Search

Surface plasmon resonance (SPR) type ultrasonic sensors enable local measurements and have a flat frequency response in a wide frequency range. They are expected to be used in photoacoustic microscopy (PAM) and other applications that require broadband ultrasonic detection. In this study, we focus on the precise measurement of ultrasound pressure waveforms via a Kretschmann-type SPR sensor. The noise equivalent pressure was estimated to be 52 Pa [Formula: see text], and the maximum amplitude of the wave measured via the SPR sensor linearly responded to the pressure until 427 kPa [Formula: see text]. Furthermore, the observed waveform for each applied pressure agreed well with the waveforms measured via the calibrated ultrasonic transducer (UT) in the MHz range. Moreover, we focused on the effect of the sensing diameter on the frequency response of the SPR sensor. The results show that the beam diameter reduction improved the frequency response at high frequencies. Evidently, we found that the sensing diameter of the SPR sensor must be carefully selected in consideration of the measurement frequency.

A varifocal concave-convex lens using ultrasound and transparent viscoelastic gel is reported. The configuration of the lens is simple and thin, consisting of four pieces of a piezoelectric ultrasound transducer, a glass disk, and a transparent silicone gel film. It uses a combination of the ultrasound resonant flexural standing- and traveling-wave modes excited by in-phase and four-phase drives so that the lens can change its shape to both concave and convex by switching the resonance mode with the same structure. The acoustic radiation force (ARF) originated from the resonant flexural vibration modes changed the surface profile of the gel. Convex and concave deformation were generated at the center of the lens at the resonance frequencies of 38 and 60 kHz, respectively, indicating that a varifocal concave-convex lens could be fabricated by controlling the driving frequency, voltage amplitude, and phase differences among the ultrasound transducers. The deformational displacement on the lens surface and the change in the focal length increased with the input voltage amplitude. The optical microscopic images observed through the lens were enlarged 1.28× (reduced 0.92× ) in the convex (concave) mode with 20 [Formula: see text]. The response time for focusing and the temperature stability under operation were evaluated. By switching the resonance vibration modes of the lens through the input signals to multiple ultrasound transducers, the variable-focus function with both concave and convex lenses was achieved in the same configuration.

A pump-probe system with a subnanosecond pulsed laser is expected to be a compact and inexpensive transient spectroscopic system that enables nondestructive and noncontact evaluations of the physical properties. However, an improvement in the sensitivity and a theoretical model to complement the measurement signal are necessary to obtain the transient signal precisely because of the low sensitivity and large time resolution. We have developed a highly sensitive pump-probe system with a subnanosecond pulsed laser that combines signal amplification based on surface plasmon resonance (SPR) in this study. An integrated theoretical model of the transient response obtained by a subnanosecond pump-probe under the SPR condition was proposed. Our model consisted of the profile descriptions of the used pulse source, temperature change, generated thermoelastic stress, estimated permittivity change in the metal film, and estimated reflectivity change. The theoretical estimations in the time domain and the incident angle dependence were compared with those of the experimental results to verify our theory. As a result, the estimations were well in agreement with the experimental results. Moreover, the signal-amplification mechanism based on SPR was discussed using our theory. The amplification was caused by the broadening of the resonant curve of SPR and the shift of the resonant angle, which seemed to come from the increase in the electron-phonon scattering rate and the thermal expansion of the metal film, respectively. A clear mechanism of SPR-based signal amplification of the subnanosecond pump-probe was identified through experimental and theoretical approaches.

Diabetic patients have a higher risk of bone fracture than those without diabetes, despite a normal bone mineral density. This higher riskmay result fromthe deterioration of collagen because of glycation. The objective of this study was to investigate the elastic properties of glycated collagen using the micro-Brillouin scattering technique. Using single-layer uniaxial collagen films with a thickness of approximately [Formula: see text], the longitudinal wave velocities, propagating in the parallel andperpendiculardirectionswith respect to the collagen fiber orientation, were measured in dry and wet film specimens. The wave velocities in the glycated collagen specimens decreased as a function of glycation time. This decrease depended on the direction of collagen fiber alignment and wave propagation. The lowest velocity due to glycation in thewet filmswas foundwhen the ultrasound propagated perpendicular to the fiber direction. These results indicate that the glycation of collagen in the bone may also reduce bone elasticity and suggest that the effects of glycation on collagen films may be anisotropic.

