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20 Aug 2012

Volume 101, Issue 8, Articles (08xxxx)

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Appl. Phys. Lett. 101, 081102 (2012); http://dx.doi.org/10.1063/1.4745791 (3 pages)

M. K. Wu, M. Feng, and N. Holonyak, Jr.
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High responsivity near-infrared photodetectors in evaporated Ge-on-Si

V. Sorianello, A. De Iacovo, L. Colace, A. Fabbri, L. Tortora, E. Buffagni, and G. Assanto

Appl. Phys. Lett. 101, 081101 (2012); http://dx.doi.org/10.1063/1.4747213 (4 pages) | Cited 1 time

Online Publication Date: 20 August 2012

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Germanium is considered the most suitable semiconductor for monolithic integration of near-infrared detectors on silicon photonic chips. Here we report on Ge-on-Si near-infrared photodetectors fabricated by thermal evaporation, demonstrating the use of phosphorus spin-on-dopant to compensate the acceptor states introduced by dislocations. The detectors exhibit 1.55 μm responsivities as high as 0.1 A/W, more than two orders of magnitude larger than in undoped devices and comparing well with state-of-the-art p-i-n photodiodes. This approach enables simple and low-cost monolithic integration of near-infrared sensors with silicon photonics.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
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Voltage modulation of a vertical cavity transistor laser via intra-cavity photon-assisted tunneling

M. K. Wu, M. Feng, and N. Holonyak, Jr.

Appl. Phys. Lett. 101, 081102 (2012); http://dx.doi.org/10.1063/1.4745791 (3 pages) | Cited 1 time

Online Publication Date: 20 August 2012

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We report the direct voltage modulated operation of a vertical cavity transistor laser via intra-cavity coherent signal photon-assisted tunneling. The reversed-bias base/collector junction of the transistor laser provides high input impedance for effective high speed direct voltage modulation. The optical L-VCE characteristics show that the emission intensity saturates and then decreases in laser intensity to half amplitude and broadens when VCE is switched from 3 to 6 V owing to intra-cavity photon-assisted tunneling at the base/collector junction. Correspondingly, the collector IC-VCE characteristics exhibit increased current at higher VCE.
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42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.60.Fc Modulation, tuning, and mode locking

Mid-infrared time-domain ellipsometry: Application to Nb-doped SrTiO3

Andrea Rubano, Lukas Braun, Martin Wolf, and Tobias Kampfrath

Appl. Phys. Lett. 101, 081103 (2012); http://dx.doi.org/10.1063/1.4746263 (4 pages) | Cited 1 time

Online Publication Date: 20 August 2012

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We present a method for determining the dielectric function of opaque materials precisely and reproducibly in the frequency range from 8 to 30 THz and higher. Our approach is based on measuring the polarization- and phase-resolved THz electrical transients reflected by the sample. This mid-infrared time-domain ellipsometry is applied to pure and Nb-doped strontium titanate SrTiO3, which allows us to infer the longitudinal and transverse optical phonon frequencies and the free-carrier plasma frequency as a function of the charge carrier concentration. We extract and discuss the value of the effective mass of the charge carriers.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
63.20.-e Phonons in crystal lattices
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons

Dual-color ultraviolet photodetector based on mixed-phase-MgZnO/i-MgO/p-Si double heterojunction

X. H. Xie, Z. Z. Zhang, C. X. Shan, H. Y. Chen, and D. Z. Shen

Appl. Phys. Lett. 101, 081104 (2012); http://dx.doi.org/10.1063/1.4746772 (3 pages) | Cited 1 time

Online Publication Date: 20 August 2012

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We report a dual-color ultraviolet (UV) photodetector based on mixed-phase-MgZnO/i-MgO/p-Si double heterojunction. The device exhibits distinct dominant responses at solar blind (250 nm) and visible blind (around 330 nm) UV regions under different reverse biases. By using the energy band diagram of the structure, it is found that the bias-tunable two-color detection is originated from different valence band offset between cubic MgZnO/MgO and hexagonal MgZnO/MgO. Meanwhile, due to the large conduction band offset at the Si/MgO interface, the visible-light photoresponse from Si substrate is suppressed.
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85.60.Gz Photodetectors (including infrared and CCD detectors)

