摘要:

Recent experimental work has shown that the contrast of near-field optical images depends on the path followed by the tip during the scan. This artifact may misguide the interpretation of the images and the estimation of the optical resolution. We provide a rigorous theoretical study of this effect based on three-dimensional perturbation theory and two-dimensional exact numerical calculations. We quantitatively study the dependence of the artifact on the illumination/detection conditions and on the scattering potential of the sample. This study should provide guidelines for future experimental work. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366270

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Scanning near-field optical microscopy is used to measure the mode profile and evanescent field of a Ge-doped D-shaped optical fiber. The structure of the fiber is determined by differential etching followed by an investigation of the resultant topography with an atomic force microscope. This information is then used to theoretically model the expected behavior of the fiber and it is shown that the theoretical results are in excellent agreement with the experimentally observed field. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365608

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Recent very promising fabrication advances in organic light emitting diodes have brought out the need for a microscopic understanding of the interaction of light emitters, hole blockers, hole transporters, dopants, and their electroluminescence. We report exploratory quantum calculations on spectra with and without the presence of an external electric field, with different “guest-host” arrangements of emitter and dopant molecules. These calculations, when suitably extended, could be used for predictive optimization of candidate materials, and for the design of optimal layer-structure and layer-ordering etc., in diodes and other proposed structures. The molecules studied are: Scandium 8-hydroxyquinoline as the light emitter, a 1,2,4-triazole molecule as a typical hole blocker, and triphenyl diamine derivatives as hole transporters. The dopant or “guest” molecule was selected to be Nile Red. Studies on the use of a highly conjugated system such as C60 as an element of layer structure are also presented here. The results give an insight into the physical processes at the molecular level and provide guidance in designing optimized material for use in “organic” light emitting diodes. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365609

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We present a method for the interference of three electron waves and its application to direct visualization of pure phase objects such as electromagnetic microfields. Using a transmission electron microscope equipped with a field-emission electron gun and two electron biprisms, an object wave and two reference waves at either side of the object wave are superposed to produce a new type of interference pattern. In this pattern, equal-phase lines of the object wave are directly displayed as intensity modulation of periodic interference fringes. An electric field around a latex particle, induced by electron-beam irradiation, has been observed. The electric charge of the particle is estimated, from observed phase shift, to be6.4×10−17 C,which is equal to about 400 electrons. A change of the electric field around charged alumina particles at high temperatures has been observed dynamically. Magnetic flux lines emerging from a barium ferrite particle are also visualized. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365610

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

This article compares switching behaviors between negative(Nn)and positive(Np)dielectro-anisotropic nematic liquid crystals driven by an in-plane electric field which is generated with interdigital electrodes. Even forNptype liquid crystals, excellent viewing angle characteristics were obtained as expected. Theoretical descriptions of the switching principle, i.e., threshold behavior and response mechanism, could be applied to both theNnandNptype liquid crystals. However, the orientational deformation of theNptype liquid crystals caused by a distorted electric field which occurred near the edges of electrodes was not the same as that of theNntype liquid crystals. The switching of theNptype liquid crystals near the edges of the electrodes by this field was followed by the in-plane switching of the liquid crystals between the electrodes. This remarkably distinctive dynamical behavior implied a difference in the response of the longitudinal axes forNnandNptype liquid crystal molecules. The longitudinal axes of the latter seemed to be sensitive to the electric field component perpendicular to the substrates when applying the in-plane electric field. Furthermore, these experimentally obtained results were supported by computer simulations which analyzed the liquid crystal director distribution and transmittance pattern from edge-to-edge of a pair of electrodes when applying the in-plane electric field. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365611

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The growth of thin films made from Samarium-doped alkaline earth fluoro halides (AEFH) of compositionSrxCa1−xFCl:Sm2+(0⩽x⩽1)is presented and the possibilities are studied to increase significantly the inhomogeneous width of theSm2+optical zero phonon transitions. The best films were obtained when grown with a molecular beam deposition (MBD) method involving two separate molecular beams: one for the alkaline earth fluoride, the other one for the alkaline earth halide (Cl or Br). The results demonstrate that the double beam MBD technique employed is able to produce pure and mixed Matlockite films with targeted composition. The results of mainly optical studies of the samariumf–ftransitions and of other complementary techniques are used to assess the composition and homogeneity of the films. With the aid of a model the composition dependence of the positions of specific opticalf–femission lines is established. Their inhomogeneous linewidth is compared with that of corresponding emission lines obtained from bulk samples of the same chemical composition. The linewidths of the films are only slightly larger (∼1.5–2times). Thus, the film morphology cannot be exploited to increase substantially the inhomogeneous broadening of the luminescence lines. A novel approach to increase this broadening was devised, theoretically modeled and successfully tested by using multilayered sandwich-type thin films in conjunction with interdiffusion. Films with cation disorder of compositionSrxCa1−xFCl(x=0.5 /0/ 0.5/ 0/..)were grown. The5D1→7F0Sm2+emission linewidth is thereby increased to70 cm−1full width half maximum. A width of100 cm−1may be obtained within the composition rangex=0,x=1.This represents an enhancement by a factor of 3–5 in comparison with the largest values obtained in appropriate mixed bulk AEFH of constant composition. A factor>50is gained in comparison with pure bulk AEFH hosts. The room temperature (RT) homogeneous linewidths, on the other hand, are similar to those found in bulk mixed crystals of constant composition. The intrafilm host cation diffusion during film growth of the sandwich structures was further studied. A diffusion constant of2⋅10−19 m2 s−1for the Sr and Ca ions was deduced from this observation. These films are among the most promising materials for optical mass data storage through RT hole burning. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365612

