摘要:

The nature of uv ablation of organic polymers is discussed in terms of a pseudo‐zeroth‐order rate law of the formdx/dt=k0e−(Eact/kT), whereEactis assumed to be the strength of the weakest bonds in the polymer andTis the local temperature increase from the incident laser pulse. Equations derived from previous models that assumed nonthermal photodecomposition were duplicated from this photothermal model. Even for the simple case of single‐photon absorption, nonideal behavior is affected by radiationless decay, pulse length, and thermal diffusion. These effects were probed. Results indicated that thermal diffusion may have a significant effect on the threshold fluence and to some degree on the shape of the etch depth versus fluence curve. Absorption dynamics (saturation and radiationless decay) appear to be the dominant factor in determining the functional dependence of etch depth on fluence. As a result of competition between absorption saturation and radiationless decay, the penetration depth is intensity dependent. High fluence as well as short temporal pulses (subnanosecond) penetrate more deeply into the polymer than predicted by simple Beer’s law absorption. The apparent existence of an optimum pulse length, for a given absorbing system, is another result of the absorption dynamics.

ISSN:0021-8979    

DOI:10.1063/1.350840

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The chemical bonding and local order in Si, Si‐N, Si‐C, and Si‐C‐N nanometric powders prepared by laser synthesis have been investigated by using two experimental methods. The local environment of Si, C, and N atoms have been studied by x‐ray‐induced photoelectron spectroscopy, from a detailed analysis of the Si 2p, C 1s, and N 1score levels. Complementary information concerning the structural arrangement around Si atoms has been deduced from extended x‐ray‐absorption fine‐structure analysis performed at the SiKedge. The results show that the Si and Si‐C powders prepared with a 600 W laser power are pure, well‐crystallized materials; the Si‐N sample prepared with the same laser power has a disordered structure, although chemical order exists. For the composite samples, the results reveal that, in particular preparation conditions, a local C‐Si‐N3structure is formed.

ISSN:0021-8979    

DOI:10.1063/1.350841

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The double‐stream cyclotron maser is a novel source of millimeter wavelength radiation in which two copropagating electron beams are caused to gyrate in a uniform axial magnetic field. The interaction of the slow cyclotron space‐charge wave on one beam with the fast cyclotron space‐charge wave on the other beam leads to high‐frequency bunching. The desired operating frequency is proportional to the electron cyclotron frequency (or a harmonic thereof) and inversely proportional to the difference in beam velocities, and can be achieved at low beam energies and axial magnetic fields. The linear instability growth rate is calculated from the fully relativistic Vlasov equation for the case of cold‐electron beams.

ISSN:0021-8979    

DOI:10.1063/1.350842

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Scattering loss characteristics of Ge‐Se binary chalcogenide glass optical fibers are investigated. It is found that there is a strong correlation between the wavelength‐independent scattering loss and the absorption caused by H2O impurities. Therefore, the amount of H2O impurity mainly determines the wavelength‐independent loss of these optical fibers. After removing the H2O impurity and adjusting the fabrication conditions, the transmission loss is less than 1 dB/m over a wide infrared wavelength region of 2–8.5 &mgr;m, except for a small peak at 4.5 &mgr;m.

ISSN:0021-8979    

DOI:10.1063/1.350843

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We fabricated thin optical layers in aluminum nitride by magnetron sputtering. Using the techniques of integrated optics, we characterized the layers and we determined the electro‐optic coefficients of AlN:r13= 0.67 pm/V andr33= −0.59 pm/V.

ISSN:0021-8979    

DOI:10.1063/1.350844

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The effects of scattering and absorbing regions which exist along the stripe of 1.3 &mgr;m InGaAsP semiconductor diode lasers are examined. A model which accounts for scattering and absorbing nonuniformities is used to explain correlations between scattering, absorption, and the spectral properties of the lasers. Normally the dominant mode of the spectrum of a semiconductor laser shifts to longer wavelength as current is increased. Occasionally, the dominant mode will shift to shorter wavelength with increasing current for a limited current range. This negative tuning can be explained by considering the effect of multiple scatterers combined with a localized absorber.

ISSN:0021-8979    

DOI:10.1063/1.350845

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We report the results of a theoretical study of the performance characteristics of an optically pumped CF4laser amplifier in terms of a rate equation model. In order to achieve the best output from such an amplifier it is necessary to optimize its operating parameters. Our calculations indicate that, for a given pump rate, there exists an optimum gain length for the amplifier at which the output energy is a maximum. This optimum gain length is a function of the amplifier pump rate as well as its operating pressure and temperature. Our results also show that proper synchronization of the CO2pump pulse and the CF4laser pulse leads to the highest amplified output energy.

ISSN:0021-8979    

DOI:10.1063/1.351387

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We report on the use of plasma hydrogenation of Si doped,p‐type GaAs crystalline samples to form infrared waveguides through acceptor passivation. Epilayers grown by liquid phase epitaxy were exposed to a deuterium plasma for ninety minutes at three different temperatures. Secondary‐ion mass spectrometry (SIMS) analysis indicated that the deuterium concentrations in the crystals after plasma exposure were nearly equal to the acceptor level and extended to depths between 2.0 and 4.0 &mgr;m. Reflectivity measurements showed that the epilayers had passivated regions whose thicknesses corresponded to those determined by SIMS analysis. Laser coupling experiments at 1.15 &mgr;m showed optical waveguiding in each sample and lowest propagation losses were on the order of 35 dB/cm. At a wavelength of 1.523 &mgr;m, only the sample processed at the highest temperature exhibited laser guiding and losses were considerably higher.

ISSN:0021-8979    

DOI:10.1063/1.350849

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Results from emission spectroscopy measurements on an Ar/SiH4plasma jet which is used for fast deposition of amorphous hydrogenated silicon are presented. The jet is produced by allowing a thermal cascaded arc plasma in argon (I=60 A,V=80 V, Ar flow=60 scc/s and pressure 4 × 104Pa) to expand to a low pressure (100 Pa) background. In the resulting plasma SiH4is injected in front of the stationary shock front. Assuming a partial local thermal equilibrium situation for higher excited atomic levels, emission spectroscopy methods yield electron densities (∼ 1018m−3), electron temperatures (∼5000 K) as well as concentrations of H+, Si+, and Ar+particles. The emission spectrum of the SiH radical, theA 2&Dgr;–X 2&Pgr; electronic transition, is observed. Numerical simulations of this spectrum are performed, resulting in upper limits for the rotational and vibrational temperatures of 4000 and 5600 K, respectively. The results can be understood assuming that, in the expansion, charge exchange and dissociative recombination are dominant processes in the formation of species in excited states, notably Si+.

ISSN:0021-8979    

DOI:10.1063/1.350850

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Observations of a vacuum spark discharge are presented using a coaxial line driver. A 120 ns, 1.5 &OHgr; coaxial line is used to give peak discharges of 90 kA. The usual line spark gap is shorted out giving a new mode of operation. The discharge is initiated once the rising sinusoidal voltage is applied by focusing a Nd:YAG laser onto the cathode front surface, peak current is reached at 250 ns after this. Reproducible hot spot formation is observed at this time. Holographic interferometry combined with time and space resolved x‐ray observations show emission from a dense anode plasma as well as from the dense plasma column in which hot spot forms at peak current.

ISSN:0021-8979    

DOI:10.1063/1.350819

出版商:AIP    

年代:1992

数据来源: AIP