Saturation gain-length product during short-wavelength plasma lasing

Davide Bleiner

Appl. Phys. Lett. 101, 081105 (2012); http://dx.doi.org/10.1063/1.4747217 (3 pages) | Cited 1 time

Online Publication Date: 21 August 2012

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A scaling-law, function of the longitudinal and the transverse aspect ratios of a laser-produced plasma gain-medium, is derived to predict the saturation gain-length product for short-wavelength lasing. Benchmarking with experimental data from the literature shows how critical is the plasma geometry and, henceforth, the focusing of the pump laser pulse.
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52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
42.55.Vc X- and γ-ray lasers
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
52.25.Os Emission, absorption, and scattering of electromagnetic radiation

Angled cavity broad area quantum cascade lasers

Y. Bai, S. Slivken, Q. Y. Lu, N. Bandyopadhyay, and M. Razeghi

Appl. Phys. Lett. 101, 081106 (2012); http://dx.doi.org/10.1063/1.4747447 (5 pages) | Cited 2 times

Online Publication Date: 21 August 2012

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Angled cavity broad area quantum cascade lasers (QCLs) are investigated with surface grating-based distributed feedback (DFB) mechanisms. It is found that an angled cavity incorporating a one dimensional DFB with grating lines parallel to the laser facet offers the simplest solution for single mode and diffraction limited emission in the facet normal direction. A room temperature single mode QCL with the highest output power for wavelengths longer than 10 μm is demonstrated. This structure could be applied to a wide range of laser structures for power scaling along with spectral and spatial beam control.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation

Phase control of bright and dark states in four-wave mixing and fluorescence channels

Peiying Li, Zhengyang Zhao, Zhiguo Wang, Yiqi Zhang, Huayan Lan, Haixia Chen, Huaibin Zheng, and Yanpeng Zhang

Appl. Phys. Lett. 101, 081107 (2012); http://dx.doi.org/10.1063/1.4747453 (4 pages) | Cited 1 time

Online Publication Date: 21 August 2012

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We first investigate the phase control of the switch between bright and dark states in the transmitted probe, four-wave mixing (FWM) and fluorescence signals in a four-level  85Rb atomic system. With the relative phase modulated from 0 to −π, pure dark state in the FWM and fluorescence channels can be switched to pure bright state, corresponding to the switch from electromagnetically induced transparency to electromagnetically induced absorption. Meanwhile, the results could be obtained in solid crystals. Such phase controlled switch could have potential applications in optical communication and quantum information processing.
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42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
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Digital optical phase conjugation of fluorescence in turbid tissue

Ivo M. Vellekoop, Meng Cui, and Changhuei Yang

Appl. Phys. Lett. 101, 081108 (2012); http://dx.doi.org/10.1063/1.4745775 (4 pages) | Cited 2 times

Online Publication Date: 22 August 2012

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We demonstrate a method for phase conjugating fluorescence. Our method, called reference free digital optical phase conjugation, can conjugate extremely weak, incoherent optical signals. It was used to phase conjugate fluorescent light originating from a bead covered with 0.5 mm of light-scattering tissue. The phase conjugated beam refocuses onto the bead and causes a local increase of over two orders of magnitude in the light intensity. Potential applications are in imaging, optical trapping, and targeted photochemical activation inside turbid tissue.
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87.63.L- Visual imaging
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
82.50.-m Photochemistry

Fabrication of a large, ordered, three-dimensional nanocup array

Joanne C. Lo, SoonGweon Hong, Richard J. Anderson, Luke P. Lee, David A. Horsley, and Jack L. Skinner

Appl. Phys. Lett. 101, 081109 (2012); http://dx.doi.org/10.1063/1.4747464 (4 pages)

Online Publication Date: 22 August 2012

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Metallic nanocups provide a unique method for redirecting scattered light by creating magnetic plasmon responses at optical frequencies. Despite considerable development of nanocup fabrication processes, simultaneously achieving accurate control over the placement, orientation, and geometry of nanocups has remained a significant challenge. Here we present a technique for fabricating large, periodically ordered arrays of uniformly oriented three-dimensional gold nanocups for manipulating light at subwavelength scales. Nanoimprint lithography, soft lithography, and shadow evaporation were used to fabricate nanocups onto the tips of polydimethylsiloxane nanopillars with precise control over the shapes and optical properties of asymmetric nanocups.
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81.07.Bc Nanocrystalline materials
81.16.Nd Micro- and nanolithography
61.46.-w Structure of nanoscale materials
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons

