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8 Jun 2009

Volume 94, Issue 23, Articles (23xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 94, 233101 (2009); http://dx.doi.org/10.1063/1.3148782 (3 pages)

E. Moyen, M. Macé, G. Agnus, A. Fleurence, T. Maroutian, F. Houzé, A. Stupakiewicz, L. Masson, B. Bartenlian, W. Wulfhekel, P. Beauvillain, and M. Hanbücken
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Imaging by silicon on insulator waveguides

Q. Song, F. Qian, E. K. Tien, I. Tomov, J. Meyer, X. Z. Sang, and O. Boyraz

Appl. Phys. Lett. 94, 231101 (2009); http://dx.doi.org/10.1063/1.3141480 (3 pages) | Cited 1 time

Online Publication Date: 8 June 2009

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We present multiphoton imaging based on semiconductor planar waveguide technology which can be used as a transmitter and receiver simultaneously. In particular, silicon on insulator waveguides with p-i-n diode structures are used to demonstrate <5 μm resolution three-photon imaging of Er3+:Y2O3 microparticles by using 1550 nm excitation. Additional theoretical study has been performed to demonstrate the proposed scheme for three-dimensional tomography of micron-sized objects, which could be realized by using multiple transmitter-detector pairs.
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07.57.Ty Infrared spectrometers, auxiliary equipment, and techniques
42.79.Gn Optical waveguides and couplers
42.82.-m Integrated optics

In-plane electric field induced by polarization and lateral photovoltaic effect in a-plane GaN

Weiguo Hu, Bei Ma, Dabing Li, Hideto Miyake, and Kazumasa Hiramatsu

Appl. Phys. Lett. 94, 231102 (2009); http://dx.doi.org/10.1063/1.3144270 (3 pages) | Cited 4 times

Online Publication Date: 8 June 2009

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A lateral photovoltaic effect was observed in a-plane GaN films grown on r-plane sapphire at room temperature. Under various light sources illuminations, contacts along the c-axis exhibited about ten times the photovoltage than those along the m-axis, which kept linear relationship with the illumination intensity. It was attributed to anisotropic in-plane electrical field induced by the intrinsic spontaneous/piezoelectric polarization, which spatially separated photogenerated carriers to produce the photovoltage.
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72.40.+w Photoconduction and photovoltaic effects
73.50.Pz Photoconduction and photovoltaic effects
77.65.-j Piezoelectricity and electromechanical effects
68.55.ag Semiconductors
77.22.Ej Polarization and depolarization

Mapping cavity modes of ZnO nanobelts

Xiulai Xu, Frederic S. F. Brossard, David A. Williams, Daniel P. Collins, Mark J. Holmes, Robert A. Taylor, and Xitian Zhang

Appl. Phys. Lett. 94, 231103 (2009); http://dx.doi.org/10.1063/1.3151866 (3 pages) | Cited 8 times

Online Publication Date: 8 June 2009

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ZnO nanostructures attract current interest because they have the potential to implement cavity quantum electrodynamics at room temperature. We report a photoluminescence mapping of ZnO nanobelts both at room temperature and 4.2 K. The multicavity modes were observed all over the belt surface, which were induced by Fabry–Pérot interference. The emission from the belt surface is enhanced at both the ends and the sides of the belt, and is highly linearly polarized in the direction perpendicular to the long axis of the belt. The results are explained using finite-difference time-domain simulations.
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78.55.Et II-VI semiconductors
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
42.55.-f Lasers
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
42.50.Pq Cavity quantum electrodynamics; micromasers

Measuring the orbital angular momentum of optical vortices using a multipinhole plate

Cheng-Shan Guo, Shu-Juan Yue, and Gong-Xing Wei

Appl. Phys. Lett. 94, 231104 (2009); http://dx.doi.org/10.1063/1.3151920 (3 pages) | Cited 15 times

Online Publication Date: 8 June 2009

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We present a method for measuring the orbital angular momentum (OAM) of optical vortices through extracting the phase values sampled by a multipinhole plate. We demonstrate that the phase of an optical vortex passing through a multipinhole plate can be directly extracted from the Fourier transform of a single diffraction intensity pattern according to a simple algorithm and thus the l state or the OAM of the photons can be measured quantitatively.
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42.50.Tx Optical angular momentum and its quantum aspects
42.25.Fx Diffraction and scattering

Dislocation density reduction in GaN by dislocation filtering through a self-assembled monolayer of silica microspheres

