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14 May 2007

Volume 90, Issue 20, Articles (20xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 90, 201101 (2007); http://dx.doi.org/10.1063/1.2739308 (3 pages)

Luca Sapienza, Angela Vasanelli, Cristiano Ciuti, Christophe Manquest, Carlo Sirtori, Raffaele Colombelli, and Ulf Gennser
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Photovoltaic probe of cavity polaritons in a quantum cascade structure

Luca Sapienza, Angela Vasanelli, Cristiano Ciuti, Christophe Manquest, Carlo Sirtori, Raffaele Colombelli, and Ulf Gennser

Appl. Phys. Lett. 90, 201101 (2007); http://dx.doi.org/10.1063/1.2739308 (3 pages) | Cited 9 times

Online Publication Date: 14 May 2007

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The strong coupling between an intersubband excitation in a quantum cascade structure and a photonic mode of a planar microcavity has been detected by angle-resolved photovoltaic measurements. A typical anticrossing behavior, with a vacuum-field Rabi splitting of 16 meV at 78 K, has been measured, for an intersubband transition at 163 meV. These results show that the strong coupling regime between photons and intersubband excitations can be engineered in a quantum cascade optoelectronic device. They also demonstrate the possibility to perform angle-resolved midinfrared photodetection and to develop active devices based on intersubband cavity polaritons.
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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
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Single radial-mode lasing in a submicron-thickness spherical shell microlaser

Sang-Bum Lee, Myoung-Kyu Oh, Jai-Hyung Lee, and Kyungwon An

Appl. Phys. Lett. 90, 201102 (2007); http://dx.doi.org/10.1063/1.2739078 (3 pages) | Cited 4 times

Online Publication Date: 14 May 2007

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The authors have investigated whispering-gallery mode (WGM) lasing characteristics in a submicron-thickness spherical shell microlaser. The dielectric shell region was doped with dye molecules at a low concentration so as to provide a gain for lasing as well as to confine WGM’s tightly. Lasing in the lowest-mode-order WGM with a quality factor as high as 107 was observed.
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42.55.Rz Doped-insulator lasers and other solid state lasers
42.55.Mv Dye lasers
42.55.Sa Microcavity and microdisk lasers
42.60.By Design of specific laser systems
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Ultrafast gain dynamics in 1.3 μm InAs/GaAs quantum-dot optical amplifiers: The effect of p doping

Valentina Cesari, Wolfgang Langbein, Paola Borri, Marco Rossetti, Andrea Fiore, S. Mikhrin, I. Krestnikov, and A. Kovsh

Appl. Phys. Lett. 90, 201103 (2007); http://dx.doi.org/10.1063/1.2739079 (3 pages) | Cited 16 times

Online Publication Date: 14 May 2007

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Ultrafast gain dynamics of the ground-state transition are measured in electrically pumped InAs/GaAs quantum-dot amplifiers emitting near 1.3 μm at room temperature. Gain recovery on a subpicosecond time scale occurs at high electrical injection. However, when comparing p-doped and undoped devices fabricated under identical conditions and operating at the same gain, faster absorption recovery but slower gain dynamics are observed in p-doped amplifiers. The slower gain dynamics is attributed to a reduced reservoir of excited-state electrons in p-doped quantum-dot devices, which limits the recovery of the electron ground-state occupation mediated by intradot carrier-carrier scattering.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.65.Re Ultrafast processes; optical pulse generation and pulse compression

Nonlinear photonic crystal waveguide structures based on barium titanate thin films and their optical properties

Zhifu Liu, Pao-Tai Lin, Bruce W. Wessels, Fei Yi, and Seng-Tiong Ho

Appl. Phys. Lett. 90, 201104 (2007); http://dx.doi.org/10.1063/1.2739083 (3 pages) | Cited 6 times

