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7 May 2012

Volume 100, Issue 19, Articles (19xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 100, 191901 (2012); http://dx.doi.org/10.1063/1.4709436 (4 pages)

Muamer Kadic, Tiemo Bückmann, Nicolas Stenger, Michael Thiel, and Martin Wegener
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Growth rate and start current in Smith–Purcell free-electron lasers

D. Li (李大治), M. Hangyo, Y. Tsunawaki, Z. Yang, Y. Wei, S. Miyamoto, M. R. Asakawa, and K. Imasaki

Appl. Phys. Lett. 100, 191101 (2012); http://dx.doi.org/10.1063/1.4711803 (4 pages)

Online Publication Date: 7 May 2012

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This letter reports a theory to calculate the growth rate and start current of a Smith–Purcell free-electron laser, which is a promising radiation source in the terahertz domain. A two-dimensional model was used to investigate the interaction between a sheet electron beam and the surface wave above a lamellar grating. After deriving the growth rate from the dispersion equation, the start current was carefully estimated by considering the power flow above the grating. The agreement between the predictions of our theory and the results from the particle-in-cell simulations is acceptable.
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41.60.Cr Free-electron lasers
42.79.Dj Gratings

Analysis of intermolecular interaction of poly(N-isopropylacrylamide) solution with attenuated total reflectance terahertz spectroscopy

H. Naito, Y. Ogawa, H. Hoshina, S. Sultana, and N. Kondo

Appl. Phys. Lett. 100, 191102 (2012); http://dx.doi.org/10.1063/1.4711804 (3 pages)

Online Publication Date: 7 May 2012

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The terahertz attenuated total reflectance spectra of Poly(N-isopropylacrylamide) (P-NIPAAm) solutions were measured in the range of 26.5–47.3 °C. The phase-change in P-NIPAAm was observed via the temperature dependence of the spectra. The collapse of hydrogen bonds in solution was confirmed by the decrease in the signal intensities around 62 cm−1, which corresponds to fast dielectric relaxation by rotation and collision of molecules, and around 166 cm−1, which corresponds to the stretching mode of the intermolecular vibration of water molecules.
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78.70.Gq Microwave and radio-frequency interactions
64.70.Ja Liquid-liquid transitions
77.22.Gm Dielectric loss and relaxation
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Grating duty-cycle induced enhancement of substrate emission from ring cavity quantum cascade lasers

Clemens Schwarzer, Elvis Mujagić, Sang Il Ahn, Aaron Maxwell Andrews, Werner Schrenk, William Charles, Claire Gmachl, and Gottfried Strasser

Appl. Phys. Lett. 100, 191103 (2012); http://dx.doi.org/10.1063/1.4712127 (3 pages)

Online Publication Date: 8 May 2012

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We report on experimental and theoretical investigation of grating parameters to promote one specific emission direction from ring-cavity quantum cascade lasers. The devices show single-mode behavior at room temperature and pulsed operation. A maximized surface output power was found for grating duty cycles around 70%, while for substrate emission, the optimized grating duty cycle is ≈50%. All tested devices emit more power through the substrate, when accounting for absorption and the reflection at the substrate air interface.
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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
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.79.Dj Gratings

High quality factor AlN nanocavities embedded in a photonic crystal waveguide

D. Sam-Giao, D. Néel, S. Sergent, B. Gayral, M. J. Rashid, F. Semond, J. Y. Duboz, M. Mexis, T. Guillet, C. Brimont, S. David, X. Checoury, and P. Boucaud

Appl. Phys. Lett. 100, 191104 (2012); http://dx.doi.org/10.1063/1.4712590 (3 pages) | Cited 2 times

Online Publication Date: 9 May 2012

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We present a spectroscopic study of nanocavities obtained by small modifications of a W1 waveguide in an AlN photonic crystal membrane. The AlN film containing GaN quantum dots is grown on silicon. The photonic crystal structure is defined by e-beam lithography and etched by inductively coupled plasma reactive ion etching, while the membrane is released by selective etching of the silicon substrate. The room temperature photoluminescence of the embedded quantum dots reveals the existence of even-symmetry and odd-symmetry confined cavity modes and guided modes. Cavity mode quality factors up to 4400 at 395 nm and 2300 at 358 nm are obtained.
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42.79.Gn Optical waveguides and couplers
42.82.-m Integrated optics
42.70.Qs Photonic bandgap materials

