摘要:

Rare‐gas–halide excimer lasers are suitable for the production of extremely high peak brightness output in the ultraviolet. The basic properties of these systems are examined and the various techniques employed to produce high power (multiterawatt) operation are described. A specific system using short‐pulse injection is examined and its ability to focus to intensities above ∼1019W cm−2is discussed. Various applications of such high intensity subpicosecond light sources are also considered.

ISSN:0021-8979    

DOI:10.1063/1.347665

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Number or density autocorrelation functions can sometimes be obtained in terms of electron emission current autocorrelation functions if the current fluctuations are related to the number fluctuations. This is the case in field emission where the fluctuations in adsorbate number in a small probed region, caused by surface diffusion, are essentially proportional to the field emission current fluctuations. The autocorrelation function of the current fluctuations then mirrors that of the adsorbate number fluctuations. When correlation intervals are very short they may contain an average number of electrons so small that the rms electron fluctuation resulting from adsorbate number fluctuations is less than 1e−and thus shows up only infrequently. The electron fluctuation can then ‘‘miss’’ most adsorbate fluctuations. We have examined this problem by Monte Carlo modeling and confirm the existence of this effect when the electron fluctuation number is truncated to integer values. Even in this case the electron fluctuation correlation images the adsorbate number correlation function quite well to values of rms electron fluctuations as low as 0.4e−per correlation interval. When shot noise is included imaging is good to the lowest value tested, 0.18e−, but the number of summations required to overcome noise goes up very steeply. The reason for the improvement in imaging is that shot noise circumvents the problem of electron fluctuation inadequacy, since very small changes in mean electron emission probability, corresponding to much less than 1e−per correlation interval, are reflected in the Poisson distribution. Thus shot noise in a way ‘‘amplifies’’ the fluctuations but at the price of vastly increased noise. This conclusion is probably not confined to the specific case examined here but seems generally valid.

ISSN:0021-8979    

DOI:10.1063/1.347663

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We present anin‐situellipsometric study of hydrogenated amorphous silicon(a‐Si:H)/hydrogenated amorphous germanium (a‐Ge:H) multilayer structures. We deposit multilayers structures with the thickness of the Si layer equal to that of the Ge layer and with thicknesses of 100 and 500 A˚. We find that whereas thea‐Ge:H grown on top ofa‐Si:H follows a uniform growth model, the initial stage of growth ofa‐Si:H ona‐Ge:H must be modelled with a hemispherical nucleation model. These growth mechanisms are observed for both the case of the 100 A˚ layers and the 500 A˚ layers. We also find that there are no long‐term effects of the deposition on the optical properties of the layers. This is observable visually in the cases where the thickness of the layers allow the real‐time trajectories to reach a saturated value and thus guaranteeing the same starting ellipsometric point for the next layer. This assures us that each material at the beginning of the superlattice growth is the same as that grown at the end. The implication of the present analysis on quantum size effects of superlattices is discussed.

ISSN:0021-8979    

DOI:10.1063/1.347750

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The profile, density, and temperature of a supersonic rare‐gas jet was determined applying methods of nuclear physics and optical spectroscopy. The jet was produced in the windowless gas target facility RHINOCEROS and the heavy‐ion particle beams were provided by the Stuttgart DYNAMITRON accelerator. Maximum densities in the range of 1017to some 1018particles/cm3could be achieved in a gas jet of about 2.6 mm diam (FWHM). In a supersonic helium jet a cooling of the gas down to 45 K was observed. These results were compared with theoretical calculations.

ISSN:0021-8979    

DOI:10.1063/1.347743

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Computer programs developed recently at Los Alamos have been used to calculate the transmission of x rays through an aluminum plasma. Theoretical energy levels, oscillator strengths, and photoionization cross sections were combined with the local thermodynamic equilibrium population model to calculate the plasma absorption coefficient as a function of photon energy. The transmission spectrum is simulated by accounting for the plasma depth and spectrometer characteristics. These results are compared to spectra observed during recent experiments; excellent agreement is obtained.

