Computational tasks such as the calculation and characterization of the optical force acting on a sphere are relatively straightforward in a Gaussian beam trap. Resulting properties of the trap such as the trap strength, spring constants, and equilibrium position can be easily determined. More complex systems with non-spherical particles or multiple particles add many more degrees of freedom to the problem. Extension of the simple methods used for single spherical particles could result in required computational time of months or years. Thus, alternative methods must be used. One powerful tool is to use dynamic simulation: model the dynamics and motion of a particle or particles within the trap. We demonstrate the use of dynamic simulation for non-spherical particles and multi-particle systems. Using a hybrid discrete dipole approximation (DDA) and T-matrix method, we find plausible equilibrium positions and orientations of cylinders of varying size and aspect ratio. Orientation landscapes revealing different regimes of behaviour for micro-cylinders and nanowires with different refractive indices trapped with beams of differing polarization are also presented. This investigation provides a solid background in both the function and properties of micro-cylinders and nanowires trapped in optical tweezers. This method can also be applied to particles with other shapes. We also investigate multiple-particle trapping, which is quite different from single particle systems, as they can include effects such as optical binding. We show that equilibrium positions, and the strength of interactions between particles can be found in systems of two and more particles.
While a variety of different optically-driven micromachines have been demonstrated by a number of groups
around the world, there is a striking similarity in the designs used. The typical optically-driven rotor consists
of a number of arms attached to a central hub, or elongated stalk in the case of free-floating rotors. This is a
consequence of the relationship between the symmetry of a scattering object and the transfer of optical angular
momentum from a beam to the object.
We use a hybrid discrete-dipole approximation/T-matrix method algorithm to computationally model the
scattering by such optically-driven rotors. We systematically explore the effects of the most important parameters
of rotors, such as the thickness, length, and width of the arms, in order to maximize the torque efficiency.
We show that it is possible to use computational modelling to optimize the design of such devices. We also
compare the computational results with experiment.
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.
Optical forces and torques acting on microscopic objects trapped in focussed laser beams promise flexible methods of driving micromachines through a microscope cover slip or even a cell wall.
We are endeavouring to engineer special purpose micro-objects for a range of tasks. Colloidal self assembly of calcium carbonate provides birefringent spheres which can exert considerable torque, while two photon polymerisation allows us to fabricate objects of arbitrary shape that can be designed to exchange both spin and orbital angular momentum. Numerical calculations of forces and torques can allow an optimal design, and optical measurements provide us with certain knowledge of the forces and torques which are actually exerted.
Two-photon polymerization of optically curing resins is a powerful method to fabricate micron sized objects which can be used as tools to measure properties at small scales. These microdevices can be driven by means of externally applied focused laser beams (optical tweezers) through angular momentum exchange, giving rise to a net torque. The advantage of the optical drive is that no contact is required, therefore making the microdevices suited to non-invasive biological applications.
The fabrication method is versatile and allows building objects of any 3D shape.
We discuss the design and modelling of various optically driven rotors. In particular, we consider fabrication of microspheres with an internal shape birefringence in order to obtain rotation in an optical trap. The reason for fabricating this type of object is that they are well-suited for studies of mechanical properties of single biomolecules such as the torsional stiffness of DNA or torque generated by molecular motors.
The microspheres fabricated are able to transduce torques of 2000 pNnm with optical powers of 500 mW and could be rotated with frequencies up to 40 Hz in circularly polarized light.
Building on the ability to exert torques in optical tweezers,
optically-driven rotating micromachines have reached the verge of practical application. Prototype devices have been made, and useful applications are being sought. We outline some general principles that can be applied to the design of optically-rotated devices, and describe a method for rigorous computational modelling that is well-suited to the optimization and engineering of such micromachines. Finally, we describe a method for rapid microfabrication of prototypes for testing, and some results of such tests.
A strongly focused laser beam can be used to trap, manipulate and exert torque on a microparticle. The torque
is the result of transfer of angular momentum from the laser beam. The laser could be used to drive a rotor,
impeller, cog wheel, etc. of a few microns in size, perhaps fabricated from a birefringent material. We review our
methods of computationally simulating the torque and force imparted by a laser beam. We introduce a method
of hybridizing the T-matrix with the finite difference frequency domain (FDFD) method to allow the simulation
of materials that are anisotropic and inhomogeneous, and structures that have complex shapes. We also employ
an alternative discrete dipole approximation method. The high degree of symmetry of a microrotor, such as
rotational periodicity, could be exploited to reduce computational time and memory requirements by orders of
magnitude. This is achieved by performing calculations for only a given segment that is repeated across the
whole structure. This can demonstrated by modeling the optical trapping and rotation of a cube.
The ability to exert optical torques to rotationally manipulate microparticles has developed from an interesting curiosity to seeing deployment in practical applications. Is the next step to genuine optically-driven micromachines feasible or possible? We review the progress made towards this goal, and future prospects.