Optical tweezers use the forces exerted by a strongly focused beam of light to trap and move objects ranging in size from tens of nanometres to tens of micrometres. Since their introduction in 1986, the optical tweezer has become an important tool for research in the fields of biology, physical chemistry and soft condensed matter physics. Recent advances promise to take optical tweezers out of the laboratory and into the mainstream of manufacturing and diagnostics; they may even become consumer products. The next generation of single-beam optical traps offers revolutionary new opportunities for fundamental and applied research.

http://physics.nyu.edu/grierlab/nreview2c/nreview2c.pdf

A revolution in optical manipulation

As they travel through space, some light beams rotate. Such light beams have angular momentum. There are two particularly important ways in which a light beam can rotate: if every polarization vector rotates, the light has spin; if the phase structure rotates, the light has orbital angular momentum (OAM), which can be many times greater than the spin. Only in the past 20 years has it been realized that beams carrying OAM, which have an optical vortex along the axis, can be easily made in the laboratory. These light beams are able to spin microscopic objects, give rise to rotational frequency shifts, create new forms of imaging systems, and behave within nonlinear material to give new insights into quantum optics.

/pdf/orbital-angular-momentum-origins-behavior-and-applications-1sv8qfuhrs.pdf

Orbital angular momentum: origins, behavior and applications

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

https://physics.nyu.edu/grierlab/dynamic5b/dynamic5b.pdf

Dynamic holographic optical tweezers

Dark optical solitons: physics and applications

Summary form only given. We demonstrate that a low-index dielectric particle can be stably trapped in three-dimensions using a stationary, focused Gaussian beam containing an optical vortex. This work is, to our knowledge, the first demonstration of three-dimensional trapping of a spherical low-index particle using a single, stationary beam. Vortex traps allow the trapping of low- and high-index particles with less risk of damage and better isolation.

https://www.cis.rit.edu/~grovers/Doc/Publications/VortexTrap.OptLett.96.pdf

Optical vortex trapping of particles

We report the first observation of the simultaneous three-dimensional confinement of both a low-index particle and a high-index particle within a single-beam optical trap by using a strongly focused laser beam containing an optical vortex. Experimental and theoretical investigations of the trap stability are described.

https://www.cis.rit.edu/~grovers/Doc/Publications/SimulTrapping.JOSAB.99.pdf

Simultaneous trapping of low-index and high-index microparticles observed with an optical-vortex trap

The equilibrium position of a low-index particle in an optical-vortex trap was experimentally measured for two different systems: a buoyant hollow glass sphere in water and a density-matched water droplet in acetophenone. Vortex traps are the only known static, single-beam configurations allowing three-dimensional trapping of such particles in the size range of 2–50 μm. The trap consists of a strongly focused Gaussian laser beam containing a holographically produced optical vortex. Using experimental and theoretical techniques, we also explored changes in the trapping efficiency owing to the vortex core size, the relative refractive index, and the numerical aperture of the focusing objective.

https://www.cis.rit.edu/~grovers/Doc/Publications/LowIndexTrap.JOSAB.98.pdf

Trapping of low-index microparticles in an optical vortex

Comparison of the optical trapping efficiency of Agrobacterium rhizogenes and A. tumefaciens strains indicates the A. rhizogenes strain, ATCC 11325, is significantly less efficiently trapped than A. rhizogenes A4, ATCC 15834, and the A. tumefaciens strain LBA4404. Differences were also found in capsule generation, growth media viscosity, and transmission electron microscopy negative staining. These observations imply a difference in surface structure exists. Calcofluor fluorescence suggests the difference involves an exopolysaccharide.

Differences in optical trapping prompt investigations of agrobacterium surface characteristics

filters. The noise level in the images corresponds to one standard deviation above the observed background fluorescence noise level due to chromophore leakage into the tissue embedding medium. The images are not shown at full dynamic range in order that most of the histological components of tissue samples may be visualized. Eosinophils (arrows) were recognized in the H&E histology of normal mucosa by their reddish cytoplasm and accurately identified in correlated LIF microscopy. The cytoplasm of eosinophils were found to have P lop4 cm-'. Although fluorescence was observed in normal epithelial cell boundaries in the 530-610 nm spectral region, markedly increased fluorescence emission was observed in the cytoplasm of dysplastic cells in the same spectral region. We describe quantitative LIF microscopy results in normal, premalignant, and malignant colonic tissues in this and other spectral emission ranges associated with important tissue fluorescent chromophores. *Department of Pathology, University Hospitals of Cleveland, Cleveland, Ohio 44106 **Department of Medicine, Case Western Reserve University, Cleveland, Ohio 44106 t Departments of Biomedical Engmeering and Medicine, Case Western Reserve University, Cleveland, Ohio 44106 1. R. M. Cothren, M. V. Sivak, J. Van Dam, R. E. Petras, et al., Gastrointest. Endos-

K.T. Gahagan

Papers

Optical vortex trapping of particles

Simultaneous trapping of low-index and high-index microparticles observed with an optical-vortex trap

Trapping of low-index microparticles in an optical vortex

Differences in optical trapping prompt investigations of agrobacterium surface characteristics

Transformation of Ginkgo biloba using optical tweezers and scissors