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22 Jul 2002

Volume 81, Issue 4, pp. 571-782

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Real-time optoacoustic imaging using a Schlieren transducer

J. J. Niederhauser, D. Frauchiger, H. P. Weber, and M. Frenz

Appl. Phys. Lett. 81, 571 (2002); http://dx.doi.org/10.1063/1.1495539 (3 pages) | Cited 22 times

Online Publication Date: 16 July 2002

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Optoacoustics, which combines the merits and most compelling features of light and sound, is a technique for diagnosing optical tissue properties and for performing noninvasive medical imaging. We present a detection method based on Schlieren optical imaging used to record optoacoustic waves generated with 6 ns long laser pulses. These waves result from irradiation of optically absorbing targets hidden deep inside highly scattering media. Since they are generated from one single shot, Schlieren images contain all the information necessary for the reconstruction of the absorbing structures. The detection method allows on-line imaging with a spatial resolution of about 30 μm with high contrast. The proposed real-time imaging system not only enhances the level of visible detail but it is ideally suited for monitoring dynamic tissue changes. © 2002 American Institute of Physics.
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43.35.Ud Thermoacoustics, high temperature acoustics, photoacoustic effect
43.38.Zp Acoustooptic and photoacoustic transducers
42.79.Mt Schlieren devices
43.80.Qf Medical diagnosis with acoustics
87.63.L- Visual imaging

Highly efficient organic light-emitting diodes with a silole-based compound

H. Y. Chen, W. Y. Lam, J. D. Luo, Y. L. Ho, B. Z. Tang, D. B. Zhu, M. Wong, and H. S. Kwok

Appl. Phys. Lett. 81, 574 (2002); http://dx.doi.org/10.1063/1.1495542 (3 pages) | Cited 93 times

Online Publication Date: 16 July 2002

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Efficient light emission was obtained in a silole-based organic light-emitting diode. A high luminous current efficiency of 20 cd/A, corresponding to an external quantum efficiency of 8%, was achieved. The apparent violation of the upper theoretical limit of 5.5% for the external quantum efficiency of a singlet emitter is discussed. With a suitably designed cathode, a high power efficiency of ∼14 lm/W was obtained. A strong dependence of the power efficiency on the thickness of Alq3 layer is also observed and explained. © 2002 American Institute of Physics.
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85.60.Jb Light-emitting devices
85.30.De Semiconductor-device characterization, design, and modeling
78.66.Qn Polymers; organic compounds

Highly bright blue organic light-emitting devices using spirobifluorene-cored conjugated compounds

C. C. Wu, Y. T. Lin, H. H. Chiang, T. Y. Cho, C. W. Chen, K. T. Wong, Y. L. Liao, G. H. Lee, and S. M. Peng

Appl. Phys. Lett. 81, 577 (2002); http://dx.doi.org/10.1063/1.1493669 (3 pages) | Cited 55 times

Online Publication Date: 16 July 2002

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An efficient and morphologically stable pyrimidine-containing spirobifluorene-cored oligoaryl, 2,7-bis[2-(4-tert-butylphenyl)pyrimidine-5-yl]-9,9′-spirobifluorene (TBPSF), as an emitter or a host for blue organic light-emitting devices (OLEDs), is reported. The steric hindrance inherent with the molecular structure renders the material a record-high neat-film photoluminescence (PL) quantum yield of 80% as a pure blue emitter (PL peak at 430 nm) of low molecular weight, and a very high glass-transition temperature (Tg) of 195 °C. Blue OLEDs employing this compound as the emitter or the emitting host exhibit unusual endurance for high currents over 5000 mA/cm2. When TBPSF is used as a host for perylene in a blue OLED, maximal brightness of ∼80 000 cd/m2 had been achieved, representing the highest values reported for blue OLEDs under dc driving. © 2002 American Institute of Physics.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
78.66.Qn Polymers; organic compounds
64.70.P- Glass transitions of specific systems
64.70.Q- Theory and modeling of the glass transition
78.55.Kz Solid organic materials
78.55.Bq Liquids