To understand the in-plane elastic character of ultrasonic waves in the skull, longitudinal wave velocities were studied in the MHz range using a conventional pulse technique. Taking advantage of the thickness of swine skulls, anisotropic in-plane wave velocity changes in the outer and diploe layers were experimentally investigated using structural information measured by X-ray computer tomography (CT). The velocities in the thin inner layer were difficult to measure. The main trabecular alignment (MTA) in the thick swine diploe layer was almost perpendicular to the thickness direction and changed with position inside the skull. The degree of anisotropy of in-plane longitudinal wave velocity ranged 1.07-1.33 in both outer and diploe layers, depending on position and swine sample. The angle of the fastest velocity in the outer layer was different from that in most parts of the diploe layer. Anisotropic character in the diploe layer gradually changed with position in the thickness direction.

Piezoelectricity in bone is thought to be a mechanism by which ultrasound promotes the healing of bone fractures. However, a few studies have been conducted in the more clinically relevant megahertz range. To understand the piezoelectricity in bone, we fabricated ultrasound transducers using bone samples as piezoelectric materials and identified the longitudinal ultrasound radiation and reception in the megahertz range. The maximum transmitting sensitivity of the bone transducer was 140 mPa/V, which was nearly 1/1000 of a polyvinylidene difluoride (PVDF) transducer that has better electrical properties and piezoelectricity. The resonance frequencies of the transducer depend on the plate thickness and angle between the bone axis (alignment direction of the hydroxyapatite crystallites) and ultrasound propagation direction, reflecting the anisotropic character of the bone. The reception and transmission sensitivities of the bone transducers also depend on the plate thickness and angle, showing maximum values at off-axis angles. These results indicate the existence of both piezoelectricity and inverse piezoelectricity in bone, which may be key factors in understanding the bone healing by low-intensity biophysical (electrical or mechanical) stimulation.

The viscosity and the surface profile of a thixotropic gel were controlled via ultrasound vibrations. The ultrasound device consisted of two piezoelectric lead zirconate titanate transducers glued to each end of a 60-mm-long rectangular glass substrate. It generated a 71-kHz flexural standing wave along the length. A 0.9-mm-thick thixotropic gel film made from silicone oil and hydrophobic fumed silica was formed on the glass substrate. The ultrasound vibration decreased the film viscosity, and its surface profile could be changed by the acoustic radiation force acting from the gel to the surrounding air, generated by the flexural vibration of the substrate. The transient response of the surface profile was observed by switching the ultrasound excitation ON and OFF. When the excitation was switched OFF, the gel displacement gradually decreased, but the surface profile did not return to that of the initial steady state, indicating that the viscosity recovered in time to preserve the surface profile. When a pulsed megahertz (MHz) signal was used to control the gel deformation, the surface profile rapidly approached the initial steady-state profile after the excitation.

This paper investigates acoustic levitation and noncontact transportation techniques for use with planar objects. An acoustic levitation system was developed which consists of a 1-mm-thick and 400-mm-long bending plate along with two bolt-clamped Langevin-type transducers (BLTs) that have stepped horns. A plane reflector was installed parallel to the vibrating plate to generate an ultrasound standing wave between the reflector and the plate. The sound pressure distribution in the ultrasound waveguide was calculated via finite-element analysis to investigate the effects of levitation of a planar object in the standing-wave field. A 1-mm-thick polystyrene plate was levitated along the nodal line of the acoustic standing wave in the waveguide. By controlling the driving phase difference between the two BLTs, the position at which flexural vibration occurs on the vibrating plate could be shifted along the length direction, and the trapped planar object could be moved by 9 mm along the same direction when the phase difference was varied from 0° to 360°.

It is difficult to perform 2-D imaging of elastic properties using the Brillouin scattering technique because the weak thermal phonon signal in the sample leads to low measurement accuracy and long measurement times. To improve the phonon signal, we artificially induced acoustic phonons using a ScAlN thin-film piezoelectric transducer, which has a giant piezoelectricity. The film was grown using RF magnetron sputtering of a ScAl alloy target on a silica glass bar sample. Using a microwave probe, the electric power applied to the film was 1 mW at 875 MHz. We obtained the enhancement of the Brillouin scattering signal in the silica glass bar sample due to the induced phonons. Compared with and without the induced phonons from the ScAlN film transducer, the peak intensity improved by nearly 3 orders of magnitude. This technique can significantly shorten the time required for the Brillouin scattering measurements.

