Volume 86, Issue 24, 13 June 2005
- 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:
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We have investigated, by means of atomic force microscopy, the complete evolution of quantum dots as a function of deposited InAs. Direct evidence is found for step erosion by quantum dots nucleated onto the step edge and an estimate of the eroded volume is provided. By studying the quantum dots volume as a function of InAs coverage, we show that the wetting layer contribution is confined within a narrow range of coverage around the two- and three-dimensional transition.
- LASERS, OPTICS, AND OPTOELECTRONICS
86(2005); http://dx.doi.org/10.1063/1.1941458View Description Hide Description
Improvements in the channel drop efficiency of an in-plane drop filter in a two-dimensional photonic crystal slab are presented, using a device consisting of two photonic crystal slabs with different lattice constants. It is theoretically shown that drop efficiencies much higher than the maximum of 25% for a conventional configuration are achievable when utilizing reflections at the photonic crystalheterostructureinterface. Additionally, the higher drop efficiency is found to be less sensitive to structural fluctuations. Drop operations with efficiencies of more than 80% are experimentally demonstrated by the fabricated devices.
Fabrication of large area two- and three-dimensional polymer photonic crystals using single refracting prism holographic lithography86(2005); http://dx.doi.org/10.1063/1.1947369View Description Hide Description
We demonstrate an approach for easy fabrication of two-dimensional (2D) hexagonal and three-dimensional (3D) face-centered-cubic (fcc)-type photonic crystal(PhC)microstructures in a photosensitive polymer by applying a simple single refracting prism. This prism enables the splitting and recombining of a single incoming laser beam to form multiple-beam interference pattern simultaneously. Thus, antivibration equipment and complicated optical alignment system are not required, leading to a much more simple optical setup than previously reported laser holographic lithography techniques. Large-scale (over ) 2D hexagonal and 3D fcc-type PhCs have been produced. Reflection/transmission measurements performed on the fabricated 3D fcc-type PhCstructures agree well with the corresponding band structure calculation.
86(2005); http://dx.doi.org/10.1063/1.1947379View Description Hide Description
We describe the fabrication and operation of an optical power monitor, monolithically integrated with a silicon-on-insulator rib waveguide. The device consists of a structure with a detection volume coincident with the single-mode supporting waveguide.Detection of optical signals at wavelengths around is significantly enhanced by the introduction of midband-gap generation centers, which provide partial absorption of the infrared light. The most efficient device extracted 19% of optical power from the waveguide and showed a responsivity of . These devices are fabricated using current standard processing technology and are fully compatible with siliconwaveguidetechnology and integrated operational amplifier circuits.
Nanocrystal-based microcavity light-emitting devices operating in the telecommunication wavelength range86(2005); http://dx.doi.org/10.1063/1.1947888View Description Hide Description
Highly luminescent colloidally prepared HgTe nanocrystals (NCs) are used to fabricate microcavitylight-emitting devices operating around . They consist of a Bragg interference mirror from standard optical materials deposited on glass substrates, an active layer embedding the nanocrystals, and a metallic top mirror. These devices give highly directional narrow single-mode emission with a beam divergence below 3° and a spectral width smaller by a factor of 8 than that of a NC reference sample. The emission wavelength can be tuned between 1.4 and by changing the cavity length and thus, the cavity finesse. The influence of the latter on output power and beam divergence is discussed. Furthermore, operation up to 75 °C is demonstrated without degradation of the NCs, which is promising for potential applications.
86(2005); http://dx.doi.org/10.1063/1.1947891View Description Hide Description
We report the observation of fluorescence in an optically transparent : perfluorocyclobutyl-based fluoropolymer composite. Under 980 nm excitation, fluorescence was observed at 1560 nm with a bandwidth of 93 nm. A quantitative analysis of the radiative properties yielded a radiative quantum efficiency of 29% corresponding to a measured lifetime of 4 ms and theoretical radiative decay time of 13.8 ms. Further, the estimated stimulated emission cross section was calculated to be , and the maximum optical gain from the composite was estimated to be 1.78 dB∕cm with a pump threshold of 1.1 mW. This estimate demonstrates that it is possible to use polymernanocomposites for active optical devices.
