Volume 91, Issue 12, 17 September 2007
- lasers, optics, and optoelectronics
- plasmas and electrical discharges
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic transport and semiconductors
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- device physics
- applied biophysics
- interdisciplinary and general physics
Index of content:
The finite difference time domain method is employed to study the crystalline structure’s influence on the propagation of a local excitation along metallic nanowires of subwavelength cross section. The metallic nanowires are elongated cylinders deposited on a transparent substrate. A tightly focused gaussian beam illuminates one end of the nanowires. According to recent experimental studies, the authors show that the propagation length of the localized surface plasmon excitations depends on the crystalline structure of the nanowire. Thus, they are able to determine the effective permittivity of metals in such a nanostructure versus its crystalline properties. The authors also demonstrate that the field of optical information transport could greatly benefit from the care of the subwavelength optical waveguide’s crystallinity.
- LASERS, OPTICS, AND OPTOELECTRONICS
Confinement of enhanced field investigated by tip-sample gap regulation in tapping-mode tip-enhanced Raman microscopy91(2007); http://dx.doi.org/10.1063/1.2785115View Description Hide Description
The authors developed a tip-enhanced near field Raman microscope that can precisely regulate longitudinal distance between a metallic tip and sample molecules. This was done by employing a time-gated photoncounting scheme that enabled us to observe exponentially decaying near field Raman intensity with the tip-sample distance. The exponential decay shows a characteristic of the enhanced field generated by the localization of the surface plasmon polaritons near the tip apex. This microscope was applied to evaluate metal-coated tips and also to investigate confinement of the field generated at a gap between two metal nanostructures from the decay curves.
91(2007); http://dx.doi.org/10.1063/1.2785955View Description Hide Description
The emission spectra of microring lasers fabricated from -conjugated polymer films casted on nylon microfibers with diameters in the range of were studied upon application of uniaxial stress with strain up to . The laser emission spectra substantially change with the applied stress, showing enhanced sensitivity to stress over changes induced in the fiber diameter alone. This is explained as due to the induced change in the polymerrefractive indexspectrum upon stress, causing an unexpected increase in the refractive indexdispersion and, consequently, also in the effectiverefraction index for lasing at emission wavelengths.
91(2007); http://dx.doi.org/10.1063/1.2785988View Description Hide Description
We demonstrate postprocessed and reconfigurable photonic crystal double-heterostructure cavities via selective fluid infiltration. We experimentally investigate the microfluidic cavities via evanescent probing from a tapered fiber at telecommunication wavelengths. Fabry-Pérot fringes associated with modes of the induced cavity are in good agreement with the theory. We also demonstrate a cavity with quality factor . Our defect-writing technique does not require nanometer-scale alterations in lattice geometry and may be undertaken at any time after photonic crystal waveguide fabrication.
91(2007); http://dx.doi.org/10.1063/1.2786587View Description Hide Description
Tunable quantum cascade lasers operating in the terahertz frequency range are demonstrated. By using an external cavity based on the reflection from a movable mirror, both broad and fine tuning of the frequency are achieved by varying the cavity length. Coarse tuning up to is obtained near the center frequency of , and continuous mode-hop-free tuning is observed over , nearly corresponding to the cavity free spectral range.
91(2007); http://dx.doi.org/10.1063/1.2786590View Description Hide Description
The authors numerically and experimentally demonstrated highly directional emission from photonic crystals. This was achieved by first splitting the incident electromagnetic wave into multiple beams using photonic crystal waveguidestructures. The beams were then emitted out of the surface of a photonic crystal with the same phase, which resulted in a highly directional radiation pattern. The measured half power beam width was 4.8°, which was in good agreement with the calculated value of 4.1°. In contrast to the traditional beaming structures, their design did not involve gratinglike structures, which resulted in a wider operation bandwidth.
Self-sustained pulsation in the oxide-confined vertical-cavity surface-emitting lasers based on submonolayer InGaAs quantum dots91(2007); http://dx.doi.org/10.1063/1.2784937View Description Hide Description
The authors report the observation of strong self-pulsations in molecular-beam epitaxy–grown oxide-confined vertical-cavity surface-emitting lasers based on submonolayerInGaAsquantum dots. At continuous-wave operation, self-pulsations with pulse durations of and repetition rates of were measured. The average optical power of the pulsations was at the laser continuous-wave current values of .
