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In microwave-induced thermo-acoustic tomography, the quality of reconstructed images is seriously influenced by the speed of sound distribution, which is usually set according to the speed of sound (SoS) of the background coupling medium or an empirical value. To reduce defocusing and distortions induced by acoustic inhomogeneity in microwave-induced thermo-acoustic tomography, a method is proposed. We classify the initial reconstructed pressure by the difference in dielectrics and update the acoustic property accordingly. A spare spectrum is obtained by the proposed method and partitions the initial pressure into several clusters, and then SoS will be updated based on clinical statistics with which the pressure is reconstructed again. The proposed method is first validated in the anatomically realistic breast phantom; an ex vivo experiment based on a prototype system is carried out for further validation. The results prove that the proposed method has a reliable effect on reducing distortions than the state-of-art methods under the condition of a low signal to noise ratio.

In this paper, we report a non-contact and quantitative method to evaluate the permittivity of semiconductor nanomaterials. A microwave atomic force microscopy (M-AFM) was used to obtain the topography and microwave images of nanomaterials in one scanning process. Morphology and microwave images of ZnO and CuO nanowires, and SnO2 nanobelts with high spatial resolution were obtained in the non-contact mode of M-AFM. The local relative permittivity of these one-dimensional metal oxide nanomaterials was quantitatively evaluated.

We present a MHz frequency-division dc-SQUID multiplexer that is based on flux ramp modulation and a series array of $N$ identical current-sensing dc-SQUIDs with tightly coupled input coil. By running a periodic, sawtooth-shaped current signal through an additional modulation coil being tightly, but non-uniformly coupled to the individual SQUIDs, the voltage drop across the array changes according to the superposition of the flux-to-voltage characteristics of the individual SQUIDs within each cycle of the modulation signal. In this mode of operation, an input signal injected in the input coil of one of the SQUIDs and being quasi-static within a time frame adds a constant flux offset and leads to a phase shift of the associated SQUID characteristics. The latter is inherently proportional to the input signal and can be inferred by channelizing and down-converting the sampled array output voltage. Using a prototype multiplexer as well as a self-developed high-speed readout electronics for real-time phase determination, we demonstrate the simultaneous readout of four signal sources with MHz bandwidth per channel., The article has been submitted to Applied Physics Letters

Since shear waves in soft human tissues have been used in clinics, little is known about their upper frequency behavior except that they fade out before the ultrasonic regime due to viscoelastic dissipation. Studying this upper bound has become a need in micro-elastography that aims at characterizing a single cell using shear waves. We report through phantom experiments the observation of ultrasonic shear waves at 20.4 kHz. Moreover, it is shown that the Kelvin–Voigt model can predict a frequency upper bound in good agreement with cell experiments. At last, according to this simple rheological model, the maximum error of elasticity estimation when viscosity is neglected (wide spread perfectly elastic assumption) is 40%.

Dielectric resonators made of high permittivity materials with low losses have been extensively studied for application in magnetic resonance imaging (MRI) and magnetic resonance spectroscopy. They can focus, redistribute, and enhance the radio frequency magnetic field in a controlled way. In this Letter, we investigate coupled very-high permittivity dielectric resonators for clinical bilateral breast MRI. The resonators are tuned to support a TE mode within the frequency of 3 T MRI and implemented as a pair of coaxial hollow cylinders consisting of ceramic rings with extremely high permittivity (e ∼ 870) and low loss. We study the electromagnetic coupling between two resonators placed in the near field and analyze the impact of symmetric and antisymmetric mode excitation on the sensitivity of the MRI machine. We experimentally verify that the coupling of the mode with symmetric field distribution to the radio frequency body birdcage coil allows us to enhance the receive sensitivity substantially and to decrease the excitation power needed to provide the optimal transmit efficiency, thus making the MRI procedure safer. Our work offers a practical method to realize a simple yet very effective system based on dielectric resonators for bilateral breast imaging.

Single-walled carbon nanotube (SWNT) has attracted widespread attention for its unique one-dimensional atomic structure and outstanding physical and chemical properties. For both fundamental research and practical applications, it is critical to figure out the chirality of SWNT because the electronic band structure and the consequent electrical and optical properties are determined by its chirality. Here, we found that the chirality of SWNT on an insulating hexagonal boron nitride (h-BN) substrate can be obtained in a quick and concise manner through measuring the aligning direction and the diameter of SWNT. Additionally, Rayleigh scattering spectroscopy is conducted to confirm the obtained chirality. The developed technique for direct detection of chirality of SWNT on an insulating h-BN substrate could advance the study of chirality-dependent functional SWNT devices and the applications of SWNT related to chirality.

