  A recently emerging concept for quantum phase discovery is the controlled
gapping of linear band crossings in topological semimetals. For example,
achieving topological superconducting and charge-density-wave (CDW) gapping
could introduce Majorana zero modes and axion electrodynamics, respectively.
Light engineering of correlation gaps in topological materials provides a new
avenue of achieving exotic topological phases inaccessible by conventional
tuning methods such as doping and straining. Here we demonstrate a light
control of correlation gaps and ultrafast phase switchings in a model CDW and
polaron insulator (TaSe$_4$)$_2$I recently predicted to be an axion insulator.
Our ultrafast terahertz photocurrent spectroscopy reveals a two-step,
non-thermal melting of polarons and electronic CDW gap via studying the fluence
dependence of a {\em longitudinal} circular photogalvanic current. The
helicity-dependent photocurrent observed along the propagation of light reveals
continuous ultrafast switchings from the polaronic state, to the CDW (axion)
phase, and finally to a hidden Weyl phase as the pump fluence increases. Other
distinguishing features corroborating with the light-induced switchings
include: mode-selective coupling of coherent phonons to polaron and CDW
modulation, and the emergence of a {\em non-thermal} chiral photocurrent above
pump threshold of CDW-related phonons. The ultrafast chirality control of
correlated topological states revealed here is important to realize axion
electrodynamics and quantum computing.

Strongly Correlated Electrons;Mesoscale and Nanoscale Physics;Materials Science