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7 Feb 2005

Volume 86, Issue 6, Articles (06xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 86, 063101 (2005); http://dx.doi.org/10.1063/1.1861133 (3 pages)

Choongho Yu, Qing Hao, Sanjoy Saha, Li Shi, Xiangyang Kong, and Z. L. Wang
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Tunable distributed feedback lasing with narrowed emission using holographic dynamic gratings in a polymeric waveguide

Naoto Tsutsumi and Arata Fujihara

Appl. Phys. Lett. 86, 061101 (2005); http://dx.doi.org/10.1063/1.1861956 (3 pages) | Cited 13 times

Online Publication Date: 31 January 2005

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The letter presents reliable tunable distributed feedback (DFB) lasing achieved by holographic dynamic gratings in a polymeric waveguide including conjugated organic compound. Tunable lasing from 484 to 508 nm was simply achieved by rotating a Lloyd mirror configuration. Lasing spectra indicate that the present DFB lasing has the contribution of periodic spatial variation of the refractive index in the gain medium in addition to that of periodic spatial variation of the gain itself.
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42.55.Rz Doped-insulator lasers and other solid state lasers
42.40.Eq Holographic optical elements; holographic gratings
42.79.Gn Optical waveguides and couplers
42.60.Fc Modulation, tuning, and mode locking

Ultrafast spin-polarized electrical currents injected in a strained zinc blende semiconductor by single color pulses

M. Bieler, N. Laman, H. M. van Driel, and Arthur L. Smirl

Appl. Phys. Lett. 86, 061102 (2005); http://dx.doi.org/10.1063/1.1855426 (3 pages) | Cited 23 times

Online Publication Date: 1 February 2005

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Ultrafast ballistic electrical currents are generated through interband excitation of strained InGaAs/GaAs quantum well samples at 295 K by 810 nm, 60 fs circularly polarized optical pulses. These currents are generated through quantum interference of optical absorption pathways associated with the orthogonally polarized components of the light field, and although the spin of the currents is not directly measured, they should also be spin polarized. The femtosecond transient currents are detected via the emitted tetrahertz (THz) radiation which has a bandwidth of ∼ 8 THz. The THz amplitude scales linearly with injected carrier density up to ∼ 1018 cm−3 beyond which state filling and carrier–carrier scattering cause a sublinear dependence. A peak current density of 9 kA/cm2 is obtained for a peak pump intensity of 250 MW/cm2.
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72.25.Dc Spin polarized transport in semiconductors
73.63.Hs Quantum wells
73.23.Ad Ballistic transport
78.70.Gq Microwave and radio-frequency interactions

Studies of band alignment and two-dimensional electron gas in InGaPN/GaAs heterostructures

J. S. Hwang, K. I. Lin, H. C. Lin, S. H. Hsu, K. C. Chen, Y. T. Lu, Y. G. Hong, and C. W. Tu

Appl. Phys. Lett. 86, 061103 (2005); http://dx.doi.org/10.1063/1.1855406 (3 pages) | Cited 10 times

Online Publication Date: 1 February 2005

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Room-temperature photoreflectance (PR) and photoluminescence (PL) spectra are measured for a series of In0.54Ga0.46P1−yNy/GaAs heterostructures grown on GaAs (100) substrate. Redshifts of the PR and PL peaks indicate that the band gap of In0.54Ga0.46P1−yNy is dramatically reduced as nitrogen is incorporated. The emergence of additional peaks in PR spectra as nitrogen is incorporated indicates that the band alignment switches from type I to type II, due to the lowering of the conduction band, thus forming a two-dimensional electron gas (2DEG) in the interface region between In0.54Ga0.46P1−yNy and GaAs. The band gap energy and transition energies between the confined levels in the 2DEG are determined for samples with various nitrogen concentrations y. The number of confined levels in the 2DEG is found to increase with y; the composition-dependent bowing parameter is determined.
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73.21.Fg Quantum wells
78.67.De Quantum wells
73.20.At Surface states, band structure, electron density of states
78.55.Cr III-V semiconductors

