Modal liquid crystal microaxicon array

A novel tunable liquid crystal microaxicon array is proposed and experimentally demonstrated. The proposed structure is capable of generating tunable axicons (thousands of elements) of micrometric size, with simple control (four control voltages) and low voltage, and is totally reconfigurable. Depending on the applied voltages, control over the diameter, as well as the effective wedge angle, can be achieved. Controls over the diameter ranging from 107 to 77 μm have been demonstrated. In addition, a control over the phase profile tunability, from 12π to 24π radians, has been demonstrated. This result modifies the effective cone angle. The diameter tunability, as well the effective cone angle, results in a control over the nondiffractive Bessel beam distance. The RMS wavefront deviation from the ideal axicon is only λ∕3. The proposed device has several advantages over the existing microaxicon arrays, including being simple having a low cost. The device could contribute to developing new applications and to reducing the fabrication costs of current devices.

High-Performance and Traditional Multicrystalline Silicon: Comparing Gettering Responses and Lifetime-Limiting Defects

In recent years, high-performance multicrystalline silicon (HPMC-Si) has emerged as an attractive alternative to traditional ingot-based multicrystalline silicon (mc-Si), with a similar cost structure but improved cell performance. Herein, we evaluate the gettering response of traditional mc-Si and HPMC-Si. Microanalytical techniques demonstrate that HPMC-Si and mc-Si share similar lifetime-limiting defect types but have different relative concentrations and distributions. HPMC-Si shows a substantial lifetime improvement after P-gettering compared with mc-Si, chiefly because of lower area fraction of dislocation-rich clusters. In both materials, the dislocation clusters and grain boundaries were associated with relatively higher interstitial iron point-defect concentrations after diffusion, which is suggestive of dissolving metal-impurity precipitates. The relatively fewer dislocation clusters in HPMC-Si are shown to exhibit similar characteristics to those found in mc-Si. Given similar governing principles, a proxy to determine relative recombination activity of dislocation clusters developed for mc-Si is successfully transferred to HPMC-Si. The lifetime in the remainder of HPMC-Si material is found to be limited by grain-boundary recombination. To reduce the recombination activity of grain boundaries in HPMC-Si, coordinated impurity control during growth, gettering, and passivation must be developed.