Three-dimensional photoacoustic imaging using a two-dimensional CMUT array

In this paper, we describe using a 2-D array of capacitive micromachined ultrasonic transducers (CMUTs) to perform 3-D photoacoustic and acoustic imaging. A tunable optical parametric oscillator laser system that generates nanosecond laser pulses was used to induce the photoacoustic signals. To demonstrate the feasibility of the system, 2 different phantoms were imaged. The first phantom consisted of alternating black and transparent fishing lines of 180 μm and 150 μm diameter, respectively. The second phantom comprised polyethylene tubes, embedded in chicken breast tissue, filled with liquids such as the dye indocyanine green, pig blood, and a mixture of the 2. The tubes were embedded at a depth of 0.8 cm inside the tissue and were at an overall distance of 1.8 cm from the CMUT array. Two-dimensional cross-sectional slices and 3-D volume rendered images of pulse-echo data as well as photoacoustic data are presented. The profile and beamwidths of the fishing line are analyzed and compared with a numerical simulation carried out using the Field II ultrasound simulation software. We investigated using a large aperture (64 x 64 element array) to perform photoacoustic and acoustic imaging by mechanically scanning a smaller CMUT array (16 x 16 elements). Two-dimensional transducer arrays overcome many of the limitations of a mechanically scanned system and enable volumetric imaging. Advantages of CMUT technology for photoacoustic imaging include the ease of integration with electronics, ability to fabricate large, fully populated 2-D arrays with arbitrary geometries, wide-bandwidth arrays and high-frequency arrays. A CMUT based photoacoustic system is proposed as a viable alternative to a piezoelectric transducer based photoacoustic systems.     

Visible light emission from silicon dioxide with silicon nanocrystals

An electroluminescent MOS structure was developed using silicon wafers covered by thermal silicon dioxide containing silicon nanocrystals. Efficiency of the structure was sufficient for observation to be possible with the naked eye in daylight conditions under DC polarization. Silicon nanocrystals were produced using silicon ion implantation followed by subsequent annealing at 1100 °C in a nitrogen atmosphere. Three separate bands of emitted light at wavelengths of ∼400-500 nm (blue), ∼500-600 nm (green), and ∼650-850 nm (red) were observed and found to be related to specific regions of the implanted silicon concentration profile. For a single energy implant, each of the emitted light bands originated from a separate depth region of the silicon dioxide layer containing silicon nanocrystals. The spectrum of the emitted light was found to depend on the excess silicon concentration profile. For practical applications, the color of the emitted light can be controlled by adjustment of the implantation parameters and MOS structuring process.     

A silicon counter array for 2He detection

A multicounter array consisting of four Si detector telescopes was developed for the coincidence measurement of two protons with small relative energy (²He). Silicon detectors were designed and fabricated so that they have large solid angles when they are assembled to form a counter array. A total effective solid angle of 5–7 msr was achieved for the detection of 20–50 MeV ²He with relative energy lower than 1 MeV. A compact amplifier system was developed using low-noise hybrid ICs to treat many signals from the counter array.     

Performance of a long-wave infrared hyperspectral imager using a Sagnac interferometer and an uncooled microbolometer array

Field and laboratory measurements using an interferometer spectrometer based on the Sagnac interferometer using a microbolometer array detector are presented. Remotely obtained signatures collected with this instrument and with a cryogenic IR spectrometer are compared and shown to closely correspond. Ground-to-ground and air-to-ground image products are presented that demonstrate the image quality of the sensor. Signal-to-noise measurements are presented and compared with a simple parametric performance model that predicts the sensor performance. The performance model is used to predict the performance of this technology when equipped with cooled detectors.     

Fluid flow through an array of fixed particles

Slow flow of an incompressible viscous fluid is studied in an array of a great number of small fixed solid particles. The particles size ? and the distance η between two neighbouring solids are such that $\text{??η?1}$. Using perturbation methods it is proved that Brinkman's law occurs really for a critical size of particles; for larger particles the fluid filtration is governed by the Darcy's law and smaller solids do not influence the flow. The 3 and 2-dimensional cases are studied.     