91(2007); http://dx.doi.org/10.1063/1.2786863View Description Hide Description
Nanoscale ripple structures with spatial periods of up to are generated by a single beam ultrashort pulsed laser structuring of surface. The period of nanoripples does not depend on the laser fluence and the number of pulses. Optical absorption and photocurrent measurements on SiC samples are investigated before and after laser nanostructuring. Nearly 40% enhancement of optical absorption and a factor of 3 increase of photocurrent are achieved after laser nanostructuring.
The impact of thermal effects on emission characteristics of asymmetrical AlGAO-waveguide microdisks based on quantum dots91(2007); http://dx.doi.org/10.1063/1.2785127View Description Hide Description
Microphotoluminescence studies of AlGaO-based asymmetric waveguide microdisk structures incorporating a single sheet of self-organizedInAsquantum dots(QD) under photoexcitation are performed. The authors have observed a strong redshift and broadening of the QD optical transition energy at high excitation densities due to excitation-density-induced overheating and many-particle effects and monitored the impact of these effects on the whispering gallery modes (WGMs) in spontaneous and stimulated emission regimes. They have observed strongly nonmonotonic dependencies of the linewidth and intensity of the WGMs with excitation density increase and underline possible reasons for the observed behavior.
Enhancement of the light output performance for GaN-based light-emitting diodes by bottom pillar structure91(2007); http://dx.doi.org/10.1063/1.2786015View Description Hide Description
A three-dimensional model with finite difference and time domain was established to investigate the enhancement of the light output intensity of GaNlight-emitting diodes(LEDs) with bottom pillar (BP)structure. Through comparing the normalized light extraction intensity of GaNLEDs with or without BP in different dimensions, the theoretical results show that the light output intensity in the LED with BPstructure involved could be enhanced by about 30%. The influence of BPstructure on the light output intensity of a LED could be explained by the physical model of light interaction. In addition, the experimental results also show the same trend to the theoretical calculations.
91(2007); http://dx.doi.org/10.1063/1.2786838View Description Hide Description
The authors investigate the dependence of the photoluminescence intensity ratio between the near-band-edge and the parasitic blue emission as a function of silane flow in -type . It is found that when the silane flow rate is higher than , the UV-to-blue ratio is a constant. When the silane flow rate is lower than , the UV-to-blue ratio increases rapidly as the silane flow rate decreases. A theoretical model is proposed, which assumes that the parasitic blue emission is caused by an acceptorlike compensating native defect. The model is fully consistent with the experimental results.
91(2007); http://dx.doi.org/10.1063/1.2783270View Description Hide Description
Multiphoton absorption of 2,5-bis[4-(2--diphenylaminostyryl)phenyl]-1,3,4-oxadiazole was experimentally studied by using femtosecond laser pulses. This material demonstrates a very broad multiphoton absorption band of around width with two peaks of 1250 and . The first peak results from the three-photon absorption process while the second is attributed to the vibrational resonance enhanced four-photon absorption process. Combination of these two processes provides a much broader multiphoton absorption band. In this letter, the analytical solution to nonlinear transmission of a three-photon absorption process is also given when the incident beam has a Gaussian transverse spatial profile.
91(2007); http://dx.doi.org/10.1063/1.2786600View Description Hide Description
The authors report on the growth of organometallic dots for optical applications using electron beam induced golddeposition on a transparent substrate. The effects of the substrate material and the deposition parameters, such as beam current and water vapor pressure, on both the deposition rate and gold purity are investigated. Ex situannealing of the sample is used as a purity improvement method. Scattering optical measurements on the purified dots reveal that they support localized surface plasmon resonances. This technique opens new perspectives in the fabrication of substantial height-base aspect ratio plasmonicnanostructures and may become particularly relevant for nonflat substrates.
91(2007); http://dx.doi.org/10.1063/1.2786603View Description Hide Description
The ZnO-based metal-insulator-semiconductor(MIS)diode was fabricated by using an insulator ZnO layer and an layer grown by radio frequency magnetron sputtering. The current-voltage of the ZnOMISdiodes showed a good diodecharacteristic with a threshold voltage of and a band-edge emission at at room temperature. The electroluminescence emission of ZnOMIS was attributed to the generation of holes in the insulating ZnO layer at the high threshold voltage of via an impact ionization process.
