High Wavenumber Resolution Research Articles

Structural Rearrangement of Organic Semiconductor Molecules with an Asymmetric Shape in Thin Films

2-Decyl-7-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C10) exhibits excellent performances as an active layer in organic thin-film transistors, and its performances are greatly influenced by the molecular packing, i.e., the crystalline polymorphs. This compound has the so-called thin-film phase in a vapor-deposited film, which is different from the single-crystal structure (the bulk phase). In this work, thin films of Ph-BTBT-C10 are prepared by spin coating, and the effect of aging on the film structure is investigated by high-resolution infrared (IR) spectroscopy in combination with X-ray diffraction. The results show that the as-spun film has the same thin-film phase as the vapor-deposited film and that the thin-film phase is rapidly converted to the bulk phase by aging at room temperature. In addition, the spectral analysis also reveals conformational changes in the decyl chain during the crystalline transition process. This work highlights the importance of using IR spectroscopy with a high-wavenumber resolution for structural analysis of molecular thin films.

Low-frequency multiplex CARS microscopy with a high-repetition near-infrared supercontinuum laser

We have developed a multiplex coherent anti-Stokes Raman scattering microscope effective for low-wavenumber measurement by combining a high-repetition supercontinuum light source of 1064 nm and a near-infrared high-sensitivity InGaAs diode array. This system could observe the low-wavenumber region down to 55 cm−1 with high sensitivity. In addition, using spectrum shaping and spectrum modulation techniques, we simultaneously realized a wide bandwidth (<1800 cm−1), high wavenumber resolution (9 cm−1), high efficiency, and increasing signal-to-noise ratio by reducing the effect of the background shape in the low-wavenumber region. Spatial variation of a sulfur crystal phase transition with metastable states was visualized.

Sparse SVD Method for High-Resolution Extraction of the Dispersion Curves of Ultrasonic Guided Waves.

The 2-D Fourier transform analysis of multichannel signals is a straightforward method to extract the dispersion curves of guided modes. Basically, the time signals recorded at several positions along the waveguide are converted to the wavenumber-frequency space, so that the dispersion curves (i.e., the frequency-dependent wavenumbers) of the guided modes can be extracted by detecting peaks of energy trajectories. In order to improve the dispersion curve extraction of low-amplitude modes propagating in a cortical bone, a multiemitter and multireceiver transducer array has been developed together with an effective singular vector decomposition (SVD)-based signal processing method. However, in practice, the limited number of positions where these signals are recorded results in a much lower resolution in the wavenumber axis than in the frequency axis. This prevents a clear identification of overlapping dispersion curves. In this paper, a sparse SVD (S-SVD) method, which combines the signal-to-noise ratio improvement of the SVD-based approach with the high wavenumber resolution advantage of the sparse optimization, is presented to overcome the above-mentioned limitation. Different penalty constraints, i.e., l1 -norm, Frobenius norm, and revised Cauchy norm, are compared with the sparse characteristics. The regularization parameters are investigated with respect to the convergence property and wavenumber resolution. The proposed S-SVD method is investigated using synthetic wideband signals and experimental data obtained from a bone-mimicking phantom and from an ex-vivo human radius. The analysis of the results suggests that the S-SVD method has the potential to significantly enhance the wavenumber resolution and to improve the extraction of the dispersion curves.

Resolution limits for wave equation imaging

Formulas are derived for the resolution limits of migration-data kernels associated with diving waves, primary reflections, diffractions, and multiple reflections. They are applicable to images formed by reverse time migration (RTM), least squares migration (LSM), and full waveform inversion (FWI), and suggest a multiscale approach to iterative FWI based on multiscale physics. That is, at the early stages of the inversion, events that only generate low-wavenumber resolution should be emphasized relative to the high-wavenumber resolution events. As the iterations proceed, the higher-resolution events should be emphasized. The formulas also suggest that inverting multiples can provide some low- and intermediate-wavenumber components of the velocity model not available in the primaries. Finally, diffractions can provide twice or better the resolution than specular reflections for comparable depths of the reflector and diffractor. The width of the diffraction–transmission wavepath is approximately λ at the diffractor location for the diffraction–transmission wavepath.

Simultaneous wavenumber measurement and coherence detection using temporal phase unwrapping

Wavelength scanning interferometry and swept-source optical coherence tomography require accurate measurement of time-varying laser wavenumber changes. We describe here a method based on recording interferograms of multiple wedges to provide simultaneously high wavenumber resolution and immunity to the ambiguities caused by large wavenumber jumps. All the data required to compute a wavenumber shift are provided in a single image, thereby allowing dynamic wavenumber monitoring. In addition, loss of coherence of the laser light is detected automatically. The paper gives details of the analysis algorithms that are based on phase detection by a two-dimensional Fourier transform method followed by temporal phase unwrapping and correction for optical dispersion in the wedges. A simple but robust method to determine the wedge thicknesses, which allows the use of low-cost optical components, is also described. The method is illustrated with experimental data from a Ti:sapphire tunable laser, including independent wavenumber measurements with a commercial wavemeter. A root mean square (rms) difference in measured wavenumber shift between the two of ~4 m⁻¹ has been achieved, equivalent to an rms wavelength shift error of ~0.4 pm.

