Δn Values Research Articles

In‐plane birefringence and elongation behavior of uniaxially drawn aromatic polyimide films

AbstractSix kinds of aromatic poly(amic acid) films, the precursor of polyimides, were uniaxially drawn by constant loads during thermal curing with varying the final curing temperature and the load. The in‐plane birefringence (Δn) generated by uniaxial orientation of polyimide molecules in the film plane was investigated in relation with their molecular structures and the elongation behavior of polyimide films. The polyimides derived from pyromellitic dianhydride (PMDA) showed much larger values of Δn than the polyimides derived from dianhydrides having rotatable diphenyl linkages. The Δn of the former polyimides increases with increasing the final curing temperature and the elongation between the most shrunken and the most elongated states during curing. In particular, the polyimides having rod‐like structures show the largest value of Δn. In contrast, the latter polyimides take maximal values of Δn at much lower temperatures. The introduction of hexafluroisopropiridene (C(CF3)2) group into the main chain effectively lowers the values of Δn. This result can be related to the inherent low polarizability anisotropy and the decreased molecular packing constant that are characteristic of highly fluorinated groups. These experimental results are qualitatively explained by the calculations of polarizability anisotropies of model compounds using FPT‐MNDO‐PM3 theory. Copyright © 2001 John Wiley & Sons, Ltd.

On a Question of Mordell and a Spectrum of Linear Forms

Let L1(x), L2(x), …, LN(x) be N real linear forms in N variables defined byformula heresuch that the associated N × N matrix of coefficients C = (cmn) is unimodular. In the classical theory of geometry of numbers, Minkowski's well-known linear forms theorem asserts that for any positive numbers e1, e2, …, eN satisfying e1, e2, …, eN [ges ] 1, there exists a lattice point p ∈ ℤN, p ≠ 0, such thatformula hereIn 1937, Mordell [7] posed the following question which may be viewed, in some sense, as a converse to Minkowski's result. Does there exist a constant ΔN such that for each N × N unimodular real matrix C, there exist positive real numbers e1, e2, …, eN satisfyingformula heresuch that the only integer solution to the inequalities of (1.1) is p = 0? Moreover, if ΔN does exist, what is the best possible value for ΔN, that is, what is the supremum of all such admissible values of ΔN? Clearly, by Minkowski's theorem, if ΔN exists, then ΔN < 1.

Optimization of “Guest-Host” Liquid Crystal Display

Using MOUSE-LCD software we analyze, how the parameters of the “guest-host” cell effects such basic characteristics of LCDs as the contrast, brightness and magnitude of the viewing angles. The basic construction of “guest-host” LCD includes one input polarizer and reveals a negative contrast. We showed, that to increase the average contrast, transmission and viewing angles of “guest-host” LCDs we have to use the LC cells with the twist angle of 90°, while the thickness of the LC layer should be sufficiently high. The concentration of the dye must be optimized for the applied values of LC cell thickness and twist angle. The higher values of LC optical anisotropy Δn results in the increase of the display contrast in our case, contrary to the non-polaroid variant of “guest-host” LCDs. One-polarizer construction of a transmissive “guest-host” LCD with a phase retardation plate was also analyzed. The application of a phase compensator in GH-LCDs allows to use thin LC cells with a high concentration of dichroic dye to get a maximum contrast and transmission for minimum response times. Our calculations may be helpful for the production of the new efficient “guest-host” LCD configurations

Antibacterial Activity of 3-Methylcyclopentanone Derivatives in Relation to Methylenomycins

3-ethylcyclopentanone derivatives were synthesized for examination of antibacterial activity, and it was evaluated by molecular orbital calculation.This parameter at minimum inhibitory concentration (MIC) was found closely correlated with absolute electronegativity (χ) derived from the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The values of ΔN and ΔE calculated from χ and absolute hardness (η) of 3-methylcyclopentanones and amino acids also showed good correlation with MIC. In relation to methylenomycins, cyclopentanone antibiotics, antibacterial activity was investigated by comparing the χ, ΔN and ΔE.2, 3-Epoxy-3-methyl-5-methylenecyclopentanone 4 whose antibacterial activity was the strongest, may possibly have potential as a cyclopentanone antibiotic based on χ, ΔN and ΔE data.

