Developments in low-cost laser detection: wide field of view implementation and direction determination

The availability of low cost but relatively high power laser pointers (hundreds of mW) has led to misuse with potentially dangerous consequences, such as dazzling aircraft which has raised concerns about aircraft safety. A low cost laser detection system based on coherence detection has been developed and is able to detect weak, continuous laser sources even against bright background light. In this paper, we introduce the use of a cone mirror to extend the horizontal field of view of the detector (originally at 3°) to 360° to detect incoming beams from different directions. With the additional use of a camera in the system, we also determine the direction of the incoming laser beam. Finally, the sensitivity between the original system and the cone mirror system are compared: the new system showed promising results with a sensitivity below 100nW

Interstitial Fractionalization and Spherical Crystallography

Finding the ground states of identical particles packed on spheres has relevance for stabilizing emulsions and a venerable history in the literature of theoretical physics and mathematics. Theory and experiment have confirmed that defects such as disclinations and dislocations are an intrinsic part of the ground state. Here we discuss the remarkable behavior of vacancies and interstitials in spherical crystals. The strain fields of isolated disclinations forced in by the spherical topology literally rip interstitials and vacancies apart, typically into dislocation fragments that combine with the disclinations to create small grain boundary scars. The fractionation is often into three charge-neutral dislocations, although dislocation pairs can be created as well. We use a powerful, freely available computer program to explore interstitial fractionalization in some detail, for a variety of power law pair potentials. We investigate the dependence on initial conditions and the final state energies, and compare the position dependence of interstitial energies with the predictions of continuum elastic theory on the sphere. The theory predicts that, before fragmentation, interstitials are repelled from 5-fold disclinations and vacancies are attracted. We also use vacancies and interstitials to study low energy states in the vicinity of "magic numbers" that accommodate regular icosadeltahedral tessellations.Comment: 21 pages, 9 figure