Transformation acoustics correlates complex material properties in physical space with distorted wave manipulations in virtual space, such that wave propagation patterns can be determined by mathematical coordinate transformations. These transformations allow for accurate modeling of acoustic propagation in complex materials. Such models are relevant for both biomedical ultrasound therapies and integrated on-chip systems, where muscle fibers and piezoelectric substrates act as effective anisotropic media, respectively. Without considering the anisotropic density of these sophisticated media, attempts to beamform acoustic patterns by phase engineering result in a heavily distorted signal. This distortion is detrimental to the performance of high intensity focused ultrasound acoustic tweezers for noninvasive surgeries, cell trapping, and cell sorting. Here, we demonstrate that the distortion effects can be corrected by transformation acoustics in which the phased array profile is adjusted to account for the corresponding anisotropy. We perform experiments to verify this transformation acoustic correction for arbitrary focused and self-bending beams with two-dimensional anisotropic spoof surface acoustic waves. The benefit of transformation acoustics in suppressing undesired anisotropic effects on beamformed waves improves the precision and efficacy of medical treatments that facilitate noninvasive ultrasound therapies and integrated on-chip applications.

van der Waals (vdW) engineering of magnetism is a topic of increasing research interest in the community at present. We study the magnetic properties of quasi-two-dimensional layered vdW Mn3Si2Te6 (MST) crystals upon proton irradiation as a function of fluences of 1 × 1015, 5 × 1015, 1 × 1016, and 1 × 1018 H+/cm2. We find that the magnetization is significantly enhanced by 53 % and 37 % in the ferrimagnetic phase (at 50 K) when the MST crystal was irradiated with the proton fluence of 5 × 1015, both in ab and c planes, respectively. The ferrimagnetic ordering temperature and magnetic anisotropy are retained even after proton irradiation. From the fluence dependence of magnetization, electron paramagnetic resonance spectral parameters (g-value and signal width), and Raman data, we show that the magnetic exchange interactions (Mn–Te–Mn) are significantly modified at this fluence. This work shows that it is possible to employ proton irradiation in tuning the magnetic properties of vdW crystals and provide many opportunities to design desired magnetic phases.

It is shown that, at helium temperatures, relaxed CdTe films reveal isolated emitters, the properties of which indicate their relation with a weakly perturbed fragment of the Shockley dislocation core. The polarization and spatial distribution of such emitters in CdTe/Si and CdTe/GaAs virtual substrates contain information on the structure of extended defects in a relaxed CdTe layer, which are important for HgCdTe epitaxy.It is shown that, at helium temperatures, relaxed CdTe films reveal isolated emitters, the properties of which indicate their relation with a weakly perturbed fragment of the Shockley dislocation core. The polarization and spatial distribution of such emitters in CdTe/Si and CdTe/GaAs virtual substrates contain information on the structure of extended defects in a relaxed CdTe layer, which are important for HgCdTe epitaxy.

Polarization shaped terahertz sources play a key factor in terahertz wireless communications, biological sensing, imaging, coherent control in fundamental sciences, and so on. Recently developed spintronic terahertz emitters have been considered as one of the next-generation promising high performance broadband terahertz sources. However, until now, polarization control, especially for twisting the circularly polarized terahertz waves at the spintronic terahertz source, has not yet been systematically explored and experimentally achieved. In this work, we not only demonstrate the generation of circularly polarized terahertz waves in cascade spintronic terahertz emitters via delicately engineering the amplitudes, applied magnetic field directions, and phase differences in two-stage terahertz beams but also implement the manipulation of the chirality, azimuthal angle, and ellipticity of the radiated broadband terahertz waves. We believe our work can help with further understanding of the ultrafast optical magnetic physics and may have valuable contributions for developing advance terahertz sources and optospintronic devices.

Magnetic Resonance Imaging (MRI) is universally acknowledged as an excellent tool to extract detailed spatial information with minimally invasive measurements. Efforts toward ultra-low-field (ULF) MRI are made to simplify the scanners and to reduce artefacts and incompatibilities. Optical Atomic Magnetometers (OAMs) are among the sensitive magnetic detectors eligible for ULF operation, however they are not compatible with the strong field gradients used in MRI. We show that a magnetic-dressing technique restores the OAMs operability despite the gradient, and we demonstrate sub-millimetric resolution MRI with a compact experimental setup based on an in situ detection. The proof-of-concept experiment produces unidimensional imaging of remotely magnetized samples with a dual sensor, but the approach is suited to be adapted for 3-D imaging of samples magnetized in loco. An extension to multi-sensor architectures is also possible., Comment: 5 pages, 3 figures, 36 refs, 3 pages supplemental material (+ 3refs). Accepted for publication in Appl.Phys.Lett

