Ultrabroadband polarization analysis of terahertz pulses.

We describe the characteristics of a sensitive photoconductive detector that simultaneously measures orthogonal electric field components of electromagnetic transients with bandwidths up to 30 THz. The device consists of an As(+) implanted GaAs photoconducting region at the centre of a pair of perpendicular bow-tie antennas. The performance is illustrated by studies of optical rectification in GaSe, retardation in a birefringent polymer film and THz emission from impulsively excited optical phonons in GaN.

Modal coupling in surface-corrugated long-period-grating fiber tapers.

We describe the properties of a surface-corrugated long-period-grating fiber taper fabricated using contact optical lithography and wet etching techniques. The preservation of cylindrical symmetry in this device facilitates investigation of the modal behavior. Comparison of the measured and calculated transmission spectra reveals that the widely used coupled-mode theory is not applicable. Instead, a mode-projection model, in which modal propagation and coupling are treated separately within the grating, explains the experiments very well.

Surface corrugation Bragg gratings on optical fiber tapers created via plasma etch postprocessing.

We describe a reflection-based fiber filter fabricated by plasma etching a surface corrugation Bragg grating on a tapered single-mode fiber. The taper waist with the grating forms the functional part of the filter, and the adiabatic taper transition removes unwanted higher-order modes. The spectral response is controlled by varying the taper diameter while maintaining a constant grating period. Reflection spectra have been investigated theoretically and experimentally and found to be in good agreement.

Modal coupling in fiber tapers decorated with metallic surface gratings.

An interference-based scheme for fabricating periodic metal gratings on one side of the uniform waist of optical fiber tapers has been developed. Optical characterization of a 5 mm long, 511 nm period gold grating fiber taper with a 10 microm diameter reveals backward coupling to both guided and radiation modes that is explained by using an analytical mode-coupling analysis. A refractometer based on this grating taper has a high and constant sensitivity over a large refractive index operating range of 1 to 1.41.

Optimization of photoconducting receivers for THz spectroscopy.

We have studied the sensitivity and noise of optically gated dipole receivers made from ion implanted Si and GaAs in an optimized time domain THz spectrometer. The spectrometer uses a room temperature, dc biased, semi-insulating GaAs stripline source capable of generating up to 30 microW average power. The 10% amplitude system bandwidth for 10 microm (50 microm) dipole receivers is 3 THz (1.5 THz). A dynamic range of 4 x 10(5) Hz(-1/2) is achieved using a 10 microm dipole GaAs receiver and 2 x 10(6) Hz(-1/2) using a 50 microm dipole for a total laser power of 110 mW and THz beam power of 20 microW. The dynamic range achieved with comparable silicon receivers is a factor of 2 smaller.

Influence of the aglycone region of the substrate binding cleft of Pseudomonas xylanase 10A on catalysis.

Pseudomonas cellulosa xylanase 10A (Pc Xyn10A) contains an extended substrate binding cleft comprising three glycone (-1 to -3) and four aglycone (+1 to +4) subsites and, typical of retaining glycoside hydrolases, exhibits transglycosylation activity at elevated substrate concentrations. In a previous study [Charnock, S. J., et al. (1997) J. Biol. Chem. 272, 2942-2951], it was demonstrated that the -2 subsite mutations E43A and N44A caused a 100-fold reduction in activity against xylooligosaccharides, but did not influence xylanase activity. This led to the proposal that the low activity of these mutants against xylooligosaccharides was due to nonproductive complex formation between these small substrates and the extended aglycone region of the active site. To test this hypothesis, key residues at the +2 (Asn182), +3 (Tyr255), and +4 (Tyr220) subsites were substituted for alanine, and the activity of the mutants against polysaccharides and oligosaccharides was evaluated. All the aglycone mutants exhibited greatly reduced or no transglycosylating activity, and the triple mutants, E43A/Y220A/Y255A and E43A/N182A/Y255A, had activity against xylotriose similar to that of E43A. The aglycone mutations caused an increase in both k(cat) and K(m) against xylan, with N182A/Y220A/Y255A and N182A/Y255A exhibiting 25- and 15-fold higher k(cat) values, respectively, than wild-type Pc Xyn10A. These data indicate that Glu43 plays a role in binding xylooligosaccharides, but not xylan, suggesting that the mechanisms by which Pc Xyn10A binds polysaccharides and oligosaccharides are distinct. The increased k(cat) of the mutants against xylan indicates that the aglycone region of wild-type Pc Xyn10A restricts the rate of catalysis by limiting diffusion of the cleaved substrate, generated at the completion of the k(2) step, out of the active site.

Mechanism of THz emission from asymmetric double quantum wells.

Impulsive interband optical excitation of the lowest two conduction subbands of a suitably engineered GaAs/AlGaAs double quantum well can lead to coherent THz emission. We demonstrate that, contrary to previous expectations, the dominant emission mechanism can involve the beating of either continuum or exciton states, depending on excitation conditions. The coherence of the continuum beats persists for several picoseconds even for excitation an optical phonon energy above the band edge. We attribute this to the small energy difference between the component eigenstates, which substantially reduces the number of relevant scattering events.

