Visible vertical cavity surface emitting laser

We have designed and fabricated the visible vertical-cavity surface-emitting lasers (VCSEL's) by using metalorganic vapor phase epitaxy (MOVPE). We use the 8 lambda optical cavities with 3 quantum wells in AlGaInP/AlGaAs red VCSEL's to reduce the drift leakage current and enhance the model gain in AlGaInP active region. The structure has a p-type stack with 36 DBR pairs on the top and an n-type with 55-1/2 pairs on the bottom. Using micro-area reflectance spectrum, we try to get a better concordance between the center wavelength of DBR and the emitting wavelength of the active region. We used a component graded layer of 0.05 lambda thick (x = 0.5 similar to 0.9) at the p-type DBR AlGaAs/AlAs interface to reduce the resistance of p-type DBR. We use selective oxidation to define the current injection path. Because the oxidation rate of a thick layer is faster than a thinner one, we grown a thick AlAs layer close to the active region. In this way, we got a smaller active region for efficient confinement of injected carriers (the aperture area is 3 x 3 mu m) to reduce the threshold and, at the same time, a bigger conductive area in the DBR layers to reduce the resistance. We employ Zn doping on the p-side of the junction to improve hole injection and control the Zn dopant diffusion to get proper p-i-n junction. At room temperature, pulse operation of the laser has been achieved with the low threshold current of 0.8mA; the wavelength is about 670nm.

Spatial Mode Selection by the Phase Modulation of Subwavelength Plasmonic Grating

The extraordinary transmission of the subwavelength gold grating has been investigated by the rigorous coupled-wave analysis and verified by the metal-insulator-metal plasmonic waveguide method. The physical mechanisms of the extraordinary transmission are characterized as the excitation of the surface plasmon polariton modes. The subwavelength grating integrated with the distributed Bragg reflector is proposed to modulate the phase to realize spatial mode selection, which is prospected to be applied for transverse mode selection in the vertical cavity surface-emitting laser.

Microphotoluminescence investigation of InAs quantum dot active region in 1.3 mu m vertical cavity surface emitting laser structure

Microphotoluminescence (mu-PL) investigation has been performed at room temperature on InAs quantum dot (QD) vertical cavity surface emitting laser (VCSEL) structure in order to characterize the QD epitaxial structure which was designed for 1.3 mu m wave band emission. Actual and precise QD emission spectra including distinct ground state (GS) and excited state (ES) transition peaks are obtained by an edge-excitation and edge-emission (EEEE) mu-PL configuration. Conventional photoluminescence methods for QD-VCSELs structure analysis are compared and discussed, which indicate the EEEE mu-PL is a useful tool to determine the optical features of the QD active region in an as-grown VCSEL structure. Some experimental results have been compared with simulation results obtained with the aid of the plane-wave admittance method. After adjustment of epitaxial growth according to EEEE mu-PL measurement results, QD-VCSEL structure wafer with QD GS transition wavelength of 1300 nm and lasing wavelength of 1301 nm was obtained.

Study on the structure change and oxygen vacation shift for Ce1-xSmxO2-y solid solution

A series of solid electrolytes Ce1-xSmxO2-y (x=0similar to0.6) were prepared by sol-gel method. XRD measurement showed that single-phase solid solution was formed in all investigated ranges at 160 degreesC, which is a significantly lower synthesis temperature compared to traditional solid state reaction. High temperature X-ray, ESR, and Raman scattering were used to characterize the samples. ESR measurement showed that ESR with sample irradiated by high-energy particle is an effective way to study the defect structure. These changes in the Raman spectrum are attributed to O vacancies, which are introduced into the lattice when tetravalent Ce4+ is substituted by trivalent Sm3+.

Theoretical representation of shear and pressure influence on the phase separation behavior of PVME/PS mixture

In the framework of lattice fluid model, the Gibbs energy and equation of state are derived by introducing the energy (E-s) stored during flow for polymer blends under shear. From the calculation of the spinodal of poly(vinyl methyl ether) (PVME) and polystyrene (PS) mixtures, we have found the influence of E., an equation of state in pure component is inappreciable, but it is appreciable in the mixture. However, the effect of E, on phase separation behavior is extremely striking. In the calculation of spinodal for the PVME/PS system, a thin, long and banana miscibility gap generated by shear is seen beside the miscibility gap with lower critical solution temperature. Meanwhile, a binodal coalescence of upper and lower miscibility gaps is occurred. The three points of the three-phase equilibrium are forecasted. The shear rate dependence of cloud point temperature at a certain composition is discussed. The calculated results are acceptable compared with the experiment values obtained by Higgins et at. However, the maximum positive shift and the minimum negative shift of cloud point temperature guessed by Higgins are not obtained, Furthermore, the combining effects of pressure and shear on spinodal shift are predicted.

