Use of shape induced birefringence for rotation in optical tweezers

Since a light beam can carry angular momentum (AM) it is possible to use optical tweezers to exert torques to twist or rotate microscopic objects. The alignment torque exerted on an elongated particle in a polarized light field represents a possible torque mechanism. In this situation, although some exchange of orbital angular momentum occurs, scattering calculations show that spin dominates, and polarization measurements allow the torque to be measured with good accuracy. This phenomenon can be explained by considering shape birefringence with an induced polarizability tensor. Another example of a shape birefringent object is a microsphere with a cylindrical cavity. Its design is based on the fact that due to its symmetry a sphere does not rotate in an optical trap, but one could break the symmetry by designing an object with a spherical outer shape with a non spherical cavity inside. The production of such a structure can be achieved using a two photon photo-polymerization technique. We show that using this technique, hollow spheres with varying sizes of the cavity can be successfully constructed. We have been able to demonstrate rotation of these spheres with cylindrical cavities when they are trapped in a laser beam carrying spin angular momentum. The torque efficiency achievable in this system can be quantified as a function of a cylinder diameter. Because they are biocompatible and easily functionalized, these structures could be very useful in work involving manipulation, control and probing of individual biological molecules and molecular motors

The difficulty of measuring orbital angular momentum

Light can carry angular momentum as well as energy and momentum; the transfer of this angular momentum to an object results in an optical torque. The development of a rotational analogue to the force measurement capability of optical tweezers is hampered by the difficulty of optical measurement of orbital angular momentum. We present an experiment with encouraging results, but emphasise the difficulty of the task

Fabrication of micro-structures for optically driven micromachines using two-photon photopolymerization of UV curing resins

Two-photon photopolymerization of UV curing resins is an attractive method for the fabrication of microscopic transparent objects with size in the tens of micrometers range. We have been using this method to produce three-dimensional structures for optical micromanipulation, in an optical system based on a femtosecond laser. By carefully adjusting the laser power and the exposure time we were able to create micro-objects with well-defined 3D features and with resolution below the diffraction limit of light. We discuss the performance and capabilities of a microfabrication system, with some examples of its products.Comment: 12 pages, 10 figure

Optical paddle-wheel

As an optically trapped micro-object spins in a fluid, there is a consequent flow in the fluid.. Since a free-floating optically-driven microrotor can be moved to a desired position, it can allow the controlled application of a directed flow in a particular location. Here we demonstrate the control and rotation of such a device, an optical paddle-wheel, using a multiple-beam trap. In contrast to the usual situation where rotation is around the beam axis, here we demonstrate rotation normal to this axis

Symmetry and the generation and measurement of optical torque

A key element in the generation of optical torque in optical traps, which occurs when electromagnetic angular momentum is transferred from the trapping beam to the trapped particle by scattering, is the symmetries of the scattering particle and the trapping beam. We discuss the effect of such symmetries on the generation and measurement of optical torque in optical tweezers, and some consequent general principles for the design of optically-driven micromachines.Comment: 28 page

Following the Birth of a Nanoplasma Produced by an Ultrashort Hard-X-Ray Laser in Xenon Clusters

X-ray free-electron lasers (XFELs) made available a new regime of x-ray intensities, revolutionizing the ultrafast structure determination and laying the foundations of the novel field of nonlinear x-ray optics. Although earlier studies revealed nanoplasma formation when an XFEL pulse interacts with any nanometer-scale matter, the formation process itself has never been decrypted and its timescale was unknown. Here we show that time-resolved ion yield measurements combined with a near-infrared laser probe reveal a surprisingly ultrafast population (similar to 12 fs), followed by a slower depopulation (similar to 250 fs) of highly excited states of atomic fragments generated in the process of XFEL-induced nanoplasma formation. Inelastic scattering of Auger electrons and interatomic Coulombic decay are suggested as the mechanisms populating and depopulating, respectively, these excited states. The observed response occurs within the typical x-ray pulse durations and affects x-ray scattering, thus providing key information on the foundations of x-ray imaging with XFELs

Real-time observation of X-ray-induced intramolecular and interatomic electronic decay in CH2I2

The increasing availability of X-ray free-electron lasers (XFELs) has catalyzed the development of single-object structural determination and of structural dynamics tracking in realtime. Disentangling the molecular-level reactions triggered by the interaction with an XFEL pulse is a fundamental step towards developing such applications. Here we report real-time observations of XFEL-induced electronic decay via short-lived transient electronic states in the diiodomethane molecule, using a femtosecond near-infrared probe laser. We determine the lifetimes of the transient states populated during the XFEL-induced Auger cascades and find that multiply charged iodine ions are issued from short-lived (similar to 20 fs) transient states, whereas the singly charged ones originate from significantly longer-lived states (similar to 100 fs). We identify the mechanisms behind these different time scales: contrary to the short-lived transient states which relax by molecular Auger decay, the long-lived ones decay by an interatomic Coulombic decay between two iodine atoms, during the molecular fragmentation

Optically Fabricated and Driven Micromachines

Two-photon photopolymerization of optically curable resins is a powerful tool for fabricating micrometer-sized objects of potentially any shape in three dimensions. This method relies on the fact that certain liquid organic resins can be hardened by exposure to UV light. After full cure, they have very good transmission over a wide spectral range from near UV to short-wavelength infrared. Due to their transparency, these objects are ideal for optical trapping experiments. By using an infrared femtosecond pulsed laser instead of UV light for curing, exploiting the two-photon absorption cross section of the resin, objects with high spatial resolution can be fabricated, even beyond the diffraction limit. My thesis describes the design, fabrication and testing of a range of optically driven micromachines, microprobes and microtools, including micromotors smaller than 10 microns across fabricated by this method. There is an ongoing effort in our group in characterizing optical micromachines theoretically, through computational modelling, and experimentally. More specific, n-fold rotational symmetry micro-objects are being investigated in terms of torque exerted on them by laser beams. My thesis provides an experimental insight into the angular momentum (both orbital and spin) exchange between the laser light and the micro-objects, with quantitative measurements for the exerted torques. I also investigate the possible use of the microfabricated objects for biological applications, in particular integration of micromachines with biological molecules (DNA, molecular motors). I will begin by explaining the principles of optical tweezers, the optical drive used for the produced micromachines in this thesis. Next I introduce the notion of “micromachine” and the available driving methods as well as symmetry considerations for optimal optical drive. I will then describe the photopolymerization process and our method of producing microstructures. The spatial resolution achieved by the two-photon photopolymerization process is discussed and I will show that we can achieve lateral resolution in the order of 150 nm. In the last part of my thesis I describe the types of micromachines elements I fabricated with examples illustrating transfer of spin and orbital angular momentum by measuring the spin and orbital torque exerted on the microfabricated particles

Three-dimensional complex-shaped photopolymerized microparticles at liquid crystal interfaces

Microparticles of arbitrary shapes immersed in the bulk of nematic fluids are known to produce dipolar or quadrupolar elastic distortions that can mediate long-range colloidal interactions. We use two-photon photopolymerization to obtain complex-shaped surface-bound microparticles that are then embedded into a nematic liquid crystal host with a uniform far-field director. By means of three-dimensional imaging with multi-photon excitation fluorescence polarizing microscopy, we demonstrate low-symmetry, long-range elastic distortions induced by the particles in the liquid crystal director field. These director distortions may provide a means for controlling elastic interactions in liquid crystals between custom-designed photopolymerized microparticles attached to confining solid substrates and nematic fluid-borne colloids, thus enabling elasticity-mediated templated self-assembly