摘要:

The use of a high-repetition-rate (250 kHz) regenerative amplified laser with &mgr;J pulse energy in a free-space THz beam electro-optic sampling system has produced a significant improvement in both the signal-to-noise ratio(>105)and absolute probe beam photomodulation depth (>19&percent;). Focal plane images of the electric field distribution (strength and polarity) in dipole and quadrupole planar photoconductive emitters are presented. Preliminary results of real-time 2D THz images of moving objects have also been obtained. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119803

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We investigate the switching bistability based on the interaction between dynamic flow and director rotation in twisted nematic liquid crystal cells. Numerical calculation shows that there exists a general type of bistable twisted director configuration. Two specific cases are verified experimentally. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119804

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

With the acousto-optic nondestructive testing technique, phase shifts between the incident and the reflected ultrasound are measured at critical angles of the investigated material. This letter is focusing theoretically as well as experimentally on the influence of a small surface roughness (some micrometers) of a thick material on the phase information. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119805

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The carrier transport of a new calamitic liquid crystal, i.e.,2-(4′-octylphenyl)6-dodecyloxynaphthalene (8-PNP-O12) was investigated by the time-of-flight technique. The fast ambipolar electronic conduction was observed in two smectic phases of 8-PNP-O12. The carrier mobilities were determined to be1.6×10−3 cm2/V⋅sin the lower temperature smectic phase (Sm1) between 79 and 100 °C and to be2.5×10−4 cm2/V⋅sin the higher temperature phase (Sm2) between 100 and 121 °C, regardless of carrier signs, i.e., electron and hole. The slower ambipolar transport was observed in isotropic phase as well, whose mobility was8×10−5 cm2/V⋅s.These mobilities were independent of applied electric fields and temperature. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119806

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

An Arrhenius-like behavior of the silane dissociation rate constant as a function of the power actually fed in a rf discharge system is reported. The rate constant is calculated using mass-spectrometric measurements of silane consumption for 50 and 75 mTorr discharges, while the power consumed in the discharge is determined by using detailed voltage and current measurements. The slope of the natural logarithm of the dissociation rate constant as a function of the inverse of the discharge power increases with pressure. This is attributed to a change in the shape and/or the mean energy of the electron energy distribution function, while the linearity indicates an analogous increase of the active electron concentration with increasing discharge power, excluding a significant change in the shape of the electron energy distribution function. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119807

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We have established, through time correlated plasma emission and electrode and plasma potential measurements, that the near electrode emission observed in asymmetric capacitively coupled 13.56 MHz-driven hydrogen plasmas is caused by field reversal that leads to sheath collapse. Near-electrode emission has now been observed in Ar and He. The field reversal appears to be due to collision-induced electron drag. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119808

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We present the investigation of novel metal/organic monolayer/metal heterostructure diodes. Our technique provides well-defined, stable, and reproducible metallic contacts to a self-assembled monolayer of 4-thioacetylbiphenyl with nanoscale area. Electronic transport measurements show a prominent rectifying behavior arising from the asymmetry of the molecular heterostructure. Variable-temperature measurements reveal that thermal emission of electrons over a barrier of 0.22 eV dominates for electron injection from Ti into the organic layer while the transport for electron injection from Au into the organic layer satisfies the formula for hopping conduction. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.120195

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The normalizedwidth=standarddeviation of island radius/mean island radius(&sgr;r/⟨r⟩)of molecular beam epitaxy grown Ge on Si(100) coherent island quantum dot size distributions is analyzed for various deposition conditions. It is found that this width decreases as substrate temperature increases independent of deposition flux. This result is interpreted in the context of models which suppose that the energy barrier for edge atom detachment decreases with island size. The faster diffusion kinetics at higher growth temperatures allow these detached edge atoms to more rapidly find the smaller islands producing sharper island size distributions. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119809

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The near-room temperature current-voltage (I-V) characteristics of small structures made from the metal-cluster materialAu55[P(C6H5)3]12Cl6were studied. It is shown that these electron-beam defined structures have highly nonlinear characteristics with features, including a threshold voltage and scaling behavior, which are consistent with Coulomb charging of individualAu55cores in a disordered array. Applied radio frequency signals introduce plateaus in theI-Vcharacteristics, which demonstrates the presence of coherent tunneling in these cluster systems. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.120568

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Curved surface morphologies resulting from disclinations are unusual in inorganic phases because the relatively large lattice energies generally permit only translational dislocations. In this letter, we show, however, that disclinations can be readily observed and identified in composite organic/inorganic phases. Disclinations and dislocations in MCM-41, a two-dimensional hexagonal (p6mm) mesophase crystal synthesized using silicate and surfactant precursors, were investigated by using transmission electron microscopy. We observed two types of dislocations and two types of disclinations. All dislocations observed are of the smallest Burgers vectora(the basis vector). Dislocation dipoles and disclination quadrupoles appear more frequently than isolated ones. ©1997 American Institute of Physics.

ISSN:0003-6951    

DOI:10.1063/1.119810

出版商:AIP    

年代:1997

数据来源: AIP