Enlargement of the micropores of a caking bituminous coal by controlled oxidation

In a study of the enlargement of pores of coals it has been found that treatment of a bituminous coal (PSOC No. 371, from the Pennsylvania State University Coal Section) with a 5:95 O₂:N₂ stream 4 h at 400 °C increases the surface area as measured by nitrogen adsorption at 77K by a factor of at least 50 to a value 52 m² g^?1. The increase in pore size was accompanied by a 9.7% weight loss. Simultaneously, the area as measured by carbon dioxide at 195K increased from 61 to 136 m² g^?1 and that measured by carbon dioxide at room temperature increased from 125 to 237 m² g^?1. Attempts to enlarge the pores by oxidation with hydrogen peroxide or ozone were unsuccessful. A Pittsburgh coal subject to a small percentage of oxygen in nitrogen or steam at 300 to 400 °C showed a surface area as measured by nitrogen adsorption of less than 1 m² g^?1 both before and after such pretreatment. This same coal with a 5:95 O₂:N₂ stream for 4 h at 450 °C showed a surface area of 110 m² g^?1 measured by nitrogen adsorption at 77K.     

Carrier dynamics in high-speed (f/sub -3 dB/>40 GHz) 0.98-/spl mu/m multiquantum-well tunneling injection lasers determined from electrical impedance measurements

Carrier capture times in InGaAs-AlAs-GaAs 0.98-/spl mu/m multiquantum-well tunneling injection lasers with f/sub -3 dB//spl sim/43-48 GHz have been determined from analysis of high frequency electrical impedance measurements. The capture times range from 14 ps, at biases around threshold, to about 1 ps, at 50-mA bias. The small capture times agree well with tunneling times obtained directly from pump-probe measurements. The impedance measurements also suggest that the carrier lifetime in the well is much less than the escape time from the well, consistent with the cold carrier distribution associated with a tunneling injection mechanism.     

Measurement of Electron Mobility in Alq3 From Optical Modulation Measurements in Multilayer Organic Light-Emitting Diodes

The dynamic characteristics of multilayer organic light-emitting diodes (OLEDs) determine the refresh rate in display applications, and are of great importance for practical organic displays. They also serve as an important tool in studying the transport mechanisms in organic conductors. Here, the modulation characteristics of several conventional small-molecule OLED structures [consisting of ITO/PEDOT:PSS(50 nm)/TPD(50 nm)/Alq₃(various)/LiF(1 nm)/Al(90 nm)] are measured and analyzed in terms of mobility in and thickness of the Alq ₃ layer. Their optical response was shown to be limited by electron transport across the Alq₃. Extracted electron mobilities were about 2-4times10^-6 cm²/Vmiddots (consistent with that reported in the literature) and near-identical values for mobility were obtained from devices of different thicknesses, suggesting that this method measures mobility independent of interface trap charging. This novel technique is a complement to large signal time of flight or delay time measurements (which can include interface and trap charging during the measurement) and can serve as a flexible method to study transport in actual devices     

Photoluminescence and lasing from deoxyribonucleic acid (DNA) thin films doped with sulforhodamine

Thin solid films of salmon deoxyribonucleic acid (DNA) have been fabricated by treatment with a surfactant and used as host for the laser dye sulforhodamine (SRh). The DNA films have an absorption peak at approximately 260 nm owing to absorption by the nitrogenous aromatic bases. The SRh molecules in the DNA films have absorption and emission peaks at 578 and 602 nm, respectively. The maximum emission was obtained at approximately 1 wt. % SRh in DNA, equivalent to approximately 100 DNA base pairs per SRh molecule. A distributed feedback grating structure was fabricated on a SiO(2)-Si substrate using interference lithography. The grating period of 437 nm was selected, corresponding to second-order emission at the amplified spontaneous emission wavelength of 650 nm. Lasing was obtained by pumping with a doubled Nd:YAG laser at 532 nm. The lasing threshold was 3 microJ, corresponding to approximately 30 microJ/cm(2) or 4 kW/cm(2). The emission linewidth decreased from approximately 30 nm in the amplified spontaneous emission mode to <0.4 nm (instrument limited) in the lasing mode. The slope efficiency of the lasing was approximately 1.2%.     