Radio-frequency atomic magnetometers offer attractive alternatives to standard detection methods in nondestructive testing, which are based on inductive measurements. We demonstrate a magnetometer in the so-called spin maser configuration, which addresses two important challenges of the technique: shifts in the radio frequency resonance position caused by magnetically permeable samples and the sensor bandwidth. Key properties of the self-oscillating sensor are presented in both a magnetically shielded and an open environment. Demonstration of defect detection via magnetic induction tomography in a ferromagnetic carbon steel sample is presented. The configuration discussed paves the way for a simple, rapid, and robust nondestructive material defect detection system based on an atomic magnetometer.

© 2017 Author(s). Shear wave measurements for the determination of tissue elastic properties have been used in clinical diagnosis and soft tissue assessment. A shear wave propagates as a transverse wave where vibration is perpendicular to the wave propagation direction. Previous transverse shear wave measurements could detect the shear modulus in the lateral region of the force; however, they could not provide the elastic information in the axial region of the force. In this study, we report the imaging and quantification of longitudinal shear wave propagation using optical coherence tomography to measure the elastic properties along the force direction. The experimental validation and finite element simulations show that the longitudinal shear wave propagates along the vibration direction as a plane wave in the near field of a planar source. The wave velocity measurement can quantify the shear moduli in a homogeneous phantom and a side-by-side phantom. Combining the transverse shear wave and longitudinal shear wave measurements, this system has great potential to detect the directionally dependent elastic properties in tissues without a change in the force direction.

Acoustic tomography developed and investigated before can only recover either the temperature or the velocity field from the time-of-flight measurements along a single direction of sonic rays. However, the sonic propagation time is a nonlinear function of temperature and velocity; thus, in principle, the measurements allow us to reconstruct both simultaneously. In order to achieve this, we propose a modality of acoustic tomography based on the recently proposed concept of nonlinear tomography, which can take full advantage of the aforementioned nonlinear dependences. Proof-of-concept numerical demonstrations are presented using representative two-dimensional temperature and flow velocity phantoms. The obtained simulative results confirmed the feasibility of nonlinear acoustic tomography in recovering the distributions of temperature and velocity fields.

We describe surface wave propagation in soft elastic media at speeds exceeding the bulk shear wave speed. By linking these waves to the elastodynamic Green's function, we derive a simple relationship to quantify the elasticity of a soft medium from the speed of this supershear evanescent wave (SEW). We experimentally probe SEW propagation in tissue-mimicking phantoms, human cornea ex vivo, and skin in vivo using a high-speed optical coherence elastography system. Measurements confirm the predicted relationship between SEW and bulk shear wave speeds, agreeing well with both theoretical and numerical models. These results suggest that SEW measurements may be a robust method to quantify elasticity in soft media, particularly in complex, bounded materials where dispersive Rayleigh-Lamb modes complicate measurements.

Focusing of ultrasonic waves in water plays an important role in various scenarios ranging from biomedical imaging to nondestructive testing. Acoustic metasurfaces have been largely explored for acoustic focusing, but they are generally narrowband and mainly implemented for airborne sound because of their structural complexity. Nevertheless, our previous development of metasurfaces provides a great opportunity to solve the challenges. Here, we present numerically and experimentally the broadband focusing of ultrasonic waves in water with a metasurface lens consisting of an array of deep-subwavelength sized and spaced slots. The slot widths of the metasurface are optimized based on microscopic coupled-wave theory. Due to the non-resonant arrangement, the focusing effect is demonstrated over a broad band of frequencies. The metasurface lens with simplicity and an ultra-compact size provides a feasible means for the design of thin and lightweight ultrasonic devices and is suitable for practical applications in biomedical and industrial fields MOE (Min. of Education, S’pore) Published version

Ultrasound imaging transducers possess serious trade-off design in performances between the penetration depth and the imaging resolution, which loses detection accuracy of diseases in large and deep-depth organs. To ease this problem, this study combines a piezoelectric transducer with the waveguide technique to realize invasive imaging. However, conventional configurations with waveguides suffer from low input energy or low energy transmission efficiency, thus leading to the proposal of the double-parabolic-reflector focusing and guiding mechanism to achieve high power transmission. In this letter, three different physical configurations of waveguides for ultrasound propagation are compared in terms of acoustic pressure, power, and propagation modes. The proposed mechanism with double parabolic reflectors enhances the acoustic pressure at the tip of the cylindrical waveguide to over 17 times compared to the conventional mechanism and increases the total power flow to around 213 times. Due to the superior performances in powerful ultrasound transmission, the proposal is expected to widen the applications in ultrasound imaging, ultrasound therapeutics, and ultrasound microscopy.

