We propose a novel method enabling to create a high-contrast dark resonance in the 87Rb vapor D2-line. The method is based on an optical pumping of atoms into the working states by a two-frequency, linearly-polarized laser radiation propagating perpendicularly to the probe field. This new scheme is compared to the traditional scheme involving the circularly-polarized probe beam only, and significant improvement of the dark resonance parameters is found. Qualitative considerations are confirmed by numerical calculations.
Nonlinear magneto-optical resonances on the hyperfine transitions belonging to the
D2 line of rubidium were changed from bright to dark resonances by changing the laser power density of the single exciting laser field or by changing the vapor temperature in the cell. In one set of experiments atoms were excited by linearly polarized light from an extended cavity diode laser with polarization vector perpendicular to the light's propagation direction and magnetic field, and laser-induced fluorescence was observed along the direction of the magnetic field, which was scanned. A low-contrast bright resonance was observed at low laser power densities when the laser was tuned to the ${F}_{g}=2\ensuremath{\rightarrow}{F}_{e}=3$ transition of
87Rb and near to the ${F}_{g}=3\ensuremath{\rightarrow}{F}_{e}=4$ transition of
85Rb. The bright resonance became dark as the laser power density was increased above 0.6 mW/cm
2 or 0.8 mW/cm
2, respectively. When the ${F}_{g}=2\ensuremath{\rightarrow}{F}_{e}=3$ transition of
87Rb was excited with circularly polarized light in a second set of experiments, a bright resonance was observed, which became dark when the temperature was increased to around 50${}^{\phantom{\rule{0.16em}{0ex}}\ensuremath{\circ}}$C. The experimental observations at room temperature could be reproduced with good agreement by calculations based on a theoretical model, although the theoretical model was not able to describe measurements at elevated temperatures, where reabsorption was thought to play a decisive role. The model was derived from the optical Bloch equations and included all nearby hyperfine components, averaging over the Doppler profile, mixing of magnetic sublevels in the external magnetic field, and a treatment of the coherence properties of the exciting radiation field.
Abstract Magnetically induced Fg = 2 → Fe = 4 transitions corresponding to the 85Rb D2 line in the case of the σ+ circularly polarized radiation have been used for the first time to form optical dark resonances in strong magnetic fields up to 1 kG under electromagnetically induced transparency conditions. A 1.5-μm-thick cell filled with Rb atomic vapor has been used. The probabilities of two of five magnetically induced transitions, which are efficiently formed only with the σ+-polarized radiation, in the magnetic field range of 0.2–1 kG exceed the probabilities of “ordinary” atomic transitions. For this reason, it is appropriate to use them in Λ systems for the formation of the dark resonance. The following rule has been established: for the formation of the dark resonance in a Λ system with the use of the σ+-polarized probe radiation at the frequencies of magnetically induced transitions, the radiation of the coupling laser should also be σ+ polarized; the dark resonance is not formed if the radiation of the coupling laser is σ– polarized, which is confirmed by the calculated theoretical curve. A significant advantage of magnetically induced resonances for electromagnetically induced transparency compared to ordinary atomic transitions corresponding to the 85Rb D2 line has been demonstrated. The formation of dark resonances in strong magnetic fields, when their frequency is shifted by several gigahertzs, can be applied in practice.
Abstract We present the results of spectroscopic and polarization studies of dilute rubidium vapor exposed to a single-frequency linearly polarized diode laser radiation in a spectral range of atomic D2 line. We report the origin of a circularly polarized radiation on V-type transitions of 87Rb F g = 2 → F e = 3 and 85Rb F g = 3 → F e = 4, and amplification of this radiation in backward direction caused by a partial population inversion among magnetic sublevels of the ground and excited levels. This is confirmed experimentally by high directivity of backward radiation, absence in its spectrum of 85Rb F g = 2 → F e = 1 (Λ-type) radiation, as well as by different nature of intensity dependences of backward and fluorescence radiations.
We analyze perspectives for creation of a novel, stable 87Rb-based frequency standard. Although the stability ~10-14 is challenging, achieving this goal looks ultimately possible. We considers three different all-optical repumping schemes aimed to improve the contrast of the dark resonance in thermal 87Rb vapor: excitation by a σ+ -polarized bichromatic laser radiation, and σ+ - σ- bithromatic co-propagating fields, and a novel excitation scheme with transversal laser pumping. Qualitative considerations are confirmed by numerical calculations.