Surface Acoustic Wave Induced Birefringence in In0.21Ga0.79As-GaAs Multiple Quantum Wells for Optical Modulation ≈1000 nm under Normal Incidence

This paper presents observation of quantum Confined Stark effect induced by the surface acoustic waves (SAW) propagating in multiple quantum well (MQW) layers. It also presents the evidence for the first time of SAW induced birefringence under surface normal mode operation in MQWs. MQW layers usually do not exhibit significant birefringence when the light is normally incident. The launching of surface acoustic waves in In0.21Ga0.79As-GaAs MQW layers produces significantly different strain along the direction of propagation (y) as compared to the lateral direction (x), and this in turn results in differential absorption coefficient Δα (= αx - αy) and birefringence (Δnbir). The enhanced birefringence due to excitonic effect depends on the magnitude of RF power exciting the SAW transducer. An interdigitated transducer operating at 119 MHz with an aperture (w) to wavelength (Λ) ratio of 25 was used to launch the surface acoustic waves. Typical values of Δn are in the range of 0.01-0.02 for polarized light at ≈1.01 μm wavelength when the RF power is varied between 0.5-1.2 Watt. Computations of Δn are in agreement with experimental data. The SAW propagation also induces a perpendicular component Ez of electric field which results in conventional Stark effect producing absorption and index changes in MQWs that vary as a function of SAW power.

Orientational Elastic Deformations of Main-Chain LC Aromatic Polyesters and Low Molecular Weight Nematics in a Magnetic Field

Orientational elastic deformations in a magnetic field were investigated in the nematic phase of polyphenylterephthaloyl-bis-(4-oxybiphenyl-4′-carbonyl), polylterephthaloyl-bis-(4-oxybiphenyl-3′-carbonyl), and their low molecular weight analogues. Temperature dependences of birefringence Δn were obtained and used to evaluate the degree of orientational order S. The splay and bend elasticiy constants, K1and K3, were determined. The elongation of the rigid mesogenic core in polymer molecules leads to an increase in the K3/K1 ratio, just as in the case of low molecular weight nematics. No effect of molecular weight on the values of Δn and Ki was detected.

Diet shift in fish following competitive release

We determined the diet preference of roach (Rutilus rutilus), perch (Perca fluviatilis), and bream (Abramis brama) before and after a fish removal program in Lake Finjasjön, Sweden. The biomass of roach and bream was reduced to 33 and 10%, respectively, after the program. The predominate diet change following this major reduction in fish biomass was an increased use of benthic organisms by most size-classes of roach. Also, bream shifted to benthos at an earlier ontogenetic stage. These diet shifts were most probably attributed to the drastic reduction in biomass of the benthivorous bream, resulting in underexploited benthic invertebrates. In order to test if alterations in diet choice were reflected in the composition of stable isotopes of consumer tissue, we determined the temporal changes in the 15N/14N ratio (δN) of potential fish prey as well as in fish tissue. No temporal trends were found for δN of fish, possibly due to the high temporal variability in δN of zooplankton. However, minimum and maximum δN values of major food organisms (chironomids and zooplankton) were generally reflected in the δN of both small roach and perch one sampling occasion later (3 months) and in piscivorous perch (exclusively feeding on small fish) two sampling occasions later. Hence, the stable isotope composition could be followed through food links, providing that the consumer mainly fed on one specific food item. However, several other predictions regarding connections between diet and stable isotope composition were not corroborated. We conclude that stable isotope analysis of consumer tissue cannot replace traditional methods of diet determination, but might well provide complementary data.

Holographic storage media based on optically active bistable defects

We describe a family of reversible holographic storage materials which exploit the bistability of the crystal defect known as the “DX” center. Crystals containing these defects have the characteristics of local photorefractive materials in that their refractive index is modified in proportion to the local optical energy absorbed. This refractive index change, which results from the release of electrons from the DX deep trap states into the conduction band, is persistent at low temperatures due to a capture barrier, Ecap, which limits reformation of the DX centers. The effect is reversed by heating above an annealing temperature, which scales with Ecap and varies with the crystal host and active dopant. A number of DX materials have now been identified with long-term persistence temperatures ranging from 50 to 180 K. In this paper, we briefly review the physics of the DX center and present theoretical estimates of several important optical properties of these materials based on a simple model. We calculate spatial resolution, maximum refractive index shift, and sensitivity, and compare our predictions with measurements on one member of the DX family, AlGaAs:Te. In a 345 μm thick sample of this material doped at 9×1017 cm−3, we find a refractive index shift, Δn, of 2×10−3 and an exposure sensitivity, S, of 0.012 cm3/J. Our expectation that the maximum refractive index change scales linearly with the doping density is consistent with our previous measurement of Δn=1.1×10−2 obtained for a sample of AlGaAs:Si doped at 4×1018 cm−3. The measured values of Δn and S, are, respectively, two and three orders of magnitude larger than corresponding values for the photorefractive material LiNbO3, and are shown here to be independent of exposing irradiance from 10−3 to 108 W/cm2. At the latter irradiance, the refractive index shift is shown to occur with a material response time shorter than our measurement limit of several picoseconds. Thus, this material exhibits high sensitivity, large refractive index change, and fast write time, all desirable properties of an optical holographic storage medium. Phase gratings written in AlGaAs:Te using low-power (mW) beams from infrared diode lasers give diffraction efficiencies from 30% to 55% for grating periods from 0.13 to 15 μm. No degradation of sensitivity is observed after large numbers of exposure–erasure cycles. Experiments with multiple-hologram exposures show that the DX materials require no exposure schedule: equal strength holograms are obtained using equal exposures. Binary data have been stored in the form of multiplexed two-dimensional arrays of pixel bits. Required material and system parameters are estimated for a 1 Tbyte holographic storage device based on angle multiplexing in a DX material.

