Assessment of a Rubidium ESFADOF Edge-Filter as ... - tuprints

116 5 Discussion of the Experimental Results
orate this further by taking the different reaction chains, which give rise to and
maintain the laser induced plasma, into consideration.
5.3.2 Laser-Induced Plasma
This section is intended to elaborate the physics behind the laser-induced plasma
as the main limiting factor of the ESFADOF operation in alkali-metal vapors. The
fluorescence spectrum, which emanates from the vapor cell, confirms the plasma
formation and a comparison with the literature allows to pinpoint the involved
reaction chains.
Fig. 5.17a shows a fluorescence spectrum recorded after the collapse of the
ESFADOF spectra and Fig. 5.17b shows a spectrum after decreasing the pump
power, i.e. when the ESFADOF spectra has recovered. They have been recorded
far away from the pumped volume on the opposite side of the vapor cell. The
fluorescence has been captured by an optical fiber and examined by an optical
spectrum analyzer (AvaSpec-1024-2, cf. Fig. 4.4). Examining the fluorescence of
the Rb vapor reveals the following general behavior:
1. Below the ESFADOF breakdown threshold, the fluorescence of the direct
pumped transition as well as energy-pooling processes (cf. Sec. 3.6.2) dominate
the spectrum and the fluorescence emerging from the energy-pooling
process can only be observed within the pumped volume. This fluorescence
results from collisions of two excited Rb atoms, which transfer one atom to the
ground state and the second atom to an even higher level [140], from where
the atom decays spontaneously. Thus, the fluorescence from these levels is
an indicator of energy-pooling. A possible energy defect is provided by the
kinetic energy of the collision partners. As the energy-pooling processes consume
the population of the lower ESFADOF state, a reduction in ESFADOF
transmission is the direct consequence.
2. The fluorescence spectrum of the Rb vapor gains significantly in complexity
and extends over the whole vapor cell, when the plasma formation threshold
has been exceeded. Thus, an important amount of Rb atoms far away from the
pumped volume are excited to high lying states.
3. Reducing the pump power or the spectral overlap between the pump laser
and the D2 absorption line by detuning the pump laser wavelength, recovers
the ESFADOF spectra and limits the energy-pooling processes again to the
pumped volume.
Tab. 5.1 summarizes the observed Rb transition lines. Striking is the fact that
every Rb spectral line within the sensitivity range of the employed spectrometer
can be observed above the plasma threshold and that high lying Rydberg-states as
high as the 11s 1/2 state (line 7) located at 3.957502 eV, just 0.2195 eV below the
Rb ionization limit of E Rb
Ion = 4.1771 eV [199], have been excited [144]. However,
when allowing only energy-pooling processes such highly excited states, as the
11S 1/2 level, can not be populated. A comparison of the measured fluorescence
spectra to those measured by Saha et al. , who give a detailed characterization of