Non-critically phase-matched,Ti:Sapphire-pumped picosecond optical parametric oscillator using LiB3O5

We report a new source of high-repetition rate and widely tunable picosecond pulses for the near infrared. A singly resonant, cw, picosecond optical parametric oscillator based on temperature-tuned LiB₃O₅ and synchronously pumped by 1.8 ps pulses from a self-mode-locked Ti:Sapphire laser is demonstrated. The oscillator can provide average output powers of up to 90 mW under non-critical type-I phase matching at a pulse repetition rate of 81 MHz. Without dispersion compensation, transform-limited signal pulses with 720 fs durations have been generated at 1.2 times threshold. With the available mirror set, signal tuning over 1.374–1.530 µm and idler tuning over 1.676–1.828 µm is demonstrated for a range of pump wavelengths and phase-matching temperatures. With additional mirrors, continuous tuning throughout 1–2.7 µm should be readily attainable with a single LiB₃O₅ crystal.     

The combinatorics of all regular flexagons

Flexagons were discovered in 1939 by topologist Arthur Stone. A regular flexagon is one that contains 9n equilateral triangular regions on a straight strip of paper. This paper is then rolled into smaller strips of paper and finally into a hexagon with 6 triangular regions called pats, producing one mathematical face. The pinch flex removes the uppermost triangular regions and replaces them with a new set producing a new face. The flexagon is said to have order 3n because you can color 3n of the faces with 3n different colors. It is well known that when only the pinch flex is used, a flexagon of order 3n is a möbius band with 3(3n−2) half-twists, and has 6n−3 different mathematical faces. Even though a colored face appears more than once, the uppermost triangles might be rotated producing a different mathematical face. When T. Bruce McLean described the V-flex on the flexagon of order 6 in 1979, he showed that it now had 3420 mathematical faces and provided a graph that demonstrated how to reach all of the different faces. This flex scrambles the colors similar to the way the Rubik’s cube does except that a flexagon is flat. It is the purpose of this paper to provide an algorithm that counts the number of mathematical faces for flexagons of order 3n for all n, once the V-flex is included.A theorem in this paper gives a recursive formula that counts the number of different pats of a given thickness (or degree). To start the count for the number of mathematical faces of a regular flexagon of order 3n an ordered set of 6 degrees that add to 9n is considered. The adjacent degrees must add to a multiple of three according to the axioms of a flexagon using both flexes. Two sets are equivalent if you can rotate one six-tuplet into the other. Then for each case, the number of pats given by this Theorem that have degrees of those 6 numbers can be multiplied by the fundamental theorem of counting and these are called initial faces after rotations are removed. The last step in the count is to allow for translations of the flexagon and when there is no symmetry, you can multiply the number of initial faces by 9n. You can only multiply by 3n when there is complete symmetry.The Java applets provided only work for the traditional integer data type.     

Design and construction of a home-made and cheaper argon arc lamp

The authors report on the design and construction of an argon arc lamp which provides noticeably a cheaper instrument for laser and medical applications. Cesium-doped tungsten and pure tungsten rods were used, respectively, for the lamp cathode and anode. To seal the glassy tube, a 50–50 Fe–Ni alloy was successfully used as a medium to attach the tungsten electrodes to the borosilicate glass tube. Starting voltage of the lamp versus the gas pressure, operation voltage–current diagram at various gas pressures, and lamp spectrum in the various pressures were measured. A comparison was made with krypton arc lamp. The lamp operation was satisfactory without any crack or fracture during lightening operation. The results showed that the lamp-lightening threshold voltage depends linearly on the pressure and arc length in such a way that there is an increase in the voltage by raising these two parameters. We have also observed that by increasing the argon pressure, there is a shifting in emission spectrum from the ultraviolet to the visible region. Comparison with krypton arc lamp indicated that argon lamp needs a higher threshold lightening voltage.     

A new Orthonormal Polynomial Series Expansion Method in vibration analysis of thin beams with non-uniform thickness

In this article, OPSEM (Orthonormal Polynomial Series Expansion Method) is developed as a new computational approach for the evaluation of thin beams of variable thickness transverse vibration. Capability of the OPSEM in assessing the free vibration frequencies and mode shapes of an Euler–Bernoulli beam with varying thickness is discussed. Multispan continuous beams with various classical boundary conditions are included. Contribution of BOPs (Basic Orthonormal Polynomials) in capturing the beam vibrations is also illustrated in numerical examples to give a quantitative measure of convergence rate. Furthermore, OPSEM is adopted for the forced vibration of a thin beam caused by a moving mass. Dynamics of beams supported by flexible elastic base like free to free beam on elastic foundation are also regarded. Verifications are made via eigenfunction expansion method and GMLSM (Generalized Moving Least Square Method). The very close observed agreement between the results of the two recently mentioned methods and that of OPSEM can be regarded as a guarantee of validity for the newly introduced technique. In comparison with eigenfunction expansion method, the simplicity and handiness of OPSEM in coping with different boundary conditions of the beam can be considered as its benefit for engineering practitioners.     