Fourier transform analysis method for modeling the positions and properties of cavity defects in Fabry–Pérot laser diodes

P. J. Bream, S. Bull, I. Harrison, S. Sujecki, and E. C. Larkins

Appl. Phys. Lett. 86, 061104 (2005); http://dx.doi.org/10.1063/1.1863438 (3 pages) | Cited 3 times

Online Publication Date: 1 February 2005

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A simple and fast model is presented, which allows the determination of defect positions within a high-power laser emitter cavity and an estimation of their transmission properties. The model is based upon the calculation of the cavity transmission spectrum below threshold and the analysis of its fast Fourier transform. Modeled and experimental results are compared, showing good correlation. The speed and simplicity of the model means that it is applicable as a screening process for the detection and characterization of defects in manufactured lasers.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
78.60.Fi Electroluminescence

Improved superlensing in two-dimensional photonic crystals with a basis

X. Wang, Z. F. Ren, and K. Kempa

Appl. Phys. Lett. 86, 061105 (2005); http://dx.doi.org/10.1063/1.1863413 (3 pages) | Cited 17 times

Online Publication Date: 1 February 2005

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We study propagation of light in square and hexagonal two-dimensional photonic crystals. We show, that slabs of these crystals focus light with subwavelength resolution. We propose a systematic way to increase this resolution, at an essentially fixed frequency, by employing a hierarchy of crystals of the same structure, and the same lattice constant, but with an increasingly complex basis.
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42.79.Bh Lenses, prisms and mirrors
42.70.Qs Photonic bandgap materials
84.40.-x Radiowave and microwave (including millimeter wave) technology

Efficient blue organic light-emitting diodes based on an oligoquinoline

Abhishek P. Kulkarni, Angela P. Gifford, Christopher J. Tonzola, and Samson A. Jenekhe

Appl. Phys. Lett. 86, 061106 (2005); http://dx.doi.org/10.1063/1.1855415 (3 pages) | Cited 40 times

Online Publication Date: 1 February 2005

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An oligoquinoline 6,6′-bis(2,4-diphenylquinoline), with excellent thermal stability and a high glass transition temperature (Tg = 133 °C), was used as a blue-emitting and electron-transport material to achieve efficient blue electroluminescence. The diodes based on poly(N-vinylcarbazole) hole-transport layer and LiF/Al cathode had a stable blue emission with a peak at 453 nm, chromaticity coordinates at (0.15, 0.12), and a maximum brightness of 925 cd/m2 in air. The external quantum efficiency of 4.1% and luminous efficiency of 3.33 cd/A at 100 cd/m2 are comparable to the best blue organic light-emitting diodes (OLEDs) in the literature. These results demonstrate that oligoquinolines are promising as both blue emitters and electron-transport materials for developing high-efficiency OLEDs with a simpler architecture.
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85.60.Jb Light-emitting devices
85.65.+h Molecular electronic devices
78.60.Fi Electroluminescence
78.66.Qn Polymers; organic compounds
78.55.Qr Amorphous materials; glasses and other disordered solids

Dispersion of thermo-optic coefficient in porous silicon layers of different porosities

Luigi Moretti, Luca De Stefano, Andrea Mario Rossi, and Ivo Rendina

Appl. Phys. Lett. 86, 061107 (2005); http://dx.doi.org/10.1063/1.1857077 (3 pages) | Cited 2 times

Online Publication Date: 1 February 2005

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The thermo-optic coefficient (dn/dT) of porous silicon has been measured. The measurements are based on a simple technique. The experimental data, in the wavelength range between 500 and 1100 nm and at different porosities, have been compared with the Bruggeman and Landau–Lifshitz models. The results show good agreement in case of the Bruggeman model for porosities lower than 0.7, while for higher porosities the Landau model is in better agreement. An estimation of dn/dT as a function of the porosity at the fiber optic communication wavelength of 1.55 μm has also been reported.
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78.66.Db Elemental semiconductors and insulators
78.20.N- Thermo-optic effects
78.20.nb Photothermal effects
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
65.40.De Thermal expansion; thermomechanical effects