Ultra-fast coherent magnetization reversal, which is indispensable for high-speed spintronic devices, is expected to be realized by irradiation with a nearly monocyclic terahertz pulse. In this letter, using MnAs ferromagnetic nanoparticles embedded in a GaAs host matrix, we observe a large modulation up to 20% of the magnetization of these nanoparticles with terahertz pump pulse irradiation. This modulation ratio is the largest value ever reported for terahertz magnetization modulation. We reveal that the modulation is induced by the electric-field component of the terahertz pulse via spin-carrier interactions. Ferromagnetic nanoparticle systems are promising, and material designing is the key for ultrafast magnetization reversal using terahertz pulses.

We present a technique on focusing laser-driven proton beams in an array pattern by employing a copper mesh irradiated by a separate, intense laser pulse. Transient fields are generated on the mesh following the intense interaction. Under the combined effect of collisional scattering and electrical deflections from the mesh, a laser-driven proton beam is split into multiple focused beams with a high density of ∼4 × 109/cm2 after propagation through the charged-up mesh. The particle density within the focused beamlets is up to ∼11 times the initial density of the proton beam. Multiple beam focusing through this approach may open routes for proton beam conditioning, leading to opportunities for multi-beam applications, such as tomographic radiography and proton implantation.We present a technique on focusing laser-driven proton beams in an array pattern by employing a copper mesh irradiated by a separate, intense laser pulse. Transient fields are generated on the mesh following the intense interaction. Under the combined effect of collisional scattering and electrical deflections from the mesh, a laser-driven proton beam is split into multiple focused beams with a high density of ∼4 × 109/cm2 after propagation through the charged-up mesh. The particle density within the focused beamlets is up to ∼11 times the initial density of the proton beam. Multiple beam focusing through this approach may open routes for proton beam conditioning, leading to opportunities for multi-beam applications, such as tomographic radiography and proton implantation.

We propose a simple and efficient method of color-encoded Stokes parameters to visualize the polarization states for polarization-sensitive optical coherence tomography (PS-OCT) of biological tissue. In this method, polarization states of the probing light are detected and described in the Stokes domain. Three primary colors of red, green, and blue are used to code Stokes parameters of Q, U, and V, respectively, which can be used to represent and visualize each unique polarization state. A strategy that uses the polarization state as the PS-OCT imaging parameter is first introduced to obtain high contrast images of the birefringent samples. Then, color-based automatic segmentation of birefringent components from 3D scanned tissue volume is proposed to isolate the network of the 3D nerve bundles in a mouse brain without cutting the brain into slices. Experimental validation and demonstrations are given by imaging ex vivo mouse tail and brain tissues to show the usefulness of proposed polarization state imaging and segmentation methods.We propose a simple and efficient method of color-encoded Stokes parameters to visualize the polarization states for polarization-sensitive optical coherence tomography (PS-OCT) of biological tissue. In this method, polarization states of the probing light are detected and described in the Stokes domain. Three primary colors of red, green, and blue are used to code Stokes parameters of Q, U, and V, respectively, which can be used to represent and visualize each unique polarization state. A strategy that uses the polarization state as the PS-OCT imaging parameter is first introduced to obtain high contrast images of the birefringent samples. Then, color-based automatic segmentation of birefringent components from 3D scanned tissue volume is proposed to isolate the network of the 3D nerve bundles in a mouse brain without cutting the brain into slices. Experimental validation and demonstrations are given by imaging ex vivo mouse tail and brain tissues to show the usefulness of proposed polarization state ima...

Proton therapy, thanks to the dose characteristics of the Bragg peak, according to which most of the radiation energy is delivered at the end of the beam with a very high dose gradient at the distal edge, can deliver a highly conformal radiation dose to the treatment volume. Currently, however, the benefit of this high dose gradient is not fully utilized in clinical practice due mainly to the dose-distribution uncertainty in the beam direction (i.e., the uncertainty of the beam range in the patient). In this paper, we present an imaging system based on gamma electron vertex imaging (GEVI), which is suitable for high-energy (1–30 MeV) gammas, and test its performance for therapeutic proton beams. GEVI images prompt gamma vertices, which are closely correlated with the dose distribution at the distal edge, by converting prompt gammas to electrons via Compton scattering and then tracking the recoiled electrons. Our experimental results show that the GEVI system can image the 2D vertices of the prompt gammas and, thus, can be utilized for the measurement of proton-beam ranges in patients. We believe, indeed, that GEVI makes possible real-time monitoring of in-vivo proton-beam ranges, whose utility significantly improves treatment effectiveness and enhances patient safety. We also expect that the GEVI system will find applications in other fields (e.g., gamma-ray astronomy, nuclear engineering, and high-energy physics) requiring high-energy-gamma (1–30 MeV) imaging.

