Saturation Spectroscopy and Related Techniques

The half-width of the signal increases with pump beam intensity according to 

A particularly simple form of saturation spectroscopy is obtained if a sample that is sufficiently dense to completely block a probe beam is used. Only at the line centre does the bleached path induced by a strong counter-propagating pump beam allow the probe beam to emerge from the cell and hit the detector. A simple set-up and a schematic curve are shown in Fig. 9.43 for this type of high-contrast transmission spectroscopy [9.161]. 

In normal saturation spectroscopy the detector essentially looks straight into the laser and a substantial absorption is needed in order to observe a signal. This means that only strong spectral lines, originating in 

In Figs.9.45 a so-called cross-over resonance is shown. This is an inherent phenonomenon in saturation spectroscopy and occurs when two lower or upper state sublevels have transitions to a common level in the other state. The two oppositely propagating laser beams can then interact at frequencies half-way between the normal resonances. Then atoms moving with a certain velocity along the laser beams are utilized [9.170,171]. 

An experimental set-up similar to the one used in polarization spectroscopy is employed in certain parity-violation experiments. A small optical rotation is induced by interference between neutral weak and electromagnetic interactions in atoms [9.172-174]. 

In normal saturation spectroscopy the detector essentially looks straight into the laser and a substantial absorption is needed in order to observe a signal. This means that only strong spectral lines, originating in well-populated states, can be investigated by this method. In cases where less favourable conditions prevail, the so-called polarization spectroscopy method can instead be used [9.348]. The experimental set-up is shown in Fig. 9.57. 

The set-up is similar to the one used in normal saturation spectroscopy but the probe beam is now blocked by crossed polarizers that are placed at opposite sides of the absorption ceU. The polarizer in front of the cell is used to increase the linear polarization of the laser beam, which is frequently already well polarized. A matched pair of polarizers (frequently Gian-Thompson prisms. Fig. 6.46) can have a rejection ratio of 10 to 10 in the crossed position. The pump beam is circularly polarized and induces an anisotropy 

A fascinating aspect of precision spectroscopy is that the constancy of the natural constants will ultimately be challenged  

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