91(2007); http://dx.doi.org/10.1063/1.2783134View Description Hide Description
The structural influences of the laser lift-off (LLO) techniques on the created (0001) GaNsurface region are characterized by cross-sectional high-resolution transmission electron microscopy and fitted using the model of stress waves caused by a longitudinal impact at the end of a cylindrical bar extending to infinity. The authors study reveals that, in addition to the superficial damage caused by laser absorption, the stress saltation in GaN crystal where the shock waves come into being induces deformation of the lattices and generates a cluster of half loops above the LLO interface. After that, the lattice deformation will be induced every time the partial dissipation of the steady-state shock waves takes place until the shock wave is dissipated to elastic mode.
91(2007); http://dx.doi.org/10.1063/1.2785111View Description Hide Description
The authors present a Mach-Zehnder-type interferometer with orthogonally polarized arms that passively, noninvasely, and optically amplifies Faraday rotation. At ambient temperatures, the interferometer, with an optical gain factor of , outperforms the optical bridge (OB) technique with a detection limit of approximately , compared to for the OB, when both techniques are used for time resolved faraday rotation under similar experimental conditions. Isolation and active stabilization of the interferometer have reduced sources of noise so that the interferometer’s signal is also quieter than the OB.
91(2007); http://dx.doi.org/10.1063/1.2787970View Description Hide Description
In this paper, the authors propose a magneto-optical tunable filter based on a long-period fiber grating (LPG) coated with magnetic fluids (MFs) as the ambient media. By applying a tunable magnetic field, the center wavelength shift of the attenuation band of LPG is found as large as . The refractive index dependence of MF on the external magnetic intensity is measured and the simulation results show that it is well agreeable with the experimental observations.
91(2007); http://dx.doi.org/10.1063/1.2789182View Description Hide Description
Photoluminescence(PL) spectroscopy and x-ray diffraction measurements were employed to study biaxial strain in AlN epilayers grown on different substrates. X-ray diffraction revealed that AlN epilayers grown on AlNbulk substrates (or homoepilayers) have the same lattice parameters as AlN bulk crystals and are almost strain-free. Compared to the free exciton (FX) transition in an AlN homoepilayer, the FX line was higher in AlN/sapphire due to a compressive strain and 55 lower in due to a tensile strain. A linear relationship between the FX transition energy peak position and in-plane stress was obtained, and a value of for the linear coefficient of the stress-induced bandgap shift in AlN epilayers was deduced. The work here establishes PL as another simple and effective method for monitoring the biaxial stress in AlN epilayers.
91(2007); http://dx.doi.org/10.1063/1.2789290View Description Hide Description
In this letter, the authors demonstrate reversible color switching from blue to red in polymer stabilized chiralnematic liquid crystals when viewed at oblique angles. A series of unusual colored scattering states, different from those noted in polymer-free samples, was observed in cells that had been prepared using a technique of cooling the sample from the isotropic phase in the presence of a low frequency applied electric field and then photopolymerizing the sample on both sides of the cell. This process “freezes-in” a texture whereby the helix axis lies preferentially in the plane of the cell.
91(2007); http://dx.doi.org/10.1063/1.2789179View Description Hide Description
An all semiconductor swept laser source demonstrating continuous sweeping over a range with a linewidth suitable for optical coherence tomography is proposed and demonstrated. The operation of this device relies upon state filling in a multiple contact laser utilizing quantum dots with strongly overlapping ground and excited states.
Optical gain measurements in porous silicon planar waveguides codoped by erbium and ytterbium ions at91(2007); http://dx.doi.org/10.1063/1.2789185View Description Hide Description
The on-off optical gain measurements as a function of the pump power were performed on porous siliconplanar waveguides codoped by erbium and ytterbium ions. These measurements were obtained for different ratios of Yb concentration to Er concentration. The highest value of the gain was reached when the Yb concentration is three times higher than that of Er at a moderate pump power value equal to . Optical losses measurements have been performed on these waveguides and were equal to and an internal gain of about was obtained.