摘要:

The interaction of plasmon surface polaritons (PSPs) with the tunneling junction of a scanning tunneling microscope (STM) has been investigated by measuring the current response to a modulated plasmon excitation. From the dependence on modulation frequency, bias voltage, steady‐state current, and metal film structure it is concluded that it originates mainly from PSP‐induced heating and the associated expansion of the tunneling tip. The modulated current signal (or the equivalently modulatedz‐piezo voltage after the feedback loop) can be recorded while scanning the surface by STM. The resulting PSP maps reflect the polycrystalline metal film structure on the scale of a few nanometers convoluted to some extent with the tip shape and the PSP field distribution. The latter can be used to differentiate between different metal films on the micrometer scale.

ISSN:0021-8979    

DOI:10.1063/1.352030

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A compact laser synchrotron source (LSS) is proposed as a means of generating tunable, narrow bandwidth, ultra‐short pulses of hard x rays. The LSS is based on the Thomson backscattering of intense laser radiation from a counterstreaming electron beam. Advances in both compact ultra‐intense solid‐state lasers and high brightness electron accelerators make the LSS an attractive compact source of high brightness pulsed x rays, particularly at photon energies beyond ∼30 keV. The x‐ray wavelength is &lgr;[A˚]=650 &lgr;0[&mgr;m]/Eb2[MeV], where &lgr;0is the laser wavelength andEbis the electron beam energy. ForEb=72 MeV and &lgr;0=1 &mgr;m, x rays at &lgr;=0.12 A˚ (100 keV) are generated. The spectral flux, brightness, bandwidth, and pulse structure are analyzed. In the absence of filtering, the spectral bandwidth in the LSS is typically ≲1% and is limited by electron beam emittance and energy spread. Two configurations of the LSS are discussed, one providing high peak power and the other moderate average power x rays. Using present day technology, the LSS can generate picosecond pulses of x rays consisting of ≳109photons/pulse with a peak brightness of ≳1020photons/s mm2 mrad2(0.1% BW) and photon energies ranging from 50 to 1200 keV.

ISSN:0021-8979    

DOI:10.1063/1.352031

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

An asymptotic treatment of power transport by guided modes in two parallel, identical planar dielectric waveguides is presented in the weak coupling limit. The small parameter is the factor by which the field amplitude decays from one waveguide to the other. The maximum power is transferred from one waveguide to the other periodically in the propagation direction, the spatial period being known as the interaction length. When the power in one waveguide is a maximum, there is a small amount of residual, undesirable power in the other waveguide. This cross talk is intrinsic to a coupled system. It is shown that the product of the intrinsic cross talk and the interaction length is a constant depending only on the parameters of the corresponding uncoupled waveguide. Also, this constant is identical to the transverse electric mode and the transverse magnetic mode. This cross‐talk relation shows that the intrinsic cross talk can be reduced only at the expense of increasing the device dimensions.

ISSN:0021-8979    

DOI:10.1063/1.352032

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The gain‐current coefficient and current density at transparency of GaAs/AlGaAs and InGaAs/GaAs/AlGaAs quantum‐well (QW) laser structures have been calculated as a function of the QW thickness by a straightforward numerical calculation. The optimum QW thicknesses are determined to be 100 and 105 A˚ for typical GaAs/AlGaAs and InGaAs/GaAs/AlGaAs QW laser structures, respectively, using the widely accepted semilogarithmic expression for threshold current density of QW lasers. These calculated optimum QW thicknesses agree with the reported experimental data very well. The reduction of the laser threshold current density as a result of using the optimum QW thickness is estimated to be 15% typically.

ISSN:0021-8979    

DOI:10.1063/1.352033

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

When a photoconducting polymer is illuminated by light with an intensity that varies sinusoidally with position, the space‐charge electric field, hole density, ionized photosensitizer density, hole trap density, and other properties, will also vary with position. We derive an expression for the zero‐order Fourier component of the hole density and a linear system of equations for the first‐order Fourier component of a number of variables. The system of equations is solved for the steady‐state first‐order Fourier component of the electric field for the case in which hole untrapping is negligible or no hole traps are present. Next, the equations are simplified for the case in which the initial trap density is far greater than the density of holes, and for the case of no hole traps. Finally, we compare the zero‐order hole density and first‐order electric field calculated from our equations to values obtained by Fourier transforming the results of numerical calculations.

