Volume 91, Issue 6, 06 August 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:
Recent advances in nanotechnology have yielded materials and structures that offer great potential for improving the sensitivity, selectivity, stability, and speed of next-generation chemical gas sensors. To fabricate practical devices, the “bottom-up” approach of producing nanoscale sensing elements must be integrated with the “top-down” methodology currently dominating microtechnology. In this letter, the authors illustrate this approach by coupling a single-crystalnanowire sensing element with a microhotplate gas sensor platform. The sensing results obtained using this prototype sensor demonstrate encouraging performance aspects including reduced operating temperature, reduced power consumption, good stability, and enhanced sensitivity.
- LASERS, OPTICS, AND OPTOELECTRONICS
91(2007); http://dx.doi.org/10.1063/1.2767182View Description Hide Description
An -activated AlNphosphor was synthesized by firing the powder mixture of AlN,, and at for under . This nitride phosphor emits a strong blue color with the chromaticity coordinates of and at an accelerating voltage of . The cathodoluminescence properties of was evaluated by utilizing it in the Spindt-type field emission display panel. It shows that the nitride phosphor exhibits higher brightness, higher color purity, lower saturation, and longer lifetime than the currently used , indicative of the suitability of the blue phosphor in field emission displays.
91(2007); http://dx.doi.org/10.1063/1.2762286View Description Hide Description
A planar device is described, which combines an oscillator (microlaser) based on a dye doped cholesteric liquid crystal and an amplifier made of another laser dye solution in glycerin. Both the oscillator and amplifier are pumped longitudinally with the same pump beam. With that simple thick structure, the oscillator beam was amplified seven times.
91(2007); http://dx.doi.org/10.1063/1.2759475View Description Hide Description
Fabrication of a polymerlight-emitting device was achieved by a laser forward transfer technique using the decomposition of a thin triazene polymer film by a XeCl excimer laser. The dry deposition process allows transfer of a bilayer consisting of the electroluminescent polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] covered with an aluminum electrode onto a receiver substrate. The soft transfer results in laterally well resolved pixels , whose fluorescence as well as electroluminescence spectra remain unaltered. The rectifying and smooth current-voltage characteristics add to the merits of this laser-based transfer method that opens up the possibility of direct-writing heat- and UV-sensitive materials.
Composition dependence of structural and optical properties for sol-gel derived (100)-oriented thin films91(2007); http://dx.doi.org/10.1063/1.2767986View Description Hide Description
Highly (100) oriented (BST) thin films were grown on coated silicon substrate by modified sol-gel process. X-ray diffraction analysis shows that the out-of-plane lattice constant decreases linearly with increase of Sr concentration. The energyband gaps of BST thin films exhibit strong dependence on Sr content by analyzing the results of the spectroscopic ellipsometer (SE) measurement. The smallest has been obtained at , which is at the phase boundary of cube phase and tetragonal phase. The refractive index and thickness of BST thin films were obtained by fitting SE data with a multiphase model.
91(2007); http://dx.doi.org/10.1063/1.2767990View Description Hide Description
A simple lithographic method is developed to generate large-area antireflective subwavelength structures (SWSs), in which the metal island films are used as masks. Using magnetron sputter deposition, stochastically arranged Ag islands were fabricated on Si substrates with dimensions controlled in the range of . After reactive ion etching with , Si SWSs were formed, with the same arrangement and density as those of Ag islands. The measured reflectivity was decreased from for polished Si to for Si SWS surfaces. The residual reflection was thought to be mainly from the bottoms of “U”-shape grooves.
Efficient spatial redistribution of quantum dot spontaneous emission from two-dimensional photonic crystals91(2007); http://dx.doi.org/10.1063/1.2757134View Description Hide Description
The authors investigate the modification of the spontaneous emission dynamics and the external quantum efficiency for self-assembled InGaAsquantum dots coupled to extended and localized photonic states in GaAs two-dimensional photonic crystals. The photonic band gap is shown to give rise to a five to ten times enhancement of the external quantum efficiency, while the spontaneous emission rate is simultaneously reduced by a comparable factor. The findings are quantitatively explained by a modal redistribution of spontaneous emission due to the modified local density of photonic states. The results suggest that quantum dots embedded within photonic crystals are suitable for practical single photon sources with high external efficiency.
