  Searching for Kitaev quantum spin liquid (QSL) is a fascinating and
challenging problem. Much effort has been devoted to honeycomb lattice
candidates with strong spin-orbit coupling in 5d-electron iridates and
4delectron RuCl3. Recently, theoretical studies suggested that the 3d7 Co-based
honeycomb materials with high spin state S=3/2 and effective orbital angular
momentum L=1 could also be promising candidates of Kitaev QSL. One of the
candidates, BaCo2(AsO4)2, was revisited recently. The long range magnetic order
in BaCo2(AsO4)2 can be suppressed by very weak in-plane magnetic field,
suggesting its proximity to Kitaev QSL. Here we perform time domain terahertz
spectroscopy measurement to study the magnetic excitations on BaCo2(AsO4)2. We
observe different magnon excitations upon increasing external magnetic field.
In particular, the system is easily driven to a field-polarized paramagnetic
phase, after the long range magnetic order is suppressed by a weak field Hc 2.
The spectra beyond Hc2 are dominated by single magnon and two magnon
excitations without showing signature of QSL. We discuss the similarity and
difference of the excitation spectra between BaCo2(AsO4)2 and the widely
studied Kitaev QSL candidate RuCl3.