Qiming Li, Jeffrey J. Figiel, and George T. Wang

Appl. Phys. Lett. 94, 231105 (2009); http://dx.doi.org/10.1063/1.3152012 (3 pages) | Cited 8 times

Online Publication Date: 8 June 2009

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We demonstrate the use of self-assembled monolayers of silica microspheres as selective growth masks for significant threading dislocation density reduction in GaN on sapphire epilayers. During GaN regrowth through the close-packed monolayer, the silica microspheres effectively terminate the propagation of threading dislocations. As a result, the threading dislocation density, measured by large area atomic force microscopy and cathodoluminescence scans, is reduced from 3.3×109 to 4.0×107 cm−2. This nearly two orders of magnitude reduction is attributed to dislocation blocking and bending by the unique interface between GaN and silica microspheres.
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61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
68.37.Ps Atomic force microscopy (AFM)
78.60.Hk Cathodoluminescence, ionoluminescence

Coherent beam combination of two-dimensional high power fiber amplifier array using stochastic parallel gradient descent algorithm

Pu Zhou, Zejin Liu, Xiaolin Wang, Yanxing Ma, Haotong Ma, and Xiaojun Xu

Appl. Phys. Lett. 94, 231106 (2009); http://dx.doi.org/10.1063/1.3152282 (3 pages) | Cited 17 times

Online Publication Date: 8 June 2009

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We demonstrate coherent beam combination of two-dimensional high power fiber amplifier array using stochastic parallel gradient descent (SPGD) algorithm. Four polarization-maintained fiber amplifiers are tiled side by side into a 2×2 laser array with a fill factor of 54% in the near-field. Phase control on the fiber amplifiers are performed by running SPGD algorithm on a digital dignal processor with updating rate of 50 000 Hz/channel. Coherent beam combination of the four fiber amplifiers with a total output power of 60.1 W using SPGD algorithm is demonstrated. Beam quality of the combined beam is computed to be BQ<1.4.
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42.55.Wd Fiber lasers
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation
02.50.Fz Stochastic analysis
42.81.Gs Birefringence, polarization

Low-threshold ablation of dielectrics irradiated by picosecond soft x-ray laser pulses

A. Ya. Faenov, N. A. Inogamov, V. V. Zhakhovskii, V. A. Khokhlov, K. Nishihara, Y. Kato, M. Tanaka, T. A. Pikuz, M. Kishimoto, M. Ishino, M. Nishikino, T. Nakamura, Y. Fukuda, S. V. Bulanov, and T. Kawachi

Appl. Phys. Lett. 94, 231107 (2009); http://dx.doi.org/10.1063/1.3152290 (3 pages) | Cited 8 times

Online Publication Date: 8 June 2009

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Ablation of LiF crystal by soft x-ray laser (XRL) pulses with wavelength λ = 13.9 nm and duration TL = 7 ps is studied experimentally and theoretically. It is found that a crater appears on a surface of LiF for XRL fluence, exceeding the ablation threshold Fa ∼ 10.2 mJ/cm2 in one shot, or 5 mJ/cm2 in each of the three XRL shots. This is substantially below the ablation thresholds obtained with other lasers having longer pulse duration and/or longer wavelength. A mechanism of thermomechanical ablation in large bandgap dielectrics is proposed. The theory explains the low Fa via small attenuation depth, absence of light reflection, and electron heat conductivity.
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61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
81.65.-b Surface treatments
61.82.-d Radiation effects on specific materials
77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials
79.20.Ds Laser-beam impact phenomena

Excitation and focusing of terahertz surface plasmons using a grating coupler with elliptically curved grooves

Gwenael Gaborit, Damien Armand, Jean-Louis Coutaz, Maxim Nazarov, and Alexander Shkurinov

Appl. Phys. Lett. 94, 231108 (2009); http://dx.doi.org/10.1063/1.3153125 (3 pages) | Cited 3 times

Online Publication Date: 8 June 2009

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Using a diffraction grating with elliptically curved grooves engraved at the top of an aluminum slab, surface plasmons in the terahertz frequency range are efficiently excited with an incoming parallel beam and focused over the metal surface down to the diffraction limit. At 0.5 THz, the lateral confinement of the surface plasmon is 1 mm while its extension in air is 0.74 mm, much smaller than the calculated value for a semi-infinite substrate having the conductivity of bulk aluminum.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.68.+m Optical properties of surfaces
78.70.Gq Microwave and radio-frequency interactions

