Trapping and guiding microscopic particles with light-induced forces

Contact-free trapping and manipulation of light absorbing micrometer and nanometer-scale particles in air and in vacuum utilises radiation pressure, which results from momentum transfer from photons, and a pressure-dependent thermal force, caused by momentum transfer from gas molecules to the confined particles. Both forces are linearly proportional to the illuminating laser intensity, and both push the particles towards regions of lower intensity. While the radiation pressure of light was predicted and described more than a century ago, the theory of thermal forces, the so called photophoretic force, is still under development. It depends on a number of poorly described factors, such as the temperature gradient across the illuminated particle and thermal creep of heated gas along the particle surface due to temperature and pressure gradients. In this thesis I use doughnut-shaped structured laser beams to levitate and guide light-absorbing micron-size particles aiming to uncover the optically induced forces in air at variable pressure ranging from 10-2000 millibar. First, I designed and built a counter-propagating optical pipeline to uncover the influence of polarisation on the particle movement. Second, I designed and constructed a vertically directed diverging vortex beam trap, a `funnel' trap, to conduct a quantitative evaluation of the photophoretic force and trapping stiffness by levitating graphite particles and carbon-coated glass shells of calibrated sizes in a carefully characterised vortex beam. Third, from the measured size of the particles and the position of the particle in the beam on the one hand, and the known density of the particles and the intensity distribution of the funnel trap on the other hand, I characterised the optically induced thermal forces in the axial and transverse directions. Fourth, I compared the contribution of thermal force to the light-pressure force and their dependence on atmospheric pressure. Based on the results of my experiments I determined the parameter space for guiding particles with hollow-core vortex and Bessel beams, taking into account the particle speed, size, and offset from the laser axis, all linked to the optical beam properties such as beam divergence, optical polarisation and power. The results of this thesis are used in the development of a touch-free optical system for pin-point delivery of macromolecules to the X-ray focal spot at the Free Electron Laser facility at the DESY (Deutsches Elektronen-Synchrotron) synchrotron in Hamburg, Germany, for coherent diffractive imaging experiments on nanometer-scale morphology. I conclude with a discussion of avenues for future work in contact-free manipulating of particles with structured laser beams to enhance significantly the efficiency of nanometer-scale morphology of proteins and biomolecules

Visualizing aerosol-particle injection for diffractive-imaging experiments

Delivering sub-micrometer particles to an intense x-ray focus is a crucial aspect of single-particle diffractive-imaging experiments at x-ray free-electron lasers. Enabling direct visualization of sub-micrometer aerosol particle streams without interfering with the operation of the particle injector can greatly improve the overall efficiency of single-particle imaging experiments by reducing the amount of time and sample consumed during measurements. We have developed in-situ non-destructive imaging diagnostics to aid real-time particle injector optimization and x-ray/particle-beam alignment, based on laser illumination schemes and fast imaging detectors. Our diagnostics are constructed to provide a non-invasive rapid feedback on injector performance during measurements, and have been demonstrated during diffraction measurements at the FLASH free-electron laser.Comment: 15 page

Polarization-sensitive photophoresis

We photophoretically trap spherical airborne particles using a single radially or azimuthally polarized laser beam and show that the trapping efficiency is significantly higher for the radial polarization. The demonstrated polarization sensitivity of the photophoretic force, which is caused by polarization-dependent reflection from the particles, adds additional flexibility to the optical micromanipulation of light absorbing particles in gaseous media

Dynamic axial control over optically levitating particles in air with an electrically-tunable variable-focus lens

Efficient delivery of viruses, proteins and biological macromelecules into a micrometer-sized focal spot of an XFEL beam for coherent diffraction imaging inspired new development in touch-free particle injection methods in gaseous and vacuum environments. This paper lays out our ongoing effort in constructing an all-optical particle delivery approach that uses piconewton photophoretic and femtonewton light-pressure forces to control particle delivery into the XFEL beam. We combine a spatial light modulator (SLM) and an electrically tunable lens (ETL) to construct a variable-divergence vortex beam providing dynamic and stable positioning of levitated micrometer-size particles, under normal atmospheric pressure. A sensorless wavefront correction approach is used to reduce optical aberrations to generate a high quality vortex beam for particle manipulation. As a proof of concept, stable manipulation of optically-controlled axial motion of trapped particles is demonstrated with a response time of 100ms. In addition, modulation of trapping intensity provides a measure of the mass of a single, isolated particle. The driving signal of this oscillatory motion can potentially be phase-locked to an external timing signal enabling synchronization of particle delivery into the x-ray focus with XFEL pulse train.This work has been supported by the Australian Research Council under DP110100975. W. M. Lee acknowledges the support of Australian Research Council Early Career Researcher Award, DE160100843

