Use of harmonics for femtosecond micromachining in pure dielectrics

A controllable reproducible modification of dielectrics within the bulk of material by focused ultrashort laser pulses provides the opportunity for writing three-dimensional (3D) photonic structures including bit-wise 3D information recording. Here a nonlinear response of material is employed, the multi-photon absorption being the first step. The present article concerns the expedience of the use of harmonics,. first of all, second harmonic, for 3D micromodification of transparent dielectrics. If modification is provided by multi-photon transition to the first electronic excited level either due to subsequent photochemical reaction or material heating, then the reduction of the order of multi-photon transition provided by the second harmonic appears to be effective. If modification is connected with ionization of material and creation of free electrons, i.e., by breakdown, then an advantage of second harmonic is less obvious because impact ionization is more effectively provided by the fundamental frequency. Here a combined effect of both the second harmonic and fundamental frequency beams can be effective. Impact ionization is described by the Fokker-Planck equation. A relation between exponent of avalanche development and impact ionization delay time is discussed. 0(C) 2003 American Institute of Physics. [DOI: 10.1063/1.1536023].