ISSN:0021-8979    

DOI:10.1063/1.352034

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A periodically domain‐inverted structure in LiTaO3has been fabricated for quasiphase‐matched (QPM) second‐harmonic generation (SHG). Uniform proton exchange is achieved with pyrophosphoric acid. Using this technique with a Ta mask followed by heat treatment, the periodically domain‐inverted structure for quasiphase matching is realized over large areas. The effect of fabrication conditions on the domain‐inverted profile is investigated to form a periodic structure having a considerable depth and a short inversion period. Consequently, a third‐order QPM‐SHG device can be fabricated by using this periodically domain‐inverted structure and proton‐exchanged waveguide. To investigate period regularity of the domain‐inverted structure and nonlinearly of the inverted region, phasematching characteristics and conversion efficiency of this device are examined. The formation mechanism of the domain inversion is also discussed.

ISSN:0021-8979    

DOI:10.1063/1.352035

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A theory as the basis for solving nonlinear boundary value problems is described. A new analysis is presented for nonlinear reflection, which reveals that there is a two‐dimensional spatial increase of the second‐harmonic wave. The accretion along the propagation direction is the result of the self‐interaction of the primary wave; the one in the wave front is due to the boundary restriction.

ISSN:0021-8979    

DOI:10.1063/1.352036

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The neutral heterogeneous chemistry between reactive species formed in a glow discharge plasma and a polymer surface is investigated. Oxygen and fluorine atom concentrations in an O2/CF4plasma afterglow are measured by gas phase electron resonance spectroscopy. These atom concentrations are correlated to etch rate data via a simple heterogeneous model which divides the etching process into three steps: initiation, etching, and passivation. The model predicts etch rate data well using measured atom concentrations. When the substrate temperature is changed, the etching step is affected most, presumably limited by product desorption. Dilution of the discharge with argon enhances the etch rate in two ways. First, the homogeneous chemistry is altered, most notably affecting the fluorine atom concentration. The homogeneous effects are taken into account through measured atom concentrations. Second, the argon metastables play a role in the heterogeneous chemistry, enhancing the etching step. From an estimation of this enhancement based on the shift in etch rate maximum, the magnitude of etching in an argon diluted discharge can be predicted.

ISSN:0021-8979    

DOI:10.1063/1.352037

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Strong microwave absorption was observed when a 10 GHz source illuminated an underdense collisional plasma that had a density gradient scale length several wavelengths long. Significant reductions in angular scattering and cross‐polarized components were also observed. These experiments confirm that absorption was the dominant process. The plasma was created by the photoionization of tetrakisdimethylaminoethylene molecules seeded into atmospheric pressure helium. Sparkboard arrays provided the intense vacuum ultraviolet ionizing radiation. Plasma density profiles were measured using transverse scans of 9.7 GHz probe microwaves and were found to approximate an Epstein profile. The absorption at 10 GHz by this plasma was as large as 28 dB in direct backscattering and 15–20 dB when orthogonally polarized microwaves were launched and detected. The peak absorption scales with sparkboard energy in a way that suggests that electron‐ion recombination is the dominant electron‐loss mechanism at high plasma densities.

ISSN:0021-8979    

DOI:10.1063/1.352038

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Planar laser‐induced fluorescence (PLIF) is used to monitor Cu atoms and Cu2produced by excimer laser ablation of a copper target (308 nm, ≳10 J/cm2, 1–3 GW/cm2) expanding rapidly into helium background gas at pressures ranging from 10 to 100 Torr. The Cu2results from gas phase condensation of the copper atoms ablated from the target in the regions of highest Cu atom density as expected, but the maximum Cu2laser‐induced fluorescence (LIF) signal occurs significantly after the maximum of the Cu signal. Rotationally resolved excitation scans of Cu2utilizing theA–X(0,0) transition indicate that the Cu2has reached equilibrium with the 300 K background gas. An extensive search for Cu3via LIF failed, indicating that Cu3is present only in very low ‘‘steady state’’ number density in the plume. This data is explained qualitatively by a simple kinetic model. In addition to the kinetic information, it is clear from the PLIF images that viscous eddy formation becomes more pronounced as the backing gas pressure increases; however, we see no evidence of turbulence in the plume even at the highest backing gas pressure studied. The PLIF technique allows us to observe the onset of condensation directly as well as to obtain information about the expansion dynamics of the plume not easily obtainable by other means.

ISSN:0021-8979    

DOI:10.1063/1.352039

出版商:AIP    

年代:1992

数据来源: AIP