Capturing light pulses into a pair of coupled photonic crystal cavities

Clayton R. Otey, Michelle L. Povinelli, and Shanhui Fan

Appl. Phys. Lett. 94, 231109 (2009); http://dx.doi.org/10.1063/1.3141485 (3 pages) | Cited 1 time

Online Publication Date: 9 June 2009

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We describe finite-difference time-domain simulations of a two-dimensional photonic crystal implementation of a two-resonator system capable of capturing light pulses from a waveguide. As much as 99.61% of incident pulse energy is captured in simulations. The release of near-perfect Gaussian pulses is also demonstrated.
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42.55.Tv Photonic crystal lasers and coherent effects
42.70.Qs Photonic bandgap materials
42.65.Re Ultrafast processes; optical pulse generation and pulse compression

Linewidth properties of active-passive coupled monolithic InGaAs semiconductor ring lasers

Muhan Choi, Tomoko Tanaka, Satoshi Sunada, and Takahisa Harayama

Appl. Phys. Lett. 94, 231110 (2009); http://dx.doi.org/10.1063/1.3148673 (3 pages) | Cited 3 times

Online Publication Date: 9 June 2009

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We report linewidth properties of active-passive coupled monolithic InGaAs semiconductor ring lasers with various length of passive waveguide. It is experimentally confirmed that the linewidth of the lasers is proportional to the square of the ratio of the length of active part of the cavity over the total length of the cavity. The lasers are applicable for communication and sensing devices, which need the narrow linewidth.
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42.55.Px Semiconductor lasers; laser diodes
42.55.Wd Fiber lasers
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.82.Et Waveguides, couplers, and arrays

Understanding the dispersion of coaxial plasmonic structures through a connection with the planar metal-insulator-metal geometry

Peter B. Catrysse and Shanhui Fan

Appl. Phys. Lett. 94, 231111 (2009); http://dx.doi.org/10.1063/1.3148692 (3 pages) | Cited 15 times

Online Publication Date: 9 June 2009

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We elucidate the dispersion behavior of deep-subwavelength propagating modes in coaxial plasmonic structures by making an explicit connection with the planar metal-insulator-metal geometry. We provide an intuitive picture that allows for a qualitative understanding and a quantitative prediction of the entire dispersion behavior, which includes the number of modes at every frequency, the modal propagation constants, the propagation losses, and the cutoff frequencies of propagating modes supported by these technologically important structures. We validate our analytical approach by comparing its predictions to first-principles finite-difference frequency-domain simulations.
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42.79.Gn Optical waveguides and couplers
42.82.Et Waveguides, couplers, and arrays
84.40.Az Waveguides, transmission lines, striplines

Photon drag effect in carbon nanotube yarns

Alexander N. Obraztsov, Dmitry A. Lyashenko, Shaoli Fang, Ray H. Baughman, Petr A. Obraztsov, Sergei V. Garnov, and Yuri P. Svirko

Appl. Phys. Lett. 94, 231112 (2009); http://dx.doi.org/10.1063/1.3151834 (3 pages) | Cited 4 times

Online Publication Date: 9 June 2009

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We demonstrate that in graphitic nanocarbon materials, combination of ballistic conductivity and strong electron photon coupling opens a unique opportunity to observe transfer of momentum of the electromagnetic radiation to free carriers. The resulting drag of quasiballistically propagating electrons can be employed, in particular, to visualize the temporal profile, polarization, and propagation direction of the laser pulse. In this letter, we report the giant photon drag effect in yarns made of multiwall carbon nanotubes.
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78.67.Ch Nanotubes
71.38.-k Polarons and electron-phonon interactions
73.23.Ad Ballistic transport
73.63.Fg Nanotubes

Ultrashort dead time of photon-counting InGaAs avalanche photodiodes

A. R. Dixon, J. F. Dynes, Z. L. Yuan (袁之良), A. W. Sharpe, A. J. Bennett, and A. J. Shields

Appl. Phys. Lett. 94, 231113 (2009); http://dx.doi.org/10.1063/1.3151864 (3 pages) | Cited 17 times

