Low-threshold 1.5 μm compressive-strained multiple- andsingle-quantum-well lasers

Design considerations for low-threshold 1.5-μm lasers using compressive-strained quantum wells are discussed. Parameters include transparency current density, maximum modal gain, bandgap wavelength, and carrier confinement. The optical confinement for a thin quantum well in the separate-confinement heterostructure (SCH) and the step graded-index separate-confinement heterostructure (GRINSCH) are analyzed and compared. 1.5-μm compressive-strained multiple- and single-quantum-well lasers have been fabricated and characterized. As a result of the compressive strain, the threshold current density is loss limited instead of transparency limited. By the use of the step graded-index separate-confinement heterostructure to reduce the waveguide loss, a low threshold current density of 319 A/cm² was measured on compressive-strained single-quantum-well broad-area lasers with a 27 μ oxide stripe width     

Evaluation of single ohmic metallisations for contacting both p- and n-type GaInAs

The contact resistivity of various non-alloyed ohmic contact metallisations on both n- and p-type Ga/sub 0.47/In/sub 0.53/As has been investigated. Pd/AuGe metallisation was found to be most suitable when layers of both doping types were to be contacted in a single step, having lower contact resistivities on p/sup +/-GaInAs than Ti/Pt/Au and AuBe/Pt/Au. On n/sup +/-GaInAs, the contact resistivity was found to be almost independent of the metallisation used.<>     

Submilliampere-threshold 1.5-μm strained-layer multiple quantumwell lasers

The effect of high-reflection facet coatings on strained-layer multiple-quantum-well lasers was studied and submilliampere-threshold lasers were made in the 1.5-μm wavelength region with a short cavity and high-reflection-coated facets. As a result of the compressive strain, the threshold current density is loss-limited instead of transparency-limited. By the use of the step-graded-index separate confinement heterostructure to reduce the waveguide loss, a threshold current density of 550 A/cm² was measured on 30-μm wide broad area lasers with 1-mm long cavity     

An outbreak of Burkholderia (formerly Pseudomonas) cepacia respiratory tract colonization and infection associated with nebulized albuterol therapy

OBJECTIVE: To investigate an outbreak of Burkholderia (formerly Pseudomonas) cepacia respiratory tract colonization and infection in mechanically ventilated patients. DESIGN: A retrospective case-control and bacteriologic study. SETTING: Veterans Affairs medical center. PATIENTS: 42 mechanically ventilated patients who developed respiratory tract colonization or infection with B. cepacia and 135 ventilator-dependent controls who were not colonized and did not develop infections. MEASUREMENTS: Clinical and demographic data; benzalkonium chloride concentrations and pH levels in albuterol sulfate solutions; repetitive-element polymerase chain reaction (PCR)-mediated molecular fingerprinting on eight patient isolates and three environmental B. cepacia isolates that were available for study. RESULTS: 42 patients had B. cepacia respiratory tract colonization or infection. Observation of intensive care unit and respiratory care personnel showed faulty infection control procedures (for example, the same multiple-dose bottle of albuterol was used for many mechanically ventilated patients). More case patients (39 [92.9%]) than controls (95 [70.4%]; P = 0.006) received nebulized albuterol, and case patients (67.5 treatments) received more treatments than controls (18 treatments; P < 0.001). In-use albuterol solutions had pH values that were unstable, and benzalkonium chloride concentrations declined over time to levels capable of supporting bacterial growth. Medication nebulizers and in-use bottles of albuterol harbored B. cepacia. Molecular fingerprints of patient isolates and environmental B. cepacia isolates were identical using repetitive-element PCR. No further isolates of B. cepacia were identified after institution of appropriate infection control procedures. CONCLUSIONS: Multiple-dose medications and reliance on benzalkonium chloride as a medication preservative provide a mechanism for nosocomial spread of microorganisms, particularly if infection control procedures are not carefully followed. Repetitive-element PCR is a useful fingerprinting technique for molecular epidemiologic studies of B. cepacia.     

Wavelength accuracy and output power of multiwavelength DFB laser arrays with integrated star couplers and optical amplifiers

In this letter, we report the static performance of multiwavelength DFB laser arrays with integrated star couplers and optical amplifiers built for the reconfigurable optical network testbed. By the use of wavelength redundancy and proximity effect, wavelength deviations of /spl plusmn/0.2 nm or less from the designated eight-wavelength comb have been achieved with high yield. Simultaneous operation of eight wavelengths has also been demonstrated. In spite of the inherent splitting loss of 13 dB, high output powers of about -13 dBm and 0 dBm per wavelength have been measured into a single-mode fiber without and with on-chip optical amplification, respectively.     

