicated to a direct reconstruction algorithm for directional-derivative projections of computed tomography of diffraction enhanced imaging. It is a “one-step” algorithm and does not require any restoration processing compared with the current “two-step” methods. The actual values of the sample’s refractive index decrement can be estimated from its reconstruction images directly. The algorithm is proven by the actual experiment at the Beijing Synchrotron Radiation Facility and the reconstructed images are described finally.
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87.57.N- Image analysis
87.59.bd Computed radiography

Trapped whispering-gallery optical modes in white light-emitting diode lamps with remote phosphor

Hong Luo, Jong Kyu Kim, Yangang Andrew Xi, E. Fred Schubert, Jaehee Cho, Cheolsoo Sone, and Yongjo Park

Appl. Phys. Lett. 89, 041125 (2006); http://dx.doi.org/10.1063/1.2221747 (3 pages) | Cited 13 times

Online Publication Date: 27 July 2006

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Three-dimensional ray tracing simulations show that a significant fraction of the phosphorescence emitted in high-power white light-emitting diode lamps with a remote phosphor is trapped as whispering-gallery modes propagating along the circumference of the encapsulant. The whispering-gallery modes, which are a significant optical loss mechanism and occur for multiple shapes of the encapsulation dome, are shown to be sensitively dependent on the diffusivity of the reflector cup employed in the lamp. By employing a diffuse reflector cup, up to 86% of the trapped modes is extracted out from the encapsulant. In addition, it is experimentally demonstrated that the phosphorescence efficiency is improved by up to 12.2% as the diffusivity of the reflector increases. The experimental results are consistent with theoretical ray tracing simulations.
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85.60.Jb Light-emitting devices
85.30.De Semiconductor-device characterization, design, and modeling
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