Online Publication Date: 9 June 2009

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We report a 1.036 GHz gated Geiger mode InGaAs avalanche photodiode with a detection dead time of just 1.93 ns. This is demonstrated by full recovery of the detection efficiency two gate cycles after a detection event, as well as a measured maximum detection rate of 497 MHz. As an application, we measure the second order correlation function g(2) of the emission from a diode laser with a single detector that works reliably at high speed owing to the extremely short dead time of the detector. The device is ideal for high bit rate fiber wavelength quantum key distribution and photonic quantum computing.
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85.60.Dw Photodiodes; phototransistors; photoresistors
03.67.Dd Quantum cryptography and communication security
03.67.Lx Quantum computation architectures and implementations
85.60.Gz Photodetectors (including infrared and CCD detectors)
42.50.Ar Photon statistics and coherence theory

Temperature stabilization of optofluidic photonic crystal cavities

Christian Karnutsch, Cameron L. C. Smith, Alexandra Graham, Snjezana Tomljenovic-Hanic, Ross McPhedran, Benjamin J. Eggleton, Liam O’Faolain, Thomas F. Krauss, Sanshui Xiao, and N. Asger Mortensen

Appl. Phys. Lett. 94, 231114 (2009); http://dx.doi.org/10.1063/1.3152998 (3 pages) | Cited 11 times

Online Publication Date: 9 June 2009

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We present a principle for the temperature stabilization of photonic crystal (PhC) cavities based on optofluidics. We introduce an analytic method enabling a specific mode of a cavity to be made wavelength insensitive to changes in ambient temperature. Using this analysis, we experimentally demonstrate a PhC cavity with a quality factor of Q ≈ 15 000 that exhibits a temperature-independent resonance. Temperature-stable cavities constitute a major building block in the development of a large suite of applications from high-sensitivity sensor systems for chemical and biomedical applications to microlasers, optical filters, and switches.
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42.70.Qs Photonic bandgap materials
42.55.Tv Photonic crystal lasers and coherent effects

Electromagnetically induced transparency-like effect in a single polydimethylsiloxane-coated silica microtoroid

Yun-Feng Xiao, Lina He, Jiangang Zhu, and Lan Yang

Appl. Phys. Lett. 94, 231115 (2009); http://dx.doi.org/10.1063/1.3149697 (3 pages) | Cited 12 times

Online Publication Date: 10 June 2009

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We study both experimentally and theoretically the coupling between a fiber taper and two whispering-gallery modes, which are simultaneously excited in a single pilydimethyl-siloxane-coated silica microtoroid system. The transmission spectrum of the fiber-coupled two-mode microresonator shows a sharp electromagnetically induced transparency-like window within the resonant absorption region. This line shape results from destructive interference between two optical pathways associated with two distinct coexisting modes in a single resonator. The sharp transparency peak has great potential applications in light modulation and highly sensitive biochemical sensors.
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78.66.Qn Polymers; organic compounds
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

Phase tunable holographic fabrication for three-dimensional photonic crystal templates by using a single optical element

Di Xu, Kevin P. Chen, Ahmad Harb, Daniel Rodriguez, Karen Lozano, and Yuankun Lin

Appl. Phys. Lett. 94, 231116 (2009); http://dx.doi.org/10.1063/1.3149705 (3 pages) | Cited 7 times

Online Publication Date: 10 June 2009

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This paper demonstrates a phase tunable holographic fabrication of three-dimensional photonic lattice structures using a single optical element. A top-cut four-side prism is employed to generate five-beam three-dimensional interference patterns. A silica glass slide is inserted into the optical path to adjust the phase of one interfering beam relative to other four beams. The phase control of the interfering laser beam renders the lattice of the interference pattern from a face-center tetragonal symmetry into a high contrast, interconnecting diamondlike symmetry. This method provides a flexible approach to fabricating three-dimensional photonic lattices with improved photonic band structures.
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42.82.Cr Fabrication techniques; lithography, pattern transfer
42.70.Qs Photonic bandgap materials
42.55.Tv Photonic crystal lasers and coherent effects
42.40.Kw Holographic interferometry; other holographic techniques

Post-fabrication fine-tuning of photonic crystal quantum well infrared photodetectors

S. Schartner, S. Kalchmair, A. M. Andrews, P. Klang, W. Schrenk, and G. Strasser

Appl. Phys. Lett. 94, 231117 (2009); http://dx.doi.org/10.1063/1.3152769 (3 pages) | Cited 4 times

