Optical Trapping: Method Used in Atomic Cooling Experiments

Optical trapping is a way of cooling atoms to the point where they are no longer in motion—or are trapped. One may instantly think of optical tweezers when hearing the mention of optical trapping. While optical tweezers are a form of optical trapping, tweezers tend to be more specifically used in biological applications. Optical trapping, in a broader sense, is one of the primary methods used in Bose-Einstein condensate experiments and is a method that preceded and influenced the design of optical tweezers.

What is Optical Trapping?

Optical trapping was worthy of the Nobel Prize in Physics in 1997. It was awarded to Steven Chu, Claude Cohen-Tannoudji, and William D. Phillips. They used Doppler cooling to trap atoms in three dimensions. This was achieved by cooling the atoms and then using six intersecting laser beams configured in three sets of two beams. The counterpropagating beam pairs were oriented orthogonally in three directions. These laser beams slowed atoms akin to a fly getting stuck in honey. This slowing effect was referred to as optical molasses. While the beams were able to slow the atoms, they did not effectively trap the atoms. Gravity prevents complete entrapment, as its pull causes the slowed atoms to escape the molasses. To actually trap the atoms, the beams had to be used in conjunction with magnetic coils that varied the magnetic field at the beams’ intersection. The magnetic field from the coils could then position the atoms in the center of the trap. This kind of trap was dubbed a magneto-optical trap (MOT). Once the atoms are trapped, they can be studied further.

Diagram of a typical magneto-optical trap arrangement. Courtesy of Patrick Gill

Evanescent Wave Optical Trapping

A different method of optical trapping employs the use of evanescent waves. A paper entitled Gravitational Laser Trop for Atoms with Evanescent-Wave Cooling published in Optics Communications describes this method succinctly: “An evanescent light wave, formed by total internal reflection of a laser beam on the surface of a dielectric substrate can act as a mirror for neutral atoms. In the field of gravity, an atom can bounce on such a mirror many times and, if the surface is appropriately formed, a stable confinement is possible”.

One such optical trapping design involves a pyramidal mirror that reflects atoms from a magneto-optical trap. While this design had a pyramidal geometry, the shape of the trap does not affect the ability of the evanescent-wave cooling mechanism. For the trap’s operation, a laser pumps the atoms into a lower state. Atoms that enter the evanescent wave in this lower state transition to a higher state and fall back to the lower state. Through this process, the atoms lose kinetic energy and the system cools.

Trapping for Bose-Einstein Experiments

Not surprisingly, optical trapping is popular for ultracold atom experiments. Such experiments typically involve creating Bose-Einstein condensates (BECs). The Bose-Einstein condensate is one of the five states of matter (the remaining four being solid, liquid, gas, and plasma). This occurs when atoms virtually reach absolute zero and the atom’s wavelength is close to the atom spacing. Under these conditions, the atoms “occupy the same quantum state and act in unison as a superfluid – so bringing otherwise microscopic wavelike properties into the macroscopic realm” (physicsworld). The atoms are no longer interacting under classical forces. Optical trapping allows for the facilitation of atoms to reach this state.

A paper out of Tuebingen, Germany published in Applied Physics B describes such an experiment. They created an optical trap that allowed them to study the interactions between ultracold atoms and evanescent waves. This trap used a combination of a magnetic trap and the repulsive potential of an evanescent dipole. Their findings revealed that a trap with a flat bottom would probably be the most ideal for reaching Bose-Einstein condensation so the atoms could be cooled more effectively.

Another proposition to improve BEC creation has been to perform optical trapping in microgravity. This would mean the optical traps would no longer be confined in their ability to trap atoms by the asymmetric pull of gravity. This led to the thought that the ideal condition for obtaining observations on the macroscopic level would be to combine both ultracold temperatures with minimal effect from gravity. The proposition was tested in parabolic flight 100 km above sea level. BECs were produced and able to be manipulated with encouraging results in such an environment.

Trapping Using Waveguides

One experiment using a yet another method of optical trapping involves trapping the atoms in evanescent fields of wave guides. In such experiments, the wave guides are laser written onto the surface of an integrated optical atom chip. The proposed setup involves a scalable light-matter interface design that is written onto silica glass chips. The waveguides are designed to minimize light loss and maximize the trap depth. These types of traps are used with both a detuned blue and a detuned red lasers to create a stable two-dimensional trap.

The calculations reveal that the trapping using the evanescent fields of the waveguides would be possible. With respect to the goal to minimize loss, it was found that the more powerful the input laser, the less loss would occur. This is because the higher power is farther from atomic resonance, thus leading to lower losses. They also found that as the trap becomes shallower, the amount of loss increases. This was attributed to the potential barrier being too small to trap atoms effectively at that depth. The findings also determined that blue laser would need to support two propagating modes and the red a single propagating mode.

Conclusion

Optical trapping comes in a variety of designs and geometries. Despite the range of setups used and tested, still more is being explored. It has been an important technic for advancing science and will continue being in the arsenal of the modern physicists in the foreseeable future.

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