Low-loss GaAs/AlGaAs optical waveguides on InP substrates

It is shown that the waveguide losses in lattice-mismatched GaAs-on-InP structures can be significantly reduced using an appropriate buffer layer. An AlGaAs buffer layer sequence was used for this purpose. A thin (400 nm) layer of Al_0.7Ga_0.3As, with an index below that of InP, was placed adjacent to the GaAs guiding layer both to maximize optical confinement in the guide and to increase the allowable guide dimensions for a single planar waveguide mode. Additional separation between guide and mismatched interface was achieved by inserting an Al_0.5Ga_0.5As layer with an index nearly equal to that of InP between the low-index buffer and InP. The final waveguide structure also included a thin (<40 nm) GaAs layer which was used to initiate growth and did not affect waveguide performance. Low losses (typically 3 dB/cm, with best results below 1 dB/cm) were achieved at a 1.52-μm wavelength for samples grown by organometallic chemical vapor deposition     

Antimony-Based Compounds with the Anti-Th<Subscript>3</Subscript>P<Subscript>4</Subscript> Structure as Potential High-Temperature Thermoelectric Materials

We present a combined experimental and theoretical study of thermoelectric properties and lattice dynamics of the R₄Sb₃ (R = La, Yb) compounds with the anti-Th₃P₄ structure. Bi alloying of Yb₄Sb₃ was found to improve the thermoelectric properties but not as well as La or Sm alloying. From the band structure, we are able to explain the origin of the relatively large and positive thermopower in Yb₄Sb₃ and of the small and negative thermopower found in La₄Sb₃ at high temperatures. Since the contribution of the 4f level of Yb atoms must be negative, as in other intermediate valence Yb-based compounds, the positive thermopower in Yb₄Sb₃ is due to the conduction electrons. We conjecture that the origin of the low thermal conductivity is similar to that of skutterudites, i.e., due to hybridization between acoustic and low-energy optical modes of the same symmetry.     

Effects of impurities on the lattice dynamics of nanocrystalline silicon for thermoelectric application

Doped silicon nanoparticles were exposed to air and sintered to form nanocrystalline silicon. The composition, microstructure, and structural defects were investigated with TEM, XRD, and PDF and the lattice dynamics was evaluated with measurements of the heat capacity, of the elastic constants with resonant ultrasound spectroscopy and of the density of phonon states (DPS) with inelastic neutron scattering. The results were combined and reveal that the samples contain a large amount of silicon dioxide and exhibit properties that deviate from bulk silicon. Both in the reduced DPS and in the heat capacity a Boson peak at low energies, characteristic of amorphous SiO₂, is observed. The thermal conductivity is strongly reduced due to nanostructuration and the incorporation of impurities.     

Influence of Doping on Structural and Thermoelectric Properties of AgSbSe<Subscript>2</Subscript>

A series of samples with nominal compositions of AgSb_1−x Sn _x Se₂ (with x = 0.0, 0.1, 0.2, and 0.3) and AgSbSe_2−y Te _y (with y = 0.0, 0.25, 0.5, 0.75, and 1.0) were prepared. The crystal structure of both single crystals and polycrystalline samples was analyzed using x-ray and neutron diffractometry. The electrical conductivity, thermal conductivity, and Seebeck coefficient were measured within the temperature range from 300 K to 700 K. In contrast to intrinsic AgSbSe₂, samples doped with Sn and Te exhibit apparent semiconducting properties (E _g = 0.3 eV to 0.5 eV), lower electrical conductivity, and higher values of the Seebeck coefficient for a small amount of Sn (x = 0.1). Further doping leads to decrease of the thermoelectric power and increase of the electrical conductivity. In order to explain electron transport behavior observed in pure and doped AgSbSe₂, electronic structure calculations were performed by the Korringa–Kohn–Rostoker method with coherent potential approximation (KKR–CPA).     

Fabrication of InP/InGaAs photodiodes for high bit rate communication by reactive ion etching

It is shown that reactive ion etching can be used for mesa etching and selfaligned contact cap removal in InP/InGaAs photodiode fabrication, without inducing unacceptable diode leakage currents. Mesa diodes with approximately 100 nA leakage, 90% quantum efficiency, and 12 GHz bandwidth at 5 V bias and 1.3 mu m wavelength, and pairs and quadruplets of such detectors for coherent lightwave systems are demonstrated.<>