A strongly-coupled $\Lambda$-type micromechanical system
  We study a classical $\Lambda$-type three-level system based on three high-$Q$ micromechanical beam resonators embedded in a gradient electric field. By modulating the strength of the field at the difference frequency between adjacent beam modes, we realize strong dynamic two-mode coupling, via the dielectric force. Driving adjacent pairs simultaneously, we observe the formation of a purely mechanical 'dark' state and an all-phononic analog of coherent population trapping --- signatures of strong three-mode coupling. The $\Lambda$-type micromechanical system is a natural extention of previously demonstrated 'two-level' micromechanical systems and offers new perspectives on the architecture of all-phononic micromechanical circuits and arrays. 
Hajime Okamoto, Ryan Schilling, Hendrik Sch\"utz, Vivishek Sudhir,   Dalziel J. Wilson, Hiroshi Yamaguchi, Tobias J. Kippenberg
cond-mat.mes-hall