The Developments In Cavity Optomechanics
Since their invention, optical traps have devolved to the point that they are an extremely useful tool in all areas of science, allowing the manipulation of tiny particles. Both experiments described below involved suspending a microparticle, only a few micrometres wide, in a laser beam and making measurements on it. The beam works by focusing a very narrow beam of light at the particle which holds the particle in place from the forces produced as the particle interacts with the light.
Tongcan Li, Simon Kheifits, David Medillin and Mark Raizen produced an incredible method for observing the ballistic motion of a Brownian Particle on timescales that Einstein once declared were impossible to measure. Their breakthrough came in using air as the medium rather than a fluid, hence why the optical trap was used. The gaps between measurements in a fluid would have to be around 10 nanoseconds, which is not yet experimentally possible. This is due to the fact that fluids are much denser than air, so the random collisions between the particle and the medium, which cause Brownian motion, occur much more frequently. The results of the experiment confirmed the ballistic motion that Einstein predicted at the tiny timescales between collisions. The instantaneous velocity was found to be almost identical to that predicted by the equipartition theorem. This experiment therefore provides direct confirmation the theory for Brownian particles is correct, which has the potential for huge steps forward in systems affected by this motion, such as living cells.
Another experiment run by Y Arita, M Mazilu and K Dholakia also makes use of the optical trap, but this time in the field of laser cooling. Laser cooling in general works due to the fact that as an object absorbs and then re-emits a photon, its overall momentum will decrease. As temperature is a function of the average movement of an object, the more photons the laser supplies for the object to absorb and then emit, the lower the temperature of the object will become. These methods of cooling are known as “active” cooling methods and are at the forefront of atomic physics due to the fact that the super-low temperatures enhance the quantum properties of the material.
In this experiment, they did not use the conventional active cooling method as described above but instead cooled the microparticle by reducing the number of degrees of freedom. As the number of degrees of freedom decreases, so does the overall energy due to the Equipartition Principle. This has the result of decreasing the overall temperature of the system. They did this by trapping the microparticle with a laser beam and forcing it to rotate at a controlled rate. The particle was held in a vacuum so that heat could not leak out of the system. This had the effect of cooling the particle down to 40 Kelvin, without using any traditional cooling methods. The results of this experiment represent a significant step forward for the field of cavity optomechanics and better understanding the underlying forces involved in rotational quantum forces.
