This paper is currently in preparation for submission
(anticipated Nov. 2011)
Preliminary results can be found in our recent SPIE Proceedings:
D. R. Carlson, John Mongelli, E. M. Wright and R. J. Jones, "Numerical
simulations of high intensity pulse trains and plasma dynamics in passive
femtosecond enhancement cavities", Proc. SPIE 8132, 813205 (2011).
Time-resolved measurements of ionization dynamics using the nonreciprocal resonance of pump-probe pulse trains in a femtosecond enhancement cavity
Abstract: We demonstrate a new technique for making time-resolved precision measurements of optical nonlinearities. Pump-probe intracavity phase spectroscopy utilizes a femtosecond enhancement cavity to translate small nonlinear phase changes into a frequency shift of the resonant cavity. The nonlinear response of a sample induced by a strong pump pulse is probed by a weak counter-propagating pulse at various delays. The measured shift of the cavity resonance is recorded to precisely measure the nonlinear phase shift. We demonstrate this new approach by measuring the decay of the plasma formed inside the fsEC due to the ionization of a xenon gas target by the pump pulse.
Preliminary Results:
Experimental schematic: A time-delayed probe pulse is used to probe the shift in cavity resonance induced by the plasma generated in a xenon gas target from a strong pump pulse.
Nonlinear cavity resonance: Under these conditions, the cavity resonance measured for the probe pulse will be diffreent than that of the pump pulse as a function of the delay time between pump and probe arriving at the position of the gas jet. The figure below shows the measured intracavity signal for each as the cavity length is scanned. In the abscence of the gas target, the line profiles would be identical.
Precision measurement of resonance shift and time dynamics: For a more precise measurement of the cavity shift, we use a modulation technique to obtain an error signal, providing a voltage proportional to the shift of the cavity resonance. By modulating the amplitude of the pump pulse (~85 kHz), the resulting modulation of the plasma density can be measured as a shift of the cavity resonance frequency (or phase) of the probe as a function of delay. The data below shows this modulation of the probe cavity resonance at 2 different delays. Note that in the abscence of the gas target (ie no ionization) the modulation of the pump pulse has no affect on the probe pulse (not shown).
The difference in the modulation amplitude for the two time delays gives us information on the plasma lifetime. Clearly, in this case, the plasma still remains 20 nsec after the pump pulse. This corresponds to the round trip time of the pulse in the cavity, which means there will be a residual plasma remaining in the cavity that does not fully decay.
Future work can examine more carefully the evolution of the on-axis plasma density for comparison with models taking into account both the spatial evolution and recombination mechanisms of the plasma. This approach can be used to study a large class of ultrafast optical nonlinearities as well.