  Ultrafast laser excitation provides a means to transiently realize long-range
ordered electronic states of matter that are hidden in thermal equilibrium.
Recently, this approach has unveiled a variety of thermally inaccessible
ordered states in strongly correlated materials, including charge density wave,
ferroelectric, magnetic, and intertwined charge-orbital ordered states.
However, more exotic hidden states exhibiting higher multipolar ordering remain
elusive owing to the challenge of directly manipulating and detecting them with
light. Here we demonstrate a method to induce a dynamical transition from a
thermally allowed to a thermally forbidden spin-orbit entangled quadrupolar
ordered state in Ca$_2$RuO$_4$ by coherently exciting a phonon that is strongly
coupled to the order parameter. Combining probe photon energy-resolved coherent
phonon spectroscopy measurements with model Hamiltonian calculations, we show
that the dynamical transition is manifested through anomalies in the
temperature, pump excitation fluence, and probe photon energy dependence of the
strongly coupled phonon. With this procedure, we introduce a general pathway to
uncover hidden multipolar ordered states and to control their re-orientation on
ultrashort timescales.