Design and fabrication of an integrated polarized light guide for liquid-crystal-display illumination

An integrated polarized light guide was designed and fabricated for use as a liquid-crystal backlight with emphasis on uniformity of the light and conversion of p-polarized to s-polarized light. Two different micro-optical structures were fabricated both on the top and the bottom surfaces of the light guide. On the top surface, a subwavelength grating separates s-polarized and p-polarized light to achieve a polarization-conversion efficiency of 69%. A 1.7 gain factor of polarization efficiency was obtained to increase the utility of light for liquid-crystal illumination.     

Diffraction of transmission light through triangular apertures in array of retro-reflective microprisms

An array of microprisms was described by a model of multiperiod blazed gratings consisting of triangular apertures. The origins of hexagram-shaped diffraction patterns were interpreted based on multiple-beam interference and diffraction array theorem. The relation between zonal/line ghost fringes and imperfectly fabricated array structures was analyzed. Geometrical performance (e.g., the dihedral angle of the microprism) was tested by measuring the features of diffraction patterns of samples from three retroreflective sheeting manufacturers.     

A method to synchronize high-intensity, focused ultrasound with an arbitrary ultrasound imager

Ultrasound imaging is useful for monitoring high-intensity, focused ultrasound (HIFU) therapy; however, interference on the ultrasound image, caused by HIFU excitation, must be avoided. A method to synchronize HIFU excitation with ultrasound imaging is described here. Synchronization was tested with two unmodified, commercial imagers and two tissue phantoms.     

An integrated circuit with transmit beamforming flip-chip bonded to a 2-D CMUT array for 3-D ultrasound imaging

State-of-the-art 3-D medical ultrasound imaging requires transmitting and receiving ultrasound using a 2-D array of ultrasound transducers with hundreds or thousands of elements. A tight combination of the transducer array with integrated circuitry eliminates bulky cables connecting the elements of the transducer array to a separate system of electronics. Furthermore, preamplifiers located close to the array can lead to improved receive sensitivity. A combined IC and transducer array can lead to a portable, high-performance, and inexpensive 3-D ultrasound imaging system. This paper presents an IC flip-chip bonded to a 16 x 16-element capacitive micromachined ultrasonic transducer (CMUT) array for 3-D ultrasound imaging. The IC includes a transmit beamformer that generates 25-V unipolar pulses with programmable focusing delays to 224 of the 256 transducer elements. One-shot circuits allow adjustment of the pulse widths for different ultrasound transducer center frequencies. For receiving reflected ultrasound signals, the IC uses the 32-elements along the array diagonals. The IC provides each receiving element with a low-noise 25-MHz-bandwidth transimpedance amplifier. Using a field-programmable gate array (FPGA) clocked at 100 MHz to operate the IC, the IC generated properly timed transmit pulses with 5-ns accuracy. With the IC flip-chip bonded to a CMUT array, we show that the IC can produce steered and focused ultrasound beams. We present 2-D and 3-D images of a wire phantom and 2-D orthogonal cross-sectional images (Bscans) of a latex heart phantom.     

Water-wave propagation through an infinite array of floating structures

The frequency-domain problem of water-wave propagation through a periodically arranged infinite array of structures is solved using the method of matched asymptotic expansions for both shallow and deep water; the structures are assumed to be small relative to the wavelength and the array periodicity, and may be fixed or float freely. Explicit approximations to the frequencies of propagating modes are obtained, and used to illustrate how the frequencies are affected by geometrical and other parameters.     

Analysis of heterodyne and confocal microscopy for illumination with broad-bandwidth light

Abstract

An imaging equation is derived which describes heterodyne microscopy with broad-band light illumination. The impact of spherical aberration resulting from imaging into thick samples is investigated. As in confocal microscopy, the lateral resolution is found to be only weakly affected by moderate spherical aberration. The loss in depth resolution can be strongly reduced by using broad-band light with a short coherence length.     

Uniform theory of the Talbot effect with partially coherent light illumination

A uniform formulation for the self-imaging of gratings with any kind of partially coherent illumination is developed in terms of the cross mutual spectral density of the partial coherence theory. The formulation includes the time diffractive intensity distribution and the averaged diffractive intensity distribution at self-imaging distances and can be applied to both continuous and temporal illuminations with any kind of spectra. It is found that the averaged intensity distribution is related only to the intensity spectrum of illumination. The continuous polychromatic illumination and the ultrashort laser pulses with or without frequency chirp are then studied by a numerical stimulation. It is shown that the ultrashort laser pulse and the continuous polychromatic illuminations have similar averaged self-image distributions. Thus the Talbot effect may help in the study of the temporal and spectral characteristics of ultrashort laser pulses. An experiment with an LED is given, as well.     

