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1. An efficient 2D array of blue-detuned optical traps

   Graham, Trent; Xiaoyu, Jiang; Poole, Cody; Sun, Yuan; Lichtman, Martin; Saffman, Mark (May 2018, Bulletin American Physical Society, DAMOP 2018)

   We demonstrate a 2D lattice of blue-detuned optical traps which uses laser power efficiently, is tolerant to perturbations in beam alignment, and is insensitive to interferometric phases. Blue traps have several advantages over red traps despite requir- ing a more complicated beam geometry. Since atoms in a blue trap sit at an intensity minimum, Stark shift noise and site-to-site calibrations are minimized. However, constructing a blue lattice which efficiently con- verts laser power into trap depth, is challenging. For example, a lattice of bottle beams is inefficient because neighboring sites are separated by two walls, limiting the number of traps that can be formed. An array of tightly spaced Gaussian beams is a more efficient blue trap, but the trap potentials are susceptible to alignment perturbations. We demonstrate an array which uses diffractive optical elements to create a cross-hatched pattern of lines in the focal region where the atoms are trapped in up to 121 sites. This "line array" is almost twice as efficient as the Gaussian beam array and is more resilient to perturbations in beam alignment. 

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2. Scaling of silicon nanoparticle growth in low temperature flowing plasmas

   https://doi.org/10.1063/5.0062255  

   Lanham, Steven J.; Polito, Jordyn; Shi, Xuetao; Elvati, Paolo; Violi, Angela; Kushner, Mark J. (October 2021, Journal of Applied Physics)

   Low temperature plasmas are an emerging method to synthesize high quality nanoparticles (NPs). An established and successful technique to produce NPs is using a capacitively coupled plasma (CCP) in cylindrical geometry. Although a robust synthesis technique, optimizing or specifying NP properties using CCPs, is challenging. In this paper, results from a computational investigation for the growth of silicon NPs in flowing inductively coupled plasmas (ICPs) using Ar/SiH4 gas mixtures of up to a few Torr are discussed. ICPs produce more locally constrained and quiescent plasma potentials. These positive plasma potentials produce an electrostatic trap for negatively charged NPs, which can significantly extend the residence time of NPs in the plasma, which in turn provides a controllable period for particle growth. The computational platforms used in this study consist of a two-dimensional plasma hydrodynamics model, a three-dimensional nanoparticle growth and trajectory tracking model, and a molecular dynamics simulation for deriving reactive sticking coefficients of silane radicals on Si NPs. Trends for the nanoparticle growth as a function of SiH4 inlet fraction, gas residence time, energy deposition per particle, pressure, and reactor diameter are discussed. The general path for particle synthesis is the trapping of small NPs in the positive electrostatic potential, followed by entrainment in the gas flow upon reaching a critical particle size. Optimizing or controlling NP synthesis then depends on the spatial distribution of plasma potential, the density of growth species, and the relative time that particles spend in the electrostatic trap and flowing through higher densities of growth species upon leaving the trap. 

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3. Scalable trapping of single nanosized extracellular vesicles using plasmonics

   https://doi.org/10.1038/s41467-023-40549-7  

   Hong, Chuchuan; Ndukaife, Justus C. (August 2023, Nature Communications)

   Abstract Heterogeneous nanoscale extracellular vesicles (EVs) are of significant interest for disease detection, monitoring, and therapeutics. However, trapping these nano-sized EVs using optical tweezers has been challenging due to their small size. Plasmon-enhanced optical trapping offers a solution. Nevertheless, existing plasmonic tweezers have limited throughput and can take tens of minutes for trapping for low particle concentrations. Here, we present an innovative approach called geometry-induced electrohydrodynamic tweezers (GET) that overcomes these limitations. GET generates multiple electrohydrodynamic potentials, allowing parallel transport and trapping of single EVs within seconds. By integrating nanoscale plasmonic cavities at the center of each GET trap, single EVs can be placed near plasmonic cavities, enabling instant plasmon-enhanced optical trapping upon laser illumination without detrimental heating effects. These non-invasive scalable hybrid nanotweezers open new horizons for high-throughput tether-free plasmon-enhanced single EV trapping and spectroscopy. Other potential areas of impact include nanoplastics characterization, and scalable hybrid integration for quantum photonics. 

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4. A compact and open-source microcontroller-based rapid auto-alignment system

   https://doi.org/10.1063/5.0211005  

   Geng, Yanda; Tsidilkovski, Alan; Weber, Kevin; Mukherjee, Shouvik; Restelli, Alessandro; Subhankar, Sarthak (September 2024, Review of Scientific Instruments)

   Maintaining stable and precise alignment of a laser beam is crucial in many optical setups. In this work, we present a microcontroller-based rapid auto-alignment system that detects and corrects for drifts in a laser beam trajectory using a pair of two-dimensional duo-lateral position sensing detectors (PSDs) and a pair of mirror mounts with piezoelectric actuators. We develop hardware and software for interfacing with the PSDs and for controlling the motion of the piezoelectric mirror mounts. Our auto-alignment strategy—implemented as a state machine on the microcontroller by a real-time operating system kernel from FreeRTOS—is based on a simple linearized geometrical optical model. We benchmark our system using the standard case of coupling laser light efficiently into the guided mode of a single-mode fiber optic patch cable. We can recover the maximum fiber coupling efficiency in ∼10 seconds, even for a laser beam misaligned to the point of zero fiber coupling efficiency. 

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5. Vortex splitting in two-dimensional fluids and non-neutral electron plasmas with smooth vorticity profiles

   https://doi.org/10.1063/5.0201712  

   Hurst, N_C; Tran, A.; Wongwaitayakornkul, P.; Danielson, J_R; Dubin, D_H_E; Surko, C_M (May 2024, Physics of Plasmas)

   Initially, elliptical, quasi-two-dimensional (2D) fluid vortices can split into multiple pieces if the aspect ratio is sufficiently large due to the growth and saturation of perturbations known as Love modes on the vortex edge. Presented here are experiments and numerical simulations, showing that the aspect ratio threshold for vortex splitting is significantly higher for vortices with realistic, smooth edges than that predicted by a simple “vortex patch” model, where the vorticity is treated as piecewise constant inside a deformable boundary. The experiments are conducted by exploiting the E × B drift dynamics of collisionless, pure electron plasmas in a Penning–Malmberg trap, which closely model 2D vortex dynamics due to an isomorphism between the Drift–Poisson equations describing the plasmas and the Euler equations describing ideal fluids. The simulations use a particle-in-cell method to model the evolution of a set of point vortices. The aspect ratio splitting threshold ranges up to about twice as large as the vortex patch prediction and depends on the edge vorticity gradient. This is thought to be due to spatial Landau damping, which decreases the vortex aspect ratio over time and, thus, stabilizes the Love modes. Near the threshold, asymmetric splitting events are observed in which one of the split products contains much less circulation than the other. These results are relevant to a wide range of quasi-2D fluid systems, including geophysical fluids, astrophysical disks, and drift-wave eddies in tokamak plasmas. 

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