Alexander Lvovsky, Sven Hartmann

This is my first major independent project, resulted in a number of publications.  The main one is here.


In 1995, Brownell et al. [1] have observed time delayed second harmonic (SH) generation. This experiment was performed by two-photon exciting the 6S_1/2 - 6D_3/2 transition in atomic cesium vapor, in the presence of transverse magnetic field. It is known from symmetry considerations that optical SH generation is impossible in gases unless an external field is applied to break the symmetry. In this experiment it was shown that even in the presence of such a field, immediately after the two-photon laser excitation, the quadrupole radiation pattern has a null in the forward (phase matched) direction and no second harmonic is emitted. It takes time before the quadrupole moment, precessing around the field direction, transforms itself in such a way that second harmonic is enabled. The authors of [1] have demonstrated this time delay by measuring the displacement of the SH signal oscilloscope trace with respect to that of an undelayed signal.
 

Although the results obtained in [1] have clearly proven the presence of the delay, they exhibited a number of puzzling features. For example, the magnitude of the delay appeared to be more than a factor of two greater than calculated.  After taking over the lab, I have performed refined measurements and analysis of SH generation in cesium vapor (Fig. 1, 2). Besides having resolved the aforementioned discrepancy (which was a result of eddy currents induced in the metal sample cell when subject to the pulsed magnetic field), we have extended our measurements to the 6S_1/2 - 6D_5/2 transition, analyzed the effect of superfluorescence on the development of SH, measured and explained the magnitude of the SH signal and introduced a more precise technique of delay determination.

1. J. H. Brownell, X. Lu, and S. R. Hartmann, Phys. Rev. Lett. 75, 3657 (1995).