Volume 91, Issue 5, 30 July 2007
- lasers, optics, and optoelectronics
- 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:
Microstructures of coherent phase decomposition near morphotropic phase boundary in lead zirconate titanate are investigated by using computer modeling and simulation. The model is based on the recently proposed incoherent equilibrium phase diagram [G. A. Rossetti, Jr. et al., Appl. Phys. Lett.88, 072912 (2006)] and takes into account the coherency strain energy and electrostatic energy. It reveals characteristic multidomain microstructures, where nanoscale lamellar domains of tetragonal and rhombohedral phases coexist with well-defined crystallographic orientation relationships and produce coherent diffractioneffects. The simulated microstructures provide important information for interpretation of diffraction data and identification of phases near morphotropic phase boundary.
- LASERS, OPTICS, AND OPTOELECTRONICS
Simultaneous optical trapping of microparticles in multiple planes by a modified self-imaging effect on a chip91(2007); http://dx.doi.org/10.1063/1.2767148View Description Hide Description
The authors propose a three-dimensional (3D) optical trapping of microparticles in multiple planes simultaneously based on a modified self-imaging effect. Similar to the Talbot self-imaging effect, the modified self-imaging effect is induced by a layer of trapped particles and it is subsequently used as a periodic grating structure to generate its own self-imaging pattern in 3D. Based on this secondary layer-by-layer self-imaging effect,optical trapping of silica and polystyrene colloidal particles at different planes in a microchamber are demonstrated experimentally.
91(2007); http://dx.doi.org/10.1063/1.2767185View Description Hide Description
A double-barrier quantum well infrared photodetector was grown by molecular beam epitaxy and fabricated by standard device processes. The growth structure of the as-grown sample was verified by x-ray diffraction measurement. The photoluminescence emission peak, which is related to the interband transition in the GaInNAs well, was observed at . After annealing at , a large blueshift of was observed. The photocurrent peak at is associated with the intersubband transitions in the conduction band of the GaInNAs quantum well. The ten-band calculations agree with the above observations.
Generation of high-quality optical vortex beams in free-space propagation by microfabricated wedge with spatial filtering technique91(2007); http://dx.doi.org/10.1063/1.2760183View Description Hide Description
The authors propose a low-pass spatial filter for improving optical vortex beams in terms of circular geometry generated by a microfabricated wedge in free space. Compared with a conventional wedge, they are able to realize high-quality vortex beams with value close to 1 in free-space propagation. Physical mechanism behind the technique is found that it removes the diffraction artifacts as well as higher spatial frequency components of the quasivortex beams based on a wedge and hence the filtered beam is reconstructed with minimized distortions.
91(2007); http://dx.doi.org/10.1063/1.2761298View Description Hide Description
A combination of low numerical aperture focusing optics, in order to minimize spherical aberration, and beam shaping with a slit has been used to produce waveguides in fused silica by femtosecond laser writing. Waveguides with circular cross section and low losses are produced over a large depth window without changing any experimental parameter. Diffraction induces beam divergence along the axis perpendicular to the slit, leading to a shift of the focal plane. The focal intensity distribution can be predicted by a hybrid model combining Gaussian beam propagation with imaging of the slit into the material.
91(2007); http://dx.doi.org/10.1063/1.2761484View Description Hide Description
The authors report on the realization of an electro-optical amplitude modulator based on (BBO) waveguide operating in the ultraviolet wavelength range down to . The ridge waveguides are produced by implantation, photolithography masking, and plasma etching. Electrodes are deposited by the standard lift-off technique. A minimum half-wave voltage times modulator length of has been demonstrated at in a -long modulator. Because of BBO’s broad transparency range , such device could be employed in the deep ultraviolet with estimated to reach at .
91(2007); http://dx.doi.org/10.1063/1.2757596View Description Hide Description
The authors report an achievement of high quality AlN ultraviolet photonic crystal nanocavities. Convex AlN air-bridge structures with embedded GaNquantum dots have been formed by utilizing photoelectrochemical etching of substrates. Room-temperature microscopic photoluminescence measurements reveal the high quality of the nanocavities. For the lowest-order cavity mode of a -period nanocavity with seven missing holes, the highest factor ever reported in nitride-based photonic crystals has been obtained.