In this letter, we present a near-field method for mapping the micro-architecture of complex media based on the measurement of the diffusion constant D using multiple backscattering of ultrasound waves. Simulation results are reported and validated with experiments. For both simulations and experiments, a linear array of ultrasound transducers is used. The coherent and incoherent intensities are separated using a matrix manipulation of the inter-element response matrix. We demonstrate that no beamforming is necessary to extract the diffusion constant. Acquiring sub-inter-element response matrices by using subsets of elements and calculating the growth of the diffusive halo for each sub-matrix provides an estimate of a semi-local diffusion constant, enabling a 1-D mapping of the scatterer density or volume fraction in a strongly heterogeneous medium. This methodology is then applied to quantitatively characterize the lung parenchyma and detect the presence of solitary pulmonary nodules. The semi-local diffusion constant is mapped along the transducer axis to search for high D values, which correspond to the nodule/lesion location.

We report an ablation enhancement (both deepening and widening in ablation profiles) on nickel by ultrashort double-pulse laser irradiation, an exception of the typical reduction trend observed in ablation rates for metals irradiated with double-pulse excitations. Maximized at a pulse-separation (τs) of approximately 4 ps, the enhancement was observed before τs was increased to approximately the electron-lattice energy relaxation time. An achievement of ablation, by temporal shaping a weak pulse with energy well below the single-pulse ablation threshold into two considerably weaker pulses, was demonstrated. It is proposed that the enhanced electron-phonon coupling and drop of electronic heat conductivity, both of which result from the reductions in electron temperature (by pulse shaping), co-contribute to the widening in ablation profiles, while the former one is responsible for the deepening. This letter proposed an ablation enhancement approach to ultrashort laser processing of nickel.

In this paper, a technique for reflection mode immersion 2D laser-ultrasound tomography of solid objects with piecewise linear 2D surface profiles is presented. Pulsed laser radiation was used for generation of short ultrasonic probe pulses, providing high spatial resolution. A piezofilm sensor array was used for detection of the waves reflected by the surface and internal inhomogeneities of the object. The original ultrasonic image reconstruction algorithm accounting for refraction of acoustic waves at the liquid-solid interface provided longitudinal resolution better than 100 μm in the polymethyl methacrylate sample object.

Anisotropic fibrous networks, especially transverse isotropic fibrous networks, are widely used to model the microstructures of biological tissues, polymer gels, fibrous thermal insulations, and other fibrous materials. In this letter, we build a three-dimensional transverse isotropic fibrous network model and study its mechanical properties along the through-thickness direction. We propose a measurement of anisotropy for transverse isotropic fibrous networks and then study the influence of anisotropy on the networks' mechanical properties, including its elastic modulus, maximum elongation, and stress–strain curve, by means of finite-element simulation. We also study theoretically the influence of anisotropy on maximum elongation. We find that as the anisotropy of the networks becomes stronger, the elastic modulus decreases and the maximum elongation increases, indicating a transition in mechanical properties from brittle to ductile. We identify this transition as the “ductile–brittle transition.” This tr...

Using a hydrodynamic two-temperature model, we simulate multi-pulse laser ablation of an aluminum bulk target. The results of modeling demonstrate that the effectiveness of the multi-pulse ablation increases an order of magnitude in comparison to a single-pulse ablation of the same energy, while the repetition rate grows up to several GHz because the material surface does not cool down substantially between successive pulses. To prevent the shielding and suppression effects, the fluence of each pulse in the burst should have a subthreshold value to avoid the generation of slow moving ablated condensed-phase nanolayers. The obtained results are consistent with recent experiments on ablation by ultrafast bursts of ultrashort pulses.

A detailed analysis of defect assisted transport properties of GaN after irradiation is carried out. Unintentionally doped GaN samples were irradiated by a 100 MeV oxygen ion beam and a 200 MeV silver ion beam at various fluences between 1 × 1010 ions/cm2 and 5 × 1011 ions/cm2. Thermopower and Hall effect measurements were done on pristine and irradiated GaN samples at variable temperatures. An increase in the thermopower with temperature as well as on irradiation (with respect to pristine GaN) was observed. Electrical conductivity and Hall mobility decreased on irradiation, suggesting scattering of charge carriers due to the formation of defects. A qualitative study of activation energy due to thermopower and conductivity is done by the scattering mechanism using different theoretical models to determine the transport path of the carriers. The observed increase in thermopower of GaN after irradiation suggests a probable hopping mechanism due to the presence of energy levels in the bandgap.