We have experimentally investigated wave velocities and refractive indices in bulk and film samples [a GaN single crystal plate and c-axis-oriented ScxAl(1-x)N (x = 0.00-0.63) films] by Brillouin scattering. All of the piezoelectrically unstiffened elastic constants and the ordinary refractive index of the GaN single crystal plate were determined from the reflection induced A (RIA) scattering geometry and the combination of 90R and 180° scattering geometries. The uncertainties of the measured wave velocities were approximately 0.17% (RIA) and 2.5% (combination technique). In addition, the longitudinal wave velocities of ScxAl(1-x)N films propagating in the normal direction were obtained by the combination technique. The maximum uncertainty was approximately 3.3%. The shear wave velocities and refractive indices of ScxAl(1-x)N films were also investigated by the 90R scattering geometry using velocities measured by high-overtone bulk acoustic resonators. The softening trends of the elasticity were obtained from the measured longitudinal and shear wave velocities, although there were large uncertainties in the Brillouin measurement system owing to thermal instability.

This paper investigates a two-dimensional ultrasonic manipulation technique for small objects in air. The ultrasonic levitation system consists of a rectangular vibrating plate with four ultrasonic transducers and a reflector. The configuration of the vibrator, the resonant frequency, and the positions of the four transducers with step horns were determined from finite element analysis such that an intense acoustic standing-wave field could be generated between the plates. A lattice flexural vibration mode with a wavelength of 28.3 mm was excited on the prototype plate at 24.6 kHz. Small objects could get trapped in air along the horizontal nodal plane of the standing wave. By controlling the driving phase difference between the transducers, trapped objects could be transported without contact in a two-dimensional plane. When the phase difference was changed from 0° to 720°, the distance moved by a small particle in the orthogonal direction was approximately 29 mm, which corresponds with the wavelength of the flexural vibration on the vibrating plate.

The resistivity-frequency characteristics of longitudinal wave velocities propagating parallel to the c-axis in a GaN single crystal were theoretically estimated by considering the piezoelectric acousto-electric effect. The temperature and frequency dependences of longitudinal and shear wave velocities in conductive and semiconductive GaN single-crystal samples were experimentally investigated by Brillouin scattering. The temperature dependence of longitudinal and shear wave velocities had a linear tendency in the conductive sample, whereas in the semiconductive sample, those had a similar tendency to the predicted velocity changes resulting from the piezoelectric stiffening effect. However, the temperature dependence of shear wave velocity, which does not possess piezoelectric coupling, had a tendency similar to that of the longitudinal wave in the semiconductive sample, unexpectedly. The frequency dependence of longitudinal wave velocities in the semiconductive sample had a tendency similar to the predicted velocity changes resulting from the piezoelectric stiffening effect.

A junction for noncontact ultrasonic transportation paths in which small objects can be manipulated is proposed. The junction consists of a vibrating disc and a reflector. The reflector is installed parallel to the vibrator to generate an acoustic standing wave in the cavity between the vibrating disc and the reflector. The resonance modes of the acoustic field in the disc cavity between the two plates are calculated theoretically. The distributions of the sound pressure amplitude and the acoustic radiation force in air are calculated using finite element analysis. The flexural vibration modes with one nodal circle and four nodal lines at 45.4 kHz and two nodal circles and three nodal lines at 58.1 kHz are used to trap and eject small objects, respectively. The transportation velocity and the thrust force in the radial direction for a polystyrene particle with a diameter of 2 mm and a weight of 0.3 mg are 812 mm/s and 24 μN, respectively. The ejection direction of the trapped object can be controlled by the driving condition of the vibrating disc.

