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Remote sensing of ground vibration is a key tool in the detection of shallowly buried objects, such as land mines. Millimeter wave systems show promise for replacing laser Doppler vibrometers as the key sensing technology on the grounds of reduced cost and because they provide a larger (10 cm versus 1 mm), more robust sensor footprint. We have developed a double-conversion interferometer operating at 39 GHz that can be built for about $10 000. The system employs quadrature detection in order to provide reliable sensitivity over wavelength-sized changes in target range. Laboratory tests show that it can estimate sinusoidal ground motion from a 1 s time series with an uncertainty of about 5 μm. [ABSTRACT FROM AUTHOR]

The presence and nature of fractures can have a profound impact on the elastic and mechanical properties of materials. Whether it is a fracture-induced loss of strength or the creation of a long-wavelength anisotropy, being able to infer the presence and properties of fractures in a material has many applications. Detecting changes in the spectra of mechanical resonances due to the presence of fractures is not difficult. A more interesting issue is which elastic moduli are sensitive to the presence of these fractures and how are these moduli related to the stiffness of the material? Resonant ultrasound spectroscopy was performed on acrylic cylinders with internal cracks. Inversions were performed for both isotropic and anisotropic moduli. Good agreement was obtained between isotropic moduli and projected (optimal isotropic moduli obtained from anisotropic moduli) for nonverlapping cracks. The same projections yielded poorer agreement for the sample with overlapping cracks. The results also show that the degree of anisotropy depends upon the amount of crack overlap, and that the isotropic shear modulus is a reliable measure of decreasing stiffness. [ABSTRACT FROM AUTHOR]

The electrical conductivity of a CdS thin film controlled by grain structures is essential to enhance its photoconductivity to be able to be fit as a window material in CdS/CdTe heterojunction solar cells. In order to characterize a thin film, electromagnetically, we employed an open cavity resonator with a sub-millimeter Vector Network Analyzer. Our technique is capable of measuring complex dielectric permittivity, ε~, of a photovoltaic film as thin as 0.1 μm. We measured the real part of the complex dielectric permittivity, εre, and electrical conductivity, σre (derived from the imaginary part, εim), of unannealed and annealed CdS films with thicknesses ∼0.15 μm on ∼3 mm thick-borosilicate glass substrates, at room temperature. We obtain the (thermally activated) electrical conductivity between 100 and 312GHz, which is less in annealed samples than in unannealed ones by ∼2 orders of magnitude. Contrary to our expectations, the carrier concentrations extracted from these data by fitting a Drude model are ∼1016 cm-3 (unannealed) and ∼1014 cm-3 (annealed). We investigate the connection between the grain size and carrier concentration. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we analyze a monochromatic plane wave scattering from an infinite homogeneous cylindrical annulus. In particular, we study the effect that the inner part of the cylindrical annulus (cylindrical void, if you will) has on the scattered field. This is done by isolating the cylindrical void's contribution to the scattered field. We show that if the cylindrical void is small, then its contribution to the scattered field may be approximated by the “screened cylindrical void” (SCV) approximation. We first develop the SCV approximation in a physically intuitive manner, and then show that it could also be obtained in a more mathematically rigorous manner. Numerical results comparing the SCV approximation to the exact solution are also presented. [Copyright &y& Elsevier]

Linear response laws and causality (the effect cannot precede the cause) are of fundamental importance in physics. In the context of classical electrodynamics, students often have a difficult time grasping these concepts because the physics is obscured by the intermingling of the time and frequency domains. In this paper, we analyse the linear response laws and causality in the time and frequency domains with the aim of pedagogical clarity. We will show that it is easy to violate causality in the frequency domain by making a vanishing absorption approximation. Furthermore, we will show that there can be subtle differences between Fourier transformingMaxwell equations and using a monochromatic source function. We discuss how these concepts can be obscured and offer some suggestions to improve the situation. [ABSTRACT FROM AUTHOR]

Abstract: The S-matrix algorithm for the propagation of an electromagnetic wave through planar stratified media has been implemented in a modern object-oriented programing language. This implementation is suitable for the study of such applications as the Anderson localization of light and super-resolution (perfect lensing). For our open-source code to be as useful as possible to the scientific community, we paid particular attention to the pathological cases that arise in the limit of vanishing absorption. [Copyright &y& Elsevier]

Examines the data and model uncertainty estimation for linear inversion. Model-based estimates of the noise characteristics from a single realization data; Use of noise estimates to characterize the set of reasonable models; Examples of vertical seismic profiling and cross-well tomography.