Study of Strain Induction for Metal–Oxide–Semiconductor Field-Effect Transistors using Transparent Dummy Gates and Stress Liners

Strain induction was studied on a sample that had a dummy gate tetraethyl orthosilicate–silicon dioxide (TEOS–SiO2) and SiN film by UV-Raman spectroscopy with high spatial and high wave-number resolution. The UV laser penetrated through the dummy gate that was transparent to UV light, which enabled us to evaluate strain in the channel of the metal–oxide–semiconductor field-effect transistor (MOSFET) model. Furthermore, we compared stress profiles obtained by finite element (FE) calculations with those obtained by UV-Raman measurements. There was a difference between the stress profiles in the line-and-space pattern sample and in the dummy-gate sample; large compressive (tensile) strains were concentrated at the channel edges in the dummy-gate sample with the compressive (tensile) stress liner, although both tensile and compressive strains existed at the channel edge in the line-and-space pattern sample. The results from UV-Raman spectroscopy were consistent with those obtained by the FE calculation.

Three‐state model for femtosecond broadband stimulated Raman scattering

AbstractStimulated Raman scattering (SRS) is analyzed with a three‐state model. Using a diagrammatic density‐matrix formalism, SRS by a pair of Raman pump and probe pulses, with observation along the probe direction, is described principally by eight terms. The eight terms can be grouped into four sets, which are labeled as SRS or IRS (inverse Raman scattering): SRS(I), SRS(II), IRS(I), and IRS(II). Specializing to the case of femtosecond SRS (FSRS) by a picosecond (ps) Raman pump pulse and a femtosecond (fs) probe pulse, the spectra for the four sets of terms under off‐resonance and resonance conditions were calculated. The results obtained can explain the FSRS experimental observations from a (decaying) stationary vibrational state, such as (1) high wavenumber resolution (determined by the narrow bandwidth Raman pump pulse) and high time resolution (determined by the fs probe pulse), (2) Stokes gain vs anti‐Stokes loss in off‐resonance FSRS, and (3) dispersive lineshapes in resonance FSRS. Copyright © 2008 John Wiley &amp; Sons, Ltd.

A Constrained‐Transport Magnetohydrodynamics Algorithm with Near‐Spectral Resolution

Numerical simulations including magnetic fields have become important in many fields of astrophysics. Evolution of magnetic fields by the constrained-transport algorithm preserves magnetic divergence to machine precision and thus represents one preferred method for the inclusion of magnetic fields in simulations. We show that constrained transport can be implemented with volume-centered fields and hyperresistivity on a high-order finite-difference stencil. In addition, the finite-difference coefficients can be tuned to enhance high-wavenumber resolution. Similar techniques can be used for the interpolations required for dealiasing corrections at high wavenumber. Together, these measures yield an algorithm with a wavenumber resolution that approaches the theoretical maximum achieved by spectral algorithms. Because this algorithm uses finite differences instead of fast Fourier transforms, it runs faster and is not restricted to periodic boundary conditions. In addition, since the finite differences are spatially local, this algorithm is easily scalable to thousands of processors. We demonstrate that, for low-Mach number turbulence, the results agree well with a high-order, non-constrained-transport scheme with Poisson divergence cleaning.

On the Utilization of a Flexible Bar as a Continuous Spatial Filter

In order to determine empirically the wavenumber-frequency spectra of the wall pressure fluctuations beneath a turbulent boundary layer at nonconvective wavenumbers, a spatial filter with high wavenumber resolution is necessary. Such a filter can be realized by using a continuous system such as a flexible bar or by using a discrete array of finite-sized transducers. The bar offers the advantage that high wavenumber resolution can be achieved without the use of the elaborate equipment necessary for the transducer arrays. In order to use the bar as a spatial filter all that is required is a measurement of the modal displacement response at some spatial position. In this paper, the theoretical formulation of the bar as a continuous spatial filter is developed. Calibration procedures and measurement problems such as those associated with vibrational noise are discussed. Finally, measurements of the low-wavenumber (nonconvective) spectra of the wall pressure fluctuations obtained by using the bar as a spatial filter are reported. One important result of this study is that it demonstrates that the primary wavenumbers associated with the pressure fluctuations contributing to the bar response are not the convective wavenumbers. This implies that Corcos's model, which has been empirically verified only for wavenumbers near the convective wavenumbers [D. M. Chase, J. Acoust. Soc. Amer. 46, 1350–1365 (1969)], may not be adequate for predicting panel response. The measured low-wavenumber (nonconvective) spectra indicate the limitations of Corcos' model in this region. [Work supported by grant from the National Aeronautics and Space Administration.]