All-optical format conversion using a periodically poled lithium niobate waveguide and a reflective semiconductor optical amplifier91(2007); http://dx.doi.org/10.1063/1.2761513View Description Hide Description
In the present letter, the authors report on the realization of all-optical format conversion by using the cascaded sum- and difference-frequency generation in a periodically poled lithium niobate waveguide and the active mode locking in a reflective-semiconductor-optical-amplifier-based fiber ring laser. Tunable format conversions from nonreturn-to-zero pseudorandom binary sequence (PRBS) signal to return-to-zero PRBS idler at 10 and are observed in the experiment.
91(2007); http://dx.doi.org/10.1063/1.2757599View Description Hide Description
Germanium vertical photodetectors grown on silicon with molecular beam epitaxy are investigated. The photocurrent of a high speed detector structure is analyzed at infrared wavelengths of around . The dark current and photocurrent were measured for reverse and forward biased detectors. It is clearly shown that the photocurrent is proportional to the width of the depletion layer. This means that the fast carriers from the depletion layer generate mainly the photocurrent and by this the high speed operation of the germaniumdetector is explained. Nearly abrupt junctions and low background doping allow zero bias operation of these detectors.
91(2007); http://dx.doi.org/10.1063/1.2757600View Description Hide Description
The authors demonstrate an optically pumped surface emitting polymerdye laser fabricated by nanoimprint lithography. The laser is based on an organic dye hosted within a poly(methylmethacrylate) matrix coated on a transparent substrate, and the laser cavity consists of a second order circular grating distributed feedback structure. The authors achieved lasing at with linewidth and pump threshold. The nanoimprinted solid-state dye laser offers a low-cost coherent light source for laboratory-on-chip spectroscopy systems. The laser also has a low pump threshold and a geometry well matched to light-emitting diode pump sources, which provide an interesting alternative for constructing portable polymer laser devices.
91(2007); http://dx.doi.org/10.1063/1.2757601View Description Hide Description
The present work focuses on the question of localizing single object by hard x-ray phase contrast projection imaging. The authors present a setup where an x-ray channel waveguide defines a “quasi-point source” used to illuminate and image an object in a highly coherent cone beam. Knife edge fluorescence scans revealed a beam diameter of at a distance of behind the guide. The recorded image corresponds to an in-line hologram of the object which can be reconstructed numerically. Object translations and associated shifts in the hologram allow for the localization accuracy.
Single-step fabrication of Fresnel microlens array on sapphire substrate of flip-chip gallium nitride light emitting diode by focused ion beam91(2007); http://dx.doi.org/10.1063/1.2757602View Description Hide Description
A simple one-step focused ion beammilling technique was used to fabricate a Fresnel microlens array on the backside of sapphire substrate of a gallium nitride blue light emitting diode. The optical output power from the flip-chipgallium nitride blue light emitting diode is enhanced to about 1.68 times at the injection current of . The spatial light distribution from the backside of flip-chipgallium nitride blue light emitting diode with a Fresnel microlens array shows a uniform and stronger light emission.
91(2007); http://dx.doi.org/10.1063/1.2757606View Description Hide Description
GaN UV sensors with an in situgrown thick low-temperature aluminium nitride (LT-AlN) cap layer were fabricated. Scanning probe microscopy showed that surface pits of threading dislocations were almost invisible when the GaN active layer was capped with a LT-AlN layer. The dark leakage current of these sensors was comparably much smaller than that of conventional sensors without the LT-AlN cap layer, owing to a thicker and higher potential barrier of the LT-AlN cap layer and effective surface passivation. With applied bias, it was found that the UV-to-visible rejection ratio was for the sensor with the LT-AlN cap layer. In brief, incorporating a LT-AlN cap layer into the sensors can beneficially suppress the dark leakage current and improve the UV-to-visible rejection ratio.
Observation of femtojoule optical bistability involving Fano resonances in high- silicon photonic crystal nanocavities91(2007); http://dx.doi.org/10.1063/1.2757607View Description Hide Description
The authors observe experimentally optical bistability enhanced through Fano interferences in high- localized siliconphotonic crystal resonances ( and modal volume cubic wavelengths). This phenomenon is analyzed through nonlinear coupled-mode formalism, including the interplay of effects such as two-photon absorption and related free-carrier dynamics, and optical Kerr as well as thermal effects and linear losses. Experimental and theoretical results demonstrate Fano resonance based bistable states with switching thresholds of and internally stored cavity energy ( consumed energy) in silicon for scalable optical buffering and logic.
