  The non-linear response of dielectrics to intense, ultrashort electric fields
has been a sustained topic of interest for decades with one of its most
important applications being femtosecond laser micro/nano-machining. More
recently, renewed interests in strong field physics of solids were raised with
the advent of mid-infrared femtosecond laser pulses, such as high-order
harmonic generation, optical-field-induced currents, etc. All these processes
are underpinned by photoionization (PI), namely the electron transfer from the
valence to the conduction bands, on a time scale too short for phononic motion
to be of relevance. Here, in hexagonal boron nitride, we reveal that the
bandgap can be finely manipulated by femtosecond laser pulses as a function of
field polarization direction with respect to the lattice, in addition to the
field's intensity. It is the modification of bandgap that enables the ultrafast
PI processes to take place in dielectrics. We further demonstrate the validity
of the Keldysh theory in describing PI in dielectrics in the few TW/cm2 regime.
