High-power vertical-cavity surface-emitting laser (VCSEL) arrays, which can serve as the light source in modern lidar and three-dimensional optical sensing systems, have recently attracted a lot of attention. In these types of systems, the time-of-flight (ToF) technique, based on the round-trip time of short optical pulses is usually adopted. Further enhancement of the ranging distance and depth resolution in these ToF driven systems by the incorporation of a VCSEL array with a high available power, high brightness (narrow divergence angle), and fast response time is highly desirable. However, a large number of light emission apertures (several hundreds) in the VCSEL array is usually necessary to raise the output power level to several watts. This leads to a large parasitic capacitance and the RC-limited bandwidth may become the dominant limiting factor of the speed of the high-power VCSEL array. In this work, Zn-diffusion and oxide-relief apertures are used to manipulate the optical modes and reduce the parasitic capacitance, respectively, in a unit device for a 940 nm VCSEL array. The demonstrated VCSEL array has a quasi-single-mode output, high available power (4 W; 1% duty cycle), narrow divergence angle ( ∼ 14 ∘ at 1 / e 2 ) under maximum output power, and a fast rise time ( <1997
The 1,3-dipolar cycloaddition reactions of benzonitrile oxide with 5-substituted adamantane-2thiones (2-X) and 2-methyleneadamantanes (3-X) produced two geometrically isomeric ∆ 2 -1,4,2oxathiazolines (5-Xs) and two ∆ 2 -isoxazolines (6-Xs), respectively. The substituent was varied from fluoro, chloro, bromo, to phenyl. X-ray single-crystal analysis confirmed the configuration of (Z)-5-F. The product formation bias resulting from the favored attack of nitrile oxide on the zu-face is discussed in terms of transition-state hyperconjugation and frontier molecular orbital theory.1,3-Dipolar cycloadditions offer a convenient one-step route for the construction of a variety of complex fivemembered heterocycles that are synthetically useful compounds. 1 Nitrile oxide cycloadditions to terminal alkenes proceeded regioselectively to give 5-substituted ∆ 2 -isoxazolines as single products. 1b The cycloaddition of nitrones and nitrile oxides to thiones leading to ∆ 2 -1,4,2-oxathiazolidines and ∆ 2 -1,4,2-oxathiazolines, respectively, have also drawn much attention recently. 2 Because of their extremely high reactivity toward 1,3dipoles, thiones have been called superdipolarophiles by Huisgen. 2a,b 5-Substituted adamantan-2-ones 1-X and their derivatives have proven to be useful probes in research aimed at understanding the electronic factors in face selection. 3 Studies by le Noble et al. 3a of a variety of reactions indicate that the reagent prefers to attack the face that is antiperiplanar to the more electron-rich vicinal bonds (zu and en face preference in 1 when X equals an electronwithdrawing and electron-donating group, respectively). Their results have been reconciled with Cieplak's transition-state hyperconjugation model. 4 The Diels-Alder reaction of 2,3-dimethylbuta-1,3-diene with 5-fluoroadamantane-2-thione (2-F) has been re-ported to follow Cieplak's prediction. 5 When frontier molecular orbital (FMO) theory is applied to this Diels-Alder reaction, the diene functions as the donor, and the reaction is controlled by the HOMO (diene)-LUMO (dienophile) interaction. In other words, this is a normal Diels-Alder-type reaction. 1a,2d,6 On the other hand, 1,3dipolar cycloadditions of benzonitrile oxide with thiones 2, or terminal alkenes 3, are controlled mainly by the LUMO (dipole)-HOMO (dipolarophile) interaction (an inverse electron-demand type of reaction). Thus, 1,3dipolar cycloaddition reactions to 2 and 3 provide an important test of the transition-state hyperconjugation model because Cieplak's model stresses that, regardless of the type of reaction (nucleophilic, electrophilic, radical addition, etc.), the newly developing σ * orbital should attract electron density with the same directional preference provided the transition states are electron deficient. 3a,4c We report here our study of the 1,3-dipolar cycloaddition reactions of benzonitrile oxide with 5-substituted adamantane-2-thiones (2-X) and 2-methyleneadamantanes (3-X). We find that the favored approach is indeed syn as predicted, in all in...
A microwave treatment method different from thermal annealing and low-energy electron beam irradiation was proposed to activate Mg dopants in p-type GaN epitaxial layer. From photoluminescence spectra and Hall effect measurements, it was shown that microwave treatment is a very effective way to activate the acceptors in Mg-doped p-type GaN layer. The activation of Mg dopant in p-type GaN layer may be explained as the breaking of magnesium-hydrogen bonding due to the microwave energy absorption.
The 1,3-dipolar cycloaddition reactions of para-substituted benzonitrile oxides (5-Y) with 5-fluoroadamantane-2-thione (2-F) and -2-methyleneadamantane (3-F) as well as with variously 5-substituted-N-benzyladamantyl-2-imines (4-X) were examined. They produce two geometrically isomeric Δ2-1,4,2-oxathiazolines (7-F,Y), Δ2-isoxazolines (8-F,Y), and Δ2-1,2,4-oxadiazolines (11-X,Y), respectively. The face selectivity in the latter reaction was found to be ∼1:1 regardless of the variations in 5-substituent and the temperature. For the former two reactions, the para-substituent was varied from electron-withdrawing (Y = F, Cl, Br, CN, or NO2) to -releasing (Y = Me, or OMe). The face selectivity was measured in all cases. The differences Δρ for the reactions of 2- and 3-F with 5-Y were obtained from linear Hammett plots; they are +0.12 and 0.0, respectively. These low values and information from previous studies imply a concerted one-step mechanism with very little charge distribution differences in the transition states. These effects of temperature on the Z/E product ratios provide us, for the first time, with activation parameter differences between the syn- and anti transition states; their values are discussed. The product bias resulting from the favored attack of nitrile oxide on the zu face is discussed in terms of transition state hyperconjugation based on the experimental results and AM1 calculations.
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