Optically-pumped long-wavelength vertical-cavity surface-emitting laser with high modulation bandwidth

M. V. Ramana Murty, D. Xu, C.-C. Lin, C.-L. Shieh, J. Y. Tsao, and Julian Cheng

Appl. Phys. Lett. 86, 061108 (2005); http://dx.doi.org/10.1063/1.1861118 (3 pages)

Online Publication Date: 2 February 2005

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Optically-pumped long-wavelength vertical-cavity surface-emitting lasers (VCSELs) have been fabricated exhibiting more than 2 mW over 20–70 °C, and 1.25 mW at 85 °C. Small-signal modulation measurements on the VCSEL at 20 °C indicate a 3 dB bandwidth of 14.8 GHz at an output power of 2 mW. The VCSELs have a narrow spectral width suitable for data transmission over single-mode fiber. The good temperature performance of the VCSEL is maintained in a suspended configuration that would allow monolithic integration.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Fc Modulation, tuning, and mode locking
42.60.By Design of specific laser systems
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Adiabatic compression of parallel-plate metal waveguides for sensitivity enhancement of waveguide THz time-domain spectroscopy

Jiangquan Zhang and D. Grischkowsky

Appl. Phys. Lett. 86, 061109 (2005); http://dx.doi.org/10.1063/1.1863439 (3 pages) | Cited 18 times

Online Publication Date: 2 February 2005

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We demonstrate the efficient coupling of THz pulses into and out of an adiabatically compressed parallel-plate metal waveguide. The plate separation of the 63.5-mm long, compressed waveguide was fixed at 103 μm at the entrance and exit ends for efficient coupling, but could be reduced in the middle. The experimental results show strong transmission of the THz signal through the compressed waveguide for a minimum midpoint separation of only 8 μm. This technique can be used to achieve efficient coupling together with higher sensitivity for waveguide THz-TDS.
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84.40.Az Waveguides, transmission lines, striplines
78.70.Gq Microwave and radio-frequency interactions
78.47.-p Spectroscopy of solid state dynamics

Efficient second-harmonic generation of tunable femtosecond pulses into the blue in periodically poled KTP

G. R. Fayaz, M. Ghotbi, and M. Ebrahim-Zadeh

Appl. Phys. Lett. 86, 061110 (2005); http://dx.doi.org/10.1063/1.1862327 (3 pages) | Cited 5 times

Online Publication Date: 2 February 2005

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We report efficient single-pass second-harmonic generation of tunable femtosecond pulses into the blue in periodically poled KTP (PPKTP). Using a 2 mm crystal with 4 μm grating period and femtosecond fundamental pulses at 76 MHz, we show that average powers of up to 320 mW in the blue at conversation efficiencies as high as 20% are attainable despite the limitations of group velocity mismatch and spectral acceptance in PPKTP. Accurate measurements of phase-matching acceptance in PPKTP have been performed and excellent agreement between experimental data and calculations is confirmed. Using temperature phase matching, a significant tuning range of 422–427 nm in the blues is also demonstrated. Cross-correlation measurements of the fundamental and second-harmonic pulses in a 500 μm thick β‐BaB2O4 result in blue pulse durations of 700–800 fs for 130 fs fundamental pulses.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.70.-a Optical materials
42.65.Re Ultrafast processes; optical pulse generation and pulse compression

Two-dimensional photonic crystals designed by evolutionary algorithms

Stefan Preble, Michal Lipson, and Hod Lipson

Appl. Phys. Lett. 86, 061111 (2005); http://dx.doi.org/10.1063/1.1862783 (3 pages) | Cited 20 times

Online Publication Date: 3 February 2005

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We use evolutionary algorithms to design photonic crystal structures with large band gaps. Starting from randomly generated photonic crystals, the algorithm yielded a photonic crystal with a band gap (defined as the gap to midgap ratio) as large as 0.3189. This band gap is an improvement of 12.5% over the best human design using the same index contrast platform.
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42.70.Qs Photonic bandgap materials
84.40.-x Radiowave and microwave (including millimeter wave) technology
63.10.+a General theory