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We present the temporal and spectral evolution of stimulated emission from (Zn,Cd)Se/Zn(S,Se)/(Zn,Mg)(S,Se) multiple quantum well lasers at 2 K after picosecond optical excitation. For pumping well above the threshold, ultrahigh-frequency oscillations of up to 100 GHz are observed. These oscillations are generated by a multimode emission through mode beating caused by the coherent superposition of longitudinal ground modes and higher lateral modes. The modelling of the experimental results shows that the modes are coupled to some extent. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365613

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The behavior of a rapidly pulsed radio-frequency capacitively coupled parallel plate reactor has been investigated using time-resolved voltage probe, microwave interferometer, and optical emission techniques. The reactor was operated with 50 mTorr of argon and 100 W rf power (measured at the generator) at 13.56 MHz supplied to the 100-mm-diam powered electrode, with pulse durations of 25 and 100 &mgr;s. For low repetition rates (50 Hz) the voltage envelope has a characteristic form which has been entitled the “Bird’s Head.” There is no plasma present at the beginning of the pulse, so that an initial breakdown phase occurs. This phase lasts about 600 ns, after which time the plasma density is sufficiently high for the Debye length to enter the gap between the electrodes and for sheaths to form on the electrodes. In asymmetric parallel plate reactors the blocking capacitor in the matching circuit charges such that the powered electrode acquires a continuous negative voltage offset (the so-called dc bias). In this system the charging time of the capacitor is longer than the rise time of the rf voltage. Consequently, for the first few &mgr;s of the pulse the discharge is symmetric (no dc bias) and confined between the rf and the adjacent earthed electrode. As the bias voltage increases the discharge fills more of the reactor and becomes asymmetric. The rate at which the blocking capacitor charges (due to net electron current from the plasma to the powered electrode) is controlled by the Bohm-criterion limited flux of ions to the earthed walls of the reactor, as shown by particle-in-cell simulations in H. B. Smith, C. Charles, and R. W. Boswell, J. Appl. Phys.82, 561 (1997). At high repetition rates (20 kHz) the plasma density is hardly modulated, there is no breakdown or symmetric phase, and only the electron temperature and dc bias are modulated. The conditions which lead to a symmetric discharge phase are defined. A simple analytical model is developed to describe the temporal evolution of the plasma density and electron temperature. The model is in good qualitative agreement with the observations, and predicts an average electron energy of 10’s of eV during the first few &mgr;s of the symmetric discharge. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365614

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A one dimensional (1D) particle-in-cell (PIC) computer simulation has been used in conjunction with a small experimental plasma reactor, to investigate the effects of pulsing on a low pressure, capacitively coupled, rf argon plasma. In particular this article investigates the time-constants involved in the development and evolution of the bias voltage in asymmetric reactor geometry. Surprisingly, the charging time for the blocking capacitor does not occur on electron time scales, but is influenced primarily by the ambipolar drift of ions to the earthed electrode. It is shown that following plasma breakdown there is a net current flow in the system which charges the blocking capacitor in the external matching circuit and produces the bias voltage. Both the PIC simulation and the experimental measurements show that a net current flow is produced by a delay in the onset of the electron current to the earthed electrode, which is correlated to the charging time of the capacitor. From the simulation it can be seen that during this period the plasma potential in the center of the discharge is higher than one would expect, preventing electrons from reaching the earthed electrode. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365615

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The effect of aspect ratio (depth/width) on charge buildup in trenches during plasma etching of polysilicon-on-insulator structures is studied by Monte Carlo simulations. Increased electron shadowing at larger aspect ratios reduces the electron current to the trench bottom. To reach a new charging steady state, the bottom potential must increase, significantly perturbing the local ion dynamics in the trench: the deflected ions bombard the sidewall with larger energies resulting in severe notching. The results capture reported experimental trends and reveal why the increase in aspect ratio that follows the reduction in critical device dimensions will cause more problems unless the geometry is scaled to maintain a constant aspect ratio. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365616

出版商:AIP    

年代:1997

数据来源: AIP