The fact that light carries both linear and angular momentum is well-known to physicists. One application of the linear momentum of light is for optical tweezers, in which the refraction of a laser beam through a particle provides a reaction force that draws the particle towards the centre of the beam. The angular momentum of light can also be transfered to particles, causing them to spin. In fact, the angular momentum of light has two components that act through different mechanisms on various types of particle. This Review covers the creation of such beams and how their unusual intensity, polarization and phase structure has been put to use in the field of optical manipulation.

Tweezers with a twist

We propose a theory to explain optical trapping by optical vortices (OVs), which are emerging as important tools to trap mesoscopic particles. The common perception is that the trapping is solely due to the gradient force and that it may be characterized by three real force constants. However, we show that the OV trap can exhibit complex force constants, implying that the trapping must be stabilized by ambient damping. At different damping levels, particles exhibit remarkably different dynamics, such as stable trapping and periodic and aperiodic orbital motions.

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Theory of optical trapping by an optical vortex beam.

CaCO3 vaterite microparticles for biomedical and personal care applications

Fabrication technology for three-dimensional microstructures with submicrometer accuracy has been needed in the fields of modern optics, such as micro mechanical system driven with photon pressure[1, 2] and laser-trapping near-filed optical microscopy[3]. However, the present accuracy with stereolithography[4] is not yet satisfactory to this purpose. Moreover, it is not so flexible to make a three- dimensional structure with the present technique. In this paper, we propose a new microfabrication method in which a point in three-dimensional volume of UV photopolymerizing resin is photopolymerized through two-photon absorption process. The microfabrication with two-photon absorption drastically improves the depth resolution due to a nonlinearlity between the power of the irradiation and that of the absorption[5].

Three-dimensional microfabrication with two-photon absorbed photopolymerization

Light-induced rotation of absorbing microscopic particles by transfer of angular momentum from light to the material raises the possibility of optically driven micromachines. The phenomenon has been observed using elliptically polarized laser beams or beams with helical phase structure. But it is difficult to develop high power in such experiments because of overheating and unwanted axial forces, limiting the achievable rotation rates to a few hertz. This problem can in principle be overcome by using transparent particles, transferring angular momentum by a mechanism first observed by Beth in 1936, when he reported a tiny torque developed in a quartz waveplate due to the change in polarization of transmitted light. Here we show that an optical torque can be induced on microscopic birefringent particles of calcite held by optical tweezers. Depending on the polarization of the incident beam, the particles either become aligned with the plane of polarization (and thus can be rotated through specified angles) or spin with constant rotation frequency. Because these microscopic particles are transparent, they can be held in three-dimensional optical traps at very high power without heating. We have observed rotation rates in excess of 350 Hz.

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Optical alignment and spinning of laser-trapped microscopic particles

This paper reports on a simple synthesis and characterization of highly birefringent vaterite microspheres, which are composed of 20–30 nm sized nanocrystalls. Scanning electron microscopy shows a quite disordered assembly of nanocrystals within the microspheres. However, using optical tweezers, the effective birefringence of the microspheres was measured to be Δn=0.06, which compares to Δn=0.1 of vaterite single crystals. This suggests a very high orientation of the nanocrystals within the microspheres. A hyperbolic model of the direction of the optical axis throughout the vaterite spherulite best fits the experimental data. Results from polarized light microscopy further confirm the hyperbolic model.

Highly birefringent vaterite microspheres: production, characterization and applications for optical micromanipulation

We describe a means for controlling the spin angular-momentum flux of a laser beam at constant power, without introducing any elliptical or linear polarization. This allows a controllable torque, acting to spin the particle uniformly, to be exerted on a birefringent particle in optical tweezers. The constant power means that transverse and axial trapping, and heating due to absorption, are unaffected by changing the torque. The torque can be computer controlled and rapidly changed. In addition, the lateral trapping is kept constant. Very low torques can be obtained such that rotational Brownian motion of birefringent particles can be observed. This has the potential to greatly extend the quantitative applications of the rotation of birefringent objects in optical tweezers.

Constant power optical tweezers with controllable torque

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.

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/7038/1/Engineering-optically-driven-micromachines/10.1117/12.798529.pdf

Engineering Optically Driven Micromachines.

Measurements of viscoelasticity in the microscopic regime are of interest in polymer solutions as well as in microscopic structures such as cells. Viscoelasticity can be studied using a localized microrheometer based on optical tweezers. We rotate a birefringent micron-sized calcium carbonate sphere crystallized in a vaterite structure. By applying a time-dependent torque or using the time-dependent thermal torque, viscoelasticity can be measured. The torque can be measured purely optically, by measuring the polarization state of the trapping beam after passing through the particle. We control the torque by controlling the relative amplitudes of two orthogonally circularly polarized components of the trapping beam with two acousto-optic modulators. This allows a wide range of oscillation frequencies to be used. We demonstrate applications of the methods on several systems.

https://espace.library.uq.edu.au/view/UQ:202744/UQ202744_OA.pdf

Vaterite Twist Microrheology with AOM Controlled Optical Tweezers

International research work for young people is common in physics. However, work experience and career plan of female workers in physics are little studied. We explore them by interviewing three international female workers in physics.

Maren Funk

Papers

Highly birefringent vaterite microspheres: production, characterization and applications for optical micromanipulation

Constant power optical tweezers with controllable torque

Engineering Optically Driven Micromachines.

Vaterite Twist Microrheology with AOM Controlled Optical Tweezers

International research work experience of young females in physics