In this paper, we report a therapeutic ultrasonic transducer with a sub-wavelength periodic structure, by which ultrasonic focusing and temperature elevation have been significantly enhanced compared to a conventional concave transducer with the same size. Enhanced acoustic focusing was demonstrated by both measuring and simulating acoustic pressure and temperature elevation. Compared to the conventional concave transducer, the proposed transducer exhibited stronger capacity in elevating acoustic pressure and temperature rise in the focal region, in which extraordinary acoustic transmission close to Wood's anomaly could be modulated by the spherically curved surface. This work is believed to possess great clinical potential in the safe and efficient application of ultrasonic therapy.

Shear wave imaging has emerged as a potential non-invasive technique for the quantitative assessment of the arterial shear modulus. Nonetheless, the arterial elasticity estimation in the transverse direction has been overlooked compared with the longitudinal direction, and the estimated transmural stiffness has rarely been evaluated. Accurate depiction of the transverse stiffness across the thin arterial wall warrants comprehensive characterization in both normal and pathological conditions. This study estimated the transmural arterial shear modulus in both the longitudinal ( μ Long) and transverse directions ( μ Trans) using group ( c T) and phase velocities ( c p h) in finite element models and hollow cylindrical tissue-mimicking phantoms with various shape factors. The results were validated against mechanical testing. Zero-order antisymmetric Lamb wave and circumferential Lamb type wave models were considered in the longitudinal and transverse directions of the thin-walled hollow cylinder, respectivel...

Microwave induced thermoacoustic tomography (TAT) images usually suffer from distortions arising from the microwave polarization effect and standing wave effect. The microwave polarization effect, resulting from linearly polarized microwave illumination, splits the image of the object along the polarization direction, while the standing wave effect, when the object size is larger than the microwave wavelength within the object, modulates the image of the object. Both effects cause non-uniform energy distribution in a uniformly absorbing object and create artifacts in the reconstructed images. To address these problems in TAT, we propose an image reconstruction method that combines multi-view Hilbert transformation with the back-projection algorithm. We experimentally validate this method by imaging breast and brain tumor phantoms, showing that the aforementioned distortions are significantly suppressed. We anticipate that this method will contribute to clinical tumor diagnosis.

Impulse generation by irradiating aluminum targets with repetitive laser pulses at normal and oblique incidence was investigated using impulse measurements with a torsion pendulum at various incidence angles under different laser beam fluence conditions. The fluence varied from 5.8–20.0 J/cm2 for normal incidence. For oblique incidence, momentum coupling is sensitive to the incident angle at fluences of 6.3 J/cm2 and 9.2 J/cm2 because of target surface reflectivity changes and plume shielding effects. At fluence of 19.3 J/cm2, the fluence on the target surface becomes dominant for impulse generation compared with the angle of incidence effect in a large angular range. Beam fluence optimization for momentum coupling at oblique incidence is discussed based on the impulse characteristics obtained.

Conventional semi-infinite analytical solutions of correlation diffusion equation may lead to errors when calculating blood flow index (BFI) from diffuse correlation spectroscopy (DCS) measurements in tissues with irregular geometries. Very recently, we created an algorithm integrating a Nth-order linear model of autocorrelation function with the Monte Carlo simulation of photon migrations in homogenous tissues with arbitrary geometries for extraction of BFI (i.e., αDB). The purpose of this study is to extend the capability of the Nth-order linear algorithm for extracting BFI in heterogeneous tissues with arbitrary geometries. The previous linear algorithm was modified to extract BFIs in different types of tissues simultaneously through utilizing DCS data at multiple source-detector separations. We compared the proposed linear algorithm with the semi-infinite homogenous solution in a computer model of adult head with heterogeneous tissue layers of scalp, skull, cerebrospinal fluid, and brain. To test the capability of the linear algorithm for extracting relative changes of cerebral blood flow (rCBF) in deep brain, we assigned ten levels of αDB in the brain layer with a step decrement of 10% while maintaining αDB values constant in other layers. Simulation results demonstrate the accuracy (errors