Post-sample aperture for low background diffraction experiments at X-ray free-electron lasers

The success of diffraction experiments from weakly scattering samples strongly depends on achieving an optimal signal-to-noise ratio. This is particularly important in single-particle imaging experiments where diffraction signals are typically very weak and the experiments are often accompanied by significant background scattering. A simple way to tremendously reduce background scattering by placing an aperture downstream of the sample has been developed and its application in a single-particle X-ray imaging experiment at FLASH is demonstrated. Using the concept of a post-sample aperture it was possible to reduce the background scattering levels by two orders of magnitude.Funding for this research was provided by: Deutsches Elektronen-Synchrotron; Deutsche Forschungsgemeinschaft (grant No. DFG-EXC1074); European Research Council (grant No. ERC614507-Kuepper); Helmholtz-Gemeinschaft (grant No. VI 419); Australian Research Council (grant No. DP170100131); National Science Foundation (grant No. STC-1231306)

Evaluation of forces levitating a particle in optical vortex beam

We study, both theoretically and in experiments, the dependence of optical forces acting on a spherical particle guided in air with an optical vortex beam, on the light polarization state, and discuss potential applications

Effect of polarization on transport of particles in air by optical vortex beam

Experiments on transport of spherical particles in air by optical vortex beam show that the speed of transport depends drastically on light polarization. There is a clear correlation between the speed of particle transport in a pipeline formed by cross-polarized vortices: a horizontally polarized beam moves particles faster than a vertically polarized one. To elucidate this effect we demonstrate, both in theory and experiments, that a radial shift of particles away from the vortex axis due to gravity results in polarization dependence of the laser intensity absorbed by the particle and thus determines the speed of transport. The results demonstrate an additional degree of freedom to control particle transport by varying the polarization of the driving vortex beams

Influence of Polarisation on Optical Trapping Forces in Air-Transport of Spherical Particles

We show, both theoretically and experimentally, that light polarization affects the driving forces through the change in absorption, and thus the speed of transport of spherical particles in air with counter-propagating optical vortex beams

Movement for women´s rights of General Pico La Pampa

Desde la vuelta de la democracia en nuestro país, muchos movimientos sociales se gestaron con diferentes fines, entre ellos los que luchan por la defensa de los derechos de las mujeres. La provincia de La Pampa, no ha estado exenta de este fenómeno. En un período de más de tres décadas aparecieron en el escenario social pampeano diversos movimientos. Pero es sobre fines del siglo XX, que emergen varios otros cuyos objetivos son claramente feministas y están ligados a la defensa expresa de los derechos de las mujeres. Este es el caso del Movimiento por los Derechos de las Mujeres de General Pico, objeto de estudio en este trabajo. Ubicándome en una perspectiva regional y de género, la presente investigación es motorizada por los siguientes interrogantes: la constitución de este movimiento ¿implicó una re-significación y reconstrucción de las identidades, individuales y colectivas, de sus integrantes? ¿En qué sentido? ¿En qué contexto deciden agruparse? ¿Cuáles fueron o son sus objetivos? ¿Qué planes de acción tienen? ¿Han obtenido logros? ¿Cuáles?Since the return to democracy in our country, many different movements, having different aims appeared. Among them, those which would strug- gle for the defence of women’s rights. The province of La Pampa has not been exempt from this fenomenon. In a period of more than three decades flourished in the pampean social scenery several movements.Nontheless, it is about the end of the 20th century that many other emerged, whose objectives are clearly feminist, and they are bound to the express defence of women’s rights. This is the case of Movement for Women’s Rights of General Pico”, subject of study in this paper. Adopting a regional perspective, from a gender viewpoint, this research is powered by the following question- ings: Did the constitution of this movement implied a resignification and reconstruction of both, individual and collective identities of its mem- bers? In what sense? In which context do they decide to group? Which were, a still are their aims? Which lines of action do they have? Have they achieved any goals? Which ones?

Optical Injector of Particles for X-ray Diffractive Imaging

We apply a high aspect-ratio first order Bessel beam, formed by imaging a vortex beam through an axicon, to guide biological macromolecules and viruses to the focus of femtosecond xray free-electron-laser for coherent diffractive imaging