Comparing Monte Carlo simulation and pseudospectral time-domain numerical solutions of Maxwell’s equations of light scattering by a macroscopic random medium91(2007); http://dx.doi.org/10.1063/1.2767777View Description Hide Description
The Monte Carlo simulation of light scattering by a cluster of dielectric spheres is compared with numerical solutions of Maxwell’sequations via the pseudospectral time-domain technique. By calculating the total scattering cross-section (TSCS) spectrum, respectively, the spectrallight scatteringcharacteristics are determined. Since the Monte Carlo simulation falls short to accurately account for coherent interference effects, it is shown that the Monte Carlo simulation yields TSCS spectra that significantly deviate from the numerical solutions of Maxwell’sequations. Therefore, it is necessary to resort to Maxwell’sequations in order to accurately determine the light scatteringcharacteristics of a macroscopic geometry.
Temperature-dependent photoresponsivity and high-temperature operation of a quantum dot infrared photodetector91(2007); http://dx.doi.org/10.1063/1.2766655View Description Hide Description
In this letter, a longwave infrared (LWIR) InAs–InGaAs quantum dot infrared photodetector with a peak detection wavelength of is reported. A large photoresponsivity of and a high peak specific photodetectivity of were obtained at the operating temperature of . The QDIP showed a strong temperature-dependent photoresponsivity over the temperature range from . This effect is shown to be attributable to temperature-dependent electron capture probability.
91(2007); http://dx.doi.org/10.1063/1.2766841View Description Hide Description
The authors report the growth of AlGaN epilayers and deep ultraviolet (UV)light emitting diodes(LEDs) on bulk AlN substrates. Heteroepitaxial nucleation and strain relaxation are studied through controlled growth interruptions. Due to a low density of preexisting dislocations in bulk AlN, the compressive strain during AlGaNheteroepitaxy cannot be relieved effectively. The built-up of strain energy eventually induces either an elastic surface roughening or plastic deformation via generation and inclination of dislocations, depending on the stressor interlayers and growth parameters used. AlGaNLEDs on bulk AlN exhibit noticeable improvements in performance over those on sapphire, pointing to a promising substrate platform for III-nitride UV optoelectronics.
Polarization of light emission by light-emitting diodes grown on (0001) oriented sapphire substrates91(2007); http://dx.doi.org/10.1063/1.2757594View Description Hide Description
Measurements on the polarization of top- and side-emitted light as a function of direction are performed for GaInN unpackaged and packaged light-emitting diode(LED) chips with a multiquantum well (MQW) active region grown on (0001) oriented sapphire substrates. Side emission is found to be highly polarized with the electric field in the plane of the MQW. Intensity ratios for in-plane to normal-to-plane polarization reach values as high as 7:1, while the total intensity for the in-plane polarization is more than twice as large compared to the normal-to-plane polarization. Despite these measuredpolarizationcharacteristics, conventional packaged LEDs are found to be virtually unpolarized due to packaging.
91(2007); http://dx.doi.org/10.1063/1.2767216View Description Hide Description
The authors report the observation of an intense conical emission from an organic microcavity. The emission is characterized experimentally and modeled as a higher order Fabry-Perot mode predicted ago by Kastler [Appl. Opt.1, 17 (1962)]. A distinct cone is only visible above the normal mode laser threshold which implies a stimulated emission nature of the off-axial light. The input/output characteristics of normal and cone emissions hint that both originate from one and the same lasing mode.
91(2007); http://dx.doi.org/10.1063/1.2767239View Description Hide Description
Phonon-assisted stimulated emission has been demonstrated by photopumping GaN stripes grown via pendeoepitaxy on (0001) substrates. Transverse-electric-polarized emission with well-defined Fabry-Pérot modes located at one longitudinal optical phononenergy below the band gap of GaN was observed at . An effective refractive index of 8.578 was obtained using a cavity length of and a mode spacing of . This value is significantly higher than the value previously reported in the literature using ellipsometry, which indicates that the absorption loss is more severe during lasing when the excess carrier concentration is very high.
91(2007); http://dx.doi.org/10.1063/1.2767241View Description Hide Description
A photodetector, comprising a layer of silicon nanocrystals that is sandwiched between two electrodes, is proposed and demonstrated in a photosensing application on low-temperature polysilicon panels. Laser annealing of silicon-rich oxide films can form nanocrystals that respond optimally to a certain absorption spectrum of a light source. These silicon nanocrystals are smaller than in diameter, which size determines the effectiveness of their quantum confinement, and promote electron-hole pair generation in the photosensing region because of their direct band gap. Besides obtaining a photosensitivity that is comparable to that of a diode, which is currently used in low-temperature polysilicon technology, the sensor maximizes the photosensing area of a pixel by its stacked structure.