Influence of a.c. magnetic field on the CPT resonance obtained on two level degenerated system in Rb E. Alipieva * , S. Gateva, and E. Taskova Institute of Electronics, Bulg.Acad.Sc., 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria ABSTRACT In this communication we present an investigation of the influence of an alternative magnetic field (a.c.MF) on the Coherent Population Trapping (CPT) resonances obtained in Hanle configuration. A.c.MF applied parallel to the sweeping magnetic field creates side-bands of the CPT resonance 1 . Their position in dependence on the a.c.MF frequency and amplitude were investigated. This phenomenon was studied for the cases of linear and circular polarized laser excitation. Coherent signal due to the hexadecapole momentum created was detected. At high amplitudes of the a.c.MF alignment to orientation conversion was recorded. Keywords: Coherent Population Trapping, Zeeman Optical Pumping 1. INTRODUCTION Coherent population trapping (CPT) resonance arises in a three-level atomic system after interaction with two coherent fields which connect the ground levels with a common upper level 2 . When the energy difference between the ground states is equal to the difference of the coherent fields, the ground states are fixed in an un-absorbing state, which does not interact anymore with the fields. If one of the coherent fields is swept around the resonance frequency, this effect manifests itself as a canceling of the fluorescence when the condition for the resonance is satisfied. In Hanle configuration the CPT resonance is due to the interference between the Zeeman sublevels of the atoms investigated which are connected in coherent superposition through the + and - components of a linearly polarized laser beam. The resonance is detected by sweeping magnetic field around its zero value. The signal is very sensitive to stray magnetic field which moves the energy position of these levels, disturbing in this way the CPT resonance. The influence of dc magnetic field on this resonance in investigated in our previous works 3, 4 . In this work we study the influence of an a.c .magnetic field on the CPT resonance. Our investigations are restricted on the simplest case, when a.c.MF is collinear to the sweeping magnetic field. This case is named also parametric resonance. Recently, the interest to this resonance is increased. Different authors report about influence of r.f. modulation of different parameters on the CPT resonance 5 . It were modulated the frequency or amplitude of the exciting laser beam or the energy of the lower levels in two- frequency CPT Resonance. 2. EXPERIMENTAL The set-up is the usual one for CPT resonances in Hanle configuration and is shown in Figure 1. It consisted of a single-mode diode laser generating 30 mW at = 794.76 nm, Rb cell and system of registration. A polarizer was inserted in front of the Rb cell to achieve better linear polarization. The frequency of the laser was controlled by observing the fluorescence from a second Rb vapor cell. The spectrum of generation was observed by a spectrum analyzer. An uncoated vacuum cell containing natural mixture of Rb isotopes at room temperature was used. The laser beam propagated along the cell’s axis (z-axis). A magnetic field, created by a solenoid, was applied collinearly to the laser beam. In the same direction an a.c.MF with different frequencies and amplitudes was applied. The gas cell and the solenoid were placed in a three-layer -metal magnetic shield. The fluorescence in x-direction was detected by a photodiode. The amplified photo-signal was stored in the PC, which also controlled the magnetic field. The fluorescence intensity of Rb atoms in a direction perpendicular to the laser beam was detected in dependence on the scanned magnetic field. * alipieva@ie.bas.bg 14th International School on Quantum Electronics: Laser Physics and Applications, edited by Peter A. Atanasov, Tanja N. Dreischuh, Sanka V. Gateva, Lubomir M. Kovachev, Proc. of SPIE Vol. 6604, 66040G, (2007) 0277-786X/07/$15 doi: 10.1117/12.726887 Proc. of SPIE Vol. 6604 66040G-1