Volume 103, Issue 13, 23 September 2013
- photonics and optoelectronics
- surfaces and interfaces
- structural, mechanical, optical, and thermodynamic properties of advanced materials
- magnetics and spintronics
- superconductivity and superconducting electronics
- dielectrics, ferroelectrics, and multiferroics
- nanoscale science and technology
- organic electronics and photonics
- device physics
- biophysics and bio-inspired systems
- energy conversion and storage
- interdisciplinary and general physics
Index of content:
We investigate beam shaping in broad area semiconductor amplifiers induced by a 2-dimensional (longitudinal and lateral) periodic modulation of the pump on a scale of several microns. The study is performed by solving numerically a (2 + 1)-dimensional model for the semiconductor amplifier. We show that, under realistic conditions, the anisotropic gain induced by the pump periodicity can show narrow angular profile of enhanced gain of less than 1°, providing an intrinsic filtering mechanism and eventually improving the spatial beam quality.
- PHOTONICS AND OPTOELECTRONICS
Highly luminescent, high-indium-content InGaN film with uniform composition and full misfit-strain relaxation103(2013); http://dx.doi.org/10.1063/1.4822122View Description Hide Description
We have studied the properties of thick In x Ga1− x N films, with indium content ranging from x ∼ 0.22 to 0.67, grown by metal-modulated epitaxy. While the low indium-content films exhibit high density of stacking faults and dislocations, a significant improvement in the crystalline quality and optical properties has been observed starting at x ∼ 0.6. Surprisingly, the In x Ga1− x N film with x ∼ 0.67 exhibits high luminescence intensity, low defect density, and uniform full lattice-mismatch strain relaxation. The efficient strain relaxation is shown to be due to a critical thickness close to the monolayer range. These films were grown at low temperatures (∼400 °C) to facilitate indium incorporation and with precursor modulation to enhance surface morphology and metal adlayer diffusion. These findings should contribute to the development of growth techniques for nitride semiconductors under high lattice misfit conditions.
103(2013); http://dx.doi.org/10.1063/1.4822093View Description Hide Description
Dielectric optical thin films, as opposed to metallic, have been very sparsely explored as good candidates for absorption-based optical field enhancement. In such materials, the low imaginary part of the refractive index implies that absorption processes are usually not predominant. This leads to dielectric-based optical resonances mainly via waveguiding modes. We show here that when properly designed, a multi-layered dielectric thin films stack can give rise to optical resonances linked to total absorption. We report here, on such dielectric stack designed to possess a theoretical optical field enhancement above 1000. Using photon scanning tunneling microscopy, we experimentally evaluate the resulting field enhancement of the stack as well as the associated penetration depth. We thus demonstrate the capability of multi-dielectric stacks in generating giant optical field with tunable penetration depth (down to few dozens of nm).
103(2013); http://dx.doi.org/10.1063/1.4822111View Description Hide Description
A ceramic electride is demonstrated to provide surface enhanced Raman scattering. The electride, an ionic crystal where the electrons serve as anions, is a conductive ceramic derived from mayenite. The textured electride surface was found to strongly enhance the Raman scattering of an organic analyte at 532 nm and 785 nm excitation wavelengths. This provides a sensitive method for monitoring the chemistry and electronic environment at the electride surface. The results are evidence of a surface electride-polariton resonance mechanism that is analogous to the surface plasmon-polariton resonance that mediates conventional surface enhance Raman scattering.
103(2013); http://dx.doi.org/10.1063/1.4822265View Description Hide Description
ZnO/Zn0.85 Mg 0.15O asymmetric double quantum well (ADQW) and multiple quantum well (MQW) were fabricated with plasma assisted molecular epitaxy on c-plane sapphire, with their optical properties and optical pumped lasing characteristics studied. Due to the good crystalline quality, the lasing threshold of the MQW is ∼20 kW cm−2. The widths of the narrow well (NW) and the wide well (WW) of the ADQW were chosen to fascinate rapid LO phonon assisted carrier tunneling from NW to WW, so as to enhance the exciton density at the WW. Very low lasing threshold of 6 kW cm−2 has been achieved.
103(2013); http://dx.doi.org/10.1063/1.4821108View Description Hide Description
We have developed a method of dispersive x-ray absorption spectroscopy with a hard x-ray free electron laser (XFEL), generated by a self-amplified spontaneous emission (SASE) mechanism. A transmission grating was utilized for splitting SASE-XFEL light, which has a relatively large bandwidth (ΔE/E ∼ 5 × 10−3), into several branches. Two primary split beams were introduced into a dispersive spectrometer for measuring signal and reference spectra simultaneously. After normalization, we obtained a Zn K-edge absorption spectrum with a photon-energy range of 210 eV, which is in excellent agreement with that measured by a conventional wavelength-scanning method. From the analysis of the difference spectra, the noise ratio was evaluated to be ∼3 × 10−3, which is sufficiently small to trace minute changes in transient spectra induced by an ultrafast optical laser. This scheme enables us to perform single-shot, high-accuracy x-ray absorption spectroscopy with femtosecond time resolution.