Assessing the sequence specificity in the binding of Co(III) to DNA via a thermodynamic approach

The interaction specificities of Co(III) with DNA were investigated via consideration of thermodynamic characteristics of the duplex to single strand transition for DNA oligomers incubated in the presence of [Co(NH3)5(OH2)](ClO4)3. It has previously been demonstrated that incubation of the DNA oligomer [(5medC-dG)4]2 with this cobalt complex leads to coordination of the cobalt center to the DNA, presumably at N7 of guanine bases [D. C. Calderone, E. J. Mantilla, M. Hicks, D. H. Huchital, W. R. Murphy, Jr. and R. D. Sheardy, (1995) Biochemistry 34, 13841]. In this report, DNA oligomers of different sequence were incubated with [Co(NH3)5(OH2)](ClO4)3 via protocols developed previously and the treated oligomers were subjected to thermal denaturation for comparison to the untreated oligomers. The DNA oligomers were designed in order to investigate the sequence specificity, if any, in the reaction of the cobalt complex with DNA. The values of Tm, delta HvH, and delta n (the differential ion binding term) obtained from the thermal denaturations were used to assess the sequence specificity of the interaction. For all oligomers, treated or untreated, Tm and delta HvH vary linearly with log [Na+] and hence the value of delta n is a function of the Na+ concentration. The results indicate no significant reaction between the cobalt complex and oligomers possessing isolated -GA- or -CG- sites; however, the thermodynamic characteristics of DNA oligomers possessing either an isolated -GG- site or an isolated -GC- site were altered by the treatment. Atomic absorption studies of the treated oligomers demonstrate that only the DNA oligomers possessing isolated -GG- or -GC- sites bind cobalt. Hence, the changes in the thermodynamic properties of these oligomers are a result of cobalt binding with a remarkable sequence specificity.

Influences of temperature and transport properties on the birefringence of CdGeAs2

We have directly measured the birefringence Δn of CdGeAs2 using polarized light interference spectra obtained in transmittance from 2.4 to 18 μm over a temperature range of 14–450 K. Four different samples, exhibiting a wide range of free carrier concentrations, were studied. It was found that free carriers can have a significant effect on the room-temperature birefringence. At 14 K, however, the data from all samples were virtually identical. The temperature dependence of Δn was obtained from a sample with low carrier concentration (≈1015/cm3) and high resistivity (9 Ω cm). Its temperature derivative d(Δn)/dT was determined at 50 K increments over the entire 14–450 K temperature range, and was found to vary from 2.6×10−5/K at 100 K to 5.1×10−5/K at 400 K. A least-squares fit to the data yielded a temperature coefficient for Δn of 8.2×10−8/K. Hall-effect measurements indicate that the resistivity, mobility, and carrier concentration all exhibit a significant temperature dependence. The resistivity for one sample increased by a factor of 108 when cooled from room temperature to 14 K. Our results suggest that previously published Δn values were probably obtained from samples with relatively high carrier concentrations, and that our results more accurately describe the true intrinsic birefringence of the material. The value of Δn decreases by 0.0004 per 1016 carriers a factor of 10 less than suggested by theory but still significantly large. The heavy to light hole effective mass ratio in the uppermost valence band was found to be 7.1.