Substrate specificity in glycoside hydrolase family 10. Tyrosine 87 and leucine 314 play a pivotal role in discriminating between glucose and xylose binding in the proximal active site of Pseudomonas cellulosa xylanase 10A.

The Pseudomonas family 10 xylanase, Xyl10A, hydrolyzes beta1, 4-linked xylans but exhibits very low activity against aryl-beta-cellobiosides. The family 10 enzyme, Cex, from Cellulomonas fimi, hydrolyzes aryl-beta-cellobiosides more efficiently than does Xyl10A, and the movements of two residues in the -1 and -2 subsites are implicated in this relaxed substrate specificity (Notenboom, V., Birsan, C., Warren, R. A. J., Withers, S. G., and Rose, D. R. (1998) Biochemistry 37, 4751-4758). The three-dimensional structure of Xyl10A suggests that Tyr-87 reduces the affinity of the enzyme for glucose-derived substrates by steric hindrance with the C6-OH in the -2 subsite of the enzyme. Furthermore, Leu-314 impedes the movement of Trp-313 that is necessary to accommodate glucose-derived substrates in the -1 subsite. We have evaluated the catalytic activities of the mutants Y87A, Y87F, L314A, L314A/Y87F, and W313A of Xyl10A. Mutations to Tyr-87 increased and decreased the catalytic efficiency against 4-nitrophenyl-beta-cellobioside and 4-nitrophenyl-beta-xylobioside, respectively. The L314A mutation caused a 200-fold decrease in 4-nitrophenyl-beta-xylobioside activity but did not significantly reduce 4-nitrophenyl-beta-cellobioside hydrolysis. The mutation L314A/Y87A gave a 6500-fold improvement in the hydrolysis of glucose-derived substrates compared with xylose-derived equivalents. These data show that substantial improvements in the ability of Xyl10A to accommodate the C6-OH of glucose-derived substrates are achieved when steric hindrance is removed.

An historical commentary on the physiological effects of music: Tomatis, Mozart and neuropsychology.

This article provides an overview of the theoretical underpinnings of the Tomatis Method, along with a commentary on other forms of sound/music training and the need for research. A public debate was sparked over the "Mozart Effect." This debate has turned out to be unfortunate because the real story is being missed. The real story starts with Alfred Tomatis, M.D., scientist and innovator. Dr. Tomatis was the first to develop a technique using modified music to stimulate the rich interconnections between the ear and the nervous system to integrate aspects of human development and behavior. The originating theories behind the Tomatis Method are reviewed to describe the ear's clear connection to the brain and the nervous system. The "neuropsychology of sound training" describes how and what the Tomatis Method effects. Since Dr. Tomatis opened this field in the mid 20th century, no fewer than a dozen offshoot and related systems of training have been developed. Though each new system of treatment makes claims of effectiveness, no research exists to substantiate their claims. Rather, each simplified system bases its "right to exist and advertise" on the claimed relationship to Tomatis and his complex Method. Research is desperately needed in this area. The 50 years of clinical experience and anecdotal evidence amassed by Tomatis show that sound stimulation can provide a valuable remediation and developmental training tool for people of all ages. Offshoot systems have watered down the Tomatis Method without research to guide the decisions of simplifying the techniques and equipment.

Cognitive dysfunction in psychiatric consultation subgroups: use of two screening tests.

Elderly patients are highly vulnerable to illness-related and drug-induced cognitive changes, especially during the acute phase of a medical or surgical illness. Using a structured cognitive screening examination enhances the accurate identification of patients with cognitive impairment. We compared the cognitive portion of the Alzheimer's Disease Assessment Scale (ADAS-COG) to the Mini-Mental State Examination (MMSE) in screening for the presence of organic brain dysfunction. Using cutoff scores, the two tests were in agreement in 94.4% of 36 cases, with total scores on the two tests correlated at r = -.90 (P less than .01). Of these 36 patients for whom psychiatric consultations were requested, 14 (38.8%) were found to be cognitively impaired. We discuss the relationship of test scores to the stated reason for the consultation, as well as variables influencing test results. Finally, we demonstrate the usefulness of cognitive testing in patients who refuse treatment.

Comparison of two cognitive rating scales in medically ill patients.

Patients with acute, complex behavioral syndromes combined with impaired comprehension and communication are difficult diagnostic challenges. Using a structured mental status interview can significantly reduce the chance of overlooking the presence of cognitive dysfunction. The authors tested the applicability of the cognitive portion of the Alzheimer's Disease Assessment Scale (ADAS-COG) as a screening instrument on a psychiatric consultation service. The ADAS-COG compared favorably with the Mini-Mental State Exam (MMSE), but appeared to be less influenced by educational level. The advantages and disadvantages of using each of these tests on a psychiatric consultation service are discussed.