Online Publication Date: 14 May 2007

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Nonlinear photonic crystal waveguide structures were fabricated from barium titanate thin films using nanolithography. A cascaded Bragg reflector using a strip waveguide was designed and analyzed. Both simulation and experimental results show that there is sufficient refractive index contrast to form a stop band by only etching through the Si3N4 strip layer. The band gap of the Bragg reflector can be engineered through control of the Bragg spacing, thickness, and etching depth of the strip layer. The transmission spectrum of the Bragg reflector waveguide was measured over the spectral range of 1500–1580 nm. A 27 nm wide stop band was obtained for a millimeter long sample. The nonlinear photonic crystal waveguides are potentially suitable as tunable filters, optical switches, and ultrawide bandwidth modulators.
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42.82.Et Waveguides, couplers, and arrays
42.70.Qs Photonic bandgap materials
42.15.Eq Optical system design
42.79.Bh Lenses, prisms and mirrors
42.79.Wc Optical coatings
42.65.Pc Optical bistability, multistability, and switching, including local field effects
42.79.Hp Optical processors, correlators, and modulators

Control of Fano line shapes by means of photonic crystal structures in a dye-doped polymer

Rik Harbers, Selim Jochim, Nikolaj Moll, Rainer F. Mahrt, Daniel Erni, John A. Hoffnagle, and William D. Hinsberg

Appl. Phys. Lett. 90, 201105 (2007); http://dx.doi.org/10.1063/1.2739334 (3 pages) | Cited 4 times

Online Publication Date: 14 May 2007

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The emission of a two-dimensional organic photonic crystal structure is investigated. A photon emitted by a molecule in this structure can take two different coupled pathways. It can either be emitted directly into the surroundings or first be transferred to a discrete state of the photonic crystal structure from where it is then emitted. This unique coupling of the pathways results in a particular asymmetric spectral line shape referred to as Fano resonance. By studying the Fano line shape as a function of the quality factor, the authors can gain insights into the coupling between the pathways within the photonic crystal structure.
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42.70.Qs Photonic bandgap materials
42.70.Jk Polymers and organics

Two dimensional dynamic focusing of laser light by ferroelectric domain based electro-optic lenses

Mahesh Krishnamurthi, Mariola O. Ramirez, Sava Denev, Venkatraman Gopalan, Thomas M. Lehecka, Jeffrey G. Thomas, and Q. X. Jia

Appl. Phys. Lett. 90, 201106 (2007); http://dx.doi.org/10.1063/1.2739368 (3 pages) | Cited 5 times

Online Publication Date: 14 May 2007

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The authors demonstrate the proof of concept of two dimensional focusing of laser light. This has been achieved by using a combination of two cylindrical electro-optic ferroelectric domain lens stacks in an orthogonal geometry. The devices were fabricated on z-cut lithium tantalate (LiTaO3) wafers and tested with helium-neon laser at 633 nm. Continuously tunable optical power ranging from −129 m−1 to 129 m−1 is obtained in both directions by varying the applied voltage.
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42.82.Gw Other integrated-optical elements and systems
42.79.Bh Lenses, prisms and mirrors

High contrast polarization sensitive quantum well infrared photodetectors

Thomas Antoni, Alexandru Nedelcu, Xavier Marcadet, Hugues Facoetti, and Vincent Berger

Appl. Phys. Lett. 90, 201107 (2007); http://dx.doi.org/10.1063/1.2739408 (3 pages) | Cited 3 times

Online Publication Date: 14 May 2007

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The responsivity of polarization sensitive quantum well infrared photodetectors with small pixel size (down to 20 μm) is investigated. It is shown that pixels suitable for integration into very large focal plane arrays (1000×1000) can discriminate the polarization of the incoming signal. A responsivity contrast higher than 60% is obtained although the grating size is reduced to only six periods. The quantum efficiency and polarimetric capabilities can be both optimized with the same grating parameters.
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07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
85.60.Gz Photodetectors (including infrared and CCD detectors)

Electro-optic properties of a homeotropic liquid crystal cell with 90° surface rubbings and a reverse-handed chiral dopant

Ju-Hyun Lee, Zhibing Ge, and Shin-Tson Wu

Appl. Phys. Lett. 90, 201108 (2007); http://dx.doi.org/10.1063/1.2739412 (3 pages)