Room-temperature Coulomb staircase in semiconducting InP nanowires modulated with light illumination

Detailed electron transport analysis is performed for an ensemble of conical indium phosphide nanowires bridging two hydrogenated n(+)-silicon electrodes. The current-voltage (I-V) characteristics exhibit a Coulomb staircase in the dark with a period of ～ 1 V at room temperature. The staircase is found to disappear under light illumination. This observation can be explained by assuming the presence of a tiny Coulomb island, and its existence is possible due to the large surface depletion region created within contributing nanowires. Electrons tunnel in and out of the Coulomb island, resulting in the Coulomb staircase I-V. Applying light illumination raises the electron quasi-Fermi level and the tunneling barriers are buried, causing the Coulomb staircase to disappear.     

A silicon/zinc 2,9,16,23-tetraaminophthalocyanine coaxial core-shell nanowire array as an efficient solar hydrogen generation photocatalyst

In this work, we have demonstrated that a silicon nanowire (SiNW) array can be an efficient visible light photocatalyst for hydrogen generation after being modified by the 2,9,16,23-tetraaminophthalocyanine of zinc (ZnTAPc). A photoelectrochemical (PEC) cell employing a ZnTAPc modified SiNW array as photoanode was found to be able to effectively produce hydrogen at a rate of 13 μmol (cm(2) h)(-1) under 100 mw cm(-2) irradiation from a xenon lamp. It is believed that the loading of ZnTAPc can enhance the efficiency of hydrogen generation and the stability of the SiNW array. This demonstrates that the ZnTAPc modified SiNW is a promising material for solar hydrogen generation.     

Electronic properties of microporous silicon under illumination and with the adsorption of ammonia

An electromotive force has been observed at contacts of porous silicon samples, its magnitude and sign varying with illumination and adsorption of ammonia. The samples exhibit characteristic nonuniformity of the porosity along the surface of the material. The physical mechanism for these effects is discussed. Pis’ma Zh. Tekh. Fiz. 23, 77–82 (June 12, 1997)     

Absorption of light in a single-nanowire silicon solar cell decorated with an octahedral silver nanocrystal

In recent photovoltaic research, nanomaterials have offered two new approaches for trapping light within solar cells to increase their absorption: nanostructuring the absorbing semiconductor and using metallic nanostructures to couple light into the absorbing layer. This work combines these two approaches by decorating a single-nanowire silicon solar cell with an octahedral silver nanocrystal. Wavelength-dependent photocurrent measurements and finite-difference time domain simulations show that increases in photocurrent arise at wavelengths corresponding to the nanocrystal's surface plasmon resonances, while decreases occur at wavelengths corresponding to optical resonances of the nanowire. Scanning photocurrent mapping with submicrometer spatial resolution experimentally confirms that changes in the device's photocurrent come from the silver nanocrystal. These results demonstrate that understanding the interactions between nanoscale absorbers and plasmonic nanostructures is essential to optimizing the efficiency of nanostructured solar cells.     

Effect of growth temperature on field emission from an array of carbon nanotubes nested into a silicon nanoporous pillar array

Large scale nest arrays of multi-walled carbon nanotubes (NA-MWCNT) were grown on silicon nanoporous pillar arrays (Si-NPA) by thermal chemical vapour deposition at 700, 800, and 900 °C respectively for 15 min. The corresponding field emission properties were also studied. It was found that growth temperature had a significant effect on the performance of CNTs, the field emission efficiency of NA-MWCNT/Si-NPA synthesized at 900 °C was higher than that of the other two types of samples. By hiring scanning electron microscope observing and high resolution transmission electron microscopy analysis, the reason was attributed to numerous nanoparticles appearance on the surface of the CNT and highly folding graphite sheets formation on both sides of the nanotubes fabricated at 900 °C. Both could enhance emission properties by lowering the effective work function and increasing the field enhancement factor as well as the number of the emission sites.