220 6.3  Optical Force Tools

kinesin protein involved in cell division (Svoboda et al., 1993), as well as a variety of proteins

that use DNA as a track. The state of the art in optical tweezers involves replacing the air

between the optics of a bespoke optical tweezers setup with helium to minimize noise effects

due to the temperature-​dependent refraction of lasers through gases, which has enabled the

transcription of single-​nucleotide base pairs on a single-​molecule DNA template by a single

molecule of the ribonucleic acid polymerase motor protein enzyme to be monitored directly

(Abbondanzieri et al., 2005).

6.3.5  NON-​GAUSSIAN BEAM OPTICAL TWEEZERS

“Standard” optical tweezers are generated from focusing a Gaussian profile laser beam

into a sample. However, optical trapping can also be enabled using non-​Gaussian pro­

file beams. For example, a Bessel beam may be used. A Bessel beam, in principle, is

diffraction free (Durnin et al., 1987). They have a Gaussian-​like central peak intensity of

width roughly one wavelength, as with a single-​beam gradient force optical trap; how­

ever, they have in theory zero divergence parallel to the optic axis. In practice, due to

finite sizes of optical components used, there is some remaining small divergence at

the ~mrad scale, but this still results in minimal spreading of the intensity pattern over

length scales of 1 m or more.

The main advantage of optical trapping with a Bessel beam, a Bessel trap, is that since

there is minimal divergence of the intensity profile of the trap with depth into the sample,

which is ideal for generating optical traps far deeper into a sample than permitted with con­

ventional Gaussian profile traps. The Bessel trap profile is also relatively unaffected by small

obstacles in the beam path, which would cause a significant distortion for standard Gaussian

profile traps; a Bessel beam can reconstruct itself around an object provided a proportion of

the light waves is able to move past the obstacle. Bessel beams can generate multiple optical

traps that are separated by up to several millimeters.

Optical tweezers can also be generated using optical fibers. The numerical aperture of a

single-​mode fiber is relatively low (~0.1) generating a divergent beam from its tip. Optical

trapping can be achieved using a pair of juxtaposed fibers separated by a gap of a few tens

of microns (Figure 6.4a). A refractile particle placed in the gap experiences a combination of

forward scattering forces and lateral forces from refraction of the two beams. This results in

an optical trap, though 10–​100 times less stiffness compared to conventional single-​beam

gradient force traps for a comparable input laser power. Such an arrangement is used to trap

relatively large single cells, in a device called the “optical stretcher.”

The refractive index of the inside of a cell is in general heterogeneous, with a mean margin­

ally higher than the water-​based solution of the external environment (see Chapter 3). This

combined with the fact that cells have a defined compliance results in an optical stretching

effect in these optical fiber traps, which has been used to investigate the mechanical

differences between normal human cells and those that have a marginally different stiffness

due to being cancerous (Gück et al., 2005). The main disadvantage with the method is that

the laser power required to produce measurable probing of cell stiffness also results in large

rises in local temperature at the NIR wavelengths nominally employed—​a few tens of degree

centigrade above room temperature is not atypical—​which can result in significant thermal

damage to the cell.

It is also possible to generate 2D optical forces using an evanescent field, similar to that

discussed for TIRF microscopy (see Chapter 3); however, to trap a particle stably in such a

geometry requires an opposing, fixed structure oriented against the direction of the force

vector, which is typically a solid surface opposite the surface from which the evanescent field

emanates (the light intensity is greater toward the surface generating the evanescent field and

so the net radiation pressure is normal to that away from the surface). This has been utilized

in the cases of nanofabricated photonic waveguides and at the surface of optical fibers. There

is scope to develop these techniques into high-​throughput assays, for example, applied in a

multiple array format of many optical traps, which could find application in new biosensing

assays.