Abstract
Control of two basic ionization processes in dielectrics i.e. photo ionization and electron-electron impact ionization on intrinsic time and intensity scales is investigated experimentally and theoretically. Temporally asymmetric femtosecond pulses of identical fluence, spectrum and pulse duration result in different final free electron densities. We found that an asymmetric pulse and its time reversed counterpart address two ionization processes in a different fashion. This results in the observation of different thresholds for surface material modification in sapphire and fused silica. We conclude that control of ionization processes with tailored femtosecond pulses is suitable for robust manipulation of breakdown and thus control of the initial steps of laser processing of high band gap materials.
Highlights
Lasers delivering ultrashort pulses have emerged as a unique tool for processing wide band gap materials for a variety of applications ranging from precision micromachining on and below the wavelength of light to new methods of medical surgery
In this work we present experimental studies with temporally asymmetric pulse shapes, enabling us to control the temporal evolution of the free electron density
We reveal a systematic dependence of the surface damage threshold on the laser pulse shape for different dielectric materials
Summary
Lasers delivering ultrashort pulses have emerged as a unique tool for processing wide band gap materials for a variety of applications ranging from precision micromachining on and below the wavelength of light to new methods of medical surgery (see for example [1, 2, 3, 4] and references therein). Dielectric material, which is transparent for light in the visible and near infrared spectral region, becomes highly absorbing when ultrashort laser pulses of sufficient high intensity are applied This allows material processing as well as implying the risk of damage of optical components. A large number of experiments has been devoted to the study of laser induced damage of dielectrics as experimental evidence for exceeding a certain critical electron density under laser interaction These involve pulse duration measurements [5, 6, 7], double pulse experiments [8, 9, 10] and pulse train experiments [11] all showing a strong dependence of the damage threshold on pulse duration and on pulse separation. We measure the damage fluence for the two temporally asymmetric pulse shapes in the two materials as a function of the pulse duration and compare the observed trends to calculations based on the multiple rate equation approach [18, 19]
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