The explosive market growth of consumer electronic devices has made a great demand for the precise processing of sapphire by using ultrafast lasers. Ultrafast lasers can induce nonlinear multiphoton absorption, and thereby leads to the precise and defined microfabrication inside transparent materials with minimum heating effects. The induction of these structural modifications by tightly focused high intensity laser pulses is a versatile technique for the fabrication of three-dimensional photonic devices. Ultrafast lasers are therefore widely used for fundamental research as well as practical applications. This paper presents the laser induced damage mechanisms and absorption phenomena that lead to structural modifications in sapphire. For the given parameters, the electron density growth as a result of plasma generation through multiphoton and avalanche processes is predicted theoretically and is found to be greater than the critical electron density required to induce breakdown in sapphire. Structural modifications, from small change of refractive index to birefringence, cracks and voids are observed when sapphire was irradiated experimentally under the same parameters. Observations revealed the creation of a highly crack region at the focal volume surrounded by local refractive index change which enhances in radial as well as longitudinal direction with increase in the incident laser power and pulse number. Followed by this highly crack region, a birefringent region associated with the nonlinear propagation of laser beam is observed, length of which increases with increasing input power however no significant effect is observed with changing number of pulses. Depending on the input beam powers, this region has length of several hundred-micron and diameter smaller than two microns.
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