Simultaneous femtosecond-pulse compression and second-harmonic generation in aperiodically poled KTiOPO(4)

We report both extracavity and intracavity simultaneous second-harmonic generation and compression of pulses at 1.25 mum from a synchronously pumped RbTiOAsO(4) -based optical parametric oscillator, using an aperiodically poled crystal of KTiOPO(4) . The 290-fs input pulses were temporally compressed to 120 fs, with average output powers as great as 120 mW. The experimental results are compared with a numerical model that uses data obtained by characterization of the input pulses by use of the frequency-resolved optical gating technique.     

Periodically poled RbTiOAsO4 femtosecond optical parametric oscillator tunable from 1.38 to 1.58 μm

4 that is continuously tunable in the signal wavelength from 1.375 to 1.575 μm and with a maximum output power of 110 mW. The signal pulses were produced with nearly transform-limited duration as short as 215 fs. Received: 27 August 1998/Revised version: 14 October 1998     

Selenium heterocycles. XLIII. Syntheses of 3,5-Diaryl-1,2,4-thiadiazoles and 3,5-Diaryl-1,2,4-selenadiazoles

Starting from readily available α-arylsulfonyl-α-bromoacetophenones 2 a series of 3,5-diaryl-1,2,4-thiadiazoles and 3,5-diaryl-1,2,4-selenadiazoles were prepared in moderate yield. Reaction of compounds 2 with thiourea or selenourea gave 2-amino-5-arylsulfonyl-4-phenylthiazole or selenazole in good yield.     

Comparison of the performance of pyridine-functionalized nanoporous silica particles for the extraction of gold(III) from natural samples

We have developed a technique for the solid-phase extraction of gold using various kinds of pyridine-functionalized nanoporous silica prior to its determination in various samples using FAAS. The effects of solution pH, sample and eluent flow rate, sample volume and of potentially interfering ions are compared. The limits of detections vary from 28 to 53?pg?mL^?1. The accuracy and precision are between 99.8% and 98.3?% and 0.7 to 1.6?% (RSD), respectively. The method was successfully applied to several standard reference materials.

Optimization of temperature-controlled ionic liquid dispersive liquid phase microextraction combined with high performance liquid chromatography for analysis of chlorobenzenes in water samples

Temperature-controlled ionic liquid dispersive liquid phase microextraction (TCIL-DLPME) combined with high performance liquid chromatography-diode array detection (HPLC-DAD) was applied for preconcentration and determination of chlorobenzenes in well water samples. The proposed method used 1-butyl-3-methylimidazolium hexafluorophosphate ([C₄mim][PF₆]) as the extraction solvent. The effect of different variables on extraction efficiency was studied simultaneously using an experimental design. The variables of interest in the TCIL-DLPME were extraction solvent volume, salt effect, solution temperature, extraction time, centrifugation time, and heating time. The Plackett-Burman design was employed for screening to determine the variables significantly affecting the extraction efficiency. Then, the significant factors were optimized by using a central composite design (CCD) and the response surface equations were developed. The optimal experimental conditions obtained from this statistical evaluation included: extraction solvent volume, 75 μL; extraction time, 20 min; centrifugation time, 25 min; heating time, 4 min; solution temperature, 50 °C; and no addition of salt. Under optimal conditions, the preconcentration factors were between 187 and 298. The limit of detections (LODs) ranged from 0.05 μg L⁻¹ (for 1,2-dichlorobenzene) to 0.1 μg L⁻¹ (for 1,2,3-trichlorobenzene). Linear dynamic ranges (LDRs) of 0.5-300 and 0.5-500 μg L⁻¹ were obtained for dichloro- and trichlorobenzenes, respectively. The performance of the method was evaluated for extraction and determination of chlorobenzenes in well water samples in micrograms per liter and satisfactory results were obtained (RSDs < 9.2%).     

Study of geometry effects in torsional rheometry of fibre suspensions

This work concerns the problem of measuring the viscosity of a suspension of fibres that are not short compared to the dimensions of the measurement device. We have examined various geometry effects in shear using parallel-plate and cone-and-plate configurations. Steady state viscosity, transient viscosity and first normal stress difference have been studied. Silicone oil with milled glass fibres at 8 vol.% was used as a model fibre suspension. The conventional parallel-plate geometry exhibits a significant gap dependence of the apparent viscous response. Too small gaps constrain the fibre rotations, thus lowering the initial stress peak and delaying the development of steady state. Too large gaps lead to a loss of liquid at the sample perimeter, which reduces the apparent steady state viscosity. The steady state response, however, seems to be correct for gaps in the range 1–2 mm when the maximum fibre length is 1.5 mm. The cone-and-plate geometry is less sensitive to large gaps. Too small a gap at the (truncated) cone apex leads to anomalous response due to bridging of the gap by fibres. The use of unusually large plates (100 mm diameter) and different cone angles and truncations was explored. It was shown that cone-and-plate configurations of large diameter and large truncation can effectively decrease the geometry disturbances both due to fibre bridging at the cone apex and loss of liquid at the edge.