Investigation of transmit and receive performance at the fundamental and third harmonic resonance frequency of a medical ultrasound transducer

In this study, the phenomenon of higher harmonic thickness resonance of a piezoelectric transducer was used to investigate potentially additional sensitivity at the third harmonic frequency for conventional medical transducers. The motivation for this research is that some applications in medical ultrasound (e.g. third harmonic transmit phasing and contrast imaging) need probes which are sensitive around both the fundamental and third harmonic frequencies, and that these higher harmonic thickness modes, although often considered as undesired, might be used beneficially. The novelty aspect in this study is the presented transmit and receive potential at both the fundamental and third harmonic of a conventional cardiac probe with modified electrical tuning. Elements of an experimental PZT-based phased-array probe (f_c = 3 MHz, 64 elements, element width = 0.3 mm, elevation aperture = 13 mm) were electrically retuned with series inductors around the third harmonic resonance frequency at 10 MHz. Hydrophone measurements with 10-MHz-tuned elements showed that, as compared to a conventionally tuned element, the transmit transfer function at the third harmonic increased more than 23 dB, while the sensitivity at the fundamental frequency was only 6 dB lower. Pulse-echo measurements showed that the two-way transfer function of a 10-MHz-tuned element resulted in 20 dB increased sensitivity around the third harmonic as compared to an untuned element. Simulated transfer functions, from both a 1D KLM and 2D finite element model of an element of the experimental array transducer, confirmed the measured sensitivity peaks at the fundamental and third harmonic. In conclusion, this study demonstrated the effect of changing the electrical tuning on a conventional array transducer which increased the sensitivity around the third harmonic resonance frequency, while maintaining good sensitivity at the fundamental frequency.     

Fabrication and study of microstructured optical fibers with elliptical holes

We report the fabrication of what are believed to be the first microstructured optical fibers with uniformly oriented elliptical holes. A high degree of hole ellipticity is achieved with a simple technique that relies on hole deformation during fiber draw. Both form and stress-optic birefringence are characterized over a broad wavelength range. These measurements are in excellent agreement with numerical modeling and demonstrate a birefringence as high as 1.0 x 10(-4) at a wavelength of 850 nm.     

Small-core single-mode microstructured polymer optical fiber with large external diameter

A preform sleeving technique is demonstrated that allows the fabrication of single-mode polymer microstructured fiber with the smallest core and hole dimensions yet reported to our knowledge. For a fixed triangular hole pattern a range of fibers is produced by adjustment to the operating conditions of the draw tower. Numerical modeling is carried out for one of the fibers produced with a 570-microm external diameter, a core diameter of 2.23 microm, an average hole diameter of 0.53 microm, and an average hole spacing of 1.38 microm. This fiber was shown to be endlessly single mode.     

Hybrid zinc oxide conjugated polymer bulk heterojunction solar cells

Bulk heterojunction photovoltaic devices based on blends of a conjugated polymer poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) as electron donor and crystalline ZnO nanoparticles (nc-ZnO) as electron acceptor have been studied. Composite nc-ZnO:MDMO-PPV films were cast from a common solvent mixture. Time-resolved pump-probe spectroscopy revealed that a photoinduced electron transfer from MDMO-PPV to nc-ZnO occurs in these blends on a sub-picosecond time scale and produces a long-lived (milliseconds) charge-separated state. The photovoltaic effect in devices, made by sandwiching the active nc-ZnO:MDMO-PPV layer between charge-selective electrodes, has been studied as a function of the ZnO concentration and the thickness of the layer. We also investigated changing the degree and type of mixing of the two components through the use of a surfactant for ZnO and by altering the size and shape of the nc-ZnO particles. Optimized devices have an estimated AM1.5 performance of 1.6% with incident photon to current conversion efficiencies up to 50%. Photoluminescence spectroscopy, atomic force microscopy, and transmission electron microscopy have been used to gain insight in the morphology of these blends.     

Evaluation of the sensitivity of miniaturized liquid chromatography-electrospray ionization-mass spectrometry for pharmaceutical analysis

LC-ESI-MS is applied frequently in pharmaceutical analysis. The sample amount is generally not restricted, however with LC-ESI-MS, a lack of sensitivity may still be observed with standard-bore LC columns in isocratic mode. Therefore, it was investigated whether increased sensitivity could be achieved by using miniaturized LC-ESI-MS. Seven columns ranging from 0.1 to 4.6 mm ID were tested using several instrument setups. For proper comparison, a sensitivity gain factor (SGF) was introduced. The SGF expresses the extra sensitivity that may be obtained on top of the normal increase of peak concentration, which can be expected when the column ID is reduced. Desogestrel, mirtazapine, and sugammadex sodium were used as test compounds. For desogestrel and sugammadex sodium, the SGF increased up to a factor of 5-13 when the column ID was reduced, indicating enhanced ionization efficiencies at lower flow rates. Optimum sensitivity was found for the 0.3 mm column coupled in combination with a microinjection valve and a dedicated low flow rate interface. For mirtazapine, no increase of SGF was observed when the column ID was decreased. Apparently, the ionization efficiency of this compound is not affected by the flow rate and the spray quality.     

Light trapping in a fiber grating defect by four-wave mixing

We propose the use of nonlinear four-wave mixing as a means of trapping light in a defect state in a nonuniform fiber grating. The amount of energy deposited is estimated by use of an approach similar to Fermi's "golden rule" and is approximately 30 fJ for realistic grating parameters and a pulsed pump of 100-ps width.     

Modal cutoff in microstructured optical fibers

We analyze the nature of modal cutoff in microstructured optical fibers of finite cross section. In doing so, we reconcile the striking endlessly single-mode behavior with the fact that in such fibers all propagation constants are complex. We show that the second mode undergoes a strong change of behavior that is reflected in the losses, effective area, and multipolar structure. We establish the parameter subspace in which the fibers are single mode and an accurate value for the limit of the endlessly single-mode regime.     

Single-molecule study of the electronic couplings in a circular array of molecules: light-harvesting-2 complex from Rhodospirillum molischianum

Applying single-molecule spectroscopic techniques allowed us to determine the mutual angles between the transition-dipole moments associated with optical transitions of the eight bacteriochlorophyll a molecules which form the so-called B800 ring of the light-harvesting-2 complex from Rhodospirillum molischianum. The orientation of the transition-dipole moment is a sensitive probe for the strength of the local intermolecular interactions because of the well-defined arrangement of the individual molecules within the B800 ring. Our data reveal that the strength of the electronic coupling between individual molecules in the ring is subjected to spatial as well as temporal variations.     

Optimal poling of nonlinear photonic crystals for frequency conversion

Nonlinear photonic crystals formed by two-dimensional periodic poling of the quadratic susceptibility chi(2) can support quasi-phase-matched harmonic generation. The frequency-conversion efficiency depends on the photonic crystal's poling pattern through certain Fourier coefficients of the poling function. A procedure is described for finding those poling patterns that are most efficient for any specific quasi-phase-matched frequency-conversion scheme.