103(2013); http://dx.doi.org/10.1063/1.4823532View Description Hide Description
We present spectral and angular transmission filters based on the guided-mode resonance (GMR) effect cooperating with the Rayleigh anomaly in a subwavelength nanograting. We theoretically and experimentally show that the onset of higher diffraction orders at the Rayleigh anomaly dramatically sharpens a GMR transmission peak in both spectral and angular domains. This unique transmission spectrum is tightly delimited in angle and wavelength as experimentally demonstrated with a precisely fabricated device.
103(2013); http://dx.doi.org/10.1063/1.4823546View Description Hide Description
We demonstrate an ultra-subwavelength surface plasmonic polaritons waveguide, which can confine light in the nano-scale region with comparable low propagation loss. The mode can be squeezed to one thousandth of the diffraction spot size with micro-meter scale propagation distance and is highly sensitive to the buffer layer materials and geometric parameters. This design improves the performance of previous surface plasmonic polaritons waveguides and lends itself to complementary metal–oxide–semiconductor compatible fabrication. These waveguides can be used as a platform for active devices as well as for nano-sensing applications.
103(2013); http://dx.doi.org/10.1063/1.4822105View Description Hide Description
We studied solar cells with periodic interface texture. For periods of 550 and 833 nm, we found that the measured photocurrent increases under oblique incidence and assumes a maximum between 20° and 30°. For the geometries used in our experiments, the initial increase occurred for conditions close to those of photonic band gaps. In addition to a forbidden band of energies where waveguide modes cannot propagate, we found that the coupling to the high-energy branches was suppressed under conditions of high symmetry such as sinusoidal interface geometry and perpendicular incidence.
103(2013); http://dx.doi.org/10.1063/1.4822433View Description Hide Description
Optical pulse durations of an InAs two-section passively mode-locked quantum dot laser with a proton bombarded absorber section reduce from 8.4 ps at 250 K to 290 fs at 20 K, a factor of 29, with a corresponding increase in optical bandwidth. Rate equation analysis of gain and emission spectra using rate equations suggests this is due to the very low emission rate of carriers to the wetting layer in the low temperature, random population regime which enables dots across the whole inhomogeneous distribution to act as independent oscillators.
103(2013); http://dx.doi.org/10.1063/1.4823537View Description Hide Description
We report on the enhancement of inverse polarization transmission of deep ultraviolet light, with the TE transmittance ηTE largely exceeding the TM transmittance ηTM, through a hybrid Al-SiO2 grating: The grating slits are conformally filled with SiO2. Strong coupling of the incident wave to surface plasmon polaritons greatly reduces the TM transmittance, whereas the coupling to the low-loss TE mode, with its effective refractive index approaching that of the substrate, leads to the enhanced TE transmittance. At resonance, a pronounced inverse polarization extinction ratio of 35 dB was obtained in our experiment, much higher than any previously reported values.
103(2013); http://dx.doi.org/10.1063/1.4822423View Description Hide Description
We report the experimental observation of quantum holographic imaging of one-dimensional object with entangled photon pairs, generated in a spontaneous parametric down-conversion process. The signal photons play both roles of “object wave” and “reference wave” in holography but are recorded by a point detector providing only encoding information, while the idler photons travel freely and are locally manipulated with spatial resolution. The holographic image is formed by the two-photon correlation measurement, although both the signal and idler beams are incoherent. Three types of quantum holography schemes are analyzed according to the detection regime of the signal photons.
Morphology-induced redistribution of surface plasmon modes in two-dimensional crystalline gold platelets103(2013); http://dx.doi.org/10.1063/1.4823533View Description Hide Description
The 2D optical field intensity distribution in sub-micron, ultrathin, and crystalline gold platelets is investigated by two-photon luminescence (TPL) microscopy. In particular, the evolution of the TPL maps as the particle morphology undergoes a transition from triangular to hexagonal reveals that the signatures of the high-order surface plasmon states sustained by the platelets follows the same C3 v to C6 v symmetry redistribution. Experimental observations are precisely accounted for by theoretical simulations based on the Green dyadic method.
103(2013); http://dx.doi.org/10.1063/1.4823538View Description Hide Description
We investigate the connection between photonic local density of states and luminescent solar concentrator (LSC) performance in two manufacturable nanocavity LSC structures: a bilayer slab and a slab photonic crystal. Finite-difference time-domain electromagnetic simulations show that the waveguided luminescence photon flux can be enhanced up to 30% for the photonic crystal design over a conventional LSC operating in the ray optic limit, assuming the same number of excited lumophores. Further photonic engineering could realize an increase of up to one order of magnitude in the flux of waveguided luminescence.
