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26 Mar 2012

Volume 100, Issue 13, Articles (13xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 100, 133701 (2012); http://dx.doi.org/10.1063/1.3696019 (3 pages)

Hewei Liu, Feng Chen, Qing Yang, Pubo Qu, Shengguan He, Xianhua Wang, Jinhai Si, and Xun Hou
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Electromagnetically induced transparency in terahertz plasmonic metamaterials via dual excitation pathways of the dark mode

Xiaojun Liu (刘小君), Jianqiang Gu (谷建强), Ranjan Singh, Yingfang Ma (马颖芳), Jun Zhu (朱俊), Zhen Tian (田震), Mingxia He (何明霞), Jiaguang Han (韩家广), and Weili Zhang (张伟力)

Appl. Phys. Lett. 100, 131101 (2012); http://dx.doi.org/10.1063/1.3696306 (4 pages) | Cited 7 times

Online Publication Date: 26 March 2012

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We observe the excitation and tuning of electromagnetically induced transparency (EIT) by the interference between different excitation pathways of the dark mode in a planar terahertz metamaterial. The EIT unit cell consists of a cut wire as the bright resonator and a pair of split ring resonators (SRRs) as the dark element. The dark mode resonance is excited by both the electric and magnetic fields when the SRR pair translates along the wire without altering the lateral distance between the resonators. The electric and magnetic pathways of exciting the dark mode allows for a giant amplitude modulation of the EIT resonance.
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42.50.Md Optical transient phenomena: quantum beats, photon echo, free-induction decay, dephasings and revivals, optical nutation, and self-induced transparency
42.50.Wk Mechanical effects of light on material media, microstructures and particles
32.30.Bv Radio-frequency, microwave, and infrared spectra
42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption
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Slow light for deep tissue imaging with ultrasound modulation

Huiliang Zhang, Mahmood Sabooni, Lars Rippe, Chulhong Kim, Stefan Kröll, Lihong V. Wang, and Philip R. Hemmer

Appl. Phys. Lett. 100, 131102 (2012); http://dx.doi.org/10.1063/1.3696307 (5 pages) | Cited 5 times

Online Publication Date: 26 March 2012

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Slow light has been extensively studied for applications ranging from optical delay lines to single photon quantum storage. Here, we show that the time delay of slow-light significantly improves the performance of the narrowband spectral filters needed to optically detect ultrasound from deep inside highly scattering tissue. We demonstrate this capability with a 9 cm thick tissue phantom, having 10 cm−1 reduced scattering coefficient, and achieve an unprecedented background-free signal. Based on the data, we project real time imaging at video rates in even thicker phantoms and possibly deep enough into real tissue for clinical applications like early cancer detection.
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87.63.L- Visual imaging
43.80.Qf Medical diagnosis with acoustics
87.63.D- Ultrasonography

Observation of copious emission at the fundamental frequency by a Smith-Purcell free-electron laser with sidewalls

J. Gardelle, P. Modin, and J. T. Donohue

Appl. Phys. Lett. 100, 131103 (2012); http://dx.doi.org/10.1063/1.3696381 (4 pages)

Online Publication Date: 26 March 2012

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An experiment at microwave frequencies confirms the recent prediction that a Smith-Purcell [S. J. Smith and E. M. Purcell, Phys. Rev. 92, 1069 (1953)] free-electron laser equipped with sidewalls can emit radiation at the frequency of the surface wave. The power output is considerably greater than for the previously observed emission at the second harmonic, in agreement with three-dimensional simulations. The dependence of frequency on beam energy and emission angle is in good agreement with three-dimensional theory and simulations. Provided that a reduction in scale can be achieved, a path is open to coherent Smith-Purcell radiation at terahertz frequency.
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41.60.Cr Free-electron lasers

A weakly coupled semiconductor superlattice as a potential for a radio frequency modulated terahertz light emitter

G. K. Rasulova, P. N. Brunkov, I. V. Pentin, A. Yu. Egorov, D. A. Knyazev, A. V. Andrianov, A. O. Zakhar’in, S. G. Konnikov, and G. N. Gol’tsman

Appl. Phys. Lett. 100, 131104 (2012); http://dx.doi.org/10.1063/1.3696673 (4 pages) | Cited 1 time