Tailoring plasmon resonances in the deep-ultraviolet by size-tunable fabrication of aluminum nanostructures

Atsushi Taguchi, Yuika Saito, Koichi Watanabe, Song Yijian, and Satoshi Kawata

Appl. Phys. Lett. 101, 081110 (2012); http://dx.doi.org/10.1063/1.4747489 (4 pages) | Cited 2 times

Online Publication Date: 22 August 2012

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Localized surface plasmon resonances were controlled at deep-ultraviolet (DUV) wavelengths by fabricating aluminum (Al) nanostructures in a size-controllable manner. Plasmon resonances were obtained at wavelengths from near-UV down to 270 nm (4.6 eV) depending on the fabricated structure size. Such precise size control was realized by the nanosphere lithography technique combined with additional microwave heating to shrink the spaces in a close-packed monolayer of colloidal nanosphere masks. By adjusting the microwave heating time, the sizes of the Al nanostructures could be controlled from 80 nm to 50 nm without the need to use nanosphere beads of different sizes. With the outstanding controllability and versatility of the presented fabrication technique, the fabricated Al nanostructure is promising for use as a DUV plasmonic substrate, a light-harvesting platform for mediating strong light–matter interactions between UV photons and molecules placed near the metal nanostructure.
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81.05.Bx Metals, semimetals, and alloys
81.07.-b Nanoscale materials and structures: fabrication and characterization
81.16.Nd Micro- and nanolithography
61.46.-w Structure of nanoscale materials
73.22.Lp Collective excitations

Light down-conversion with over 100% external quantum efficiency in bulk germanium

V. K. Malyutenko and V. V. Bogatyrenko

Appl. Phys. Lett. 101, 081111 (2012); http://dx.doi.org/10.1063/1.4748362 (3 pages)

Online Publication Date: 22 August 2012

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To bypass indirect bandstructure limitations in Ge and have it effectively emitting, we employ light down conversion process at elevated temperatures (emissivity enhancement technique). By applying short wavelength pump light (λin, interband electron transitions), we enhance thermal emission output in the spectral range of intraband electron transitions (λout, free carrier absorption band). We experimentally realized conditions (λin = 1.9 μm, λout = 2–25 μm, T = 380 K) when Ge wafers demonstrated power conversion efficiency of 21% and external quantum efficiency (photon multiplication coefficient) of 130%. Advantages of Ge over Si and ways to realize Ge-based radiative cooler are discussed.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
44.40.+a Thermal radiation

Frequency-controlled deflection of spatial solitons in nematic liquid crystals

Armando Piccardi, Alessandro Alberucci, Oleksandr Buchnev, Malgosia Kaczmarek, Iam Choon Khoo, and Gaetano Assanto

Appl. Phys. Lett. 101, 081112 (2012); http://dx.doi.org/10.1063/1.4747716 (4 pages)

Online Publication Date: 23 August 2012

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We demonstrate frequency-controlled angular steering of light-induced solitary waveguides using dual frequency nematic liquid crystals. Specially designed comb-shaped electrodes allow the maximization of the in-plane electro-optic angular deviation of the soliton Poynting vector, modulating the resulting walk-off up to an overall deflection of 13° when the frequency of the applied voltage goes from 1 to 100 kHz.
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42.70.Df Liquid crystals
42.65.Tg Optical solitons; nonlinear guided waves
42.79.Gn Optical waveguides and couplers
42.79.Hp Optical processors, correlators, and modulators
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Resonant transmission of light through ZnO nanowaveguides in a silver film

Victor L. Garcia, Mary Grace M. Velasco, Spencer B. Mamer, Kimberly R. Singh, Nabila K. Hossain, Guangyue He, Mostafa Sadoqi, and Huizhong Xu

Appl. Phys. Lett. 101, 081113 (2012); http://dx.doi.org/10.1063/1.4747718 (5 pages)