MOSSBAUER-EFFECT STUDIES OF THE PHASE-TRANSITION PROCESS IN EUROPIUM PENTAPHOSPHATE

Mossbauer spectra of europium pentaphosphate are measured at various temperatures (126 to 200-degrees-C). Some Mossbauer parameters, such as isomer shift, electric quadrupole splitting, and asymmetry parameter of the EFG at Eu-151 nuclei are derived from the experimental spectra. The lattice parameters of the crystal are determined at several temperatures. The experimental results indicate that the crystal structure of europium pentaphosphate changes from monoclinic to orthorhombic. All of the temperature dependences of the Mossbauer parameters provide evidence of a phase transition of the crystal. The phase transition temperature can be determined from the curve of the asymmetry parameter of EFG versus temperature to 165-degrees-C.

Enhanced anthocyanin production by repeated-batch culture of strawberry cells with medium shift

Repeated-batch cultures of strawberry cells (Fragaria ananassa cv. Shikinari) subjected to four medium-shift procedures (constant LS medium, constant B5 medium, alternation between LS and B5 starting from LS and alternation between LS and B5 starting from B5) were investigated for the enhanced anthocyanin productivity. To determine the optimum period for repeated batch cultures, two medium-shift periods of 9 and 14 days were studied, which represent the end of the exponential growth phase and the stationary phase. By comparison with the corresponding batch cultures, higher anthocyanin productivity was achieved for all the repeated-batch cultures at a 9-day medium-shift period. The average anthocyanin productivity was enhanced 1.7-and 1.76-fold by repeated-batch cultures in constant LS and constant B5 medium at a 9-day shift period for 45 days, respectively. No further improvement was observed when the medium was alternated between LS (the growth medium) and B5 (the production medium). Anthocyanin production was unstable at a 14-day shift period regardless of the medium-shift procedures. The results show that it is feasible to improve anthocyanin production by a repeated-batch culture of strawberry cells.

Broadband cavity-enhanced absorption spectroscopy with incoherent light

Although broadband incoherent light does not efficiently couple into a high-finesse optical cavity, its transmission is readily detectable and enables applications in cavity-enhanced absorption spectroscopy in the gas phase, liquid phase and on surfaces. This chapter gives an overview of measurement principles and experimental approaches implementing incoherent light sources in cavity-enhanced spectroscopic applications. The general principles of broadband CEAS are outlined and general “pros and cons” discussed, detailing aspects like cavity mirror reflectivity calibration or the establishment of detection limits. Different approaches concerning light sources, cavity design and detection schemes are discussed and a comprehensive overview of the current literature based on a methodological classification scheme is also presented.

Exercise elicits phase shifts and acute alterations of melatonin that vary with circadian phase

To examine the immediate phase-shifting effects of high-intensity exercise of a practical duration (1 h) on human circadian phase, five groups of healthy men 20-30 yr of age participated in studies involving no exercise or exposure to morning, afternoon, evening, or nocturnal exercise. Except during scheduled sleep/dark and exercise periods, subjects remained under modified constant routine conditions allowing a sleep period and including constant posture, knowledge of clock time, and exposure to dim light intensities averaging (±SD) 42 ± 19 lx. The nocturnal onset of plasma melatonin secretion was used as a marker of circadian phase. A phase response curve was used to summarize the phase-shifting effects of exercise as a function of the timing of exercise. A significant effect of time of day on circadian phase shifts was observed (P < 0.004). Over the interval from the melatonin onset before exercise to the first onset after exercise, circadian phase was significantly advanced in the evening exercise group by 30 ± 15 min (SE) compared with the phase delays observed in the no-exercise group (-25 ± 14 min, P < 0.05). Phase shifts in response to evening exercise exposure were attenuated on the second day after exercise exposure and no longer significantly different from phase shifts observed in the absence of exercise. Unanticipated transient elevations of melatonin levels were observed in response to nocturnal exercise and in some evening exercise subjects. Taken together with the results from previous studies in humans and diurnal rodents, the current results suggest that 1) a longer duration of exercise exposure and/or repeated daily exposure to exercise may be necessary for reliable phase-shifting of the human circadian system and that 2) early evening exercise of high intensity may induce phase advances relevant for nonphotic entrainment of the human circadian system.

