Non-absorption resonances in a J=1 to J=1 atomic system in Samarium with resonant radiation at 686nm.

This project investigates the interaction of a J=1 to J=1 atomic system (see Figure 1) in Samarium with incident resonant radiation at 686nm. Specifically this system is monitored with increasing laser exposure times to evaluate how the atom-laser interaction changes.

When the two ground state Zeeman sub-levels (m=-1,m=+1) of this atomic configuration are simultaneously excited to the single excited state level (m=0), then optical pumping can produce a coherent superposition of states. The resonant radiation creates an atomic coherence so that the atom's evolution is exactly out of phase with the incoming radiation causing non-absorption. The net result is that as the magnetic field approaches zero, the atom is `trapped' in a coherent superposition of states and is unable to absorb less and less resonant radiation until a minimum absorption occurs when the magnetic field is zero. In a three-state Λ system, this phenonmenon is known as `coherent population trapping' or a `dark-line resonance', and was first observed in 1976 by Alzetta et al. See Figure 2.

In the experiment (see Figure 3), a linearly polarized laser at 686nm excites atoms from the level 4f⁶ 6s^{2 7} F₁ to a excited level 4f⁶ 6s6p ⁹F₁ via the process of optical pumping. The resonant radiation is supplied by an external cavity diode laser; manipulation of the external cavity allows the wavelength to be tuned to the transition. When an external magnetic field is applied, a level-crossing fluorescence intensity profile can be recorded. A non-absorption resonance is an obvious feature of these level-crossing profiles.

Level-crossing profiles are recorded with different exposure lengths to resonant radiation. As non-absorption resonances are a second-order effect, these resonances are used as a sensitive indicator of how the J=1 to J=1 atomic system evolves with increasing resonant radiation.

Figure 3: Vacuum system and optical table where interaction takes place between the laser radiation and Samarium atoms.

References

Non-absorption resonances by nonlinear coherent effects in a three level system.
G. Orriols.
Il nuovo cimento, 53B:1, 1979.

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