Online Publication Date: 14 May 2007

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The electro-optic properties of a 90° twisted-homeotropic liquid crystal (LC) cell with a chiral dopant whose handedness is opposite to the LC twist are studied. In the voltage-off state, the LC directors exhibit a homeotropic alignment. However, in the intermediate voltage state, the bulk LC directors behave like a homogeneous alignment due to the balanced torques between the electric field, surface anchoring, and reversed chiral dopant. Potential applications of this mode for liquid crystal displays, especially for dual-cell-gap transflective liquid crystal displays, are emphasized.
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42.79.Kr Display devices, liquid-crystal devices
85.60.Pg Display systems

High operating temperature 320×256 middle-wavelength infrared focal plane array imaging based on an InAs/InGaAs/InAlAs/InP quantum dot infrared photodetector

S. Tsao, H. Lim, W. Zhang, and M. Razeghi

Appl. Phys. Lett. 90, 201109 (2007); http://dx.doi.org/10.1063/1.2740111 (3 pages) | Cited 18 times

Online Publication Date: 14 May 2007

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This letter reports a 320×256 middle-wavelength infrared focal plane array operating at temperatures up to 200 K based on an InAs quantum dot/InGaAs quantum well/InAlAs barrier detector grown on InP substrate by low pressure metal organic chemical vapor deposition. The device’s low dark current density and the persistence of the photocurrent up to room temperature enabled the high temperature imaging. The focal plane array had a peak detection wavelength of 4 μm, a responsivity of 34 mA/W, a conversion efficiency of 1.1%, and a noise equivalent temperature difference of 344 mK at an operating temperature of 120 K.
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07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
85.60.Gz Photodetectors (including infrared and CCD detectors)
42.79.Pw Imaging detectors and sensors

Poly(9,9-dioctylfluorene)-based light-emitting diodes with pure β-phase emission

J. Morgado, L. Alcácer, and A. Charas

Appl. Phys. Lett. 90, 201110 (2007); http://dx.doi.org/10.1063/1.2737424 (3 pages) | Cited 9 times

Online Publication Date: 14 May 2007

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The authors report on poly(9,9-dioctylfluorene) (PFO)-based light-emitting diodes exhibiting an emission spectrum which was previously attributed to the so-called β phase. These devices were prepared by drop cast from dilute toluene solutions. Photoluminescence and electroluminescence spectra, recorded at room temperature, show no evidence of the usual amorphous phase emission. Devices with pure β-phase emission exhibit a higher color stability, upon increase of the driving voltage, than those based on amorphous PFO.
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85.60.Jb Light-emitting devices
42.70.Jk Polymers and organics

Photonic band gaps and planar cavity of two-dimensional eightfold symmetric void-channel photonic quasicrystals

Guangyong Zhou and Min Gu

Appl. Phys. Lett. 90, 201111 (2007); http://dx.doi.org/10.1063/1.2739090 (3 pages) | Cited 5 times

Online Publication Date: 15 May 2007

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By using the femtosecond laser induced microexplosion method, high-quality two-dimensional eightfold photonic quasicrystals have been fabricated in a solid transparent polymer material. Multiorder band gaps have been observed in a 25-layer structure with a suppression rate of up to 72% for the fundamental gap. Polarization measurements show that the photonic quasicrystal has a strong anisotropic effect, showing that the transverse electric is the favorite polarization. Fabry-Pérot cavities have been fabricated by removing the central layer of channels. Based on the cavity mode position, the order of the mode and the effective cavity size have been determined.
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42.70.Qs Photonic bandgap materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
61.72.Qq Microscopic defects (voids, inclusions, etc.)
78.30.-j Infrared and Raman spectra
42.82.Gw Other integrated-optical elements and systems

“Blueshift” in photoluminescence and photovoltaic spectroscopy of the ion-milling formed n-on-p HgCdTe photodiodes

F. X. Zha, Jun Shao, J. Jiang, and W. Y. Yang

Appl. Phys. Lett. 90, 201112 (2007); http://dx.doi.org/10.1063/1.2740107 (3 pages) | Cited 11 times

Online Publication Date: 15 May 2007

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Photoluminescence characterization of n-on-p Hg1−xCdxTe photodiodes formed by ion-milling technique shows that the luminescence peak of the n-type conversion region shifts remarkably toward higher energy with an amount of ∼ 40 meV more than that of the p region. The photovoltaic response of the diodes was examined. The origin of the “blueshift” may be ascribed to the Burstein-Moss effect associated with the high electron concentration of the type conversion region.
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85.60.Dw Photodiodes; phototransistors; photoresistors
78.66.Hf II-VI semiconductors

