We report the study of lasing modes in broad-area, equilateral-triangular laser cavities. An alternative approach is proposed to study optical modes in equilateral triangular cavities in an analytical form. The modes were obtained by examining the simplest optical paths inside the cavity, which yields the final solution with the boundary conditions. The cavities can be fabricated from semiconductor heterostructures grown on ͑111͒oriented substrates, which can be easily cleaved into equilateral triangular shapes. Such a design takes advantage of total internal reflection at the cleaved facets of the cavity for circulating modes. Experimental results are obtained from cavities fabricated from a superlattice structure of In 0.13 Ga 0.87 As/GaAs grown on a ͑111͒ GaAs substrate.
New donor-acceptor alternating conjugated polymers were synthesized and characterized. Among them, PCPBBT exhibited a band-gap of 1.01 eV and ambipolar characteristics with μ(h) = 7.1 × 10(-4) cm(2) V(-1) s(-1) and μ(e) = 3.3 × 10(-3) cm(2) V(-1) s(-1).
Pattern size and pitch are two major factors that directly affect the ultimate capacity of a data storage unit. Electron beam lithography, because it can be used for direct writing at the nanometer scale, is a candidate for the fabrication of ultra-fine and ultra-compact patterns for the next generation of data storage media.In this paper, we present methods to reduce the pattern size and pitch written by electron beam lithography on the positive tone photo resist Poly(methylmethacrylate) (PMMA). The first method presented uses a writing dose that is much lower than the critical dose of the PMMA. However, in this technique, the distance between the adjacent writing spots (which are approximately Gaussian in spatial extent) is finely adjusted until the partially overlapping Gaussian functions form a contour with periodic dose peaks higher than the critical dose. This overlapping exposure results in a periodic exposure of the PMMA. Most importantly, this writing strategy, using the overlapping of low dose Gaussian beams, results in patterns with smaller size and pitch compared to the conventional methods that use a higher writing dose than the critical dose. To further improve the results, we use ultrasonic development in combination with cold development to greatly increase the resist contrast. The increased contrast results in smaller pattern sizes, and enables smaller pattern pitches, as well as improved pattern uniformity and reproducibility.Finally, as a demonstration of the technique using a conventional beam voltage of 15kV, we demonstrate ultra-fine and high density patterns with ~10nm linewidths and 50nm pitch, which corresponds to a storage capacity near 200 billion bits/inch 2 . By comparison, by using standard techniques (using a dose slightly higher than the critical dose and a normal development strategy), we can only obtain patterns with linewidths of about 20-30nm and pitch of about 100nm.
Hetero n-i-p-i's are one of several semiconductor quantum-well devices that use second-order electro-optic effects and that continue to attract interest for possible applications to low-power two-dimensional switching arrays and all-optical spatial light modulators. The turn-on time for a hetero n-i-p-i device is usually determined by transport perpendicular to the quantum wells. More specifically, it is determined by the time required for carriers generated in the quantum wells to escape the wells and to move to screen the built-in electric field, thus shifting the exciton. Consequently, typical turn-on times are of the order of a few ps. By contrast, when used in the conventional single-beam geometry, the recovery (or turn-off) time of hetero n-i-p-i's is determined by the slow recombination of the spatially-separated charges in the doped regions and is typically in the range of µs-ms. If instead, however, we use a two-beam mixing geometry for the device (where gratings are written in the material by the interference of the two beams), then the decay or turn off of the signal is determined by the decay of the gratings by in-plane transport over micron dimensions. Here, we use transient grating techniques to measure the recovery of such photorefractive and photoabsorptive gratings written in all-binary hetero n-i-p-i's. In this geometry, we show that the separation of photo-generated charge actually speeds the recovery by enhancing the effective in-plane ambipolar diffusion coefficient by roughly an order of magnitude (in contrast to the single beam geometry where charge separation elongates the recombination and recovery time).
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