91(2007); http://dx.doi.org/10.1063/1.2761223View Description Hide Description
The concept of optical conductance is introduced in order to characterize the transport properties of waveguides built into finite photonic crystals. The conductance is given by the integral of the transmission cross section as a function of the incoming angle. This concept is illustrated by exact calculations of the light-guiding properties of a waveguide built into a photonic crystal slab based on a square lattice of dielectric cylinders in air. In analogy with their electronic counterparts, the optical conductance of a waveguide is quantized and thus provides a direct measure of the number of guided modes supported by the system.
91(2007); http://dx.doi.org/10.1063/1.2761246View Description Hide Description
The optical response of nonparabolic quantum wells is dominated by a strong peak at the plasmon frequency. When the electrons reach the anharmonic regions, resonant absorption becomes inefficient. This limitation is overcome by using a chirped laser pulse in the autoresonant regime. By direct simulations using the Wigner phase-space approach, the authors prove that, with a sequence of just a few pulses, electrons can be efficiently detrapped from a nonparabolic well. For an array of multiple quantum wells, they can create and control an electronic current by suitably applying an autoresonant laser pulse and a slowly varying dcelectric field.
91(2007); http://dx.doi.org/10.1063/1.2768195View Description Hide Description
High power electroluminescence near was observed from borondopedsilicon devices operating at heat sink temperatures up to . This represents the highest emission temperature yet observed for silicondopant-based terahertz devices, and is a significant increase from previous reports. This letter compares the temperature dependence of the emission mechanism to the dopant occupation function and describes an empirical model that fits the variation of output power with temperature, and that can guide the design of future terahertz devices.
91(2007); http://dx.doi.org/10.1063/1.2766859View Description Hide Description
The authors demonstrate that the oxidation smoothing of sidewall roughness of dry-etched ridge structures is enabled through a modified wet thermal oxidation process which involves the addition of dilute amounts of to the water vapor ambient. High magnification cross-section and top-view scanning electron microscope imagings both before and after oxide removal clearly show a substantial reduction of photolithography- and dry-etching-induced sidewall roughness (from down to ), occurring only with the participation of added . The smoothing process provides means to realize high-index-contrast GaAs-based optical waveguides with both low bend and scattering losses.
91(2007); http://dx.doi.org/10.1063/1.2766962View Description Hide Description
Self-phase modulation is utilized to generate a continuum of wavelengths on a silicon chip. The propagation of near transform-limited, picosecond pulses through a micron sized siliconwaveguide results in the generation of a spectral continuum. This spectral continuum is then carved by microdisk resonators that are integrated three dimensionally on the same chip, resulting in multiple wavelength channels, with the farthest channel being around away from the center wavelength of the seed pulse. This technique can potentially enable wavelength division multiplexing at the chip level using a single off-chip laser.
91(2007); http://dx.doi.org/10.1063/1.2768309View Description Hide Description
This letter reports that adding nanoparticles in the negative dielectricanisotropicliquid crystal(LC) cell can exhibit the properties of vertical alignment without using alignment layers. The electro-optical properties of this nanoparticles-induced vertical alignment in the LC cell are very similar to the conventional homeotropic LC cell with alignment layers. This technique can be used to fabricate a flexible LC display requiring a low temperature process.
91(2007); http://dx.doi.org/10.1063/1.2768872View Description Hide Description
Two four-wave-mixing(FWM) processes can coexist in a four-level -type atomic system with carefully arranged coupling laser beams. The generated two FWM signal beams fall into two simultaneously opened dual electromagnetically induced transparency (EIT) windows, which can be tuned to overlap or separate by various frequency detunings. The authors report our experimental observation of competing FWM processes, especially mutual suppression of the two FWM signals when the two EIT windows merge in frequency. Controlling FWM processes can have important applications in wavelength conversion for optical communication.