Nonlinear optical and bistable properties of a wafer-fused vertical-cavity device based on InGaAsP

We report on a detailed analysis of the nonlinear characteristics at 1.55 μm wavelength of a bistable device realised by wafer fusion of an InGaAsP InP heterostructure on an AlAs GaAs Bragg mirror. A sub-milliwatt threshold power and a switching contrast up to 70 : 1 have been obtained. The bistable behaviour has been observed over more than a 2.5 nm range. The carrier-induced dispersive nonlinearity of the InGaAsP active layer has been investigated, including the saturation behaviour of the nonlinear index change. A saturating value δn s = −3.1 × 10 −2 has been obtained at the wavelength corresponding to a linear absorption of 134 cm −1. The nonlinear refractive index cross-section is found in good agreement with a two-band absorption saturation model. Similarly, the switching thresholds are well described by a simple plane-wave model including nonlinear index saturation.

Preparation of gradient-index (GRIN) polymer fibers for imaging applications

A closed extrusion process combined with a core-shell separation die design is developed in this study for preparing gradient index (GRIN) polymer fibers with a quadratic distribution of the refractive index. The material system used in this investigation is methyl methacrylate (MMA, n = 1.49) and benzyl methacrylate (BzMA, n = 1.568). The refractive index differences between the center and the periphery (Δn) of the prepared polymer fibers increase from 0.0115 to 0.020 when BzMA in the reactant mixtures increases from 17 to 28%. This finding would suggest that increasing the high refractive index monomer in the reactant mixture increased Δn. The Δn values decrease from 0.018 to 0.0135 when the diffusion zone temperature increases from 70 to 90°C. The diffusion rates of the monomers increase with the temperature, thereby causing BzMA and MMA to distribute more uniformly at a higher temperature than at a lower one. This uniform distribution leads to the decrease of Δn with an increasing temperature. The prepared GRIN polymer fibers have potential applications as imaging lenses for scanners, fax machines, and copiers. © 1996 John Wiley & Sons, Inc.

The study of [formula omitted] δ-doping resonant interband tunneling diode

The δ-doping InGaAs GaAs quantum well resonant interband tunneling diode by low pressure metal organic chemical vapor deposition has been successfully grown. The full width at half maximum of the doping profile analyzed by capacitance-voltage measurement is small. Two kinds of δ-doping resonant interband tunneling diodes were simulated in this study. One was the δn + -i-δp + resonant interband tunneling structure and the other was the δn + -i-δp + -i-δn + single barrier resonant interband tunneling structure. The simulated current-voltage characteristics exhibited the expected N-shaped negative differential resistance. The calculated peak-to-valley current ratio (PVCR) values of δn + -i-δp + and δn + -i-δp + -i-δp + structures are 6 and 5.6, respectively. The calculated PVCR value is larger than experimental PVCR values for the δn + -i-δp + structure. The PVCR values of this study are better than the published data of other authors.

Direct measurement of ZnGeP2 birefringence from 0.66 to 12.2 μm using polarized light interference

The birefringence (Δn) of ZnGeP2 has been measured directly from polarized light interference spectra obtained in transmittance over the 0.66–12 μm wavelength range from samples of six different thicknesses. The Δn values were determined from the positions of fringe maxima (Δn=kλ/t) and then compared to previously published data which were obtained by a different technique. It was found that the interference fringe method results in values of Δn accurate to ±0.00005. The data are shown to exhibit much less scatter as a function of wavelength than previous results and can lead to more accurate calculations of phase-matching angles for second-harmonic generation applications.

The linear sweep voltametric method: An application to the study of reversible and irreversible processes

AbstractThe theory of the linear sweep voltametric method has been the object of a series of articles in which useful solutions for current versus scan rate (I vs. v) relationships and for the diagnostic criteria for different kinds of reacting systems have been found. Some difficulties remain in order to clarify these concepts. This article is an attempt to introduce clearer ideas about how to deduce I vs. v and I vs. E relationships for the method mentioned above as applied to the study of reversible and irreversible electrochemical processes. Some new equations are given which have been deduced under consideration of the numerical solutions obtained by Nicholson and Shain [4]. Variations of Co(O, t) and CR(O, t) with the potential value have been found, and an unambiguous definition of k°′ for irreversible processes is given, which allows the determination of k° values from linear voltammograms recorded in the study of such processes. Also, the values of δN, the thickness of the diffusion layer, have been calculated as a function of the potential and sweep rate.

