  The formation of an electron-hole plasma during the interaction of intense
femtosecond laser pulses with transparent solids lies at the heart of
femtosecond laser processing. Advanced micro- and nanomachining applications
require improved control over the excitation characteristics. Here, we relate
the emission of low-order harmonics to the strong laser-field-induced plasma
formation. Together with a measurement of the total plasma density we identify
the contribution of two competing ionization mechanisms - strong-field and
electron-impact ionization.