Design and parallel fabrication of wire-grid polarization arrays for polarization-resolved imaging at 1.55 microm

Polarization-resolved imaging can provide information about the composition and topography of the environment that is invisible to the eye. We demonstrate a practical method to fabricate arrays of small, orthogonal wire-grid polarizers (WGPs) that can be matched to individual detector pixels, and we present design curves that relate the structure to the polarization extinction ratio obtained. The photonic area lithographically mapped (PALM) method uses multiple-exposure conventional and holographic lithography to create subwavelength patterns easily aligned to conventional mask features. WGPs with polarization extinction ratios of approximately 10 at a 1.55 microm wavelength were fabricated, and square centimeter areas of square micrometer size WGP arrays suitable for polarization-resolved imaging on glass were realized.     

High-speed modulation and switching characteristics ofIn(Ga)As-Al(Ga)As self-organized quantum-dot lasers

The dynamic characteristics, and in particular the modulation bandwidth, of high-speed semiconductor lasers are determined by intrinsic factors and extrinsic parameters. In particular, carrier transport through the heterostructure and thermalization, or quantum capture in the gain region, tend to play an important role. We have made a detailed study of carrier relaxation and quantum capture phenomena in In(Ga)As-Al(Ga)As self-organized quantum dots (QD's) and single-mode lasers incorporating such dots in the gain region through a variety of measurements. The modulation bandwidth of QD lasers is limited to 5-6 GHz at room temperature and increases to ~30 GHz only upon lowering the temperature to 100 K. This behavior is explained by considering electron-hole scattering as the dominant mechanisms of electron relaxation in QD's and the scattering rate seems to decrease with increase of temperature. The switching of the emission wavelength, from the ground state to an excited state, has been studied in coupled cavity devices. It is found that the switching speed is determined intrinsically by the relaxation time of carriers into the QD states. Fast switching from the ground to the excited state transition is observed. The electrooptic coefficients in the dots have been measured and linear coefficient τ=2.58×10^-11 m/V. The characteristics of electrooptic modulators (EOM's) are also described     

Bringing quantum dots up to speed [Breaking the phonon bottleneckwith high-speed modulation of quantum-dot lasers]

In this article, we will focus on the carrier relaxation time in quantum dots (QDs), its probable mechanism, and the implications for the performance characteristics of directly modulated QD lasers and other QD devices. The electron and hole bound states and general predictions of carrier capture time into them will be presented, followed by a discussion of intersubband carrier relaxation in QDs. The modulation characteristics of QD lasers as a function of temperature will be described, and these modulation results will be discussed in terms of the temperature, composition, and size dependence of the relaxation time in QDs, including possible methods for designing QDs to overcome this relaxation time barrier. Also, the performance characteristics of other possible QD devices, such as intersubband lasers and detectors, will be examined in terms of our current understanding of the relaxation time in QDs     

Quantum capture times at room temperature in high-speed In0.4Ga0.6As-GaAs self-organized quantum-dot lasers

The quantum capture times in high-speed single-mode self-organized quantum-dot (QD) lasers with I_th=15-30 mA, and small-signal modulation bandwidth f_{-3 dB}=4.5 GHz, have been estimated from high frequency electrical impedance measurements. The effective carrier capture times, determined from this relatively simple measurement technique, vary in the range of 20-40 ps, depending on bias current, and are in excellent agreement with theoretical predictions. The results suggest that carrier capture, not damping, may prove to be the limiting factor in the modulation bandwidths of QD lasers     

Enhanced modulation bandwidth (20 GHz) ofIn0.4Ga0.6As-GaAs self-organized quantum-dotlasers at cryogenic temperatures: role of carrier relaxation anddifferential gain

Measurements of the threshold current, slope efficiency and optical modulation characteristics of self-assembled InGaAs-GaAs quantum-dot lasers have been made in the temperature range of 20-200 K in order to understand the carrier dynamics in these devices. The dc characteristics of these devices showed a region of almost temperature independent threshold current up to 85 K (T₀=670 K) with a maximum slope efficiency at 150 K. The maximum measured bandwidth increased from 5 GHz at room temperature to 20 GHz at 80 K. This is consistent with the bandwidth being limited by carrier relaxation time through electron-hole scattering