Online Publication Date: 10 June 2009

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Photonic crystal (PC) devices require high fabrication accuracy for on demand positioning of resonances. We describe post-fabrication fine-tuning of a PC quantum well infrared photodetector (QWIP) by sidewall-deposition of silicon nitride. The PC resonance was shifted over a bandwidth of 43 cm−1. From photoresponse measurements we calculated a tuning coefficient of ∂ν/∂dSiN = −0.06 cm−1/nm. The QWIP responsivity did not suffer from nitride absorption while the PC resonance increased by a factor of 1.6. This shows that post-fabrication tuning by dielectric deposition with, e.g., silicon nitride is a feasible method to achieve precise implementations of PC devices.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors

Origin of second-order nonlinear optical response of polarity-controlled ZnO films

J. S. Park, Y. Yamazaki, Y. Takahashi, S. K. Hong, J. H. Chang, T. Fujiwara, and T. Yao

Appl. Phys. Lett. 94, 231118 (2009); http://dx.doi.org/10.1063/1.3152773 (3 pages) | Cited 4 times

Online Publication Date: 10 June 2009

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Polarity-controlled ZnO thin films grown on different buffer layers were investigated as nonlinear optical materials for second harmonic generation. The effective nonlinear optical coefficient (deff) of ZnO grown on Cr-compound buffer layers showed a higher value than that of ZnO grown on MgO buffer layers. The correlations among the grain size and surface roughness with the values of deff were found to be strong. The deff of the ZnO film increased with decrease in the grain size and surface roughness. This relationship could be explained by the increase in reaction interface and the decrease in scattering probability at the surface as decrease in grain size and surface roughness, respectively.
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42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
68.35.bg Semiconductors
78.66.Hf II-VI semiconductors

Fabrication of high-Q polydimethylsiloxane optical microspheres for thermal sensing

C.-H. Dong, L. He, Y.-F. Xiao, V. R. Gaddam, S. K. Ozdemir, Z.-F. Han, G.-C. Guo, and L. Yang

Appl. Phys. Lett. 94, 231119 (2009); http://dx.doi.org/10.1063/1.3152791 (3 pages) | Cited 29 times

Online Publication Date: 10 June 2009

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Polydimethylsiloxane (PDMS) optical microspheres are fabricated and whispering gallery modes with quality factors of 106 in the 1480 nm band are demonstrated. The dependence of the resonance shifts on the input power is investigated in both the transient (blueshift) and the steady-state (redshift) regimes. Moreover, we demonstrate that such high-Q PDMS optical resonators can be used as highly sensitive thermal sensors with temperature sensitivity of 0.245 nm/°C, which is one order of magnitude higher than conventional silica microsphere resonators. The estimated thermal resolution of the sensor is 2×10−4 °C.
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42.55.Sa Microcavity and microdisk lasers
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.70.Jk Polymers and organics
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

Experimental verification of quasiscarred resonance mode

Chil-Min Kim, Sang Hun Lee, Kwang Ryung Oh, and Jong Hoi Kim

Appl. Phys. Lett. 94, 231120 (2009); http://dx.doi.org/10.1063/1.3148801 (3 pages) | Cited 5 times

Online Publication Date: 11 June 2009

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We have experimentally verified the lasing of a quasiscarred resonance mode (QSRM) in a spiral-shaped InGaAsP microcavity laser by pumping the cavity boundary with current injection. To confirm the lasing of a QSRM, a far field pattern is obtained and six laser emission directions are found; one corresponds to the whispering gallery type modes and the other to a QSRM. By observing the spectrum and lasing thresholds, the lasing characteristics of the QSRM are analyzed.
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42.55.Sa Microcavity and microdisk lasers
42.55.Px Semiconductor lasers; laser diodes

Nanostructured black silicon and the optical reflectance of graded-density surfaces

Howard M. Branz, Vernon E. Yost, Scott Ward, Kim M. Jones, Bobby To, and Paul Stradins

Appl. Phys. Lett. 94, 231121 (2009); http://dx.doi.org/10.1063/1.3152244 (3 pages) | Cited 45 times

Online Publication Date: 11 June 2009

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We fabricate and measure graded-index “black silicon” surfaces and find the underlying scaling law governing reflectance. Wet etching (100) silicon in HAuCl4, HF, and H2O2 produces Au nanoparticles that catalyze formation of a network of [100]-oriented nanopores. This network grades the near-surface optical constants and reduces reflectance to below 2% at wavelengths from 300 to 1000 nm. As the density-grade depth increases, reflectance decreases exponentially with a characteristic grade depth of about 1/8 the vacuum wavelength or half the wavelength in Si. Observation of Au nanoparticles at the ends of cylindrical nanopores confirms local catalytic action of moving Au nanoparticles.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
81.65.Cf Surface cleaning, etching, patterning
78.66.Db Elemental semiconductors and insulators