Distributed hydrogen sensing using in-fiber Rayleigh scattering

Tong Chen, Qingqing Wang, Rongzhang Chen, Botao Zhang, Kevin P. Chen, Mokhar Maklad, and Philip R. Swinehart

Appl. Phys. Lett. 100, 191105 (2012); http://dx.doi.org/10.1063/1.4712592 (3 pages) | Cited 3 times

Online Publication Date: 9 May 2012

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This letter reports a fully distributed hydrogen sensing technique using Rayleigh backscattering in palladium (Pd) and copper (Cu) coated optical fiber. The local in-fiber strain changes due to Pd hydrogen absorptions are interrogated spatially resolved optical frequency domain reflectrometry measurements of the Rayleigh signals. Electrical power is used to induce heating in the Pd coating, which accelerates both the hydrogen response and the sensor recycling. This technique promises an inexpensive and truly distributed fiber solution for continuous hydrogen leak detection with centimeter spatial resolution at room and low temperatures.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
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Enhancement of x-rays generated by a guided laser wakefield accelerator inside capillary tubes

J. Ju, K. Svensson, A. Döpp, H. E. Ferrari, K. Cassou, O. Neveu, G. Genoud, F. Wojda, M. Burza, A. Persson, O. Lundh, C.-G. Wahlström, and B. Cros

Appl. Phys. Lett. 100, 191106 (2012); http://dx.doi.org/10.1063/1.4712594 (3 pages)

Online Publication Date: 9 May 2012

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Electrons accelerated in the nonlinear regime in a laser wakefield accelerator experience transverse oscillations inside the plasma cavity, giving rise to ultra-short pulsed x-rays, also called the betatron radiation. We show that the fluence of x-ray can be enhanced by more than one order of magnitude when the laser is guided by a 10 mm long capillary tube instead of interacting with a 2 mm gas jet. X-rays with a synchrotron-like spectrum and associated critical energy ∼ 5 keV, with a peak brightness of ∼ 1×1021 ph/s/mm2/mrad2/0.1%BW, were achieved by employing 16 TW laser pulses.
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52.38.Kd Laser-plasma acceleration of electrons and ions
52.40.Mj Particle beam interactions in plasmas
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.59.Px Hard X-ray sources
29.20.Ej Linear accelerators
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)

Effects of contact space charge on the performance of quantum intersubband photodetectors

A. V. Barve, S. Meesala, S. Sengupta, J. O. Kim, S. Chakrabarti, and S. Krishna

Appl. Phys. Lett. 100, 191107 (2012); http://dx.doi.org/10.1063/1.4712601 (4 pages)

Online Publication Date: 9 May 2012

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Highly non-uniform electric field exists in the active region of quantum intersubband devices, primarily due to the presence of PN junctions forming between heavily doped contact regions and non-intentionally doped barriers. Using a combination of experiments and theoretical simulations, we investigate the effect of this non-uniform internal electric field on the photodetector operation. Three quantum dots-in-a-well (DWELL) photodetectors have been fabricated with top spacer, bottom spacer, and no spacer around the active region, respectively, to demonstrate the effect of the non-uniform field. Drift-diffusion based calculations of the electric field provide further insight into the device operation.
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85.60.Gz Photodetectors (including infrared and CCD detectors)

Wavefront corrected light sheet microscopy in turbid media

H. I. C. Dalgarno, T. Čižmár, T. Vettenburg, J. Nylk, F. J. Gunn-Moore, and K. Dholakia

Appl. Phys. Lett. 100, 191108 (2012); http://dx.doi.org/10.1063/1.4710527 (5 pages)

Online Publication Date: 9 May 2012

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Light sheet microscopy is a powerful method for three-dimensional imaging of large biological specimens. However, its imaging ability is greatly diminished by sample scattering and aberrations. Optical clearing, Bessel light modes, and background rejection have been employed in attempts to circumvent these deleterious effects. We present an in situ wavefront correction that offers a major advance by creating an “optimal” light sheet within a turbid sample. Crucially, we show that no tissue clearing or specialized sample preparation is required, and clear improvements in image quality and depth resolution are demonstrated both in Gaussian and Bessel beam-based light sheet modalities.
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07.60.Pb Conventional optical microscopes

