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22 Feb 2010

Volume 96, Issue 8, Articles (08xxxx)

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Appl. Phys. Lett. 96, 081101 (2010); http://dx.doi.org/10.1063/1.3324885 (3 pages)

Wuzhou Song and Demetri Psaltis
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Pneumatically tunable optofluidic dye laser

Wuzhou Song and Demetri Psaltis

Appl. Phys. Lett. 96, 081101 (2010); http://dx.doi.org/10.1063/1.3324885 (3 pages) | Cited 16 times

Online Publication Date: 22 February 2010

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We presented a tunable optofluidic dye laser with integrated elastomeric air-gap etalon controlled by air pressure. The chip was fabricated with polydimethylsiloxane (PDMS) via replica molding. It comprises a liquid waveguide and microscale air-gap mirrors providing the feedback. The lasing wavelength is chosen by the interference between two parallel PDMS-air interfaces inside the internal tunable air-gap etalon, of which pneumatic tuning can be realized by inflating the air-gap etalon with compressed air. This dye laser exhibits a pumping threshold of 1.6 μJ/pulse, a lasing linewidth of 3 nm, and a tuning range of 14 nm.
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42.55.Mv Dye lasers
42.60.By Design of specific laser systems
42.70.Hj Laser materials
42.60.Fc Modulation, tuning, and mode locking
42.70.-a Optical materials
42.82.-m Integrated optics
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Improved measurement accuracy of the quadrant detector through improvement of linearity index

Song Cui and Yeng Chai Soh

Appl. Phys. Lett. 96, 081102 (2010); http://dx.doi.org/10.1063/1.3326078 (3 pages) | Cited 5 times

Online Publication Date: 22 February 2010

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This paper presents a set of formulae for estimating the beam position projected on a quadrant detector (QD). Our new formulae are designed through a systematic procedure to improve the measurement accuracy of the QD. It can be applied to different beam profiles. It is shown in simulations that our new formulae have achieved a much better measurement accuracy. It is also shown through experimental study that our approach is robust and able to achieve very good performance in practice.
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29.40.Gx Tracking and position-sensitive detectors

Effect of indium fluctuation on the photovoltaic characteristics of InGaN/GaN multiple quantum well solar cells

K. Y. Lai, G. J. Lin, Y.-L. Lai, Y. F. Chen, and J. H. He

Appl. Phys. Lett. 96, 081103 (2010); http://dx.doi.org/10.1063/1.3327331 (3 pages) | Cited 27 times

Online Publication Date: 22 February 2010

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Severe In fluctuation was observed in In0.3Ga0.7N/GaN multiple quantum well solar cells using scanning transmission electron microscopy and energy dispersive x-ray spectroscopy. The high In content and fluctuation lead to low fill factor (FF) of 30% and energy conversion efficiency (η) of 0.48% under the illumination of AM 1.5G. As the temperature was increased from 250 to 300 K, FF and η were substantially enhanced. This strong temperature-dependent enhancement is attributed to the additional contribution to the photocurrents by the thermally activated carriers, which are originally trapped in the shallow quantum wells resulting from the inhomogeneous In distribution.
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88.40.H- Solar cells (photovoltaics)
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Theoretical and experimental studies of three-photon-induced excited-state absorption

Bing Gu, Wei Ji, Hong-Zhi Yang, and Hui-Tian Wang

Appl. Phys. Lett. 96, 081104 (2010); http://dx.doi.org/10.1063/1.3327340 (3 pages) | Cited 3 times

Online Publication Date: 22 February 2010

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We theoretically and experimentally study excited-state absorption (ESA) induced by three-photon absorption (3PA) under the excitation of ultrafast laser pulses. Based on five-level rate-equation theory, we obtain an analytical expression for the effective four-photon absorption coefficient that originates from both singlet-singlet (S-S) and triplet-triplet (T-T) ESA. We validate that the T-T ESA could be justifiably neglected in the femtosecond regime. Experimentally, we verify it in acetone solution of 2,4,5-trimethoxychalcone by performing femtosecond Z-scan measurements; at low excitation intensity, only 3PA is determined; and under the excitation of high intensity, however, we observe the concurrence of 3PA and 3PA-induced S-S ESA processes.
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42.50.Hz Strong-field excitation of optical transitions in quantum systems; multiphoton processes; dynamic Stark shift
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.70.Jk Polymers and organics

