Summary: Optical trapping and manipulation of neutral particles using lasers

Introduction

Optical trapping is based on the force of radiation pressure coming from the momentum of light in a laser. Lasers enable forces large enough to stably trap small particles in continuous wave coherent light beams. These manipulation methods can be applied from particles such as atoms to larger biological particles such as viruses and living cells. For the atomic scale, it is possible to optically cool atoms to micro-kelvin temperatures and trap them at high densities, with applications in atomic clocks and interferometers. In biology, applications include applying forces on living cells without inducing detectable optical damage. Optical tweezers can measure the forces generated by single biologic motors to understand with forces of actin strands in the cytoskeleton.

Basic forces and the first optical trap

Using a focused 1 W laser beam on a particle of radius $\approx$ 1 wavelength, the incident light momentum reflects back on itself assuming the particle acts as a perfect mirror. The particle acceleration $F/m$ is $\approx 10^{5}g$. This is large and prompted Ashkin to consider the resulting dynamical effects and he considered an experiment to look for particle motion from this force. He used transparent latex spheres suspended in water. With laser power on the order of milliwatts, particle motion was observed in the direction of a Gaussian beam. This was a radiation pressure effect. What was unexpected was that the beam strongly bulled particles located in the beam’s fringes into the middle of the high intensity beam axis.

The understanding of these force components of the scattering force component, $F_{scat}$, and the gradient component $F_{grad}$ made it possible to consider the first optical trap for neutral particles. The success of these experiments led to the idea that trapping should be possible with molecules using lasers tuned to specific optical transitions. The scattering force can be given by the rate of scattering momentum $F_{scat}=hf/\lambda t$ where $f$ is the excited population fraction, and $t$ the spontaneous emission lifetime. The formula for the gradient force of an electromagnetic wave on a neutral atom is the dipole force formula $\frac{1}{2}\alpha \nabla E^2$, where $\alpha$ is the induced polarizability of the particle.

Optical levitation and trapping

Optical levitated in air was the next advancement, where a single beam is able to confine a particle where gravity and the upward scattering force balance. Levitated in high vacuum was also possible using feedback to damp particle oscillations.

The tobacco mosaic virus is an example of early optical trapping in biology, and was selected due to its rugged nature. Changing from a green 5125 Å laser (due to the death of motile bacteria) to an infrared yttrium/aluminium laser at 1.06 $\mu m$ made a huge difference, and it became possible to hold Escherichia coli and yeast. Damage-free trapping was also done in red blood cells and algae.

Conclusion

The precise degree of control that is possible by optical trapping has many applications in physics and biology. Laser advances will lead to further adoption of this manipulation method.