Research On Polarization-dependent Ablation Of Crystals After Femtosecond Laser Irradiation

Femtosecond laser, as a new type of micro-nano machining method and explorationvehicle, the nonlinear and non-equilibrium processing of interaction with ablated materialis being gradually revealed. Laser parameters, such as pulse duration, peak power,wavelength, have proven to have an enormous impact on the laser-matter interactionprocessing. But until recently, the effect of laser polarization direction on laser-matterinteraction processing is gradually realized. In this work, a new phenomenon that theablation area with the angle between laser polarization direction and crystal axis of sampleexhibits periodic oscillation, is observed and further studied.Major contributions and innovations in this thesis are summarized as followed:(1) In femtosecond laser pulse processing in ZnSe crystal, with the polarization angleincreasing, the ablation area exhibit π/2-periodic oscillation (polarization-dependentablation for short). A similar experiment phenomenon can be observed in GaAs, MgO andLiF with cubic crystal, which illustrates that the polarization-dependent ablation a universalphenomenon in cubic crystals.(2) Polarization-dependent ablation is just observed in certain fluence ranges, and withthe increase of laser fluence, the polarization-dependent ablation weakens dramatically, andtends to disappear. Furthermore, the materials’ bandgap can regulate and control the fluenceranging of appearing the phenomenon. While the polarization dependence is sustained in awide range of laser fluences for a narrow band-gap crystal, it is strongly suppressed with aslight augmentation of laser fluence in a wide band-gap material.(3) The polarization-dependent ablation in cubic crystals is explained by theorientation-dependent photonionization due to reduced-electron mass, while is independentof impact ionization. Keldysh’s photonionization theory proved that small difference inreduced electron mass can result in a contrasted rate in nonlinear absorption and found thatpolarization-dependent ablation occurs in a mixed regime between tunneling andmultiphoton ionization. The interplay between photoionization and avalanche ionization isdiscussed to interpret the influence of laser fluence on polarization-dependent ablation. (4) for the cubic crystal ZnSe, it was found that the π/2-period modulation in ablationsize was accompanied by an interesting cross-grating structure within the ablation crater,with a near-wavelength ripple (periodicity:~700nm) parallel to the polarization in theupper part of the crater and a deep subwavelength ripple (periodicity:~150nm)perpendicular to the polarization in the lower part. Through a series of experimentalresearch，a new thought of various mechanisms synergy effect is proposed to tentativelyexplain the formation of cross-grating structure. The near-wavelength ripple is priorgenerated by the incident light field and the surface scattered wave, and then, secondharmonic generation will induce the formation of the deep subwavelength ripple.Theoretical simulation based on Drude model indicates that the LIPSS period caused bysecond harmonic generation can be changed from~150nm to~400, and preliminaryverifies the possible of formation mechanism of cross-grating structure.