High-electric-field quantum yield roll-off in efficient europium chelates-based light-emitting diodes86(2005); http://dx.doi.org/10.1063/1.1948512View Description Hide Description
The europium chelates-based light-emitting-diodes (LEDs) have been fabricated showing the maximum electroluminescence(EL) quantum efficiency (QE) up to at electric fields slightly below and current density. Their line-like emission QE drops, however, rapidly within the high drive voltage range. This roll-off effect, exceeding one order of magnitude at , is shown to be underlain by the electric field-assisted dissociation of electron-hole pair precursors of europium ion-localized emissive states. The high field dependence of EL QE fits the Onsager model of geminate recombination well. This is at variance with triplet-triplet annihilation mechanism assumed previously to reduce QE in such LEDs.
Surface acoustic wave-induced electroluminescence intensity oscillation in planar light-emitting devices86(2005); http://dx.doi.org/10.1063/1.1948520View Description Hide Description
Electroluminescence emission from surface acoustic wave-driven light-emitting diodes (SAWLEDs) is studied by means of time-resolved techniques. We show that the intensity of the SAW-induced electroluminescence is modulated at the SAW frequency , demonstrating electron injection into the -type region synchronous with the SAWwave fronts.
86(2005); http://dx.doi.org/10.1063/1.1949730View Description Hide Description
High quality films on commercial -type 6H–SiC substrates have been grown by plasma-assisted molecular-beam epitaxy, and heterojunction mesa structures have been fabricated. Current-voltage characteristics of the structures had a very good rectifying diode-like behavior with a leakage current less than at , a breakdown voltage greater than , a forward turn on voltage of , and a forward current of at . Photosensitivity of the diodes was studied at room temperature and a photoresponsivity of as high as at reverse bias was observed for photonenergies higher than .
86(2005); http://dx.doi.org/10.1063/1.1949720View Description Hide Description
We present measurements of the optical absorption cross section of the transition at of ions embedded in and Si-rich oxide, using cavity ringdown spectroscopy on thin films. The peak absorption cross section for embedded in Si-rich oxide ( excess Si) was found to be at , similar to typical values for Er embedded in . The data imply that the siliconnanoclusters incorporated in Si-rich oxide do not enhance the peak cross section of the transition by 1-2 orders of magnitude, contrary to what has been reported in earlier work.
86(2005); http://dx.doi.org/10.1063/1.1949708View Description Hide Description
Lasing emission has been observed in optically pumped DCM-poly(methyl methacrylate)-based spiral-shaped micropillar cavities. By transverse ring shaping of the pump beam, unidirectional laser emission is being clearly favored. The resulting improved far-field directionality evidenced by our imaging technique is shown to originate from the notch of the spiral microcavities.
86(2005); http://dx.doi.org/10.1063/1.1947889View Description Hide Description
We report the low-temperature fabrication of heterojunctionlight-emitting diodes by filtered cathodic vacuum arc technique. Diodelike rectifying current-voltage characteristics, with turn-on voltage of and low reverse leakage current of , were measured at room temperature. In addition, ultraviolet emission with peak wavelength of and full width at half maximum of are observed at a forward biased voltage of . The ultraviolet electroluminescence from the heterojunction is originated from the exciton-exciton scattering inside the film.
2.8 and emission from diode-pumped -doped and co-doped lead lanthanum zirconate titanate transparent ferroelectric ceramic86(2005); http://dx.doi.org/10.1063/1.1944905View Description Hide Description
This work reports the observation of intense emissions at 2.8 and from optical quality diode-pumped -doped lead lanthanum zirconate titanate (PLZT) transparent ferroelectric ceramic. Radiative properties were calculated using the Judd Ofelt theory, and the respective values of and were obtained. The effect of ytterbium co-doping the PLZT: samples was investigated in an attempt to increase intensities. It was found that energy transfer processes, that favor upconversion, are detrimental to the efficiency of midinfrared emissions. Even so, the advantageous spectroscopic characteristics of PLZT: associated with its excellent thermal and mechanical properties, indicate it is a potential cost-effective laser active media.
86(2005); http://dx.doi.org/10.1063/1.1941459View Description Hide Description
A counterpropagating parametric nonlinear interaction is considered in the framework of a short one-dimensional photonic crystal. This interaction, otherwise highly inefficient, is shown to be efficient in the framework of such one-dimensional photonic crystal. The large momentum mismatch of this counterpropagating interaction is shown to be compensated for a broad range of frequencies when difference generation is considered. The numerical results presented indicate that such nonlinear photonic crystals are a very good material to consider the observation of backward parametric oscillation without mirror feedback.