Waveguide artefacts in terahertz near field imaging

M. Misra, S. R. Andrews, and S. A. Maier

Appl. Phys. Lett. 100, 191109 (2012); http://dx.doi.org/10.1063/1.4714532 (3 pages) | Cited 1 time

Online Publication Date: 9 May 2012

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We report experimental and computational studies of the behaviour of a photoconductive THz near field probe when imaging a simple planar metal-dielectric structure. We show that the excitation of waveguide modes in the gap between sample and probe, together with diffraction at the probe, must generally be taken into account when analysing images and that electromagnetic simulations provide a very useful aid to interpretation.
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84.40.-x Radiowave and microwave (including millimeter wave) technology

Enhanced THz emission from c-plane InxGa1−xN due to piezoelectric field-induced electron transport

Nathaniel Woodward, C. Gallinat, L. E. Rodak, G. D. Metcalfe, H. Shen, and M. Wraback

Appl. Phys. Lett. 100, 191110 (2012); http://dx.doi.org/10.1063/1.4707387 (4 pages)

Online Publication Date: 9 May 2012

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Enhanced terahertz emission from coherently strained InxGa1−xN epilayers on GaN is observed, which exceeds or is comparable to bulk InAs emission at pump wavelengths of 400 nm or 800 nm, respectively. The inverted terahertz waveform from the InxGa1−xN/GaN heterostructure indicates that the dominant terahertz generation mechanism is electron acceleration toward the InxGa1−xN surface in an internal electric field primarily associated with piezoelectric polarization charge at the heterointerface, rather than diffusive transport away from the surface typically observed in bulk semiconductors. The persistence of the inverted waveform for 266 nm excitation provides evidence of ultrafast electron relaxation via LO phonon emission.
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78.66.Fd III-V semiconductors
68.55.ag Semiconductors

Sub-monolayer quantum dots in confinement enhanced dots-in-a-well heterostructure

S. Sengupta, J. O. Kim, A. V. Barve, S. Adhikary, Y. D. Sharma, N. Gautam, S. J. Lee, S. K. Noh, S. Chakrabarti, and S. Krishna

Appl. Phys. Lett. 100, 191111 (2012); http://dx.doi.org/10.1063/1.4711214 (4 pages)

Online Publication Date: 9 May 2012

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We have investigated optical properties and device performance of sub-monolayer quantum dots infrared photodetector with confinement enhancing (CE) barrier and compared with conventional Stranski-Krastanov quantum dots with a similar design. This quantum dots-in-a-well structure with CE barrier enables higher quantum confinement and increased absorption efficiency due to stronger overlap of wavefunctions between the ground state and the excited state. Normal incidence photoresponse peak is obtained at 7.5 μm with a detectivity of 1.2 × 1011 cm Hz1/2 W−1 and responsivity of 0.5 A/W (77 K, 0.4 V, f/2 optics). Using photoluminescence and spectral response measurements, the bandstructure of the samples were deduced semi-empirically.
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85.60.Gz Photodetectors (including infrared and CCD detectors)

Losses from long-living photoelectrons in terahertz-generating continuous-wave photomixers

E. A. Michael and M. Mikulics

Appl. Phys. Lett. 100, 191112 (2012); http://dx.doi.org/10.1063/1.4711777 (4 pages)

Online Publication Date: 9 May 2012

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The extraction of continuous-wave terahertz (THz) power from photonic mixers is known to be hampered by input power limitations, low conversion efficiencies, and saturation effects. Using vertically illuminated low-temperature-grown GaAs travelling-wave mixers with a coplanar stripline geometry, a mechanism of illumination-dependent reabsorption of the THz-power generated by the mixer was isolated. We find evidence that it is related to a substantial density of long-living photoelectrons (several nanoseconds). The proposed mechanism is expected to impact the performance of photonic terahertz mixers at high input powers, also of those based on transit-time-dominated semiconductor structures.
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84.40.-x Radiowave and microwave (including millimeter wave) technology

