Large Signal Enhancement Research Articles

Determination of the nonlinear optical susceptibility ?(2) of surface layers by sum and difference frequency generation in reflection and transmission

The theoretical investigation of sum and difference frequency generation in thin surface layers with rotational symmetry leads to formulas which connect the generated light intensities to the surface second order nonlinear susceptibility tensor. A maximum of seven tensor components can be determined in the case of lowest symmetry. Measurements in transmission should be especially useful since they allow easy variation of both polarization and angle of incidence. On the other hand, large signal enhancements are expected for total internal reflection geometries. A consistent set of χ(2) tensor components for a thin layer of rhodamine-6G adsorbed on fused silica is found based on data from reflection and transmission measurements.

Aspects of seismic reflection prospecting for oil and gas

Summary. Seismic reflection survey is a technology which has made, and is making, rapid advances by means of continuous marginal improvement over each of its subdivisions of data acquisition, signal enhancement and geological interpretation. It is wedded to the digital computer and as long as the real cost of digital computers and their peripherals continues to fall so long, at least, will reflection seismology continue to advance - for there are many algorithms waiting only for more (at the right price) computer power before they are implemented. The essence of the technique is simple echo-sounding combined with large data redundancy and (fairly) complex signal enhancement and imaging procedures. On land the source is normally a few kilograms of high explosive and at sea it is usually an array of airguns, which is a device for releasing into the water a few litres of air at high pressure. Particle velocity detectors are used on land and pressure detectors at sea, their output is digitally recorded on magnetic tape with a total dynamic range of some 180 dB, though resolution is limited to 14 bits. Arrays of sources and detectors are used and the first 10–12 stages in the signal processing chain are devoted to producing a record as close as possible to the hypothetical record which would have been obtained if the source and detector had been coincident on a horizontal datum plane and if there had been no noise and no multiply reflected echoes. Once the best such ‘zero-offset’ record has been obtained an imaging algorithm, based on the acoustic (not elastic) wave equation, is used in order to bring into focus as sharply as possible the seismic image of the subsurface. This is normally done for vertical slices through the Earth but increasingly attempts are being made to produce proper three-dimensional images. The models of the Earth which underlie signal enhancement procedures are grossly simplified versions of reality. A major development effort in iterative and interactive model fitting is just beginning with the aim of allowing more plausible models to be used. Interpretable echoes are commonly obtained from depths in sedimentary rocks of 5 km and more. Absorption limits penetration of the higher frequencies so that it is rare for echoes from the greater depths to have appreciable energy above 25 Hz. Some information on the nature of the rocks and their depositional environment may be obtained from the reflections but essentially nothing may be deduced about whether their pores are filled with water, oil or gas. Colour graphics work stations are just being introduced to aid in the geological interpretation of the computer enhanced signals but it will be some time before they can call up fast enough and display adequately the quantity of data involved in an average survey (1010 bits).

Effect of 3-D heat flow near edges in photothermal measurements

Photothermal imaging scans often show large signal enhancement at sample edges. This effect can be attributed to 3-D heat flow within the sample. Geometric constraints which limit 3-D heat flow generally increase the sample surface temperature and the photothermal signal. The dependence of edge enhancement on sample thermal and physical dimensions is investigated for several systems containing one or more edges. Results suggest that geometrically restricted heat flow might explain the enhanced photothermal signal obtained for powders and other textured surfaces.

Far-infrared laser study of magnetic polaritons in FeF2and Mn impurity mode in FeF2: Mn

From a high-resolution, far-infrared laser study of the antiferromagnetic resonance (AFMR) of Fe${\mathrm{F}}_{2}$, it is shown that a magnetic polariton model not only reproduces the broad (\ensuremath{\sim}4 kOe) complex structures observed in transmission, but is essential to the determination of the intrinsic (radiative and nonradiative) contributions to the linewidth. For the purest Fe${\mathrm{F}}_{2}$ crystal an upper bound of 20 Oe for $\ensuremath{\Delta}H$ was found. The polariton model is extended to include the effects of Mn impurities. The theory correctly predicts the observed frequency pulling of host and impurity modes as a function of Mn concentration $c$ and the large enhancement of the impurity-mode signal. From their dependence on $c$ and sample thickness $t$ the super-radiance and impurity-impurity contributions to $\ensuremath{\Delta}{H}_{\mathrm{imp}}$ are separately obtained and compared with recently developed theories. The thermal relaxation of the host AFMR is shown to arise from fourmagnon scattering.

Experimental Results on the Dynamic Polarisation in a Solid by Variation of Temperature

A description of the apparatus for a new dynamic polarization effect by variation of temperature, and the experimental results obtained, are presented. As described earlier a temperature jump produces a very large enhancement of the dipolar signal (about 10.000) in solid y-picolene due to the dipolar couplings between the protons of the CH3-group and the modes of the CH3-rotator with the crystal phonons. A simple phenomenological model for the interpretation of the experimental data is introduced. Within the experimental errors agreement between model and data is obtained. From the shape of the measured polarization curves a dipolar polarization time Tp and the dipolar relaxation time Td is determined. From the maxima of the polarization at different temperature jumps a curve dependent only on the temperature is constructed, which together with Tp and Td characterizes completely the dynamic polarization behaviour. A comparison of the Zeeman- and dipolar relaxation with earlier measurements on similar molecules containing CH3-groups shows that the CH3-rotation in solid y-picolene is almost unhindered and that the "relaxation efficiency" is very low. These last conditions are important for a large enhancement of the dipolar polarization.

Electron spin resonance in carbon-doped boron nitride

It is shown that carbon impurity is responsible for the yellow color and paramagnetism in boron nitride which has been heated to 1800–2300°C in graphite furnaces. The yellow BN powders exhibited 10-line ESR spectra with spin concentrations up to 10 19/g. Annealing the yellow BN at 1800–2300°C in a carbon-free environment removed the yellow color and reduced the spin concentration below the observable limit of 10 16/g. Such pure white BN was unaffected by prolonged irradiation with u.v., 50 kV X-rays or 1 MeV γ-rays, whereas other less pure types of BN showed a large enhancement of ESR signal after relatively short exposures to such radiation. The ESR spectra from pure white BN doped with C 13-enriched carbon were identical to those from BN doped with normal carbon. These results suggest that the yellow color in BN is due to an F-center produced by a carbon impurity ionization mechanism. The anisotropy of the ESR was studied in carbon-doped specimens of highly oriented BN. The room temperature hyperfine splitting at 9·151 GHz was found to vary as ( a + b cos 2 θ), where a = 7·85 ± 0·07 G and b = −1·27 ± 0·07 G. The g-value obeyed a similar relation: g = g ⊥ + Δg cos 2 θ, where g ⊥ = = 2·00321 ± 0·00002 and Δg = g ∥ − g ⊥ = (−9·5 ± 0·2) × 10 −4.