Organic wall coatings for atomic clocks

Miniature atomic clocks and magnetometers are being developed in the Time & Frequency Division of the National Institute of Standards and Technology in Boulder1-3. In the NIST clock design, a local oscillator is used to lock onto a resonance of alkali metal atoms in the gas phase based on coherent population resonance. Ideally, these gas atoms should retain their spin polarization for as long as possible to ensure a stable reference signal. In typical gas cells, however, the atoms collide occasionally with the walls of the cell, changing their polarization during this interaction and drifting from resonance. As the scale of the devices becomes smaller, these collisions become more frequent and the chemistry of the walls becomes increasingly important. Ideally, the cell surfaces are perfectly covered with a material which interacts only minimally with the alkali metal atoms. In addition, as the cell becomes ever smaller the coating material should be very thin.

 

Figure 1. Chip Scale Atomic Clock. In this prototype, the vapor cell (shown at top) has dimensions of 1x1x2 mm3. (Image courtesy CSAC group at NIST/Boulder)

Figure 2. Principle of wall coating in atomic clocks. Alkali metal atoms in a gas cell provide the clock reference signal. Organic films on the cell walls reduce the depolarization and resonance detuning of the alkali atoms resulting from collisions with the walls.

It is known that alkane chains such as paraffin help to reduce the depolarization, and evaporated coatings of waxy materials have been used to create effective wall coatings in larger gas cells. In this project, we are investigating using self-assembled monolayers (SAMs)4 for molecularly thin wall coatings in miniature clocks. Silane-based SAMs are extremely thin and have proven to be quite robust. Our experiments show that such SAMs are chemically stable in rubidium (Rb) vapor and do reduce the de-polarization of the atoms. We are also exploring using thin films of liquid crystal for wall coatings.

This project is a collaboration with the Chip-Scale Atomic Clock group at NIST/Boulder.

References

[1] “A microfabricated atomic clock,” Svenja Knappe et al. Applied Physics Letters 85, 1460 (2004).
[2]  “The atomic wrist-watch,” Robert Wynands, Nature 429, 509 (2004).
[3] “Chip-scale atomic magnetometer,” Peter D. D. Schwindt et al. Applied Physics Letters 85, 6409 (2004).
[4] “Formation and Structure of Self-Assembled Monolayers,” Abraham Ulman, Chemical Reviews 96, 1533 (1996).

Text and images contributed by Youngwoo Yi.