A pulse wave is the displacement wave which arises because of ejection of blood from the heart and reflection at vascular bed and distal point. The investigation of pressure waves leads to understanding the propagation characteristics of a pulse wave. To investigate the pulse wave behavior, an experimental study was performed using an artificial polymer tube and viscous liquid. A polyurethane tube and glycerin solution were used to simulate a blood vessel and blood, respectively. In the case of the 40 wt% glycerin solution, which corresponds to the viscosity of ordinary blood, the attenuation coefficient of a pressure wave in the tube decreased from 4.3 to 1.6 dB/m because of the tube stiffness (Young's modulus: 60 to 200 kPa). When the viscosity of liquid increased from approximately 4 to 10 mPa·s (the range of human blood viscosity) in the stiff tube, the attenuation coefficient of the pressure wave changed from 1.6 to 3.2 dB/m. The hardening of the blood vessel caused by aging and the increase of blood viscosity caused by illness possibly have opposite effects on the intravascular pressure wave. The effect of the viscosity of a liquid on the amplitude of a pressure wave was then considered using a phantom simulating human blood vessels. As a result, in the typical range of blood viscosity, the amplitude ratio of the waves obtained by the experiments with water and glycerin solution became 1:0.83. In comparison with clinical data, this value is much smaller than that seen from blood vessel hardening. Thus, it can be concluded that the blood viscosity seldom affects the attenuation of a pulse wave.

Pulse wave evaluation is an effective method for arteriosclerosis screening. The pulse wave comprises two displacement components, the incident wave ε(i)(t) and the reflected wave ε(r)(t). Because the amplitude of the reflected wave changes markedly with arterial stiffness, analysis of this wave is useful for evaluation of such stiffness. In this paper, a noninvasive method for extracting the reflected component from a pulse wave is proposed. First, the pulse wave ε(i)(t) + ε(r)(t) and blood flow velocity u(i)(t) - u(r)(t) were measured at the common carotid artery. A new approach is used to estimate the displacement wave ε(i)(t) -ε(r)(t), in which a transform of the conservation of mass, an elastic tube model, and a Voigt model for a viscoelastic body are applied to blood flow velocity data. Twice the amplitude of the reflected wave [TARW; 2ε(r)(t)] was obtained by subtracting the amplitude of the calculated displacement wave from that of the observed pulse wave. This method was applied to subjects aged from their 20s to 60s to evaluate differences in the reflected component. The results indicate moderate correlation between age and TARW (R(2) = 0.65). To evaluate the validity of this method for screening arterial stiffness, we compared TARW with existing diagnostic indices pulse wave velocity (PWV) and cardio-ankle vascular index (CAVI). TARW was moderately correlated with PWV (R(2) = 0.48) and CAVI (R(2) = 0.71). Therefore, this new method has potential for diagnosing arterial stiffness.

The two-wave phenomenon reflects not only bone mass but also the complex bone structure of cancellous bone. We propose a new simple imaging technique based on the two-wave phenomenon for investigating the anisotropic structure of cancellous bone. A cylindrical specimen of cancellous bone was obtained from a bovine femur. The structure (alignment of trabeculae) of the specimen was obtained from 3-D X-ray micro computed tomography imaging. Using a conventional ultrasonic pulse technique, we rotated the receiver around the specimen to investigate the ultrasonic fields after propagation within the specimen. The ultrasonic propagation image clearly showed the effect of the bone structure. We found that the fast wave showed apparent refraction, whereas the slow wave did not. Fast-wave propagation imaging is thus a simple and convenient technique for easy interpretation of the anisotropic structure. This imaging technique has the potential to become a powerful tool to investigate the structure of trabeculae during in vivo measurements.

To improve the accuracy of velocity measurements in the Brillouin scattering technique using weak thermal phonons, we have used induced coherent phonons, which intensify the scattering. To induce phonons in the gigahertz range, we used a c-axis tilted ZnO film transducer that was developed in our laboratory. This allowed us to induce longitudinal and shear acoustic phonons effectively at hypersonic frequencies. As a result, we obtained scattered light in the silica glass sample that was much more intense than that obtained from the thermal phonons. Because the Brillouin scattering from induced phonons was measured, the shift frequency was that of the electric signal applied to the ZnO transducer. Strong peaks lead to a reduction of the measurement time. This is useful for two-dimensional mapping of thin film elasticity using Brillouin scattering. Additionally, Brillouin scattering enables the simultaneous measurement of longitudinal and shear phonon velocities in the sample plane. This opens up a potential new technique for non-destructive elasticity measurements of various materials.

A method for designing frequencies and modes in ultrasonic transducers above the very-high-frequency (VHF) range is required for ultrasonic non-destructive evaluation and acoustic mass sensors. To obtain the desired longitudinal and shear wave conversion loss characteristics in the transducer, we propose the use of a c-axis zig-zag structure consisting of multilayered c-axis 23° tilted ZnO piezoelectric films. In this structure, every layer has the same thickness, and the c-axis tilt directions in odd and even layers are symmetric with respect to the film surface normal. c-axis zig-zag crystal growth was achieved by using a SiO(2) low-temperature buffer layer. The frequency characteristics of the multilayered transducer were predicted using a transmission line model based on Mason's equivalent circuit. We experimentally demonstrated two types of transducers: those exciting longitudinal and shear waves simultaneously at the same frequency, and those exciting shear waves with suppressed longitudinal waves.

