Sub-Doppler and subnatural narrowing of an absorption line induced by interacting dark resonances in a tripod system

A tripod system consisting of three lower levels g_1-3 and an excited level e interacting with two pump lasers of frequency ω _1,3 with Rabi frequencies was investigated. It was observed that the linewidth of the sharp absorption peak that appeared in the probe absorption spectrum of a degenerate tripod system could be made subnatural and sub-Doppler for copropagating lasers. A strong and narrow absorption feature appeared at the line center in a simple nearly degenerate tripod system interacting with a σ-polarized pump and a π-polarized probe in a small magnetic field. The narrow absorption peak disappeared and the system became transparent at the line center which could thus be used as a magneto-optic switch.