RESUMO
The growth of long carbon fibers was investigated using hyperbaric-pressure laser chemical vapor deposition (HP-LCVD). Precursors included the unbranched alkenes with linear structure 1-C(x)H(2x) (where x = 2,4,5,6,7,8), the unbranched alkynes, i.e., 1-C(x)H(2)(x-2) (where x = 3,4,5,6,8), and benzene. Rate constants, reaction orders, and apparent activation energies were derived for each precursor over a range of experimental conditions. Axial growth rates from the alkenes were 1-2 orders of magnitude greater than for the alkynes, while growth rates for benzene exceeded 10 mm s(-1). Generalized expressions for the growth rate vs molecular weight were determined. For the alkenes, the growth rate was directly proportional to the square root of the precursor molecular weight, while the alkynes exhibited an inverse relationship. Two regions of differing reaction order were identified for the alkynes; at pressures less than 2.0-2.5 bar, the average reaction order was 3.07, while above 2.0-2.5 bar, reaction orders diverged. Expressions were derived for the fraction of carbon atoms deposited per alkyne molecule transported; the deposition efficiency decreased with molecular weight for the alkynes, due in part to the Soret effect. In contrast, the reaction order for the alkenes was 1.65, and for benzene was 2.25. A phase change in the deposit was observed for both the alkenes and alkynes, with the exceptions of pentene and pentyne. Complete axial rate equations for the alkenes and alkynes were derived, as well as volumetric growth equations for the alkynes. It was shown that the volumetric rate increases nonlinearly with laser power at sufficiently high pressures.
RESUMO
As previously reported [Opt. Lett. 29, 2791 (2004)], one-dimensional photonic crystals exhibit a decrease in their normal reflectivity if their interfaces are not flat. We show that the homogenization approximation accurately predicts this diminished optical response by comparing results with finite-difference time-domain (FDTD) simulations applied to the same roughened structures. Within the parameter range tested (rms roughness < 20% and rms wavelengths < 100% of the photonic crystal periodicity), the homogenization approximation accurately reproduces the reflectivities obtained by the FDTD simulations, which are much more computationally expensive.
RESUMO
Dielectric reflectors that are periodic in one dimension, also known as one-dimensional photonic crystals (1DPCs), have become extremely useful tools in the optics industry due to the presence of wavelength-tunable photonic bandgaps. However, little is known about the practical effects of manufacturing defects, such as interfacial roughness, on this technologically useful property of 1DPCs. We employ a finite-difference time-domain code to gain further insight into the effect of interfacial roughness on the reflectivity of quarter-wave-tuned 1DPCs in the center of the bandgap at normal incidence. This provides an estimate of the magnitude of the effect of the roughness for even the most-robust incidence conditions.