Al-based Ohmic reflectors with low leakage currents and high reflectance for p-GaN flip-chip processes

S. W. Chae, D. H. Kim, T. G. Kim, K. Y. Ko, and Y. M. Sung

Appl. Phys. Lett. 90, 201113 (2007); http://dx.doi.org/10.1063/1.2740173 (3 pages) | Cited 6 times

Online Publication Date: 15 May 2007

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The authors report the improvement of InGaN/GaN light-emitting diodes on Al reflectors, commonly used as n-type GaN contacts. A Cu-doped indium oxide (CIO) (5 nm)/indium tin oxide (ITO) (380 nm) interlayer was deposited and annealed at 500 °C, after which an Al (400 nm)/Ti–W (30 nm) layer was sputtered on the ITO interlayer to reflect the light. The reflectance of CIO/ITO/Al/Ti–W was ∼ 92% at 460 nm, higher than that of the popular Ni/Ag/Pt scheme, and the forward voltage was 3.2–3.3 V, similar to that of the Ni/Ag/Pt contact. Furthermore, the mean leakage current of CIO/ITO/Al/Ti–W was 0.12 μA, much lower than 0.54 μA of Ni/Ag/Pt at −5 V.
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85.60.Jb Light-emitting devices
73.40.Cg Contact resistance, contact potential

Direct imaging of a laser mode via midinfrared near-field microscopy

Virginie Moreau, Michael Bahriz, Raffaele Colombelli, Paul-Arthur Lemoine, Yannick De Wilde, Luke R. Wilson, and Andrey B. Krysa

Appl. Phys. Lett. 90, 201114 (2007); http://dx.doi.org/10.1063/1.2738189 (3 pages) | Cited 7 times

Online Publication Date: 16 May 2007

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Fabry-Pérot standing waves inside a midinfrared quantum cascade laser have been imaged using an apertureless scanning near-field optical microscope. The devices emit at λ ≈ 7.7 μm and they feature air-confinement waveguides, with the optical mode guided at the semiconductor-air interface. A consistent portion of the mode leaks evanescently from the device top surface and can be detected in the near field of the device. Imaging of the evanescent wave across a plane parallel to the device surface allows one to directly assess the effective light wavelength inside the laser material, yielding the effective index of refraction. Imaging across a plane perpendicular to the device surface allows one to directly measure the electric field decay length, which is found in excellent agreement with the numerical simulations.
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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
07.79.Fc Near-field scanning optical microscopes

Optical amplification in multilayer photorefractive liquid crystal films

Xiudong Sun, Yanbo Pei, Fengfeng Yao, Jianlong Zhang, and Chunfeng Hou

Appl. Phys. Lett. 90, 201115 (2007); http://dx.doi.org/10.1063/1.2740477 (3 pages) | Cited 4 times

Online Publication Date: 17 May 2007

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Photorefractive two-beam coupling was performed in fullerene C60-doped nematic liquid crystal cell. Photorefractive gain coefficient as high as 1386 cm−1 was obtained at an input pump power as weak as 16 mW. However, the thin film nature led to a small gain of 16. By the use of several such cells stacked together to increase the overall interaction length, the optical gain was improved greatly, and the pump power to obtain the highest optical gain was reduced. At 1.2 V, the highest gain of 141 was obtained for the 40 μW signal at an input pump power of 12 mW.
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42.70.Df Liquid crystals
42.70.Gi Light-sensitive materials
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
68.15.+e Liquid thin films

High performance asymmetric graded index coupler with integrated lens for high index waveguides

Rong Sun, Victor Nguyen, Anu Agarwal, Ching-yin Hong, John Yasaitis, Lionel Kimerling, and Jurgen Michel

Appl. Phys. Lett. 90, 201116 (2007); http://dx.doi.org/10.1063/1.2740589 (3 pages) | Cited 2 times

