Midinfrared Optical Breakdown in Transparent Dielectrics

Optical breakdown measurements for transparent dielectrics are reported for 1 ps laser pulses as a function of mid-IR wavelength from 4.7 to $7.8\mu \mathrm{m}$. For wide-gap dielectrics seed electrons are generated by tunnel ionization with subsequent avalanche ionization and laser absorption by dense plasma. For narrow-gap dielectrics tunnel ionization alone leads to dense plasma formation.