Online Publication Date: 23 August 2012

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Zinc oxide (ZnO) nanowaveguides of 40 nm in diameter are fabricated by embedding ZnO nanowires vertically grown on a glass substrate in a silver film. Resonant transmission through these nanowaveguides is observed for incident wavelengths around 500 nm. As the ZnO nanowaveguide diameter increases, the transmission resonance shifts toward longer wavelengths with increased amplitude. We attribute the observed resonant transmission to the unique material combination used enabling the excitation of localized surface plasmons.
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42.79.Gn Optical waveguides and couplers
42.86.+b Optical workshop techniques

Continuous wave terahertz radiation from an InAs/GaAs quantum-dot photomixer device

T. Kruczek, R. Leyman, D. Carnegie, N. Bazieva, G. Erbert, S. Schulz, C. Reardon, S. Reynolds, and E. U. Rafailov

Appl. Phys. Lett. 101, 081114 (2012); http://dx.doi.org/10.1063/1.4747724 (4 pages)

Online Publication Date: 23 August 2012

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Generation of continuous wave radiation at terahertz (THz) frequencies from a heterodyne source based on quantum-dot (QD) semiconductor materials is reported. The source comprises an active region characterised by multiple alternating photoconductive and QD carrier trapping layers and is pumped by two infrared optical signals with slightly offset wavelengths, allowing photoconductive device switching at the signals’ difference frequency ∼1 THz.
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85.60.-q Optoelectronic devices

Optimized optomechanical crystal cavity with acoustic radiation shield

Jasper Chan, Amir H. Safavi-Naeini, Jeff T. Hill, Seán Meenehan, and Oskar Painter

Appl. Phys. Lett. 101, 081115 (2012); http://dx.doi.org/10.1063/1.4747726 (4 pages) | Cited 8 times

Online Publication Date: 23 August 2012

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We present the design of an optomechanical crystal nanobeam cavity that combines finite-element simulation with numerical optimization, and considers the optomechanical coupling arising from both moving dielectric boundaries and the photo-elastic effect. Applying this methodology results in a nanobeam with an experimentally realized intrinsic optical Q-factor of 1.2×106, a mechanical frequency of 5.1 GHz, a mechanical Q-factor of 6.8×105 (at T = 10 K), and a zero-point-motion optomechanical coupling rate of g = 1.1 MHz.
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42.70.Qs Photonic bandgap materials
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.79.Gn Optical waveguides and couplers
46.25.-y Static elasticity
02.70.Dh Finite-element and Galerkin methods

A dual-color plasmonic focus for surface-selective four-wave mixing

Xuejun Liu, Yong Wang, and Eric O. Potma

Appl. Phys. Lett. 101, 081116 (2012); http://dx.doi.org/10.1063/1.4747798 (4 pages) | Cited 1 time

Online Publication Date: 23 August 2012

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We describe a dual-color plasmonic lens suitable for focusing two femtosecond surface plasmon polariton wavefronts to a common focal spot. We show that the overlapping evanescent fields, which are confined to (sub-) micrometer dimensions, form a surface-selective excitation volume for four-wave mixing (FWM) experiments. We demonstrate that stable and virtually background-free FWM signals from single nano-objects placed in the plasmonic focus can be generated. The plasmonic focal spot constitutes a precisely controlled excitation source for surface-selective nonlinear spectroscopic measurements.
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42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
42.79.Bh Lenses, prisms and mirrors
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)

Crystalline garnet Bragg reflectors for high power, high temperature, and integrated applications fabricated by multi-beam pulsed laser deposition

Katherine A. Sloyan, Timothy C. May-Smith, Michalis N. Zervas, and Robert W. Eason

Appl. Phys. Lett. 101, 081117 (2012); http://dx.doi.org/10.1063/1.4748107 (4 pages)

Online Publication Date: 24 August 2012

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Crystalline Bragg reflectors are of interest for high power, high temperature, and integrated applications. We demonstrate the automated growth of such structures by shuttered multi-beam pulsed laser deposition. Geometries include 145 layer stacks exhibiting >99.5% reflection and π phase-shifted designs. A crystalline grating strength-apodized sample was grown by mixing plumes to obtain layers with custom refractive indices. Peak reflection wavelength was tuneable with incident position, samples withstood temperatures of ∼750 °C, and film and substrate have been shown to withstand incident pulsed laser fluences of up to ∼33 J cm−2.
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81.15.Fg Pulsed laser ablation deposition
42.79.Dj Gratings