Online Publication Date: 26 March 2012

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The bolometer response to THz radiation from a weakly coupled GaAs/AlGaAs superlattice biased in the self-oscillations regime has been observed. The bolometer signal is modulated with the frequency equal to the fundamental frequency of superlattice self-oscillations. The frequency spectrum of the bolometer signal contains higher harmonics whose frequency is a multiple of fundamental frequency of self-oscillations.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
84.40.-x Radiowave and microwave (including millimeter wave) technology

Direct and indirect two-photon absorption in Ge within the effective mass approximation

Hernando Garcia and Kobra Nasiri Avanaki

Appl. Phys. Lett. 100, 131105 (2012); http://dx.doi.org/10.1063/1.3693389 (3 pages) | Cited 1 time

Online Publication Date: 26 March 2012

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We have calculated the two photon absorption coefficient for Ge within the effective mass approximation for the direct as well as the indirect gap region. By including all the possible contributions to the nonlinear absorption based on the Ge band structure, we found excellent agreement with the recent experimental report on the two photon absorption in Ge. It is found that even though the direct transition is stronger than the indirect process, a tail at the low end of the energy spectrum appears, and the ultimate limit for the two photon absorption is half of the indirect gap energy.
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71.20.Mq Elemental semiconductors
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Effect of internal optical loss on the modulation bandwidth of a quantum dot laser

Yuchang Wu, Robert A. Suris, and Levon V. Asryan

Appl. Phys. Lett. 100, 131106 (2012); http://dx.doi.org/10.1063/1.3697683 (4 pages) | Cited 1 time

Online Publication Date: 26 March 2012

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We show that the internal optical loss, which increases with free-carrier density in the waveguide region, considerably reduces the modulation bandwidth ω−3 dB of a quantum dot laser. At a certain optimum value j0opt of the dc component of the injection current density, the maximum bandwidth ω-3dBmax is attained and the modulation response function becomes as flat as possible. With internal loss cross-section σint increasing and approaching its maximum tolerable value, ω-3dBmax decreases and becomes zero. As with j0opt, there also exists the optimum cavity length, at which ω−3 dB is highest; the larger is σint, the longer is the optimum cavity.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Fc Modulation, tuning, and mode locking
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Quantum-dot nano-cavity lasers with Purcell-enhanced stimulated emission

N. Gregersen, T. Suhr, M. Lorke, and J. Mørk

Appl. Phys. Lett. 100, 131107 (2012); http://dx.doi.org/10.1063/1.3697702 (4 pages) | Cited 1 time

Online Publication Date: 26 March 2012

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We present a rate equation model for quantum-dot light-emitting devices that take into account Purcell enhancement of both spontaneous emission and stimulated emission as well as the spectral profile of the optical and electronic density-of-states. We find that below threshold the β-factor in a quantum-dot nanolaser depends strongly on the pump. For quantum dots with linewidth comparable to that of the cavity, we then show that an otherwise non-lasing device can lase due to Purcell enhancement of the stimulated emission. Finally, we compare the rate equation model to a microscopic model and obtain good agreement.
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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

Contactless mapping of saturation currents of solar cells by photoluminescence

Amaury Delamarre, Laurent Lombez, and Jean-François Guillemoles

Appl. Phys. Lett. 100, 131108 (2012); http://dx.doi.org/10.1063/1.3697704 (3 pages) | Cited 2 times

Online Publication Date: 26 March 2012

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We report in this letter the contactless measurement of spatially resolved photocurrent–photovoltage relationship. The method is based on hyperspectral imaging, from which we record cartography of absolute photoluminescence spectra from solar cells. Using the generalized Planck’s law, it is therefore possible to derive the quantitative value of the quasi-Fermi levels splitting, related to the voltage over the junction. It allows us to directly extract optoelectronics properties of the device with a solely optical method. As a proof of concept, we derive saturation currents of a GaAs solar cell and find a good agreement with the standard electrical measurements.
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88.40.J- Types of solar cells
88.40.hj Efficiency and performance of solar cells

Analysis of efficiency droop in nitride light-emitting diodes by the reduced effective volume of InGaN active material

