ISSN 0021-3640, JETP Letters, 2007, Vol. 85, No. 9, pp. 417–421. © Pleiades Publishing, Ltd., 2007. Original Russian Text © S.A. Zibrov, Ya.O. Dudin, A.G. Radnaev, V.V. Vassiliev, V.L. Velichansky, D.V. Brazhnikov, A.V. Taœchenachev, V.I. Yudin, 2007, published in Pis’ma v Zhurnal Éksperimental’noœ i Teoreticheskoœ Fiziki, 2007, Vol. 85, No. 9, pp. 515–519. 417 1. INTRODUCTION The nonlinear interference effects appearing due to the resonant interaction of atoms with light give rise to coherent population trapping (see, e.g., [1]), electro- magnetically induced transparency (EIT) [2], electro- magnetically induced absorption (EIA) [3, 4], etc. These phenomena were applied in nonlinear ultrahigh- resolution spectroscopy [5, 6], in metrology (atomic clocks and masers [7], in magnetometers [8]), and in laser cooling [9]. One of the kinds of the above effects is change in the transmission of the resonant optical radiation of an atomic ensemble when the magnetic sublevels are intersected in the magnetic field alternating near zero (magneto-optical effects in the Hanle configuration [10, 4, 6]). In this work, the effect of the backward optical wave on the EIA resonance observed in the cyclic tran- sition corresponding to the D 2 87 Rb line is investigated in this configuration. The probe and backward wave are formed by the same laser and have the same frequency. The transmission of a cell filled with 87 Rb is detected as a function of the longitudinal magnetic field for various fixed laser frequencies. It is known that both EIT and EIA resonances can be observed in the cell transmission in dependence on the total angular momenta of the lower, F g , and upper, F e , levels. The transmission resonance is observed for the transitions from the excited state whose total angular momentum is lower than or equal to the total angular momentum of the ground state, i.e., F g F e = F g – 1 or F g F e = F g . The absorption resonance is observed when the total angular momentum of the excited state is higher than the total angular momentum of the ground state by one (F g F e = F g + 1). For cells filled with 87 Rb without antirelaxation coating and buffer gas, the absorption contour of the D 2 line is the sum of the Doppler contributions from the cyclic tran- sition F g = 2 F e = 3 and open transitions F g = 2 F e = 2, 1. For a traveling wave, the level inter- section signal is the absorption peak (EIA) when the frequency is tuned to the optical transition, whereas the peak is continuously transformed to a dip (EIT) when the frequency is shifted toward the open transition. In the presence of the backward wave, change in the inter- section signal becomes nonmonotonic: when the laser frequency is tuned to the cross resonance (to the middle of the interval between the hyperfine components F e = 3, 2 of the excited state), the amplitude of the EIA res- onance increases. It seems that the EIA peak in this case must be lower in amplitude than that in the case of the exact tuning of the laser frequency to the cyclic transi- tion owing, first, to the competition between EIA and EIT having different signs and, second, to the optical transfer of some atoms to the nonabsorbing hyperfine Magneto-Optical Resonances in the Field of Counterpropagating Waves S. A. Zibrov a, b , Ya. O. Dudin a, b , A. G. Radnaev a, b , V. V. Vassiliev a , V. L. Velichansky a, b , D. V. Brazhnikov c , A. V. Taœchenachev c, d , and V. I. Yudin c, d a Lebedev Physical Institute, Russian Academy of Sciences, Leninskiœ pr. 53, Moscow, 117924 Russia e-mail: szibrov@yandex.ru b Moscow Engineering Physics Institute (State University), Kashirskoe sh. 31, Moscow, 115409 Russia c Institute of Laser Physics, Siberian Division, Russian Academy of Sciences, pr. Lavrent’eva 13/3, Novosibirsk, 630090 Russia d Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090 Russia Received March 11, 2007 The resonance of the intersection of sublevels in a probe laser field resonant to the cyclic transition correspond- ing to the D 2 87 Rb line has been investigated in a zero magnetic field. The strong effect of an additional laser field acting on the adjacent transition has been revealed. In a cell without the buffer gas and antirelaxation coat- ing, the amplitude of the probe-field absorption resonance can increase by more than an order of magnitude in the presence of a counterpropagating wave. The effect is observed at the laser frequency tuned to the cross res- onance, when the counterpropagating waves simultaneously act on moving atoms at the cyclic and open tran- sitions with the common lower level. The theoretical analysis of the effect of the additional field on the electro- magnetically induced absorption resonance is in agreement with the experimental data. PACS numbers: 42.50.Gy, 42.65.-k DOI: 10.1134/S0021364007090019