Chiral wave propagation manifold of the photonic quantum-ring laser

B. H. Park, J. Bae, M. J. Kim, and O’Dae Kwon

Appl. Phys. Lett. 81, 580 (2002); http://dx.doi.org/10.1063/1.1495545 (3 pages) | Cited 10 times

Online Publication Date: 16 July 2002

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A torus knot model is presented to analyze the chiral wave propagation of photonic quantum-ring lasers in a three-dimensional Rayleigh–Fabry–Perot toroidal cavity. The Rayleigh–Fabry–Perot cavity gives rise to peculiar multichromatic spectra with nonequal intermode spacings. In particular, the knot model analysis suggests a strange three-dimensional effective index ellipsoid for the cavity with a large birefringence of neff3D(z)−neff2D(x,y) ≈ 0.4–0.5. © 2002 American Institute of Physics.
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42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.55.Ah General laser theory

Current-spreading-induced bistability in bipolar cascade vertical-cavity surface-emitting lasers

T. Knödl, R. Michalzik, M. Golling, and K. J. Ebeling

Appl. Phys. Lett. 81, 583 (2002); http://dx.doi.org/10.1063/1.1494861 (3 pages) | Cited 4 times

Online Publication Date: 16 July 2002

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We present experimental evidence that current spreading in the cavity of two-stage bipolar cascade vertical-cavity surface-emitting lasers is responsible for the formation of bistability loops in the light versus current characteristics. The bistable behavior strongly varies with the detuning between cavity resonance and gain maximum and is attributed to the wavelength and carrier density dependent absorption coefficient. In order to explain the measured dependencies, an analytical model is presented that very well matches the light output characteristics. © 2002 American Institute of Physics.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.65.Pc Optical bistability, multistability, and switching, including local field effects
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
42.60.Fc Modulation, tuning, and mode locking

High performance germanium-on-silicon detectors for optical communications

Silvia Famà, Lorenzo Colace, Gianlorenzo Masini, Gaetano Assanto, and Hsin-Chiao Luan

Appl. Phys. Lett. 81, 586 (2002); http://dx.doi.org/10.1063/1.1496492 (3 pages) | Cited 47 times

Online Publication Date: 16 July 2002

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We demonstrate fast and efficient germanium-on-silicon p-i-n photodetectors for optical communications, with responsivities as high as 0.89 and 0.75 A/W at 1.3 and 1.55 μm, respectively, time response <200 ps and dark currents as low as 1.2 μA. Ge was epitaxially grown on Si by chemical vapor deposition, employing a low temperature buffer and cyclic thermal annealing to reduce the dislocation density. The overall performance is well suited for >2.5 Gb/s integrated receivers for the second and third fiber spectral windows. © 2002 American Institute of Physics.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
81.05.Cy Elemental semiconductors
85.60.Dw Photodiodes; phototransistors; photoresistors
42.79.Sz Optical communication systems, multiplexers, and demultiplexers
68.55.A- Nucleation and growth
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase

Fluorescence quantum efficiency measurements of excitation and nonradiative deexcitation processes of rare earth 4f-states in chalcogenide glasses

S. M. Lima, A. S. S. de Camargo, L. A. O. Nunes, T. Catunda, and D. W. Hewak

Appl. Phys. Lett. 81, 589 (2002); http://dx.doi.org/10.1063/1.1495879 (3 pages) | Cited 7 times

Online Publication Date: 16 July 2002

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Thermal lens spectroscopic measurements were performed to quantitatively determine the fluorescence quantum efficiency (η) of different luminescence mechanisms in Nd3+ chalcogenide glasses. Low energy (<1.7 eV) direct resonant pumping at Nd3+ infrared lines resulted in η ∼ 1. High energy ( ∼ 2.4 eV) indirect excitation via energy transfer (ET) from glass conduction band to Nd3+ 4f states, resulted in η = ∼ 0.4 (ET probability ∼ 40%). A minimum η ∼ 0.2 was obtained for intermediate excitation energies ( ∼ 2.2 eV) and attributed to back ET from the Nd3+ 4f state to the glass host. © 2002 American Institute of Physics.
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78.55.Hx Other solid inorganic materials
78.20.N- Thermo-optic effects
78.20.nb Photothermal effects
71.55.Jv Disordered structures; amorphous and glassy solids

