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14 Feb 2013

Volume 113, Issue 6, Articles (06xxxx)

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J. Appl. Phys. 113, 064301 (2013); http://dx.doi.org/10.1063/1.4789897 (11 pages)

Y. G. Marinov, G. B. Hadjichristov, A. G. Petrov, S. Marino, C. Versace, and N. Scaramuzza
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back to top Lasers, Optics, and Optoelectronics

Effect of growth temperature and post-growth annealing on luminescence properties of molecular beam epitaxy grown single layer Ge quantum dots

S. Das, S. Manna, R. K. Singha, R. Aluguri, and S. K. Ray

J. Appl. Phys. 113, 063101 (2013); http://dx.doi.org/10.1063/1.4790593 (8 pages)

Online Publication Date: 8 February 2013

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We report the detailed structural and luminescence properties of Si-embedded single layer Ge nanoislands grown via Stranski-Krastanov mechanism using molecular beam epitaxy. The luminescence energy peak positions correlated with structural properties have been studied as a function of the growth temperature and post-growth annealing. The photoluminescence peak for the annealed sample (500 °C grown) is blue shifted with increasing anneal temperature compared to the as grown sample due to intermixing of Si/Ge. Electroluminescence from the single layer Ge islands in the wavelength range 1.53 to 1.67 μm has been demonstrated using a metal-insulator-semiconductor structure, making it attractive for quantum dot light emitting devices.
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61.72.Cc Kinetics of defect formation and annealing
78.55.Ap Elemental semiconductors
78.66.Db Elemental semiconductors and insulators
78.67.Hc Quantum dots
81.07.Ta Quantum dots
68.55.ag Semiconductors

Intensity dependent spectral features in high harmonic generation

Khuong Ba Dinh, Peter Hannaford, and Lap Van Dao

J. Appl. Phys. 113, 063102 (2013); http://dx.doi.org/10.1063/1.4790708 (6 pages)

Online Publication Date: 8 February 2013

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We study the influence of the driving laser intensity on the spectral features of high harmonic generation in a semi-infinite gas cell. The effects of the harmonic dipole phase and the dispersion phase mismatches induced by the ionized medium during the harmonic generation process are revealed and the interplay between the macroscopic response and the single-atom response is discussed. We consider the conditions for generation of a narrow bandwidth, bright, and highly coherent high harmonic source.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.72.-g Optical sources and standards
42.60.Fc Modulation, tuning, and mode locking
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation

Polarization-independent rapidly responding phase grating based on hybrid blue phase liquid crystal

Yi-Ting Lin, Hung-Chang Jau, and Tsung-Hsien Lin

J. Appl. Phys. 113, 063103 (2013); http://dx.doi.org/10.1063/1.4790375 (4 pages)

Online Publication Date: 11 February 2013

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This work demonstrates a polymer-stabilized blue phase (PSBP) liquid crystal phase grating, which is made of hybrid PSBPs with two different Kerr constants. The Kerr constant of a PSBP is related to the morphology of the polymer network which can be controlled by the phase separation temperature. Owing to the non-patterned electrode and the optical isotropy of the PSBP, the diffraction effect can be completely switched off when the voltage is absent. The diffraction intensity increases when a uniform applied electrical field induces the phase difference in the hybrid PSBP. The phase grating is completely independent of the polarization of the incident light. Furthermore, the response time to switching is in the sub-millisecond range.
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42.79.Dj Gratings
78.20.Jq Electro-optical effects
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
42.70.Df Liquid crystals

Rate equation analysis of injection-locked quantum cascade lasers

Cheng Wang, Frédéric Grillot, Vassilios Kovanis, and Jacky Even

J. Appl. Phys. 113, 063104 (2013); http://dx.doi.org/10.1063/1.4790883 (6 pages)

Online Publication Date: 12 February 2013

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The modulation properties of optical injection-locked quantum cascade lasers (QCLs) are investigated theoretically via a simple low dimensional rate equation model. It is found that both strong injection level and positive optical frequency detuning increase the modulation bandwidth, while a large linewidth enhancement factor (LEF) contributes to the enhancement of the peak magnitude in the intensity modulation (IM) response. As opposed to conventional injection-locked interband lasers, it is demonstrated that no dip occurs in the QCL's IM response, which is beneficial for a series of broadband microwave photonic applications. Computations also show that the value of the LEF can critically modify both the locking and stability regions on the optical frequency detuning injection level map.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.60.Fc Modulation, tuning, and mode locking
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Effects of band non-parabolicity on cavity modes in photonic crystals

N.-Y. Lue, Y.-S. Chen, H.-S. Wei, and G. Y. Wu

J. Appl. Phys. 113, 063105 (2013); http://dx.doi.org/10.1063/1.4789945 (6 pages)

Online Publication Date: 13 February 2013

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We include the effect of band non-parabolicity on photonic defect states within the Wannier theory, which improves the quadratic approximation adopted by Painter et al. [Phys. Rev. B 68, 035214 (2003)] for large-size defects, as well as extends the theory to a wider range of defect size. A 2D hexagonal photonic crystal is considered, and analyzed for the origin and degree of non-parabolicity in the 1st TE band around J-point, and for the effect of non-parabolicity on acceptor type cavity modes. The non-parabolicity is shown to derive primarily from the inherent anisotropy of band dispersion around the point. Overall, with the inclusion of non-parabolicity, (i) mode degeneracy is lowered, (ii) the “binding energy” of a cavity mode is increased, and (iii) the cut-off of defect size for a given cavity mode is reduced, by as much as 35% in certain cases, in comparison with that calculated without the non-parabolicity. A simple “overall effective mass” picture is provided for the understanding of non-parabolicity effects.
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42.70.Qs Photonic bandgap materials

