摘要:

Crystals of &agr;‐[(4’‐methoxyphenyl)methylene]‐4‐nitro‐benzeneacetonitrile exhibit powder second‐harmonic generation (SHG) efficiencies under 1.06 &mgr;m irradiation several times that of 2‐methyl‐4‐nitroaniline. We have identified three distinct, solution‐grown polymorphs which show marked variation in SHG efficiency. Controlled transformation between these polymorphs can be made either solvent assisted or thermally. One polymorph shows a 25‐fold increase in SHG efficiency when the irradiation wavelength is changed to 1.32 &mgr;m while the others show a small or no increase.

ISSN:0003-6951    

DOI:10.1063/1.103288

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A two‐section multiple quantum well laser is passively mode locked without an external cavity at ∼108 GHz. The pulse widths average 2.4 ps and have a time‐bandwidth product of 1.1. Self‐pulsations at frequencies up to 8 GHz are also observed.

ISSN:0003-6951    

DOI:10.1063/1.103187

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Mass transport of a semiconductor alloy has been demonstrated using Ga0.51In0.49P which is lattice matched to GaAs. Buried‐heterostructure diode lasers with Ga0.51In0.49P as cladding and GaAs as the active layer have been made using this fabrication technique. Initial attempts produce devices with room‐temperature lasing thresholds of ∼33 mA and 15% differential power efficiency per facet.

ISSN:0003-6951    

DOI:10.1063/1.102813

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Quarter‐wave semiconductor mirrors of InP‐In0.53Ga0.47As for high reflectivity at 1.65 &mgr;m wavelength are epitaxially grown using metalorganic chemical vapor deposition. Doping of the In0.53Ga0.47As layers is found to be critical for high reflectivity at wavelengths corresponding to the In0.53Ga0.47As band gap.n‐type doping reduces the band‐to‐band absorption resulting in high reflectivity whilep‐type doped mirrors show reduced reflectivity.

ISSN:0003-6951    

DOI:10.1063/1.102814

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

By adjusting the carrier confining structure and the optical confining structure of strained InGaAs/AlGaAs single quantum well (QW) lasers, an improvement in performance has been obtained. First, the influence of optical confinement was examined by comparing two graded‐index confining structures. For InxGa1−xAs QWs with eitherx=0.20 orx=0.25, lasers with greater optical confinement factor had improved performance, with both lower threshold (180 A/cm2forx=0.20) and higher characteristic temperature (250 K forx=0.20), despite their reduced carrier confining potentials. Second, experiments on graded‐composition quantum wells show that thin step‐grading layers result in improved performance. In this structure, where the QW hasx=0.35, and the step layers havex=0.15, the optimum step thickness is 30–40 A˚. Thicker step layers appear to create too much strain, degrading the laser operation. These results indicate that step grading of strained QWs produces active region interfaces with lower defect density, and that step grading is especially useful in improving the performance of long‐wavelength, highly strained InGaAs/AlGaAs QW lasers.

ISSN:0003-6951    

DOI:10.1063/1.102815

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Regularly coiled carbon filaments have been obtained by the catalytic pyrolysis of acetylene at 350–750 °C using Ni plate and powder as a catalyst. Morphology and extension characteristics of the obtained coiled filaments were examined in some detail. The regularly coiled filaments have generally a 0.1–0.3 &mgr;m thickness, a 2–8 &mgr;m coil diameter, and a 0.1–5 mm coil length. The coiled filaments were always formed by the entwistness of two pair coils which grew in the same direction simultaneously from a diamond‐shaped Ni seed. We have found that the coiled filaments could be elastically extended up to about three times versus the original coil length.

ISSN:0003-6951    

DOI:10.1063/1.102816

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Containerless undercooling by electromagnetic levitation has been used to measure both growth velocity and grain size as a function of undercooling in Cu70Ni30and Cu69Ni30B1alloys. At a critical undercooling &Dgr;T* the temperature dependence of the growth velocity changes discontinuously. This is accompanied by a sudden drop of grain size in the solidified product by more than two orders of magnitude. The comparative investigations show that it is not a critical undercooling that initiates the grain refinement process, but rather a critical crystal growth velocity, which approximately equals the diffusive speed.

ISSN:0003-6951    

DOI:10.1063/1.103289

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Layered transition metal dichalcogenides (MoSe2, NbSe2) have been heteroepitaxially grown on (NH4)2Sx(x&bartil;2) treated GaAs(111)Ga, GaAs(∼(111))As surfaces in spite of the large difference in their crystal structures. Theinsituobservation of reflection high‐energy electron diffraction has shown that the grown film has its own lattice constant even from the first layer. The lattice matching condition, which is severely restricting in the usual heteroepitaxial growth case, is greatly relaxed in the present system because only weak van der Waals forces exist between the grown film and the substrate. This results from the fact that sulfur atoms regularly terminate dangling bonds on the GaAs surface after the (NH4)2Sxtreatment.

ISSN:0003-6951    

DOI:10.1063/1.102817

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Thermally stimulated discharge (TSD) behavior of native (dark rested) and irradiated amorphous selenium films (∼100 &mgr;m) has been studied. The TSD spectrum of nonirradiated films shows electron trapping levels at ∼0.99 eV and no detectable hole trapping levels. On the other hand, the irradiated films show both electron as well as hole trapping levels at 0.99 and 1.05 eV, respectively. These results have been explained in terms of trapping of charge carriers at intrinsic defect states and the photoinduced defect levels created during irradiation.

ISSN:0003-6951    

DOI:10.1063/1.102799

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Laser‐driven semiconductor photoemitters can provide the very bright beams of electrons needed in advanced accelerators. However, these semiconductors are easily degraded in operation. Photoemissive testing of the compound LaB6, which is expected to be a more environmentally rugged material, has shown that under excimer laser irradiation normalized electron beam brightnesses of 6.7×106, 2.6×106, and 1.5×105A/cm2 rad2can be achieved at photon wavelengths, respectively, of 193, 248, and 308 nm.

ISSN:0003-6951    

DOI:10.1063/1.103290

出版商:AIP    

年代:1990

数据来源: AIP