Three-dimensional bulk metamaterials operating in the terahertz range

Fumiaki Miyamaru, Shiro Kuboda, Kazuo Taima, Keisuke Takano, Masanori Hangyo, and Mitsuo Wada Takeda

Appl. Phys. Lett. 96, 081105 (2010); http://dx.doi.org/10.1063/1.3327830 (3 pages) | Cited 9 times

Online Publication Date: 22 February 2010

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Three-dimensional bulk metamaterials that operate in the terahertz (THz) frequency range were fabricated by stacking 100 two-dimensional sheets containing metallic split-ring resonators (SRR) on thin polyethylene terephthalate film substrates. The THz magnetic resonance for the incident magnetic field perpendicular to the plane of the SRR structure was measured. We also investigated the dependence of the magnetic resonant strength on the metal thickness.
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81.05.Xj Metamaterials for chiral, bianisotropic and other complex media
42.70.-a Optical materials

Pulsed terahertz time domain spectroscopy of vertically structured photoconductive antennas

R. Faulks, S. Rihani, H. E. Beere, M. J. Evans, D. A. Ritchie, and M. Pepper

Appl. Phys. Lett. 96, 081106 (2010); http://dx.doi.org/10.1063/1.3313940 (3 pages)

Online Publication Date: 22 February 2010

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We present a terahertz (THz) photoconductive emitter structure, which employs a n-doped layer underneath a low-temperature-grown GaAs region to enable the THz transient to couple vertically through a defined mesa. A nonlinear bias dependence is observed, yielding an order in magnitude improvement in power for a mesa device with a 100 μm2 area over a conventional planar control reference device at 32 V and 5 mW illumination power. We relate the bias dependence of the THz signal to the breakdown voltage observed in the current-voltage characteristic. Reducing the antenna gap size through reducing the thickness of the low temperature-GaAs region below 1 μm shows a large improvement in the bandwidth of the device, with an enhancement of the normalized intensity between 0.2 to 2 THz for a bow-tie antenna geometry.
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81.05.Ea III-V semiconductors
85.60.-q Optoelectronic devices
85.40.Ry Impurity doping, diffusion and ion implantation technology
84.40.Ba Antennas: theory, components and accessories

Deterministic tuning of slow-light in photonic-crystal waveguides through the C and L bands by atomic layer deposition

Charlton J. Chen, Chad A. Husko, Inanc Meric, Ken L. Shepard, Chee Wei Wong, William M. J. Green, Yurii A. Vlasov, and Solomon Assefa

Appl. Phys. Lett. 96, 081107 (2010); http://dx.doi.org/10.1063/1.3308492 (3 pages) | Cited 5 times

Online Publication Date: 23 February 2010

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We demonstrate digital tuning of the slow-light regime in silicon photonic-crystal waveguides by performing atomic layer deposition of hafnium oxide. The high group-index regime was deterministically controlled (redshift of 140±10 pm per atomic layer) without affecting the group-velocity dispersion and third-order dispersion. Additionally, differential tuning of 110±30 pm per monolayer of the slow-light TE-like and TM-like modes was observed. This passive postfabrication process has potential applications including the tuning of chip-scale optical interconnects, as well as Raman and parametric amplification.
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42.70.Qs Photonic bandgap materials
42.82.Cr Fabrication techniques; lithography, pattern transfer

Theoretical analysis of power swapping in quadratic nonlinear medium

Jian Wang and Qizhen Sun

Appl. Phys. Lett. 96, 081108 (2010); http://dx.doi.org/10.1063/1.3299019 (3 pages) | Cited 1 time

Online Publication Date: 24 February 2010

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Nonlinear optical effects in quadratic nonlinear mediums for the traditional frequency conversion generally involve two or three incident optical waves and transfer the power of the input signal wavelength to another new generated wavelength. The authors find that bidirectional power swapping can be carried out by proper waveguide length and power arrangement when two signals and two continuous-wave pumps simultaneously take part in the quadratic nonlinear interactions. Theoretical analyses referring to four incident optical waves are presented, taking into account the cascaded sum- and difference-frequency generation in a periodically poled lithium niobate waveguide. We apply such theory to ultrafast data information swapping and derive analytical solutions clearly indicating the swapping condition. An example of bidirectional ultrafast data information swapping is demonstrated.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.79.Gn Optical waveguides and couplers