Fluorescence emission enhanced by surface electromagnetic waves on one-dimensional photonic crystals

I. V. Soboleva, E. Descrovi, C. Summonte, A. A. Fedyanin, and F. Giorgis

Appl. Phys. Lett. 94, 231122 (2009); http://dx.doi.org/10.1063/1.3148671 (3 pages) | Cited 8 times

Online Publication Date: 12 June 2009

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An appreciable increase in the fluorescence emission of an organic chromofore is obtained by exploiting the local field enhancement at the surface of one-dimensional photonic crystals after excitation of surface electromagnetic waves (SEW). Using a properly designed photonic crystal consisting of alternating a-Si1−xNx:H layers with different nitrogen content, efficient emission of R6G dye spun on the surface of the photonic crystal is detected and the intensity spatial distribution of the SEW is visualized by means of far-field fluorescence microscopy. Our results demonstrate potential applications in enhanced fluorescence microscopy with an increased sensitivity and spectral selectivity.
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42.55.Tv Photonic crystal lasers and coherent effects
33.50.Dq Fluorescence and phosphorescence spectra
41.20.Jb Electromagnetic wave propagation; radiowave propagation
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Effect of electron blocking layer on efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes

Sang-Heon Han, Dong-Yul Lee, Sang-Jun Lee, Chu-Young Cho, Min-Ki Kwon, S. P. Lee, D. Y. Noh, Dong-Joon Kim, Yong Chun Kim, and Seong-Ju Park

Appl. Phys. Lett. 94, 231123 (2009); http://dx.doi.org/10.1063/1.3153508 (3 pages) | Cited 50 times

Online Publication Date: 12 June 2009

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The effect of an electron blocking layer (EBL) on the efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes (LEDs) is investigated. At low current density, the LEDs with a p-AlGaN EBL show a higher external quantum efficiency (EQE) than LEDs without an EBL. However, the EQE of LEDs without an EBL is higher than LEDs with an EBL as injection current density is increased. The improved EQE of LEDs without an EBL at high current density is attributed to the increased hole injection efficiency.
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85.60.Jb Light-emitting devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Efficient generation and sorting of orbital angular momentum eigenmodes of light by thermally tuned q-plates

Ebrahim Karimi, Bruno Piccirillo, Eleonora Nagali, Lorenzo Marrucci, and Enrico Santamato

Appl. Phys. Lett. 94, 231124 (2009); http://dx.doi.org/10.1063/1.3154549 (3 pages) | Cited 24 times

Online Publication Date: 12 June 2009

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We present methods for generating and for sorting specific orbital angular momentum (OAM) eigenmodes of a light beam with high efficiency, using a liquid crystal birefringent plate with unit topological charge known as “q-plate.” The generation efficiency has been optimized by tuning the optical retardation of the q-plate with temperature. The measured OAM m = ±2 eigenmodes generation efficiency from an input TEM00 beam was of 97%. Mode sorting of the two input OAM m = ±2 eigenmodes was achieved with an efficiency of 81%.
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42.79.Ci Filters, zone plates, and polarizers
42.79.Kr Display devices, liquid-crystal devices

Tilted-charge high speed (7 GHz) light emitting diode

G. Walter, C. H. Wu, H. W. Then, M. Feng, and N. Holonyak, Jr.

Appl. Phys. Lett. 94, 231125 (2009); http://dx.doi.org/10.1063/1.3154565 (3 pages) | Cited 12 times

Online Publication Date: 12 June 2009

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We demonstrate a higher speed form of light emitting diode (LED), an asymmetrical two-junction tilted-charge LED, utilizing an n-type buried “drain” layer beneath the p-type “base” quantum-well (carrier and photon) active region. The drain layer tilts and pins the charge in the manner of a heterojunction bipolar light emitting transistor (HBLET), selecting and allowing only “fast” recombination (recombination lifetime τB of the order of base transit time τt). The tilted-charge LED, simple in design and construction, is capable of operation at low current in spontaneous recombination at a 7 GHz bandwidth or even higher with more refinement.
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85.60.Jb Light-emitting devices
85.30.Pq Bipolar transistors
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