Han-Youl Ryu, Dong-Soo Shin, and Jong-In Shim

Appl. Phys. Lett. 100, 131109 (2012); http://dx.doi.org/10.1063/1.3698113 (4 pages) | Cited 7 times

Online Publication Date: 27 March 2012

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In InGaN quantum wells (QWs), effective active volume can be greatly reduced due to carrier localization in In-rich region and inhomogeneous carrier distribution. The authors investigate the efficiency droop of InGaN-based light-emitting diodes (LEDs) based on the carrier rate equation including the influence of the reduced effective active volume. It is found that efficiency droop characteristics can be modeled well without employing a large Auger recombination coefficient by assuming that only a small portion of the QWs is effectively used as active region. The presented model is expected to provide insight into the realization of droop-free operation in nitride LEDs.
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85.60.Jb Light-emitting devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Tuning asymmetry parameter of Fano resonance of spoof surface plasmons by modes coupling

F. Cheng, H. F. Liu, B. H. Li, J. Han, H. Xiao, X. F. Han, C. Z. Gu, and X. G. Qiu

Appl. Phys. Lett. 100, 131110 (2012); http://dx.doi.org/10.1063/1.3698117 (4 pages) | Cited 3 times

Online Publication Date: 27 March 2012

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We report a kind of subwavelength, compound hole arrays formed by two nested sub-lattices used to modulate the Fano resonance of spoof surface plasmons (SSPs). Experiments complemented with numerical simulations show that the asymmetry parameter (q) of the Fano line shape can be tuned artificially and continuously: the q value experiences a sign reversal and varies linearly with the variation of hole size. The coupling between different SSP modes of sub-lattices is suggested to be responsible for the tunability of Fano resonance in the compound hole arrays.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
79.60.-i Photoemission and photoelectron spectra
72.25.-b Spin polarized transport

Optical coupling of surface plasmons between graphene sheets

Bing Wang, Xiang Zhang, Xiaocong Yuan, and Jinghua Teng

Appl. Phys. Lett. 100, 131111 (2012); http://dx.doi.org/10.1063/1.3698133 (4 pages) | Cited 6 times

Online Publication Date: 27 March 2012

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In this letter, we theoretically investigate the coupling of far-infrared surface plasmon polaritons (SPPs) between spatially separated graphene sheets. By using the finite-difference frequency-domain method, we numerically illustrate the SPP propagation and modulation in the graphene sheets. The coupling of SPPs is employed to design zero insertion loss optical splitters, 1 × 2 digital optical spatial switches, and ultra-compact Mach-Zehnder interferometers with the arm length far below the diffraction limit. The study provides an effective way in designing graphene based high-speed and ultra-compact optoelectronic devices.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
07.60.Ly Interferometers
42.79.Fm Reflectors, beam splitters, and deflectors
42.79.Ta Optical computers, logic elements, interconnects, switches; neural networks
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)

Electroluminescence from strained germanium membranes and implications for an efficient Si-compatible laser

Donguk Nam, David Sukhdeo, Szu-Lin Cheng, Arunanshu Roy, Kevin Chih-Yao Huang, Mark Brongersma, Yoshio Nishi, and Krishna Saraswat

Appl. Phys. Lett. 100, 131112 (2012); http://dx.doi.org/10.1063/1.3699224 (4 pages) | Cited 3 times

Online Publication Date: 27 March 2012

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We demonstrate room-temperature electroluminescence (EL) from light-emitting diodes (LEDs) on highly strained germanium (Ge) membranes. An external stressor technique was employed to introduce a 0.76% bi-axial tensile strain in the active region of a vertical PN junction. Electrical measurements show an on-off ratio increase of one order of magnitude in membrane LEDs compared to bulk. The EL spectrum from the 0.76% strained Ge LED shows a 100 nm redshift of the center wavelength because of the strain-induced direct band gap reduction. Finally, using tight-binding and finite-difference time domain simulations, we discuss the implications for highly efficient Ge lasers.
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85.60.Jb Light-emitting devices
42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems

Sub-diffraction-limit semiconductor resonators operating on the fundamental magnetic resonance

