It is well established that a light beam can carry angular momentum and therefore when using optical tweezers
it is possible to exert torques to twist or rotate microscopic objects. Both spin and orbital angular momentum
can be transferred. This transfer can be achieved using birefringent particles exposed to a Gaussian circularly
polarized beam. In this case, a transfer of spin angular momentum will occur. The change in spin, and hence the
torque, can be readily measured optically. On the other hand, it is much more challenging to measure orbital
angular momentum and torque. Laguerre-Gauss mode decomposition, as used for orbital angular momentum
encoding for quantum communication, and rotational frequency shift can be used, and are effective methods in
a macro-environment. However, the situation becomes more complicated when a measurement is done on microscale,
especially with highly focused laser beams. We review the methods for the measurement of the angular
momentum of light in optical tweezers, and the challenges faced when measuring orbital angular momentum. We
also demonstrate one possible simple method for a quantitative measurement of the orbital angular momentum
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.
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.
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.
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.
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.
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