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Colloids that interact via a short-range attraction serve as the primary building blocks for a broad range of self-assembled materials. However, one of the well-known drawbacks to this strategy is that these building blocks rapidly and readily condense into a metastable colloidal gel. Using computer simulations, we illustrate how the addition of a small fraction of purely repulsive self-propelled colloids, a technique referred to as active doping, can prevent the formation of this metastable gel state and drive the system toward its thermodynamically favored crystalline target structure. The simplicity and robust nature of this strategy offers a systematic and generic pathway to improving the self-assembly of a large number of complex colloidal structures. We discuss in detail the process by which this feat is accomplished and provide quantitative metrics for exploiting it to modulate self-assembly. We provide evidence for the generic nature of this approach by demonstrating that it remains robust under a number of different anisotropic short-ranged pair interactions in both two and three dimensions. In addition, we report on a novel microphase in mixtures of passive and active colloids. For a broad range of self-propelling velocities, it is possible to stabilize a suspension of fairly monodisperse finite-size crystallites. Surprisingly, this microphase is also insensitive to the underlying pair interaction between building blocks. The active stabilization of these moderately-sized monodisperse clusters is quite remarkable and should be of great utility in the design of hierarchical self-assembly strategies. This work further bolsters the notion that active forces can play a pivotal role in directing colloidal self-assembly., Comment: Supplemental Material available here: https://aip.scitation.org/doi/suppl/10.1063/5.0016514

A study on the feedback stabilization of resistive wall modes in a cylindrical geometry is presented. The effect of radial separation of feedback coils from sensor loops is investigated. It is shown that there is an optimum radial position for feedback coils where sideband mode coupling parameter is minimized, hence enhancing the efficacy of a system of coils for feedback stabilization of resistive wall modes.

We present the theory and design of a Mach-Zehnder interferometer with orthogonally polarized arms that passively, noninvasively, optically amplifies polarization rotation in one arm of the interferometer. Theoretical analysis of the electric fields in the two arms is used to compare the significance of device optimizations. Design of the beam alignment, beam recombination, noise isolation, and active stabilization to enable reliable measurements are thoroughly discussed. Finally, we review a comparison between the interferometer with an optical gain factor of approximately 4.5 and the optical bridge (OB) technique for polarization rotation detection that shows the interferometer offers better performance than the OB.

Fabry-Perot interferometers typically require some form of active stabilization system for long-term operation. A stabilization system based on a microcomputer is described which is able to maintain the finesse of a plane parallel Fabry-Perot interferometer for long periods of time. The microcomputer takes the place of the multichannel analyzer, ramp generator, and stabilizer of a conventionally stabilized system. Sample data illustrating the operation of the system on the Brillouin spectrum of fused silica, with and without computer corrections, are included.

A continuous‐wave harmonically modulated Nd:LiYF4 laser has been built, which offers a high degree of mechanical, electrical, and optical stability. Different operating regimes of this system are outlined. For reliable operation, the cavity length has to be accurately matched to the frequency of the mode‐locker driver, much more so than for a fundamentally modulated system. For this purpose, a computer‐controlled length‐stabilization system is developed. This active stabilization scheme, which compensates thermal and mechanical drifts of the resonator and eliminates the warm‐up time of the laser, ensures a stable optical pulse train at the fundamental axial mode spacing of the resonator. The stabilization system is fully compatible with other more commonly implemented analog feedback mechanisms. It should find applications in most harmonically modulated solid‐state lasers, and possibly also in fundamentally modulated ones. Compared to the latter, the actively stabilized harmonically modulated laser genera...