Sensitive trace gas detection with cavity enhanced absorption spectroscopy using a continuous wave external-cavity quantum cascade laser103(2013); http://dx.doi.org/10.1063/1.4823545View Description Hide Description
Trace gas sensing in the mid-infrared using quantum cascade lasers (QCLs) promises high specificity and sensitivity. We report on the performance of a simple cavity enhanced absorption spectroscopy (CEAS) sensor using a continuous wave external-cavity QCL at 7.4 μm. A noise-equivalent absorption coefficient of 2.6 × 10–8 cm–1 in 625 s was achieved, which corresponds to a detection limit of 6 ± 1 ppb of CH4 in 15 millibars air for the R(3) transition at 1327.074 cm–1. This is the highest value of noise-equivalent absorption and among the longest effective path length (1780 m) reported to date with QCL-based CEAS.
103(2013); http://dx.doi.org/10.1063/1.4823554View Description Hide Description
Randomly textured surfaces are nowadays routinely integrated into solar cells. Nonetheless, their performance is still not optimal. This became obvious while comparing their performance to optimized surfaces. Thus far, however, these optimized surfaces suffer from being either impossible to implement or only with expensive top-down nanofabrication technologies not suitable for large scale wafers. Here, we suggest a different approach to achieve optimized randomly textured surfaces. It exploits a self-assembled monolayer of spheres with a carefully balanced size distribution to define the random texture. Existing solar cells are outperformed with such realistic textures by up to 26%.
103(2013); http://dx.doi.org/10.1063/1.4823589View Description Hide Description
Scanning near-field optical spectroscopy was applied to semipolar InGaN/GaN quantum wells (QWs) to evaluate spatial homogeneity of QW band gap and its dependence on the growth conditions. In the most uniform QW, photoluminescence (PL) spectra were found to be narrow with small peak wavelength and spectral width variations. A QW grown at reduced temperature showed sub-micrometer size PL features aligned along the a axis and caused by nonuniform In incorporation at surface undulations. At extended defects, complex and strongly varying near-field spectra were observed and tentatively assigned to QW segments of different orientations around these defects.
- SURFACES AND INTERFACES
103(2013); http://dx.doi.org/10.1063/1.4821953View Description Hide Description
We present a straightforward and reproducible method to grow stoichiometric and single phase (100) textured EuO thin films on epitaxial graphene. Depending on coverage, either separated EuO grains or fully closed layers can be prepared. Room temperature preparation followed by annealing in Eu vapor leads to a random distribution of the in-plane orientation, whereas growth under distillation conditions at 720 K induces a fixed orientation with respect to the substrate. Magneto-optical Kerr effect (MOKE) shows that the films are ferromagnetic with an enhanced Curie temperature.
103(2013); http://dx.doi.org/10.1063/1.4822266View Description Hide Description
Nano-Crystalline Diamond (NCD) thin films are a topic of recent interest due to their excellent mechanical and electrical properties. The inclusion of nitrogen is a specific interest as its presence within NCD modifies its conductive properties. The methodology adopted for the characterization of nitrogen incorporated NCD films grown on a chromium underlayer determined a correlation between the chromium and nitrogen concentrations as well as a variation in the concentration profile of elements. Additionally, the concentration of nitrogen was found to be more than three times greater for these films versus those grown on a silicon substrate.
103(2013); http://dx.doi.org/10.1063/1.4822270View Description Hide Description
The monoclinic and tetragonal phases of bismuth vanadate (BiVO4) have been found to exhibit significantly different photocatalytic activities for water splitting. To assess a possible surface effect on the phase-dependent behavior, we calculate and compare the geometries and electronic structures of the monoclinic and tetragonal BiVO4 (001) surfaces using hybrid density functional theory. The relaxed atomic configurations of these two surfaces are found to be nearly identical, while an excess hole shows a relatively stronger tendency to localize at the surface than the bulk in both phases. Possible factors for the phase-dependent photocatalytic activity of BiVO4 are discussed.
103(2013); http://dx.doi.org/10.1063/1.4822302View Description Hide Description
An approach is presented to tune the amplitude of ripple patterns using ion beam. By varying the depth location of amorphous/crystalline interface, ripple patterns of different amplitude with similar wavelength were grown on the surface of Si (100) using 50 keV Ar+ beam irradiation. Atomic force microscopy study demonstrates the tuning of amplitude of ripples patterns for wide range. Rutherford backscattering channeling measurement was performed to measure the depth location of amorphous/crystalline interface. It is postulated that the ion beam stimulated solid flow inside the amorphous layer controls the wavelength, whereas mass rearrangement at amorphous/crystalline interface controls the amplitude.