Vibrations can cause noise in scanning probe microscopies. Relative vibrations between the scanning sensor and the sample are important but can be more difficult to determine than absolute vibrations or vibrations relative to the laboratory. We measure the noise spectral density in a scanning SQUID microscope as a function of position near a localized source of magnetic field, and show that we can determine the spectra of all three components of the relative sensor-sample vibrations. This method is a powerful tool for diagnosing vibrational noise in scanning microscopies., 4 pages, 3 figures

Magnetic resonance imaging (MRI) is conventionally performed in very high fields, and this leads to some restrictions in applications. To remove such restrictions, the ultra-low field MRI approach has been proposed. Because of the loss of sensitivity, the detection methods based on superconducting quantum interference devices (SQUIDs) in a shielded room were used. Atomic magnetometers have similar sensitivity as SQUIDs and can also be used for MRI, but there are some technical difficulties to overcome. We demonstrate that MRI of the human brain can be obtained with an atomic magnetometer with in-plane resolution of 3 mm in 13 min.

Self-organized grating structures formed on Mo and Ti metal surfaces irradiated with femtosecond laser pulses at wavelengths of 800 and 400 nm are investigated by electron microscopy. We observe the formation of the self-organized grating structures on the metals irradiated with 400-nm laser pulses at low laser fluence in narrow fluence ranges. The interspaces of the grating structure depend on the wavelength and fluence of the laser. We find that the dependence of the grating interspaces on laser fluence can be explained by a simple formula for induction of a surface-plasma wave through the parametric decay of laser light.

A study of ns-laser ablation is presented, which focuses on the transient behavior of the physical processes that act in and above a copper sample. A dimensionless multiphase collisional radiative model describes the interplay between the ablation, collisional, and radiative mechanisms. Calculations are done for a 6 ns-Nd:YAG laser pulse operating at 532 nm and fluences up to 15 J/cm2. Temporal intensity profiles as well as transmissivities are in good agreement with experimental results. It is found that volumetric ablation mechanisms and photo-processes both play an essential role in the onset of ns-laser induced breakdown.

We report the conversion of optical energy into mechanical energy by naturally occurring red blood cells (RBCs) placed in an optical trap. A trapped RBC undergoes folding due to the elastic nature of its cell membrane. On use of circularly polarized light in the trap, the folded RBCs rotate, indicating their birefringence. The cellular rotation speed depends on the size of the blood cells and on laser power. Rotating RBCs have implications for naturally occurring, optically driven, rotary micromachines.

This letter discusses theoretical studies of optical properties of gold nanospheres and nanorods with various aspect ratios; these are embedded in a porous anodic alumina matrix. The nanoparticles have potential applications in optical filters and sensors.

A polarization-sensitive Fourier domain optical coherence tomography (PS-FD-OCT) system is demonstrated. This OCT system is based on a spectral interferometer, does not require mechanical axial scanning, and enables phase information to be used in an OCT image. Owing to this phase information, the system requires only two measurements for determining the Jones matrix images and Muller images of biological samples. This system reveals the birefringence properties of the inner surface of a porcine esophagus.

We have examined the origins of luminescence in N-ion-implanted epitaxial GaAs, using a combination of cross-sectional transmission electron microscopy and low-energy electron-excited nanoscale-luminescence spectroscopy. A comparison of reference, as-implanted, and implanted-plus-annealed samples reveals a variety of emissions. In all samples, we observe the GaAs fundamental band-gap emission, as well as several emissions related to GaAs native defects. In the as-implanted and implanted-plus-annealed samples, an emission related to the implantation-induced defects, is also observed. Interestingly, in the implanted-plus-annealed samples, we identify a near-infrared emission associated with GaAsN nanocrystallites.

Two key theoretical predictions for flat negative index lenses are that thicker lenses should have both internal and external foci, and that these foci should move linearly with small deviations in frequency. Two-dimensional electric field profiles of microwave transmission through rectangular slabs of composite wire and split-ring resonator material are presented. By scanning inside and outside the material, both internal and external foci are revealed. The power concentration around the internal focus is shown to have a negative curvature, consistent with the theoretical predictions.

We describe a superconducting quantum interference device (SQUID) multiplexer operated at microwave frequencies. The outputs of multiple SQUIDs are simultaneously modulated at different frequencies and summed into the input of one high electron mobility transistor (HEMT). The large bandwidth and dynamic range provided by HEMT amplifiers should make it possible to frequency-division multiplex a large number of SQUIDs in one output coaxial cable. We measure low SQUID noise (∼0.5μΦ0∕Hz at 4K) and demonstrate the multiplexed readout of two direct current (dc) SQUIDs at different resonant frequencies. In this work, dc SQUIDs are used, but this approach is equally applicable to radio-frequency SQUIDs.

