摘要:

Hamaker constants and dispersion forces interactions of materials are of increasing interest and the advent of atomic force microscopy (AFM) force measurements represents a new opportunity for quantitative studies of these interactions. A critical problem is the determination of a force–distance relation for realistic AFM probes. Due to the inadequacies of existing power‐law sphere–plane models to describe the probe–sample system, we present a new parametric tip force–distance relation (PT/FDR). A surface integration method is developed to compute the interactions between arbitrarily shaped bodies. The method is based on the Hamaker pairwise integration in a continuous fashion, reducing the six‐dimensional integration to a four‐dimensional scheme. With this method, the PT/FDR is obtained and a nonlinear fitting routine is used to extract the model parameters and the Hamaker constant from AFM force–distance data. From the sensitivity analysis of the fitting of synthesized AFM force–distance data, one finds that, for large tip radius (compared to separation), the force is proportional to the product of the Hamaker constant and tip radius. Unique determination of the Hamaker constant can be achieved if a small radius tip is used in the AFM scan. By fitting to literature data, the effectiveness of the PT/FDR is shown. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363680

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The electrical resistivity of polymer composites is studied as a function of temperature. The initial resistivity &rgr; of thermoplastic or thermoset containing a metallic filler is in the range of 1–10×10−2&OHgr; cm. Around the curing temperature of epoxies, the resistivity increases by eight to twelve orders of magnitude. For thermoplastic polymers, however, the transition temperature is related to the melting temperature at which a strong volume increase occurs. Hence, the choice of polymer and its processing determine the transition temperature from a conducting state to an insulating state. For a variety of polymers we have observed transitions between 80 and 200 °C. Due to a sharp and strong transition at a predetermined temperature, such materials can be used as temperature sensors. Since the resistivity of the cold state is low, they can also carry rather high currents. The balance between heating and cooling determines then a critical value for the current. Thus, the materials can also serve as current sensors. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363682

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

LiClO4⋅3H2O has been found to be 90° phase‐matchable for ultraviolet generation at 0.2660 &mgr;m at a crystal temperature of 30.6 °C. A peak power as high as 28 MW with an average power of 2.2 W was obtained in a 1.5‐cm‐long crystal. Sellmeier’s equations and nonlinear constant of this crystal are reported. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363683

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have observed a strong correlation between the internal fields and the infrared absorption spectra of OH−ions in congruent LiTaO3crystals. After reversal of the direction of polarization (Ps) of the crystal by an electric field at room temperature, (a) the internal field is antiparallel toPs, (b) the relative intensities of the infrared absorption bands of OH−ions (at ∼3462, ∼3476, and ∼3490 cm−1) change, and (c) the 180° domain walls are visible under a light microscope. Annealing the domain‐reversed crystal at 200 °C or above and cooling back to 25 °C has the following effects: (a) The internal field realigns parallel toPs. (b) The shape of the OH−spectrum returns to the shape observed before domain reversal. The time–temperature dependence of the shape of the infrared absorption spectrum of OH−ions also shows the same characteristics as that of the realignment process for the internal field. (c) The optical domain wall contrast becomes extremely weak. Infrared absorption studies suggest that domain inversion as well as the heating of LiTaO3crystal induce structural changes that are similar in nature. It is proposed that the origin of internal fields lies in OH−ions and/or nonstoichiometric point defects in the LiTaO3crystals. We also explain the optical 180° domain wall contrast based on internal fields. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363684

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A rigorous analytical treatment of nonradiative deexcitation and thermal‐conduction transient evolution in solid‐state laser media, resulting from intense optical pumping with rectangular pulses from a time‐gated laser source, is presented. This situation arises in rate‐window photothermal detection from laser rods with bulk and surface absorptions, the latter being due to polishing during the manufacturing of the rod. Numerical simulations of the theory show that the surface nonradiative (optical‐to‐thermal) energy conversion term is likely to dominate even at absorptances on the order of 1%–2%. Therefore, polishing optimization appears to be necessary in order to minimize laser losses at the surfaces when laser rods are active in a resonator cavity. The present theory also provides physical insights into the very different nature of the bulk‐ and surface‐originating heat‐conduction transients, as well as on the profile of the superposition photothermal wave form and its dependence on the optical, thermal, and metastable‐state relaxation parameters. As a prelude for input to the photothermal theory, a treatment of the excited‐state dynamics of a typical laser manifold pumped by an intense laser beam, away from or near saturation, is developed, and the luminescence and photothermal energy source profiles are calculated analytically. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363685