E. Strupiechonski, G. Xu, M. Brekenfeld, Y. Todorov, N. Isac, A. M. Andrews, P. Klang, C. Sirtori, G. Strasser, A. Degiron, and R. Colombelli

Appl. Phys. Lett. 100, 131113 (2012); http://dx.doi.org/10.1063/1.3697660 (4 pages) | Cited 3 times

Online Publication Date: 27 March 2012

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We demonstrate semiconductor terahertz (THz) resonators with sub-wavelength dimensions in all three dimensions of space. The maximum confinement is obtained for resonators with a diameter of 13 μm, which operate at a wavelength of ≈272 μm. This corresponds to a λeff/6 confinement, where λeff is the wavelength inside the material (or λ/20, if the free space wavelength is considered). These highly sub-wavelength devices operate on the fundamental magnetic resonance, which corresponds to the fundamental oscillation mode of split-ring resonators and is usually inactive in purely optical resonators. In this respect, these resonators are another step towards the hybridization of optics and electronics at THz frequencies. As a proof of principle for cavity quantum electrodynamics experiments, we apply these resonators to THz intersubband polaritons.
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84.40.Az Waveguides, transmission lines, striplines
85.30.-z Semiconductor devices

Positive-negative turbulence-free ghost imaging

Ronald E. Meyers, Keith S. Deacon, and Yanhua Shih

Appl. Phys. Lett. 100, 131114 (2012); http://dx.doi.org/10.1063/1.3698158 (4 pages) | Cited 3 times

Online Publication Date: 29 March 2012

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This experiment observed turbulence-free positive and negative thermal light ghost images from independently recorded event histories of a “bucket” photo-detector and a charged coupled device (CCD) array. The positive (negative) ghost image is computed from the “bucket” detector counts which are above (below) its mean, and the ghost image is not degraded by the turbulence. How do the “bucket” photon counts yield a positive or negative ghost image with a distant CCD array? This letter provides a quantum interference model which effectively explains the phenomenon.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
85.30.Tv Field effect devices

Absolute calibration of optical tweezers including aberrations

R. S. Dutra, N. B. Viana, P. A. Maia Neto, and H. M. Nussenzveig

Appl. Phys. Lett. 100, 131115 (2012); http://dx.doi.org/10.1063/1.3699273 (3 pages) | Cited 1 time

Online Publication Date: 29 March 2012

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We extend a previous proposal for absolute calibration of optical tweezers by including optical setup aberrations into the first-principles theory, with no fitting parameters. Astigmatism, the dominant term, is determined from images of the focused laser spot. Correcting it can substantially increase stiffness. Comparison with experimental results yields agreement within error bars for a broad range of bead sizes and trap heights, as well as different polarizations. Absolute calibration is established as a reliable and practical method for applications and design of optical tweezers systems.
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87.80.Cc Optical trapping
42.50.Wk Mechanical effects of light on material media, microstructures and particles
42.15.Fr Aberrations

Enhanced light emission of double heterostructured MgZnO/ZnO/MgZnO in ultraviolet blind light-emitting diodes deposited by vapor cooling condensation system

Po-Ching Wu, Hsin-Ying Lee, and Ching-Ting Lee

Appl. Phys. Lett. 100, 131116 (2012); http://dx.doi.org/10.1063/1.3698387 (3 pages) | Cited 2 times

Online Publication Date: 30 March 2012

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The MgZnO/ZnO/MgZnO double heterostructure was deposited at low temperature by a vapor cooling condensation system to enhance the light emission of the ultraviolet p-AlGaN/i-MgZnO/i-ZnO/i-MgZnO/n-ZnO:In light-emitting diodes (ULEDs). The defect and vacancy concentrations of the deposited films were effectively reduced. The peak intensity and total emission power of the ultraviolet electroluminescence (EL) spectra of the ULEDs were 3.08 times and 1.82 times higher than those of the p-AlGaN/i-ZnO/n-ZnO:In ULEDs, respectively. Furthermore, the visible EL emission intensity induced by defect and vacancy in the ULEDs was negligible due to the high performances of the deposited active i-ZnO films.
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85.60.Jb Light-emitting devices
68.55.ag Semiconductors
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
78.60.Fi Electroluminescence
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