This letter reports the results obtained by measuring the temperature dependence of the coercivity of magnetically isolated L10-FePt nanoparticles in agglomeration-free films deposited by using a dispersion stabilizer and a spin-coat technique. These measurements not only give the basic magnetic parameters of the nanoparticles but also provide information about the nanoparticle ordering process. The temperature at which isolated FePt nanoparticles start to order seems to be about 650°C.

We propose a laser resonance chirality selection method to select single-walled carbon nanotubes (SWNTs) with specific chirality from mass of SWNTs by intense laser irradiation. This technique exploits the multiple Van Hove singularities in the density of states of SWNTs expected for one-dimensional confinement. Absorption of the exciting laser beam can be strongly enhanced for SWNTs with specific chirality when the energy of the incident photon matches the energy of the allowed electronic transitions. As a result of the resonance effect, when the excitation power density of the laser beam increases in an atmosphere of air, at a certain threshold excitation power density, only these SWNTs are oxidized and are removed selectively. With Raman scattering measurements, we have demonstrated that SWNTs with specific chirality are selectively removed after intense laser irradiation.

An axial first-order gradiometer is formed by coupling the input coil of a flexible high-temperature superconducting flux transformer inductively to a directly coupled superconducting quantum interference device magnetometer. The transformer is patterned in a single-layer YBa2Cu3O7−x film on a flexible Hastelloy tape. The tape is bent such that the two outer pickup loops of the transformer are facing each other while perpendicular to the magnetometer plane resulting in a gradiometer baseline of 35mm. A superconducting shield is mechanically adjusted to reduce the gradiometer response to uniform fields applied perpendicularly to both the magnetometer plane and the plane of the transformer pickup loops, by a factor of typically 7000.

The tomographic measurement of the residual stress profile in femtosecond-laser irradiated standard SMF-28 germanium-doped telecommunication fiber is demonstrated. The fiber is irradiated with weakly focused pulses to realize long-period fiber gratings. In the irradiated grating regions, an asymmetrical increase in axial core stress up to 6.2 kg/mm2 is found. The increase in stress is attributed to a densification of the irradiated glass matrix. The stress-induced anisotropic index distribution is calculated and related to the absolute index change in the irradiated regions.

Bilayers of polycrystalline Co(37 nm)/Fe(37 nm) were deposited onto Si wafers via electron-gun evaporation and irradiated at room temperature with 200 keV Xe+ ions to fluences of 5×1014–1×1016/cm2. The magneto-optical Kerr effect, Rutherford backscattering spectroscopy and x-ray diffraction were used to characterize the magnetization and microstructure of the films. Xe fluences of up to 5×1015 ions/cm2 were found to induce a four-fold magnetic anisotropy in the originally isotropic films, as a consequence of the phase transformation of hexagonal Co to face-centered-cubic Co caused by ion irradiation. Xe fluences exceeding 7×1015 ions/cm2 produced a uniaxial magnetic texture, which is explained by strong Co/Fe interface mixing and possibly ion-induced formation of CoFe grains at the interface.

The technique of point-defect engineering (PDE), with excess vacancies produced near the surface region by MeV Si ion implantation, has been applied to form ultrashallow junctions with sub-keV B implants. PDE can reduce boride-enhanced diffusion that dominates the enhanced diffusion of ultralow energy B implants. PDE can further sharpen the dopant profile and enhance boron activation. For 1×1015/cm2, 0.5 keV B implant, B solid solubility has been enhanced over a wide temperature range of 750–1000 °C, with an enhancement factor of 2.5 at 900 °C. These features have enabled a shallower and sharper box-like boron junction achievable by PDE in combination with ultralow energy 0.5 keV B implantation.

The effect of 200 MeV Ag ion irradiation on the temperature and field dependence of critical current density (JC) of high quality MgB2 thin films is studied. Substantial increase in JC is observed over a certain field range for the film irradiated at a dose of 1012 ions/cm2. Our analysis suggests that columnar defects are not formed under irradiation conditions used in these studies, which correspond to an electronic energy loss of about 16 keV/nm. Defects clusters are likely to be responsible for the observed improvement in JC.

A technique of detecting the rolling and spinning of half-coated nanoparticles using interference ring patterns of the fluorescence has been demonstrated. Using the unique ability to measure nanoparticle rotations in multiple degrees of freedom, we are able to determine the orientation of a moving nanoparticle in real time. Ultimately, the detailed knowledge of the rotational behavior of half-coated nanoparticles is expected to produce physical insight of nano biomechanical systems.