2 GHz repetition-rate femtosecond blue sources by second harmonic generation in a resonantly enhanced cavity

Tzu-Ming Liu, Cheng-Ta Yu, and Chi-Kuang Sun

Appl. Phys. Lett. 86, 061112 (2005); http://dx.doi.org/10.1063/1.1862785 (3 pages) | Cited 4 times

Online Publication Date: 3 February 2005

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We report a 2 GHz repetition-rate, all-solid-state femtosecond blue source. Pumped by a 740 mW femtosecond Ti:sapphire laser with the same repetition rate, 150 mW femtosecond pulses at 409 nm can be efficiently generated from the external resonant cavity with a lithium triborate crystal.
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42.55.Rz Doped-insulator lasers and other solid state lasers
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation

Negative differential resistance and electroluminescence from InAs light-emitting diodes grown by liquid-phase epitaxy

A. Krier and X. L. Huang

Appl. Phys. Lett. 86, 061113 (2005); http://dx.doi.org/10.1063/1.1863446 (3 pages) | Cited 3 times

Online Publication Date: 3 February 2005

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Negative differential resistance has been observed from InAs homojunction light-emitting diodes grown using liquid-phase epitaxy at 455 °C. The devices were characterized using current–voltage (I–V) and electroluminescence spectroscopy measurements to obtain information about structure defects in InAs. Two distinct negative differential resistance regions were observed in the forward bias I–V characteristic, consistent with carriers tunnelling into defect levels within the InAs band gap. At large forward bias, carrier injection into the defect levels resulted in electroluminescence peaking at 372 meV and then at 392 meV with increasing current. Analysis based on a native lattice complex defect indicates that carriers recombine via the defect levels at temperatures up to 175 K.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
73.40.Gk Tunneling
73.61.Ey III-V semiconductors
71.55.Eq III-V semiconductors

Displacement current detection of photoconduction in carbon nanotubes

A. Mohite, S. Chakraborty, P. Gopinath, G. U. Sumanasekera, and B. W. Alphenaar

Appl. Phys. Lett. 86, 061114 (2005); http://dx.doi.org/10.1063/1.1863447 (3 pages) | Cited 24 times

Online Publication Date: 3 February 2005

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Using a capacitive photocurrent measurement technique, we demonstrate the ability of both semiconducting and metallic single wall nanotubes to function as photodetectors over a wide spectral range. We observe clear peaks in the photo induced displacement current of a nanotube-plated capacitor that correspond directly to the semiconducting and metallic transitions in the nanotube absorbance spectrum. The signal increases substantially as the carrier drift velocity is raised with applied bias. A large increase in the photocurrent observed below temperatures of 100 K suggests that the nanotube hot carrier relaxation rate decreases substantially at low temperatures.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
72.40.+w Photoconduction and photovoltaic effects
73.63.Fg Nanotubes
73.50.Fq High-field and nonlinear effects
85.35.Kt Nanotube devices

Reducing the threshold current density of AlInGaAs/GaAs double quantum well lasers with n-type delta doping

D. Fekete

Appl. Phys. Lett. 86, 061115 (2005); http://dx.doi.org/10.1063/1.1845592 (3 pages) | Cited 4 times

Online Publication Date: 4 February 2005

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It is demonstrated that by positioning n-type Te-δ doping in the vicinity of two coupled quantum wells (QWs), a threshold current density of 67 A/cm2 per QW is obtained for a strained double QW AlInGaAs/GaAs laser designed to emit light at 0.8 μm. This low value stems in part from the higher initial electron population in the QWs but is mainly due to the enhanced coupling between the two QWs resulting from the overlap of the confined fundamental electronic levels in the two QWs under the injected current density at threshold.
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42.55.Px Semiconductor lasers; laser diodes
61.72.S- Impurities in crystals
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