Examines the development of the Bayesian approach on inferences about finite-dimensional subsets of earth models. Computation of posterior marginal distributions; Derivation of the modified Bayesian formulation; Calculation of probability intervals from the marginal posterior distributions.

Presents several letters related to medicine in Great Britain. Risks of total hip replacement; Medical implications of prostaglandins and asthma; Application of the declaration of the President of the General Dental Council on operator and anesthetists.

At the meso and macro-scale, the connection between the elastic and the electromagnetic properties of granular materials such as rocks is complicated and involves, among other things, the chemical, mineralogical, and geometric features of the grains and their host medium. In the previous work, we showed how to perform rapid near-field millimeter wave scanning to obtain high-spatial resolution maps of the spatially varying dielectric permittivity of heterogeneous materials, including rocks. Here, we extend this concept to laser ultrasound and map the spatially varying mechanical properties of materials with similar resolution. [ABSTRACT FROM AUTHOR]

In analogy with optical near-field scanning methods we use tapered dielectric waveguides as probes for a millimeter wave vector network analyzer. By scanning thin samples between two such probes we are able to map the spatially varying dielectric properties of materials with subwavelength resolution. Using a 150 GHz probe in transmission mode we see spatial resolution of better than 500 μm. Here, we apply the technique to rocks map the mineralogy at the submillimeter scale. [ABSTRACT FROM AUTHOR]

Natural sedimentation processes give rise to fine layers in shales. If these layers alternate between organic-rich and organic-poor sediments, then the contrast in dielectric properties gives rise to an effective birefringence as the presence of hydrocarbons suppresses the dielectric constant of the host rock. We have measured these effects with a quasioptical millimeter wave setup that is rapid and noncontacting. We find that the strength of this birefringence and the overall dielectric permittivity provide two useful diagnostics of the organic content of oil shales. [ABSTRACT FROM AUTHOR]

One region of the electromagnetic spectrum that is relatively unexploited for materials characterization is the millimeter wave band (frequencies roughly between 40 and 300 GHz). Millimeter wave techniques involve free-space (noncontacting) measurements which have a length scale that makes them ideal for characterizing bulk properties of multicomponent composites where the scale of homogeneity is on the order of millimeters. Such composites include granular materials such as rocks, fluid mixtures, suspensions, and emulsions. Here we show measurements on partially saturated rocks and an oil/water mixture, demonstrating that millimeter wave spectroscopy is a sensitive yet rapid measure of changing composition. [ABSTRACT FROM AUTHOR]

To study the propagation of multiply scattered waves, it is useful to have a medium in which the scattering properties can be easily controlled. Here, we describe such a system; it involves the propagation of ultrasonic surface waves in a medium with an aligned, disordered pattern of grooves. Waves propagating parallel to the grooves see a homogeneous medium; waves propagating perpendicular to the grooves are strongly scattered. By varying the source–receiver distance and orientation with respect to the grooves, we are able to map out the transition from ballistic to diffusive propagation. In addition, by using an optical detection system we are able to measure the wave motion inside the scattering medium. We measure the angle-dependent macroscopic properties of the medium, such as the group velocity as well as the mean-free path and the diffusion constant in the strong-scattering regime. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Discusses measurements of ultrasonic surface wave propagation in a strong-scattering, quasiperiodic medium consisting of a grooved surface of aluminum. Evolution of surface wave pulses within the scattering medium; Comparison of the behavior of waves propagating parallel to the grooves and waves propagating normal to the grooves.

Non-contacting acoustical methods can be used to extract various material properties of liquid or solid samples without disturbing the sample. These methods are useful even in the lab since they do not involve coupling anything to the sample, which might change its properties. A forteriori, when dealing with potentially dangerous materials, non-contacting methods may be the only safe solutions to mechanical characterization. Here, we show examples of using laser ultrasound to remotely insonify and monitor the elastic properties of several granular explosive simulants. The relatively short near-infrared laser pulse length (a few hundred nanoseconds) provides a broad-band thermoelastic source of ultrasound; we intentionally stay in the thermoelastic regime to avoid damaging the material. Then, we use a scanning laser Doppler vibrometer to measure the ultrasonic response of the sample. LDV technology is well established and very sensitive at ultrasonic frequencies; atomic level motions can be measured with modest averaging. The resulting impulse response of the explosive simulant can be analyzed to determine decay rates and wave speeds, with stiffer samples showing faster wave speeds and lower decay rates. On the other hand, at the low-frequency end of the acoustic spectrum, we use an electronically phased array to couple into a freely suspended sample's normal modes. This allows us to gently heat up the sample (3 °C in just under 5 min, as shown with a thermal IR camera). In addition to the practical interest in making the sample more chemically visible through heat, these two measurements (low-frequency resonant excitation vs high-frequency wave propagation) bracket the frequency range of acoustic non-destructive evaluation methods available. [ABSTRACT FROM AUTHOR]

