Micromechanical fiber-optic attenuator with 3 μs response

Optomechanical fiber-optic attenuators are bulky and slow. The mechanical antireflection switch (MARS) modulator offers a high-speed alternative for applications including dynamic gain control in fiber amplifiers. This paper describes a compact electrically controlled variable attenuator using a micromechanical device where electrostatic deflection of a silicon nitride quarter-wave dielectric layer suspended over a silicon substrate creates a variable reflectivity mirror. This device is packaged with two fibers in one ceramic ferrule placed in contact with a gradient index (GRIN) collimation lens, so that the input light reflects from the modulator in the collimated beam plane and couples into the output fiber. Using a 300 μm diameter MARS attenuator and a 500 μm diameter collimation lens, the total insertion loss at 1550 nm was 3.0 dB with no applied voltage, increasing to 31 dB at 35.2 V. The polarization dependent loss was less than 0.06 dB. Full attenuation with more than 100 mW input power produced no damage. The response time was 2.8 μs to move from maximum to minimum transmission and 1.1 μs to return to maximum transmission     

On the operational and manufacturing tolerances of GaAs-AlAs MQWmodulators

We approach the question of optimization of surface-normal p-i(multiquantum-well, MQW)-n modulators from the viewpoint of investigating their tolerance to variations in wavelength and temperature and errors in manufacture. The reflection characteristics of two high-quality samples are carefully processed to eliminate Fabry-Perot fringes, and then their spectra at any bias are characterized with six phenomenological parameters which depend on λ₀, the zero-field exciton position. The two GaAs-AlAs samples have λ₀'s of 833.8 and 842.3 nm, and so cover a range useful for modulators designed to operate near 850 nm in the normally reflecting condition, i.e., reflection decreases with field. A linear interpolation of the parameters of these two samples is used to predict the behavior of MQW diodes with λ₀'s around this range, and so a fully comprehensive examination of normally reflecting MQW modulators is performed. The performance aspect that is examined is contrast ratio as a function of nonuniformities in the devices or operating conditions given a voltage swing of 3 V. There are two operational modes discussed. If the voltage offset of the bias is allowed to vary via a feedback circuit, a contrast of 2:1 may be maintained over an operating wavelength change (Δλ) of 17 nm with local variations of wavelength of ±1 nm, which corresponds to a temperature variation of 60°C while allowing for variations of laser driver wavelength of ±1 nm. If feedback Is not permitted, we determine that, given tolerances to manufacturing errors, a contrast of 1.5:1 may be maintained over a wavelength range of ~5 nm by either using stacked diode designs or extremely shallow quantum wells     

Mechanistic investigation of the Ru-catalyzed hydroamidation of terminal alkynes

The ruthenium-catalyzed hydroamidation of terminal alkynes has evolved to become a broadly applicable tool for the synthesis of enamides and enimides. Depending on the catalyst system employed, the reaction leads chemo-, regio-, and stereoselectively to a single diastereoisomer. Herein, we present a comprehensive mechanistic study of the ruthenium-catalyzed hydroamidation of terminal alkynes, which includes deuterium-labeling, in situ IR, in situ NMR, and in situ ESI-MS experiments complemented by computational studies. The results support the involvement of ruthenium-hydride and ruthenium-vinylidene species as the key intermediates. They are best explained by a reaction pathway that consists of an oxidative addition of the amide, followed by insertion of a π-coordinated alkyne into a ruthenium-hydride bond, rearrangement to a vinylidene species, nucleophilic attack of the amide, and finally reductive elimination of the product.     

Interleaved-contact electroabsorption modulator usingdoping-selective electrodes with 25°C to 95°C operating range

A multiple-quantum-well (MQW) modulator with multiple stacked p-i(MQW)-n-i(MQW)-p-regions is demonstrated. Electrodes are deposited such that all the n-layers are connected to one contact and all the p-layers to the other. This allows high fields to be produced in the i-regions with relatively low voltages, since the i-regions may be made thin while retaining large optical interaction because they are stacked. A large usable wavelength range which translates into a large operating temperature range is obtained because of large Stark shifts in the MQWs at high fields. For a 0 to 6 V swing >22% reflectivity change from 25°C to 95°C, or alternatively over a wavelength range of 15 nm at 25°C, is achieved     

Gas composition dependence of silicon nitride used as gallium diffusion barrier during GaAs molecular beam epitaxy growth on Si complementary metal oxide semiconductor

We have measured the effect of deposition parameters upon the utility of plasma enhanced chemical vapor depositon silicon nitride as a gallium diffusions barrier during molecular beam epitaxy growth of gallium arsenide on silicon submicron complementary metal oxide semiconductor electronics. It was found that the conditions under which the silicon nitride is deposited severely impact the ability of these films to be used as a diffusion barrier, with the most critical dependence upon the silane to nitrogen gas ratio.     

Optical bandwidth considerations in p-i-n multiple quantum-wellmodulators

We investigate the optical bandwidth of p-i(multiple quantum well [MQW])-n modulators employing various MQW designs. The optical bandwidth translates directly into an operating temperature range due to the shift of the band gap with temperature. We find that although greater maximum modulation may be obtained with narrow (~90 Å) quantum wells operating below the band edge (absorption increases with field), uniform large performance may be obtained over a larger bandwidth using wider (~110 Å) quantum wells operating at the exciton (absorption decreases with field). We obtain a usable bandwidth of 7.7 nm, which translates into a operating temperature range of 27°C     

The palladium-catalyzed cross-coupling reaction of carboxylic anhydrides with arylboronic acids: a DFT study

The mechanism of the cross-coupling of phenylboronic acid with acetic anhydride, a viable model of the widely used Suzuki reaction, has been studied by DFT calculations at the BP86/6-31G level of theory. Two alternative catalytic cycles have been investigated, one starting from a neutral Pd(0)L(2) complex, the other from an anionic "Jutand-type" [Pd(0)L(2)X](-) species. The reaction profiles are in good agreement with the experimental findings, as both pathways require only moderate activation energies. Both pathways are dominated by cis-configured square-planar palladium(II)diphosphine intermediates. Despite careful investigations, we did not find in this model reaction any evidence for five-coordinate palladium(II) intermediates, which are commonly believed to cause the profound effects of counterions in palladium-catalyzed transformations. Instead, our calculations suggest that the higher catalytic activity of anionic complexes, such as [Pd(PMe(3))(2)OAc](-), may arise from their stronger ability to coordinate to carbon electrophiles. The transmetalation sequence is the same for both catalytic cycles, involving the dissociation of one phosphine ligand from the palladium. In the decisive transition state, in which the phenyl group is transferred from boron to palladium, the acetate base is found to be in a bridging coordination between these two atoms.