Ultrasonic waves in cancellous bone change dramatically depending on its structural complexity. One good example is the separation of an ultrasonic longitudinal wave into fast and slow waves during propagation. In this study, we examined fast wave propagation in cancellous bone obtained from the head of the bovine femur, taking the bone structure into consideration. We investigated the wave propagation perpendicular to the bone axis and found the two-wave phenomenon. By rotating the cylindrical cancellous bone specimen, changes in the fast wave speed due to the rotation angle then were observed. In addition to the ultrasonic evaluation, the structural anisotropy of each specimen was measured by X-ray micro-computed tomography (CT). From the CT images, we obtained the mean intercept length (MIL), degree of anisotropy (DA), and angle of insonification relative to the trabecular orientation. The ultrasonic and CT results showed that the fast wave speed was dependent on the structural anisotropy, especially on the trabecular orientation and length. The fast wave speeds always were higher for propagation parallel to the trabecular orientation. In addition, there was a strong correlation between the DA and the ratio between maximum and minimum speeds (V(max)/V(min)) (R(2) = 0.63).

At the nanoscopic level, bone consists of calcium phosphate, which forms incomplete hydroxyapatite (HAp) crystals. The preferred orientation of the c-axis of HAp crystallites induces anisotropy and inhomogeneity of elastic properties in bone. In this study, the effect of the preferred orientation of HAp crystallites on the spatial distribution of ultrasonic wave velocity was experimentally investigated, considering bone mineral density (BMD) and microstructure. Three ring-shaped cortical bone samples were made from a 36-month-old bovine femur. Longitudinal wave velocity was measured by a conventional ultrasonic pulse system, using self-made polyvinylidene fluoride transducers. The integrated intensity of the (0002) peak obtained using X-ray diffraction was estimated to evaluate the amount of preferred orientation. The velocity distribution pattern was similar to the distribution of integrated intensity of (0002). The effect of the preferred orientation of HAp crystallites on velocity was clearly observed in the plexiform structure, despite the fact that the BMD value was almost independent of the preferred orientation of HAp crystallites. Velocity measurement of cortical bone can reveal information about HAp crystallite orientation.

Thickness pure-shear mode film bulk acoustic wave resonators (FBARs) made of (1120) textured ZnO films have been fabricated. We also have fabricated FBAR structure consisting of two layers of the (1120) textured ZnO film with opposite polarization directions. This FBAR structure operated in second overtone pure-shear mode and allowed shear-mode FBARs at higher frequency. The effective electromechanical coupling coefficients k2 of pure-shear mode FBAR and second overtone pure-shear mode FBAR in this study were found to be 3.3% and 0.8%, respectively. The temperature coefficient of frequency (TCF) of thickness extensional mode FBAR, pure-shear mode FBAR, and second overtone pure-shear mode FBAR were measured in the temperature range of 10-60 degrees C. TCF values of -63.1 ppm/degrees C, -34.7 ppm/degrees C, and -35.6 ppm/degrees C were found for the thickness extensional mode FBAR, the pure-shear mode FBAR, and the second overtone pure-shear mode FBAR, respectively. These results demonstrated that pure-shear mode ZnO FBARs have more stable temperature characteristics than the conventional thickness extensional mode ZnO FBARs.

This paper reports the fabrication and characterization of ZnO piezoelectric thin films in which the crystallite c-axis is unidirectionally aligned in the plane. The films were deposited by a conventional radio frequency (RF) magnetron sputtering apparatus without epitaxy. We have measured reflection coefficient S11 of the ZnO film/glass substrate composite shear mode resonator and confirmed that the resonator excites shear wave only in the very high frequency to ultra high frequency ranges (VHF-UHF). The crystallites c-axis orientation and alignment were determined by x-ray diffraction (XRD) patterns, phi-scan pole figure analysis, omega-scan rocking curves, and atomic force microscope (AFM) measurement. The transduction of the shear wave showed good agreement with properties of the crystallite alignment in the film.