We have designed and built an ultrasonic parametric array with an emphasis on creating an intense, collimated beam of low frequency sound. With this device, we can insonify a small area of ground or a small target at range and induce vibrations. These vibrations can be synchronously detected with any stand-off device such as a laser-Doppler vibrometer or the millimeter wave vibrometer we describe in Smith et al. [J. Appl. Physics 108, 024902 (2010)]. Despite nonlinear conversion losses, the array produces sound pressure levels in excess of 90 dB at 1 kHz, 1.5 m in front of the array using 25 low-cost 40 KHz transducers. [ABSTRACT FROM AUTHOR]

Discusses definitions of various forms of waves. Wave as a natural consequence of the stability of simple harmonic motion; Wave as an organized propagating imbalance; Nonlinearity as essential to maintain the organization of a wave; Wave spreading by dispersion in solitons; Total intensity of scattered waves; Ubiquity of waves in nature.

In a coarse-grained rock scattering plays a dominant role in determining the characteristics of the ultrasonic wavefield. Immediately after a pulse excites waves in such a sample energy propagates away from the source as a packet. As this packet travels, the individual grains scatter its energy in all directions. This scattering process continues until there are equal amounts of energy propagating in all directions. If the scattering is strong enough, this transition is completed before the signal strength has relaxed to the ambient level. By collecting a dense data set on several rock samples we are able to watch this transition and see the influence of grain size on the wavefield. More quantitatively we show, with noncontacting laboratory data, how this transition can be tracked using the symmetry of the advanced and retarded Green functions. These Green functions are estimated by cross-correlating the signal recorded at two detectors; the two Green functions have equal amplitude only when equal amounts of energy are propagating in all directions. [Work supported by the NSF (EAR-0111804), the US Army Research Office (DAAG55-98-1-0070), the Center for Wave Phenomena, and ACI 2066 of the French Department for Research.] [ABSTRACT FROM AUTHOR]

Quasi-optical (QO) methods of dielectric spectroscopy are well established in the millimeter and submillimeter frequency bands. These methods exploit standing wave structure in the sample produced by a transmitted Gaussian beam to achieve accurate, low-noise measurement of the complex permittivity of the sample [e.g., J. A. Scales and M. Batzle, Appl. Phys. Lett. 88, 062906 (2006); R. N. Clarke and C. B. Rosenberg, J. Phys. E 15, 9 (1982); T. M. Hirovnen, P. Vainikainen, A. Lozowski, and A. V. Raisanen, IEEE Trans. Instrum. Meas. 45, 780 (1996)]. In effect the sample itself becomes a low-Q cavity. On the other hand, for optically thin samples (films of thickness much less than a wavelength) or extremely low loss samples (loss tangents below 10-5) the QO approach tends to break down due to loss of signal. In such a case it is useful to put the sample in a high-Q cavity and measure the perturbation of the cavity modes. Provided that the average mode frequency divided by the shift in mode frequency is less than the Q (quality factor) of the mode, then the perturbation should be resolvable. Cavity perturbation techniques are not new, but there are technological difficulties in working in the millimeter/submillimeter wave region. In this paper we will show applications of cavity perturbation to the dielectric characterization of semi-conductor thin films of the type used in the manufacture of photovoltaics in the 100 and 350 GHz range. We measured the complex optical constants of hot-wire chemical deposition grown 1-μm thick amorphous silicon (a-Si:H) film on borosilicate glass substrate. The real part of the refractive index and dielectric constant of the glass-substrate varies from frequency-independent to linearly frequency-dependent. We also see power-law behavior of the frequency-dependent optical conductivity from 316 GHz (9.48 cm-1) down to 104 GHz (3.12 cm-1). [ABSTRACT FROM AUTHOR]

Some experiments are conveniently performed in the time domain, some in the frequency domain, and some use a hybrid approach. Does the domain make any difference in the ultimate resolution of the experiment? Or can the details of the experiment always be tweaked so that the different approaches give the same answer? We consider a simple optics experiment and consider both time and frequency measurements and take into account the influence of noise, finite dynamic range, attenuation, and the duration of the measurement. [ABSTRACT FROM AUTHOR]