Fabrication of large periodic arrays of AlGaAs microdisks by laser-interference lithography and selective etching

K. Petter, T. Kipp, Ch. Heyn, D. Heitmann, and C. Schüller

Appl. Phys. Lett. 81, 592 (2002); http://dx.doi.org/10.1063/1.1495082 (3 pages) | Cited 10 times

Online Publication Date: 16 July 2002

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By laser-interference lithography, reactive-ion etching, and selective wet-chemical etching using a citric acid-based solution, we have fabricated large periodic arrays of AlGaAs microdisks with periods of 4 μm and disk diameters between 1.5 and 2 μm. The arrays are characterized by temperature-dependent photoluminescence spectroscopy. Taking into account the below-threshold absorption of the quantum wells inside the disks, we get disk quality factors close to the theoretical maximum value. We demonstrate that our technique allows one also to produce one-dimensionally or two-dimensionally coupled arrays of microdisks. © 2002 American Institute of Physics.
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42.82.Cr Fabrication techniques; lithography, pattern transfer
42.70.Qs Photonic bandgap materials
81.65.Cf Surface cleaning, etching, patterning
78.55.Cr III-V semiconductors
78.67.De Quantum wells

Effects of absorption on coherence domain path length resolved dynamic light scattering in the diffuse regime

Anna L. Petoukhova, Wiendelt Steenbergen, Ton G. van Leeuwen, and Frits F. M. de Mul

Appl. Phys. Lett. 81, 595 (2002); http://dx.doi.org/10.1063/1.1493644 (3 pages) | Cited 8 times

Online Publication Date: 16 July 2002

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A low coherence Mach–Zehnder interferometer is developed for path length resolved dynamic light scattering in highly turbid media. The path length distribution of multiply scattered photons in Intralipid is changed by the addition of absorbing dyes. Path length distributions obtained for various absorption levels are shown to be mutually related by Lambert–Beer’s law. Furthermore, the broadening of path length resolved heterodyne light beating spectra of media with identical scattering properties is shown to be independent of the absorption coefficient. © 2002 American Institute of Physics.
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07.60.Ly Interferometers
82.70.Kj Emulsions and suspensions
78.35.+c Brillouin and Rayleigh scattering; other light scattering
87.80.-y Biophysical techniques (research methods)

Heteroepitaxial growth of a wide-gap p-type semiconductor, LaCuOS

Hidenori Hiramatsu, Kazushige Ueda, Hiromichi Ohta, Masahiro Orita, Masahiro Hirano, and Hideo Hosono

Appl. Phys. Lett. 81, 598 (2002); http://dx.doi.org/10.1063/1.1494853 (3 pages) | Cited 38 times

Online Publication Date: 16 July 2002

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High-quality epitaxial films of LaCuOS, a wide-gap p-type semiconductor, are grown on yittria-stabilized-zirconia (001) or MgO (001) single-crystal substrates by a unique method. Postannealing of a bilayer film composed of an extremely thin metallic copper layer and an amorphous LaCuOS layer at 1000 °C results in an epitaxially grown LaCuOS thin film. This thin copper layer with high orientation, which likely acts as an epitaxial initiator, is essential for epitaxial growth. The resulting epitaxial films exhibit relatively intense ultraviolet emission associated with excitons at room temperature, confirming the high crystal quality of the films. © 2002 American Institute of Physics.
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68.55.-a Thin film structure and morphology
78.66.Li Other semiconductors
61.72.Cc Kinetics of defect formation and annealing
78.55.Hx Other solid inorganic materials
71.35.Cc Intrinsic properties of excitons; optical absorption spectra
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