Online Publication Date: 17 May 2007

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The authors have demonstrated a planar, monolithically integrated lensed, asymmetric graded index fiber-to-waveguide coupler with 0.45 dB coupling loss at 1550 nm. The silicon-based coupler and the waveguides were fabricated entirely by complementary metal-oxide semiconductor compatible processes. They also observed an excellent broadband performance between 1520 and 1630 nm. The average coupling loss was 0.4 dB across the entire cathodoluminescence band (1530–1625 nm).
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42.79.Gn Optical waveguides and couplers
42.79.Ry Gradient-index (GRIN) devices
42.82.Et Waveguides, couplers, and arrays

Optical amplification in Ho3+-doped transparent oxyfluoride glass ceramics at 750 nm

F. Lahoz, S. E. Hernández, N. E. Capuj, and D. Navarro-Urrios

Appl. Phys. Lett. 90, 201117 (2007); http://dx.doi.org/10.1063/1.2741146 (3 pages) | Cited 14 times

Online Publication Date: 17 May 2007

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Positive transient optical gain has been demonstrated in Ho3+-doped transparent oxyfluoride glass-ceramics. A pump and probe experiment has been designed to show this result. High power laser pulses at 532 nm were used as the pump source to strongly populate the Ho3+ math:math level due to nonresonant ground state absorption. Low power cw laser radiation at 750 nm was used as the probe beam. The signal beam stimulates the emission associated with the Ho3+ math:math→math electronic transition at 750 nm. In addition to this, the high power pump pulses provide population inversion between the math:math and math, initial and final states of the transition, respectively, giving rise to the optical amplification of the signal beam. A gain coefficient of 3.7 cm−1 ( ∼ 16 dB/cm) was obtained for a pump energy density of about 135 mJ/cm2 and a signal beam power density of 6 μW/cm2.
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42.70.Gi Light-sensitive materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.45.+h Stimulated emission

TiO2 based metal-semiconductor-metal ultraviolet photodetectors

Hailin Xue, Xiangzi Kong, Ziran Liu, Caixia Liu, Jingran Zhou, Weiyou Chen, Shengping Ruan, and Qian Xu

Appl. Phys. Lett. 90, 201118 (2007); http://dx.doi.org/10.1063/1.2741128 (3 pages) | Cited 32 times

Online Publication Date: 18 May 2007

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Nanocrystalline TiO2 thin films were prepared by sol-gel method and were then used to fabricate metal-semiconductor-metal ultraviolet photodetectors with Au Schottky contact. It was found that dark current of the fabricated devices was only 1.9 nA at 5 V applied bias. High responsivity of 199 A/W was achieved when it was irradiated by the ultraviolet light (λ = 260 nm). The low dark current and high responsivity maybe attributed to the effect of Schottky barrier in company with neutral semiconductor owing to the wide finger gap of 20 μm. The devices show a slow time response with a rise time of 6 s and a decay time of 15 s. The authors deduced that the slow time response was caused by defect traps which were widely distributed in nanocrysal.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
73.40.Sx Metal-semiconductor-metal structures

Oxidized macroscopic-long Cu nanowire bundle photoconductor

Jia-Lin Sun, Jia Xu, and Jia-Lin Zhu

Appl. Phys. Lett. 90, 201119 (2007); http://dx.doi.org/10.1063/1.2741131 (3 pages) | Cited 6 times

Online Publication Date: 18 May 2007

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Using a solid electrolyte RbCu4Cl3I2 thin film and a controlled constant current electric field, the authors have directly fabricated macroscopic-long Cu nanowire bundles. Their structural characters have been described at different scales. The oxidation behavior of macroscopic-long Cu nanowire bundles containing thousands of straight and dendritic Cu nanowires was studied using x-ray photoelectron spectroscopy. Benefiting from the oxidized macroscopic-long Cu nanowire bundles, they have experimentally explored photoinduced conductivity in the bundle upon illumination with a 532 nm laser beam, and observed that it can be regarded as a photoconductor with high sensitivity and fast response.
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72.40.+w Photoconduction and photovoltaic effects
73.63.Nm Quantum wires
81.65.Mq Oxidation
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
61.46.-w Structure of nanoscale materials
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