Nitrogen NMR of some sulphur diimide anions and calculated (GIAO) nitrogen shielding constants

Sulphur diimide anions of the type [NSN]2− (1), [RNSN]− (1), [R = tBu (3), SiMe3 (4), tBu2P (5)] were prepared with the counter ions [nBu4N]+ or [(Me2N)3S]+ (for 1, 3, 4) and K+ (for 5) and studied in solution by 14N NMR, and for assignment purposes by 15N NMR (4). The experimentally determined 14N chemical shifts (δN) were compared with nitrogen shielding constants calculated by the gauge included atomic orbitals (GIAO) technique, and a linear relationship between σN(calc) and δN was obtained. This correlation serves to support the Z configuration of the anions [RNSN]−, analogous to the isoelectronic N-sulphinylamines. On this basis, two 14N resonance signals reported in the literature with uncertain assignment can be attributed to [HNSN]− (2), and the previously assigned δN values suggest that the anion [HNSNS]− (6) adopts the Z/E configuration.

Rotationally resolved nonresonant two-photon ionization of SH

The threshold photoelectron (PE) spectrum for nascent SH formed in the ultraviolet photodissociation of H2S has been measured using the nonresonant two-photon pulsed field ionization (N2P-PFI) technique. The rotationally resolved N2P-PFI-PE spectrum of SH indicates that photoionization dynamics favor the rotational angular momentum change ΔN≤0 with the ΔN values up to −3, an observation similar to that found in the PFI-PE spectra of OH (OD) and NO. The ionization energy for SH(X 2Π3/2) is determined to be 84 057.5 ± 3 cm−1 (10.4219 ± 0.0004 eV). The spin–orbit splitting for SH(X 2Π3/2,1/2) is 377 ± 2 cm−1, in agreement with the literature value. This study illustrates that the PFI-PE detection method can be a sensitive probe for the nascent internal energy distribution of photoproducts.

How Smooth Should Curves Be for Calibrating Radiocarbon Ages?

We show that smoothed versions of the high-resolution calibration curve should be used when 14C ages are calibrated with large (&gt; ∼30 14C yr) measurement errors (represented by standard deviation δm) or are mixtures of elements of variable age (natural sample error with standard deviation δn). The degree of smoothing should agree with the standard deviation of total sample error, δt, the square root of the quadratic sum of δm and δn. However, in most cases, δt is not well known, especially due to difficulties in quantifying δn. We present an inverse method that gives a measure of mean δt, for different materials that are widely used in (conventional) 14C dating. Calculations with large (&gt;100) data sets of wood, charcoal, ombrotrophic peat and minerotrophic peat/gyttja samples indicate that δt of such materials is generally much larger than previously assumed, mainly because of large values of δn. This means that particularly in organic deposits, strongly smoothed calibration curves should be used where medium-term 14C variations (wiggles) are completely straightened. This has especially major consequences for calibrating 14C histograms for natural 14C variations. We conclude that 14C histograms consisting of samples of organic deposits do not require correction for medium-term 14C variations and that uncalibrated 14C histograms need not be as suspect as is usually believed.

Liquid crystal birefringence for millimeter wave radar

Abstract Many liquid crystals are found to have relatively high birefringence (Δn) values in the microwave and millimeter wave regions, as calculated from the phase shift induced by their reorientation by magnetic or electric fields. At 30 GHz, Δn values were obtained in the range of 0.08 to 0.18 for eleven liquid crystal mixtures of various types. The most favourable liquid crystal structures for high millimeter wave birefringence are highly conjugated rod-like molecules containing biphenyl, terphenyl, phenylpyrimidine, biphenylpyrimidine, and tolane groups in nematics of positive dielectric anisotropy (Δε). However, other liquid crystal structures including Schiffs base, azoxybenzene, and aromatic ester groups also have substantial birefringence, including nematics with negative and crossover Δε, as well as cholesteric nematics. The Δn varied only slightly at different frequencies of microwave millimeter wave in the 15–94 GHz range. Studies on magnetic and electrical field liquid crystal orientation in specially designed waveguides provide a basis for new types of modulators and scanning array antennae in the millimeter wave region, where more compact liquid crystal modulation media can be used than in the microwave region. These scanners can be used for both sending and receiving radar signals for potentially low cost radar systems.

Bloch-function analysis of resonant arrays of antiguided diode lasers

Bloch-function analysis of antiguided laser arrays provides closed-form expressions for key parameters in array design, and allows new insights in device behavior. For resonant and near-resonant devices it is found that as the gain is preferentially placed in the low-index regions (i.e., antiguide cores) the array mode favored to lase is the resonant leaky mode: in-phase or out-of-phase type. The intermodal discrimination is found to be strongly asymmetrical with respect to resonance: the resonant mode is preferred to lase over a much wider range in index step, Δn, variation on the high-Δn side of resonance than on the low-Δn side of resonance. Devices with Talbot-type spatial filters invariably operate in-phase except for Δn values well below resonance, when out-of-phase-like modes take over.