Numerical simulation of stimulated emission and lasing in dye doped cholesteric liquid crystal films

L. Penninck, J. Beeckman, P. De Visschere, and K. Neyts

J. Appl. Phys. 113, 063106 (2013); http://dx.doi.org/10.1063/1.4790873 (8 pages)

Online Publication Date: 13 February 2013

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Dye-doped chiral-nematic liquid crystal lasers have great potential as small size, low-cost, widely tunable lasers. We present a numerical model for stimulated emission and lasing in liquid crystal films based on thin film optics. The gain threshold is modelled and the results are confirmed experimentally. The effect of the orientation of dye molecules and the matching of the photonic bandgap to the dye spectrum on the threshold for lasing is discussed.
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42.55.-f Lasers
42.70.Df Liquid crystals
42.70.Hj Laser materials
02.60.-x Numerical approximation and analysis

Enhancement of momentum coupling coefficient by cavity with toroidal bubble for underwater laser propulsion

Jun Chen, Bei-Bei Li, Hong-Chao Zhang, Hao Qiang, Zhong-Hua Shen, and Xiao-Wu Ni

J. Appl. Phys. 113, 063107 (2013); http://dx.doi.org/10.1063/1.4792041 (5 pages)

Online Publication Date: 14 February 2013

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High-speed photography method is employed to study underwater laser propulsion using targets with and without cavity as well as the effect of the cavity depth. The shapes and motions of bubbles generated from the target tail by Nd: YAG laser are recorded by high-speed camera. Then, the influence of different bubble shapes on the laser propulsion is analyzed. Besides, the velocity and momentum coupling coefficient of the targets are investigated. The results show that the bubble is hemispherical in the case of target without cavity, while it is toroidal in the case of target with cavity. Experiments verify that compared with hemispherical bubble, the toroidal bubble is more conducive to laser propulsion in water, which means the target with cavity obtains more momentum than the target without cavity. In addition, the momentum coupling coefficient increases with laser energy first, and then it levels out and decreases a bit for the target with cavity. There is little effect of the cavity depth on propulsion.
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47.55.dd Bubble dynamics
47.80.Jk Flow visualization and imaging
47.60.-i Flow phenomena in quasi-one-dimensional systems
42.62.Eh Metrological applications; optical frequency synthesizers for precision spectroscopy

Study on phosphor sedimentation effect in white light-emitting diode packages by modeling multi-layer phosphors with the modified Kubelka-Munk theory

Run Hu, Yiman Wang, Yong Zou, Xing Chen, Sheng Liu, and Xiaobing Luo

J. Appl. Phys. 113, 063108 (2013); http://dx.doi.org/10.1063/1.4792051 (6 pages)

Online Publication Date: 14 February 2013

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In this study, we studied the phosphor sedimentation effect in white phosphor-converted light-emitting diode packages by modeling the multi-layer phosphors with gradient concentrations. The essence of phosphor sedimentation can attribute to the variation of phosphor concentrations. By modifying the Kubelka-Munk theory, we built a multi-layer phosphor model with considering the light scattering, light absorption, and light conversion process simultaneously. With a brief review of Kubelka-Munk theory, multi-layer phosphors were modeled on the basis of single-layer phosphor model. The phosphor sedimentation effect was characterized by modeling multi-layer phosphors with gradient concentrations, whereas keeping the total amount of phosphors at the same level. It is found from the five calculation cases that phosphor sedimentation will cause the drop of light extraction efficiency (LEE) by 13.04%. Furthermore, the phosphor layer with inverse-gradient concentrations will enhance the LEE 16.56%. To figure out the reasons, the light losses were calculated, and it is proved that the light loss is enhanced when phosphor sedimentation happens.
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85.60.Jb Light-emitting devices

Polymeric photovoltaics with various metallic plasmonic nanostructures

Beibei Zeng, Qiaoqiang Gan, Zakya H. Kafafi, and Filbert J. Bartoli

J. Appl. Phys. 113, 063109 (2013); http://dx.doi.org/10.1063/1.4790504 (10 pages)

Online Publication Date: 14 February 2013

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Broadband light absorption enhancement is numerically investigated for the active light harvesting layer of an organic photovoltaic (OPV), which consists of a blend of poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). Periodic plasmonic nanostructures placed above and below the active layer incorporate Ag, Al, Au, or a combination of two different metals. Three dimensional (3D) full-field electromagnetic simulations are applied to determine the effect of varying the metal employed in the plasmonic nanostructures on the absorption enhancement of the OPV. In addition, the geometric parameters (e.g., film thickness, period, and diameter) of the symmetrically distributed top and bottom metal (Ag, Al, or Au) nanostructures were varied to optimize the device structure and delineate the mechanism(s) leading to the absorption enhancement. A spectrally broadband, polarization-insensitive, and wide-angle absorption enhancement is obtained using a double plasmonic nanostructure and is attributed to the combined excitation of localized and single-interface surface plasmon polariton modes. The total photon absorption of the OPV with the optimized double plasmonic Ag nanostructures was found to be enhanced by as much as 82.8% and 80.4% under normal (0°) and 60° light incidence, respectively.
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88.40.jr Organic photovoltaics
85.65.+h Molecular electronic devices
88.40.hj Efficiency and performance of solar cells
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