High-velocity dark states in velocity-selective coherent population trapping.

Abstract

We have observed long-lived dark states in velocity-selective coherent population trapping (VSCPT) in a one-dimensional lin-angle-lin configuration with metastable He atoms using the $J=1\ensuremath{\Rightarrow}1$ transition at $\ensuremath{\lambda}=1.083$ \ensuremath{\mu}m. These states are characterized by two peaks in the atomic momentum distribution at $\ifmmode\pm\else\textpm\fi{}Q\ensuremath{\hbar}k(Q=\mathrm{positive}\mathrm{integer})$, where $k$ is the magnitude of the optical wave vector. VSCPT states previously observed are characterized by $Q=1$, but we have observed long-lived states with $Q=2$. Optical Bloch equation calculations show that these high-velocity dark states appear at higher $Q$ for greater light intensity.