Enhancement of light extraction in ultraviolet light-emitting diodes using nanopixel contact design with Al reflector

N. Lobo, H. Rodriguez, A. Knauer, M. Hoppe, S. Einfeldt, P. Vogt, M. Weyers, and M. Kneissl

Appl. Phys. Lett. 96, 081109 (2010); http://dx.doi.org/10.1063/1.3334721 (3 pages) | Cited 5 times

Online Publication Date: 25 February 2010

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We report on a nanopixel contact design for nitride-based ultraviolet light-emitting diodes to enhance light extraction. The structure consists of arrays of Pd ohmic contact pixels and an overlying Al reflector layer. Based on this design a twofold increase in the light output, compared to large area Pd square contacts is demonstrated. Theoretical calculations and experiments reveal that a nanopixel spacing of 1 μm or less is required to enable current overlap in the region between the nanopixels due to current spreading in the p-GaN layer and to ensure current injection into the entire active region. Light emitted in the region between the nanopixels will be reflected by the Al layer enhancing the light output. The dependence of the light extraction on the nanopixel size and spacing is investigated.
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85.60.Jb Light-emitting devices

Electro-optically switched compact coupled photonic crystal waveguide directional coupler

Mathew J. Zablocki, Ahmed Sharkawy, Ozgenc Ebil, Shouyuan Shi, and Dennis Prather

Appl. Phys. Lett. 96, 081110 (2010); http://dx.doi.org/10.1063/1.3330927 (3 pages) | Cited 4 times

Online Publication Date: 25 February 2010

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In this paper, we present a compact photonic crystal directional coupler in a silicon on insulator platform electro-optically switched at 150 kHz with a switching time of 620 ns under a low voltage operation of 2.9 V. The switch design utilizes a coupled photonic crystal structure designed to operate in the slow light regime. Switching is attained by modulating the coupling coefficient of the coupled photonic crystal waveguide system by using a p-i-n diode to modulate the carrier concentration with a density of ∼ 104 A/cm2 across the plane of the photonic crystal.
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42.82.Et Waveguides, couplers, and arrays
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
42.65.Pc Optical bistability, multistability, and switching, including local field effects

Tunable-focus microlens arrays on curved surfaces

Difeng Zhu, Chenhui Li, Xuefeng Zeng, and Hongrui Jiang

Appl. Phys. Lett. 96, 081111 (2010); http://dx.doi.org/10.1063/1.3330965 (3 pages) | Cited 12 times

Online Publication Date: 25 February 2010

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We present a microlens array consisting of multiple liquid-based tunable-focus microlenses omnidirectionally fabricated on a hemisphere, resulting in large field of view. Polymer bridge structure is formed between microlenses to reduce the stress and deformation in each lens structure. Each microlens in the array is formed via a water-oil interface at its lens aperture. Photopatterned thermo-responsive hydrogel actuators are used to regulate the curvature of the water-oil interface, thus tuning the focal length, ranging from millimeters to infinity.
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42.79.Bh Lenses, prisms and mirrors
42.82.Cr Fabrication techniques; lithography, pattern transfer
42.15.Eq Optical system design

Coherent backscattering in optical microring resonators

F. Morichetti, A. Canciamilla, M. Martinelli, A. Samarelli, R. M. De La Rue, M. Sorel, and A. Melloni

Appl. Phys. Lett. 96, 081112 (2010); http://dx.doi.org/10.1063/1.3330894 (3 pages) | Cited 15 times

Online Publication Date: 26 February 2010

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The effects of backscattering induced by waveguide sidewall roughness in integrated ring resonators (RRs) are experimentally observed. We demonstrate that coherent backscattering, originated by multiple round trips in the RR, increases with the square of the effective group index of the cavity and can dramatically affect the behavior of integrated RRs even at moderate quality factors of 104. From our results backscattering emerges as one of the most severe limiting factors on the performance of RRs fabricated with state-of-the-art silicon-on-insulator nanowaveguides.
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42.82.Et Waveguides, couplers, and arrays
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