Theoretical study of the effects of InAs/GaAs quantum dot layer’s position in i-region on current-voltage characteristic in intermediate band solar cells

Yong-Xian Gu, Xiao-Guang Yang, Hai-Ming Ji, Peng-Fei Xu, and Tao Yang

Appl. Phys. Lett. 101, 081118 (2012); http://dx.doi.org/10.1063/1.4748161 (4 pages)

Online Publication Date: 24 August 2012

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We theoretically investigated the current-voltage characteristic of InAs/GaAs quantum dot (QD) intermediate band solar cells by changing the QD layer’s position in i-region. The open circuit voltage, short current density, fill factor, and conversion efficiency all vary with the position of QD layer. If the light generation coefficients through intermediate band (IB) are small, the IB mainly plays the role of a recombination energy level. If the light generation coefficients are improved, in order to ensure the highest QD layer performance, QD layer should be placed in an appropriate range.
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88.40.jp Multijunction solar cells
88.40.hj Efficiency and performance of solar cells

Polarization-sensitive air-biased-coherent-detection for terahertz wave

Zhihui Lü, Dongwen Zhang, Chao Meng, Lin Sun, Zhaoyan Zhou, Zengxiu Zhao, and Jianmin Yuan

Appl. Phys. Lett. 101, 081119 (2012); http://dx.doi.org/10.1063/1.4748171 (3 pages)

Online Publication Date: 24 August 2012

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Employing an orientation-modulated bias field, a polarization-sensitive scheme for terahertz air-biased-coherent-detection (THz-ABCD) is presented to directly measure the amplitude and polarization angle of THz field in the time domain. It can provide all characteristics of arbitrarily polarized THz wave with one single-scan measurement. Measuring convenience, broad bandwidth, and high angular resolution have been achieved. Polarization-sensitive THz time-domain spectroscopy can surely be developed based on this technology. Many other applications in the THz spectral region are also believed.
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84.40.-x Radiowave and microwave (including millimeter wave) technology
07.57.Pt Submillimeter wave, microwave and radiowave spectrometers; magnetic resonance spectrometers, auxiliary equipment, and techniques

Inserting a p-InGaN layer before the p-AlGaN electron blocking layer suppresses efficiency droop in InGaN-based light-emitting diodes

Ray-Ming Lin, Sheng-Fu Yu, Shoou-Jinn Chang, Tsung-Hsun Chiang, Sheng-Po Chang, and Chang-Ho Chen

Appl. Phys. Lett. 101, 081120 (2012); http://dx.doi.org/10.1063/1.4747802 (4 pages) | Cited 2 times

Online Publication Date: 24 August 2012

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In this study, we observed a dramatic decrease in the efficiency droop of InGaN/GaN light-emitting diodes after positioning a p-InGaN insertion layer before the p-AlGaN electron-blocking layer. The saturated external quantum efficiency of this device extended to 316 mA, with an efficiency droop of only 7% upon increasing the operating current to 1 A; in contrast, the corresponding conventional light-emitting diode suffered a severe efficiency droop of 42%. We suspect that the asymmetric carrier distribution was effectively mitigated as a result of an improvement in the hole injection rate and a suppression of electron overflow.
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85.60.Jb Light-emitting devices

200-fs pulse generation from a GaInN semiconductor laser diode passively mode-locked in a dispersion-compensated external cavity

Shunsuke Kono, Hideki Watanabe, Rintaro Koda, Takao Miyajima, and Masaru Kuramoto

Appl. Phys. Lett. 101, 081121 (2012); http://dx.doi.org/10.1063/1.4747808 (4 pages) | Cited 1 time

Online Publication Date: 24 August 2012

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We have demonstrated passively mode-locked operation of a GaInN bisectional laser diode in a dispersion-compensated external cavity. With negative group velocity dispersion in the external cavity, we obtained femtosecond optical pulses after spectral filtering with a bandpass filter. The pulse duration was as short as 200 fs with a time-bandwidth product of 0.41. This is the shortest pulse ever reported for a mode-locked GaInN laser diode.
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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
42.60.Fc Modulation, tuning, and mode locking
42.79.Ci Filters, zone plates, and polarizers
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