ISSN:0021-8979    

DOI:10.1063/1.347757

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Electron excitation temperatures have been measured in a low‐pressure (0.05‐Torr Cs, 2‐Torr total pressure) argon‐cesium discharge that uses a heated cathode (900–1100 K). The excitation temperature determinations are based upon a model that allows calculation of cesium excited state densities for low electron density (<1011cm−3). The model assumes that the dominant creation processes for excited states are electron impact excitation from the ground state and radiative cascade from higher levels, while destruction is by spontaneous emission. Maxwellian electron energy distributions were used and the plasmas were considered to be optically thin. The model indicates that cascade contributions to the production of excited states can be as high as 50% for some cesium levels. Predicted emission spectra with cascade contributions to spontaneous emission intensities agree well with measured spectra except for radiation trapped transitions from low nP states to the ground state. Excitation temperatures are determined by fitting measured and calculated spectra.

ISSN:0021-8979    

DOI:10.1063/1.347709

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Here, Lienard–Wiechert potentials are derived for accelerated charged particle radiation in chiral media. Radiation from moving charges in chiral media is formulated and discussed. As two examples, the radiation patterns of both linear and circular (synchrotron) acceleration in chiral media are found. We focus on the effects of chirality and examine the presence of two eigenmodes of radiation, each with a different refractive index and beam characteristics. The interplay between relative velocity and relative chirality is shown.

ISSN:0021-8979    

DOI:10.1063/1.347719

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A novel computer model of a pulsed H2/SF6nonchain first vibrational overtone HF chemical laser has been constructed to investigate the competition between the overtone and fundamental laser performances. This model including the rotational relaxation process, can simultaneously simulate both fundamental and overtone lasing processes since undesirable fundamental lasing cannot completely be suppressed during experimental overtone oscillation. The relationship between resonator conditions and laser gains was studied. A He diluent gas up to 400 Torr is added to the basic gas mixture H2/SF6=12.5/87.5 (Torr) in order to suppress the fundamental small signal gain. Under this gas mixture the overtone output energy of 117 mJ/lcan be obtained, which is reduced to 63% compared with the one without diluent gas. The overtone output energy is found to be very sensitive to the residual reflectivity of the mirrors over the fundamental lasing band.

ISSN:0021-8979    

DOI:10.1063/1.347729

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We have observed modulation of the dark current in Bi12SiO20(BSO) as a function of prior irradiation, and a time‐varying absorption of radiation that is dependent upon prior irradiation. As a result of these observations and the previously reported observations of photoconductivity, photorefractivity, photosensitivity, and mobility modulation, a model theory of transport has been developed that requires multiple sources of defect/impurity energy levels. A hopping band exists in the forbidden energy gap where these levels coincide. Transport via exchange of charge is possible here, analogous to conduction via hopping in the presence of impurity bands or dense concentrations of defects. Traps capable of containing immobile charge carriers occur where these various levels do not coincide. Large mobility variations can occur as a result of photon‐induced modulation of the Fermi level. Much of the phenomena observed in BSO can be explained through interaction between the multiple defect/impurity energy levels.

ISSN:0021-8979    

DOI:10.1063/1.347683

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We present data on strong carrier‐induced antiguiding present in strained‐layer InGaAs‐GaAs‐AlGaAs lasers. We calculate a value of 12.9 for the PetermannKfactor, and a corresponding antiguiding factor ofb=6.4. The magnitude of the carrier‐induced index depression in the well is estimated to range with drive current from 0.032 to 0.41. In all cases, the antiguiding is much stronger than reported for similar GaAs‐AlGaAs lasers. In addition, we show that emission from short cavity devices occurs from two distinct transitions, with emission from the GaAs barriers contributing a single peak to the far field, and emission from the InGaAs well contributing two lobes. The result is a three‐lobed far‐field pattern at laser threshold of the GaAs barrier emission.

ISSN:0021-8979    

DOI:10.1063/1.347656

出版商:AIP    

年代:1991

数据来源: AIP