Model for direct-transition gain in a Ge-on-Si laser

Weng W. Chow

Appl. Phys. Lett. 100, 191113 (2012); http://dx.doi.org/10.1063/1.4714540 (3 pages) | Cited 2 times

Online Publication Date: 10 May 2012

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This paper describes a laser gain model for the direct, Γ-point transition in bulk Ge. The model allows calculation of gain and spontaneous-emission spectra for arbitrary strain, as well as doping and injected carrier densities. Many-body effects are included at the level of the screened Hartree-Fock approximation to account for energy renormalization modifications, especially to the impact of n-doping on magnitude and spectral extension of laser gain. Application of the model is illustrated by predicting the net material peak gain versus injected current density under different combinations of tensile strain and n-doping density.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems

Guided-mode-resonance-coupled plasmonic-active SiO2 nanotubes for surface enhanced Raman spectroscopy

Xiaobin Xu, Dihan Hasan, Lei Wang, Swapnajit Chakravarty, Ray T. Chen, D. L. Fan, and Alan X. Wang

Appl. Phys. Lett. 100, 191114 (2012); http://dx.doi.org/10.1063/1.4714710 (5 pages) | Cited 1 time

Online Publication Date: 10 May 2012

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We demonstrate a surface enhanced Raman scattering (SERS) substrate by integrating plasmonic-active SiO2 nanotubes into Si3N4 gratings. First, the dielectric grating that is working under guided mode resonance (GMR) provides enhanced electric field for localized surface plasmon polaritons on the surface of metallic nanoparticles. Second, we use SiO2 nanotubes with densely assembled silver nanoparticles to provide a large amount of “hot spots” without significantly damping the GMR mode of the grating. Experimental measurement on Rhodamine-6G shows a constant enhancement factor of 8 ∼ 10 in addition to the existing SERS effect across the entire surface of the SiO2 nanotubes.
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78.30.Hv Other nonmetallic inorganics
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.68.+m Optical properties of surfaces
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.22.Lp Collective excitations

Terahertz coherent control of surface plasmon polariton propagation in subwavelength metallic hole arrays

Gaofang Li, Zuanming Jin, Xin Xue, Xian Lin, Guohong Ma, Shuhong Hu, and Ning Dai

Appl. Phys. Lett. 100, 191115 (2012); http://dx.doi.org/10.1063/1.4716022 (4 pages)

Online Publication Date: 10 May 2012

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We demonstrate the terahertz coherent control of extraordinary transmission through subwavelength metallic hole arrays with double terahertz pulses. The interference of excitations of surface plasmon polaritons (SPPs) by two THz pulses sequence is employed to control the propagation of SPPs in the periodical structure. As a result, the THz wave transmission is controllable by adjusting the arriving time of the control THz pulse. Our results reveal that THz extraordinary transmission in subwavelength structure arises from the excitation of SPPs of the structures, which provides a universal ultrafast means to control the SPPs motion in various THz subwavelength structures.
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71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.70.Gq Microwave and radio-frequency interactions

Improved emission efficiency of a-plane GaN light emitting diodes with silica nano-spheres integrated into a-plane GaN buffer layer

S. H. Park, J. Park, D.-J. You, K. Joo, D. Moon, J. Jang, D.-U. Kim, H. Chang, S. Moon, Y.-K. Song, G.-D. Lee, H. Jeon, J. Xu, Y. Nanishi, and E. Yoon

Appl. Phys. Lett. 100, 191116 (2012); http://dx.doi.org/10.1063/1.4716472 (4 pages)

Online Publication Date: 11 May 2012

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A simple and inexpensive technique to improve the emission efficiency of nonpolar a-plane light emitting diodes (LEDs) is proposed. The 3-dimensional growth nature of a-plane GaN was utilized to form the regrowth template of a-plane GaN. Subsequently, the controlled integration of silica nano-spheres (CIS) into the regrowth template is performed to improve the crystal quality of a-plane GaN by epitaxial lateral overgrowth method. In addition, the CIS improves light extraction by the scattering process. The light output power from the CIS a-plane GaN LEDs showed 130%–150% increase compared to that of LED without silica nano-spheres.
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85.60.Jb Light-emitting devices
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