Exploiting the fine structure of strongly scattered waves could provide a wealth of new information in seismology, ultrasonics, acoustics, and other fields that study wave propagation in heterogeneous media. Therefore, noncontacting laser-based measurements of ultrasonic surface waves propagating in a strongly disordered medium are performed in which the ratio of the dominant surface wavelength to the size of a scatterer is large, and waves that propagate through many scatterers are recorded. This allows analysis of scattering-induced dispersion and attenuation, as well as the transition from ballistic to diffusive propagation. Despite the relatively small size of the scatterers, multiple scattering strikingly amplifies small perturbations, making changes even in a single scatterer visible in the later-arriving waveforms. To understand the complexity of the measured waveforms, elastic spectral-element numerical simulations are performed. The multiple-scattering sensitivity requires precise gridding of the actual model, but once this has been accomplished, we obtain good agreement between the measured and simulated waveforms. In fact, the simulations are invaluable in analyzing subtle effects in the data such as weak precursory body-wave diffractions. The flexibility of the spectral-element method in handling media with sharp boundaries makes it a powerful tool to study surface-wave propagation in the multiple-scattering regime. [ABSTRACT FROM AUTHOR]

Resonant Ultrasound Spectroscopy (RUS) uses normal modes of elastic bodies to infer material properties such as elastic moduli and Q. In principle, the complete elastic tensor can be inferred from a single measurement. For centimeter-sized samples RUS fills an experimental gap between low-frequency stress-strain methods (quasi-static up to a few kHz) and ultrasonic time-delay methods (hundreds of kHz to GHz). We use synchronous detection methods to measure the resonance spectra of homogeneous rock samples. These spectra are then fit interactively with a model to extract the normal-mode frequencies and Q factors. Inversion is performed by fitting the normal-mode frequencies. We have successfully applied this technique to a variety of isotropic and anisotropic samples, both man-made and natural. In this paper we will show in detail the procedure applied to a cylindrical core of Elberton granite. By means of a statistical fit of the measured normal modes and an independent laser ultrasonic measurement, the granite core was inferred to have orthorhombic symmetry. A 10 per cent P-wave anisotropy was measured in the plane perpendicular to the core axis. [ABSTRACT FROM AUTHOR]

We demonstrate 260 GHz (λ=1.15 mm) near field imaging using a conical Teflon probe whose tip protrudes through an aperture in a tapered aluminum holder. The imaging system is based on a quasioptical millimeter wave vector network analyzer. We present a variety of different imaging examples of dielectrics and metals, in both reflection and transmission modes, as well as an analysis of interesting diffraction and scattering effects observed in some of the images. The probe has an approximate tip diameter of 0.17 mm and an aperture size of about 1 mm. We observe horizontal resolution ranging from 0.2–0.5 mm depending on the sample being imaged. [ABSTRACT FROM AUTHOR]

Presents several letters to the editor related to medical issues. Inquiries on the initiation of genetic counselling and genetic registers; Widespread of metabolic bone disease identified among Asians; Epidemiological suggestions relative to smallpox vaccination.

Presents several letters to the editor concerning on medicine. Assessment on the range of treatments used for medical practices; Evaluation on the care for long stay elderly patients; Effect of sulfasalazine induced in immunodeficiency patient with rheumatoid arthritis.

In coda wave interferometry, one records multiply scattered waves at a limited number of receivers to infer changes in the medium over time. With this technique, we have determined the nonlinear dependence of the seismic velocity in granite on temperature and the associated acoustic emissions. This technique can be used in warning mode, to detect the presence of temporal changes in the medium, or in diagnostic mode, where the temporal change in the medium is quantified. [ABSTRACT FROM AUTHOR]

Knowing the frequency-dependent photo-induced complex conductivity of thin films is useful in the design of photovoltaics and other semi-conductor devices. For example, annealing in the far-infrared could in principle be tailored to the specific dielectric properties of a particular sample. The frequency dependence of the conductivity (whether dark or photo-induced) also gives insight into the effective dimensionality of thin films (via the phonon density of states) as well as the presence (or absence) of free carriers, dopants, defects, etc. Ultimately, our goal is to make low-noise, phase-sensitive room temperature measurements of the frequency-dependent conductivity of thin films from microwave frequencies into the far-infrared; covering, the frequency range from ionic and dipole relaxation to atomic and electronic processes. To this end, we have developed a high-Q (quality factor) open cavity resonator capable of resolving the complex conductivity of sub-micron films in the range of 100-350 GHz (0.1-0.35 THz, or 0.4-1 meV). In this paper, we use a low-power green laser to excite bound charges in high-resistivity amorphous silicon thin film. Even at room temperature, we can resolve both the dark conductivity and photo-induced changes associated with dielectric relaxation and possibly some small portion of free carriers. [ABSTRACT FROM AUTHOR]