Non-local quantum gates: a cavity-quantum-electrodynamics implementation

The problems related to the management of large quantum registers could be handled in the context of distributed quantum computation: unitary non-local transformations among spatially separated local processors are realized performing local unitary transformations and exchanging classical communication. In this paper, a scheme is proposed for the implementation of universal non-local quantum gates such as a controlled NOT (CNOT) and a controlled quantum phase gate (CQPG). The system chosen for their physical implementation is a cavity-quantum-electrodynamics (CQED) system formed by two spatially separated microwave cavities and two trapped Rydberg atoms. The procedures to follow for the realization of each step necessary to perform a specific non-local operation are described.

Cavity-Enhanced Photoacoustic Detection Using Acoustic and Fiber-Optic Resonators

A wineglass has been used as an acoustic resonator to enhance the photoacoustic signal generated by laser excitation of absorbing dyes in solution. The amplitude of the acoustic signal was recorded using a fiber-optic transducer based on a Fabry-Pérot cavity attached to the rim of the wineglass. The optical and acoustic properties of the setup were characterized, and it was used to quantify the concentration of phosphomolybdenum blue and methyl red solutions. Detection limits of 1.2 ppm and 8 muM were obtained, respectively.

Dramatic liquid-phase dehydrogenation rate enhancements using gas-phase hydrogen acceptors

The dehydrogenation of 1,2,3,4-tetrahydrocarbazole (THCZ) to form carbazole (CZ) over supported palladium catalysts was examined in the presence of hydrogen acceptors. As expected, liquid hydrogen acceptors increased the rate of reaction but, importantly, gaseous hydrogen acceptors also have been used. Ethene, propene, and but-1-ene showed up to a fivefold increase in the rate of dehydrogenation. Moreover, compared with the analogous liquid systems, the gaseous alternatives are a potentially more economic method of enhancing the activity and provide a simpler workup. The mechanism for the increase in rate was examined by density functional theory calculations, which showed that the propene hydrogenation competes effectively with the back-hydrogenation of the intermediates formed during the THCZ dehydrogenation, resulting in a shift in the equilibrium toward to the formation of CZ. (C) 2007 Elsevier Inc. All rights reserved.

High-gain subwavelength resonant cavity antennas based on metamaterial ground planes

Planar metarnaterial Surfaces with negative reflection phase values are proposed as ground planes in a high-gain resonant cavity antenna configuration. The antenna is formed by the metarnaterial ground plane (MGP) and a superimposed metallodielectric electromagnetic band gap (MEBG) array that acts as a partially reflective surface (PRS). A single dipole positioned between the PRS and the ground IS utilised as the excitation. Ray analysis is employed to describe the functioning of the antennas and to qualitatively predict the effect of the MGP oil the antenna performance. By employing MGPs with negative reflection phase values, the planar antenna profile is reduced to subwavelength values (less than lambda/6) whilst maintaining high directivity. Full-wave simulations have been carried out with commercially available software (Microstripes (TM)). The effect of the finite PRS size on the antenna radiation performance (directivity and sidelobe level) is studied. A prototype has been fabricated and tested experimentally in order to validate the predictions.

The Preboreal climate reversal and a subsequent solar-forced climate shift

Accurate chronologies are essential for linking palaeoclimate archives. Carbon-14 wiggle-match dating was used to produce an accurate chronology for part of an early Holocene peat sequence from the Borchert (The Netherlands). Following the Younger Dryas-Preboreal transition, two climatic shifts could be inferred. Around 11 400 cal. yr BP the expansion of birch (Betula) forest was interrupted by a dry continental phase with dominantly open grassland vegetation, coeval with the PBO (Preboreal Oscillation), as observed in the GRIP ice core. At 11 250 cal. yr BP a sudden shift to a humid climate occurred. This second change appears to be contemporaneous with: (i) a sharp increase of atmospheric C-14; (ii) a temporary decline of atmospheric CO2; and (iii) an increase in the GRIP Be-10 flux. The close correspondence with excursions of cosmogenic nuclides points to a decline in solar activity, which may have forced the changes in climate and vegetation at around 11 250 cal. yr BP. Copyright (C) 2004 John Wiley Sons, Ltd.

Electrochemical synthesis of Mo2C catalytical coatings for the water-gas shift reaction

The electroreduction of CO32- ions on a molybdenum cathode in a NaCl-KCl-Li2CO3 melt was studied by cyclic voltarnmetry. The electrochemical synthesis of Mo2C on molybdenum substrates has been performed at It 23 K for 7 h with a cathodic current density of 5 mA cm(-2). If molybdenum carbide is present as a thin (ca. 500 nm) film on a molybdenum substrate (Mo2C/Mo), its catalytic activity in the water gas-shift reaction is enhanced by at least an order of magnitude compared to that of the bulk Mo2C phase.