Comparison of 87Rb N-resonances for D1 and D2 transitions
  We report an experimental comparison of three-photon-absorption resonances (known as "N-resonances") for the D_1 and D_2 optical transitions of thermal 87Rb vapor. We find that the D_2 N-resonance has better contrast, a broader linewidth, and a more symmetric lineshape than the D_1 N-resonance. Taken together, these factors imply superior performance for frequency standards operating on alkali D_2 N-resonances, in contrast to coherent population trapping (CPT) resonances for which the D_2 transition provides poorer frequency standard performance than the D_1 transition. 
Irina Novikova (1), David F. Phillips (1), Alexander S. Zibrov (1 and   2), Ronald L. Walsworth (1), Alexksei V. Taichenache...