91(2007); http://dx.doi.org/10.1063/1.2768908View Description Hide Description
Photosensitive solid-liquid compounds were fabricated by filling nanoholes of alumina or polymer films with dye solution. Although these compounds can be handled as solid in device fabrication processes, the liquid phase inside holes provides a sufficient free volume for photochromic dye to change molecular structure. Nanoholes also prevent the solution to flow disturbing an optically written image. A function of the rewritable grating was demonstrated by bleaching a polycarbonate-based compound with various interference fringes of green laser.
91(2007); http://dx.doi.org/10.1063/1.2768639View Description Hide Description
The authors report on fluctuations of the geometric scale factor of a very large ring laser situated underground in the Cashmere Cavern in Christchurch (New Zealand). Variations in temperature and atmospheric pressure cause thermoelastic deformations to the cavern, which lead to changes of the area and perimeter of the ring laser structure. In situ beam monitoring has been used to partially correct for these effects.
91(2007); http://dx.doi.org/10.1063/1.2768882View Description Hide Description
For solar cells based on thin-film microcrystalline or amorphous silicon with absorber layers in the micrometer range, highly effective light trapping and an optimal incoupling of the entire sun spectrum are essential. To investigate and optimize both effects the wave propagation in thin-film silicon solar cells is modeled in three dimensions (3D) solving the Maxwell equations rigorously. A periodic nanostructured texture is investigated as an alternative to the common randomly rough texture. Inverted 3D pyramids with a periodicity of and structure height of show promising high quantum efficiencies close to the Tiedje limit.
91(2007); http://dx.doi.org/10.1063/1.2762287View Description Hide Description
The authors present a theoretical model and analysis to gain a further insight of the broadband quantum-dot laser characteristics. A detailed analysis of the role of both inhomogeneous and homogeneous optical gain broadenings on the broad lasing emission is incorporated in the theoretical model and compared with the experimental results. The experimental data of broadband laser signature agrees well with theoretical calculation confirming that the broadband stimulated emission from the laser at room temperature is a result of the occurrence of bistate lasing in highly inhomogeneousdots.
91(2007); http://dx.doi.org/10.1063/1.2768889View Description Hide Description
The authors introduce an alternative approach for acousto-optical light control based on the interference of light propagating through several waveguides, each subjected to a periodic refractive index modulation induced by a surface acoustic wave. The feasibility of the concept is demonstrated by the realization of an optical switch for arbitrary time intervals with an on/off contrast ratio of 20.
91(2007); http://dx.doi.org/10.1063/1.2768916View Description Hide Description
The authors report here the enhanced luminescence properties of doped with Eu and (, Dy, and Mn). The Eu and Eu, Mn-codoped powders were prepared by a solid state reaction at temperatures between 1400 and under (75%) atmosphere. The Eu, -codoped phosphors have the monoclinic structure with lattice parameters , , , and . The phosphors can be efficiently excited in the UV to visible region, making them attractive as conversion phosphors for a light emitting diode application. A green-yellow emission was observed for -codoped . The addition of in the Eu site in remarkably enhances the luminescent intensity by the factor of 144%, 148%, and 168% for Ce, Dy, and Mn, respectively.
91(2007); http://dx.doi.org/10.1063/1.2769397View Description Hide Description
The authors report on InGaN microcavitylight-emitting diodes with an effective thickness of at the emission wavelength of . The starting material for the flip-chip laser lift-off device is a structure with an active region embedding six quantum wells,-thick AlGaN, and a GaN template grown on a -plane sapphire substrate. High-precision control of the final microcavity thickness was facilitated by -based selective inductively coupled plasmaetching on the flipped material with an etch rate of for , where . Pronounced microcavity effects were observed by angular measurements, in agreement with the theoretical cavity-mode dispersion characteristics.