Seminar by Deepak Rajput
PHYS 605 Advanced Topics: Laser Spectroscopy
July 10, 2007
Center for Laser Applications
University of Tennessee Space Institute
Tullahoma, TN 37388
A set of spectroscopic techniques based on polarization properties of light.
* The output from a monochromatic tunable laser is split into a ?weak? probe beam (with the intensity I1) and a ?stronger? pump beam (with the intensity I2)
* The probe beam passes through a linear polarizer (P1), the sample cell, and a second linear polarizer (P2) which is crossed with P1.
* Without the pump laser the sample is isotropic and the detector D behind P2 receives only a very small signal caused by the residual transmission of the crossed polarizer.
* After having passed through a ?/4 plate which produces a circular polarization, the pump beam travels in the opposite direction through the sample cell.
* When the laser frequency ? is tuned to a molecular transition (J?,M?) (J?,M?), molecules in the lower level (J?,M?) can absorb the pump wave.
* The quantum number M which describes the projection of J onto the direction of light propagation, follows the selection rule ?M = ?1 for the transitions M? M induced by ?+-circularly polarized light (M? M?=M?+1).
Linearly polarized probe wave as a superposition of ?+ and ?- components
* Due to saturation the degenerate M sublevels of the rotational level J? become partially or completely depleted.
* The degree of depletion depends on the pump intensity (I2), the absorption cross section ?(J?,M? -> J?,M?), and on possible relaxation processes which may repopulate the level (J?,M?).
* The cross section ? depends on J?, M?, J?, and M?.
* In the case of P or R transitions ?J=-1 or +1, not all the M sublevels are pumped.
* For example, from levels with M?=+J no P transitions with ?M=+1 are possible while for transitions the levels M?=J? are not populated.
* This implies that the pumping process produces an unequal saturation and with it a nonuniform population of the M sublevels which is equivalent to anisotropic distribution for the orientations of the angular momentum vector J.
* Such an anisotropic sample becomes birefringent for the incident, linearly polarized probe beam.
* Its plane of polarization is slightly rotated after having passed the anisotropic sample. (analogous to Faraday effect)
* For Polarization spectroscopy no magnetic field is needed.
* Contrary to the Faraday effect where all molecules are oriented, here only those molecules which interact with the monochromatic pump wave show this nonisotropic orientation.
* This is the subgroup of molecules with the velocity components:
* For ???0 the probe wave which passes in the opposite direction through the sample interacts with a different group of molecules in the velocity interval , and will therefore not be influenced by the pump.
* If, however, the laser frequency ? coincides with the center frequency ?0 of the molecular transition within its homogenous linewidth ?? (i.e., ?= ?0+ ?? -> Vz = 0??Vz), both waves can be absorbed by the same molecules and the probe wave experiences a birefringence due to nonisotropic M distribution of the absorbing molecules.
* Only in this case will the plane of polarization of the probe wave be slightly rotated by ?? and the detector D will receive a Doppler-free signal every time the laser frequency ? is tuned across the center of a molecular absorption line.
* 7.4, Polarization Spectroscopy, Laser Spectroscopy by Wolfgang Demtr?der