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Photonic integrated circuits in silicon require waveguiding through a material compatible with silicon very large scale integrated circuit technology. Polycrystalline silicon (poly‐Si), with a high index of refraction compared to SiO2and air, is an ideal candidate for use in silicon optical interconnect technology. In spite of its advantages, the biggest hurdle to overcome in this technology is that losses of 350 dB/cm have been measured in as‐deposited bulk poly‐Si structures, as against 1 dB/cm losses measured in waveguides fabricated in crystalline silicon. We report methods for reducing scattering and absorption, which are the main sources of losses in this system. To reduce surface scattering losses we fabricate waveguides in smooth recrystallized amorphous silicon and chemomechanically polished poly‐Si, both of which reduce losses by about 40 dB/cm. Atomic force microscopy and spectrophotometry studies are used to monitor surface roughness, which was reduced from an rms value of 19–20 nm down to about 4–6 nm. Bulk absorption/scattering losses can depend on size, structure, and quality of grains and grain boundaries which we investigate by means of transmission electron microscopy. Although the lowest temperature deposition has twice as large a grain size as the highest temperature deposition, the losses appear to not be greatly dependent on grain size in the 0.1–0.4 &mgr;m range. Additionally, absorption/scattering at dangling bonds is investigated before and after a low temperature electron‐cyclotron resonance hydrogenation step. After hydrogenation, we obtain the lowest reported poly‐Si loss values at &lgr;=1.54 &mgr;m of about 15 dB/cm. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363686

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A simple technique for measuring the thermal diffusivity of dielectric films on thermally sinking substrates is proposed and demonstrated. It is an outgrowth of the thermal pulse technique for measuring charge and polarization profiles. The thermal diffusivity is derived from the transient electrical response induced by a thermal pulse applied to a dc voltage‐biased sample. Because the response is proportional to the bias voltage, the signal‐to‐noise ratio is adjustable independently of the thermal pulse energy and may be made as large as required for determining the diffusivity with high precision. Measuring times of around 1 ms or shorter are sufficient for polymers with thicknesses up to 10 &mgr;m. Experimental results for two different polyimide films spin coated on crystalline silicon substrates are presented and discussed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363687

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Sound speeds, at pressure, and the overdriven Hugoniot were measured for the plastic‐bonded explosive PBX‐9501. The two curves intersect at the Chapman–Jouguet (CJ) state because of the sonic conditionD=c+u. This permitted a novel determination of the ‘‘thermodynamic’’ CJ pressure. A value of 34.8±0.3 GPa was obtained. The data permit a direct experimental determination of the isentropic gamma, &ggr;S=−(∂lnP/∂lnV)S, and the Gru¨neisen parameter, &ggr;=V(∂P/∂E)V, in the overdriven pressure range. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363681

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Oxygen rf glow discharges between parallel plates were numerically analyzed by using the relaxation continuum model. The result at a frequency of 13.56 MHz, sustaining voltage of 150–350 sin &ohgr;tV, pressure of 0.15–1.0 Torr, and stainless steel surface, shows that O2(a1&Dgr;g) has a number density that is an order of magnitude larger than that of atomic oxygen. The plasma density as a function of pressure has a maximum at about 0.2 Torr, and decreases with increasing pressure due to the increase in the net rate of associative detachment from O−by O2(a1&Dgr;g). The comparison between the discharges in two surface materials, stainless steel and copper, indicates that the number densities of O2(a1&Dgr;g) and atomic oxygen strongly depend on the surface loss probability, and that consequently the plasma density is also changed by replacing the surface material. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363649

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A study of the temporal response of a biased Fabry–Perot etalon with a nematic liquid crystal exposed to short (∼10 &mgr;s) voltage pulses is presented. It is found that the finesse is the crucial factor determining the achievable switching time. For a given maximum transmission the reflectivities of the Fabry–Perot mirrors are optimized to achieve minimum switching times. In particular, switching times of 0.5 &mgr;s with an initial transmission of 3% and a maximum value of 70% have been obtained. The experimental results are in good agreement with theory. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363688

出版商:AIP    

年代:1996

数据来源: AIP