Modelling of optical traps for aerosols

Experimental observations suggest that there are differences between the behavior of particles optically trapped in air and trapped in a liquid phase. We present a modified version of Mie Debye Spherical Aberration theory to numerically simulate such optical system in attempt to explain and predict these effects. The model incorporates Mie scattering and focussing of the trapping beam through media of stratified refractive index. Our results show a geometrical optics approach cannot correctly describe our system and that spherical aberration must be included. We successfully qualitatively explain the observed phenomena and those of other authors, before discussing the limits of our experimental techniques and methods to improve it. We draw the important conclusion that when optically trapping aerosols the system does not behave as a true `optical tweezers', varying between levitation and single beam gradient force trapping depending on particle and beam parameters

Numerically Enhanced Stimulated Emission Depletion Microscopy with Adaptive Optics for Deep-Tissue Super-Resolved Imaging

Copyright © 2019 American Chemical Society. In stimulated emission depletion (STED) nanoscopy, the major origin of decreased signal-to-noise ratio within images can be attributed to sample photobleaching and strong optical aberrations. This is due to STED utilizing a high-power depletion laser (increasing the risk of photodamage), while the depletion beam is very sensitive to sample-induced aberrations. Here, we demonstrate a custom-built STED microscope with automated aberration correction that is capable of 3D super-resolution imaging through thick, highly aberrating tissue. We introduce and investigate a state of the art image denoising method by block-matching and collaborative 3D filtering (BM3D) to numerically enhance fine object details otherwise mixed with noise and further enhance the image quality. Numerical denoising provides an increase in the final effective resolution of the STED imaging of 31% using the well established Fourier ring correlation metric. Results achieved through the combination of aberration correction and tailored image processing are experimentally validated through super-resolved 3D imaging of axons in differentiated induced pluripotent stem cells growing under an 80 μm thick layer of tissue with lateral and axial resolution of 204 and 310 nm, respectively

Editorial: Structuring doctoral education

The number of doctoral candidates is increasing worldwide..

Structures and strategy in doctoral education in the UK and Ireland

This most recent report builds from previous UKCGE publications that have charted changes in structures that support doctoral education, to include a new focus on institutional strategies, priorities and commitments which shape the work of graduate schools across the UK and Ireland. The 2022 report draws on the detailed responses from 74 higher education institutions, additional research on trends in national and international policy and practice in doctoral education and comparator data from the previous reports in this series, to offer readers an insight into the policy drivers, changing markets and the major challenges and opportunities which have driven decision-making in the doctoral space in the UK and Ireland in the last six years. The important findings, outlined in the report, cover issues related to graduate school identity, leadership, access to resource, space, new ways of working, responses to the pandemic and the evolving policy agenda related to diversity and inclusion and health and well-being. It is an essential read for Deans and Directors of Graduate Schools or equivalent, research leaders, research policy-makers and funders and anyone with an interest in contributing to the urgent work required to ensure doctoral education can play a significant role in delivering both the kind of research that can drive post-pandemic recovery and the trained, highly skilled doctoral graduates who are happy and confident to lead it

Effective magnetic fields in degenerate atomic gases induced by light beams with orbital angular momenta

We investigate the influence of two resonant laser beams on the mechanical properties of degenerate atomic gases. The control and probe beams of light are considered to have Orbital Angular Momenta (OAM) and act on the three-level atoms in the Electromagnetically Induced Transparency (EIT) configuration. The theory is based on the explicit analysis of the quantum dynamics of cold atoms coupled with two laser beams. Using the adiabatic approximation, we obtain an effective equation of motion for the atoms driven to the dark state. The equation contains a vector potential type interaction as well as an effective trapping potential. The effective magnetic field is shown to be oriented along the propagation direction of the control and probe beams containing OAM. Its spatial profile can be controlled by choosing proper laser beams. We demonstrate how to generate a constant effective magnetic field, as well as a field exhibiting a radial distance dependence. The resulting effective magnetic field can be concentrated within a region where the effective trapping potential holds the atoms. The estimated magnetic length can be considerably smaller than the size of the atomic cloud.Comment: 11 pages, 5 figures Corrected some mistakes in equation

Theoretical study of a cold atom beam splitter

A theoretical model is presented for the study of the dynamics of a cold atomic cloud falling in the gravity field in the presence of two crossing dipole guides. The cloud is split between the two branches of this laser guide, and we compare experimental measurements of the splitting efficiency with semiclassical simulations. We then explore the possibilities of optimization of this beam splitter. Our numerical study also gives access to detailed information, such as the atom temperature after the splitting

Cold atom confinement in an all-optical dark ring trap

We demonstrate confinement of $^{85}$Rb atoms in a dark, toroidal optical trap. We use a spatial light modulator to convert a single blue-detuned Gaussian laser beam to a superposition of Laguerre-Gaussian modes that forms a ring-shaped intensity null bounded harmonically in all directions. We measure a 1/e spin-relaxation lifetime of ~1.5 seconds for a trap detuning of 4.0 nm. For smaller detunings, a time-dependent relaxation rate is observed. We use these relaxation rate measurements and imaging diagnostics to optimize trap alignment in a programmable manner with the modulator. The results are compared with numerical simulations.Comment: 5 pages, 4 figure

Underdamped modes in a hydrodynamically coupled microparticle system

When micron-sized particles are trapped in a linear periodic array, for example, by using optical tweezers, they interact only through the hydrodynamic forces between them. This couples the motion of the spheres and it has been predicted that an extended system might behave as an elastic medium that could support underdamped propagating waves. In practice, these underdamped modes can be observed only with massive particles in very stiff traps and very low viscosity fluids. We have been able to realize these conditions by trapping water droplets in air. Even with a system of just two particles we were able to observe the coupled oscillatory motion predicted: underdamping of the symmetric (collective) mode and overdamping of the asymmetric (relative) mode