86(2005); http://dx.doi.org/10.1063/1.1949279View Description Hide Description
An array of droplets of photonic crystal with polystyrene colloidal particles was assembled on silicon using ink-jet printing. A microreflectance spectrum from a single droplet was collected using an optical microscope and a spectrometer. It was observed that the photonic band gap of these droplets shifts as a function of the size of the colloidal particle. The reflectance peak position predicted from the model based on Bruggeman and Maxwell–Garnett effective medium approximations agrees well with those of the experimentally observed peak.
86(2005); http://dx.doi.org/10.1063/1.1944226View Description Hide Description
We present a simulation of the temperature sensitivity of the resonance wavelength of a long-period waveguidegrating (LPWG). We find that the temperature sensitivity of an LPWG in a channel waveguide is proportional to a modal dispersion factor that depends sensitively on the size of the core of the waveguide. Measurements with experimental polymer LPWGs agree well with the simulation results. The dimension of the waveguide core is an effective parameter for the control of the thermal characteristics of an LPWG in a channel waveguide.
86(2005); http://dx.doi.org/10.1063/1.1946192View Description Hide Description
The absorptionspectrum of the explosive 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) has been measured using a conventional Fourier transform infrared spectroscopy and by terahertz pulsed spectroscopy. Seven absorption features in the spectral range of have been observed and identified as the fingerprint of RDX. Furthermore, the spatial distribution of individual chemical substances including RDX, has been mapped out using reflection terahertz spectroscopicimaging in combination with component spatial pattern analysis. This is the terahertz spectroscopy and chemical mapping of explosives obtained using reflection terahertz measurement, and represents a significant advance toward developing a terahertz pulsed imaging system for security screening of explosives.
86(2005); http://dx.doi.org/10.1063/1.1946903View Description Hide Description
The development of photonic integrated circuits lattice matched to GaAs are desirable for the manufacture of high-power, high-efficiency optical components. In this letter we investigate and describe a process technique based on quantum-well intermixing to achieve multiple band edges in the Al-free material system.
- STRUCTURAL, MECHANICAL, THERMODYNAMIC, AND OPTICAL PROPERTIES OF CONDENSED MATTER
86(2005); http://dx.doi.org/10.1063/1.1947910View Description Hide Description
High-angle annular dark-field (HAADF) imaging in scanning transmission electron microscopy was used to determine the atomic structure of interfaces between epitaxial ErAs layers with the cubic rock salt structure and and GaAs, respectively. All layers were grown by molecular-beam epitaxy. We show that the interfacial atomic arrangement corresponds to the so-called chain model, in which the zinc blende semiconductor is terminated with a Ga layer. Image analysis was used to quantify the expansion between the first ErAs plane and the terminating Ga plane. In the HAADF images, a high intensity transfer from the heavy Er columns into the background was observed in the ErAs layer, whereas the background in was of much lower intensity.
86(2005); http://dx.doi.org/10.1063/1.1941456View Description Hide Description
Electromigration in beta-Sn has shown a 10% drop of resistance due to the anisotropic properties of the material. The drop was proposed due to reorientation of grains to reduce the resistance. The driving force as well as the atomic mechanism of grain rotation under electromigration has been considered in this letter. We propose that the anisotropicresistivity causes the divergence of the vacancy fluxes at the grain boundaries and induces the vacancy fluxes to/from the free surface along the grain boundary. The vacancy gradients along the grain boundaries correspond to the gradients of stress. The opposite sign of the stress along grain boundaries generates a torque which leads to rotation of the grain by grain boundarydiffusion or creep. The rate of rotation estimated on the base of the model seems to agree well with the observed experimental data.
Enhanced annihilations of self-interstitial clusters by vacancies transported through vehicle action of Cu in Cu-implanted silicon crystals86(2005); http://dx.doi.org/10.1063/1.1947383View Description Hide Description
Thermal behaviors of intrinsic defect clusters (self-interstitials and vacancies) in silicon crystals implanted with several metals including Cu were observed by photoluminescence measurements to investigate the influences of the metals on the diffusion of the intrinsic defects. Enhanced annihilations (EAs) of the defect clusters were observed only for an oxygen-lean floating zone (FZ) crystal implanted with Cu and annealed at temperatures between 400 °C and 600 °C, while they were not observed for any types of crystals (- and -type FZ and Czochralski crystals) implanted with Fe and Cr. The EAs of the defect clusters were well explained by a vehicle action of fast diffusing Cu for vacancy transportation.