Αλληλεπίδραση βραχέων παλμών laser με διαφανή διηλεκτρικά

The present work deals with the interaction of femtosecond laser pulses at near IR with transparent dielectric materials. Such laser pulses when powerfully focused in the bulk of a transparent material can cause permanent change in the refraction index at the point of the focus of the beam without any sign of deformity in its surface. We demonstrate the fabrication of embedded strongly birefringent binary computer generated holograms in the internal of transparent materials with the use of fs laser pulses. We show for the first time that it is possible to manufacture such diffractive elements with the utilisation of ultrafast laser and concretely fs laser pulses. During the fabrication process each CGH pixel is represented by a local anisotropic variation of the refraction index while the birefringence axes are controlled by the polarization of the recording beam. The birefringent diffractive structures are sensitive to the polarization of the reading beam, thus during reconstruction the diffracted beams are elliptically polarized with their major axis rotated compared to the undiffracted beam. This results to an increase of the signal to noise ratio during reconstruction since it enables the selective strong attenuation of the undiffracted beam, without the need of any spatial filtering, when the CGH is reconstructed between crossed polarizers. Substantially, the pre-mentioned holograms present a selectivity in the entering angle of polarization and hence different behaviour for orthogonal linear polarizations. Such holographic elements manufactured in the internal of isotropic transparent materials show new impressive possibilities as they are proved to be useful in many photonic applications and optical systems. The use of ultrafast laser pulses is proved to be interesting and practical as the main difference to the method of lithography is that the foremost has the freedom of fabricating structures so in the outer as in the interior of materials while the latter deals with the fabrication of structures only on their surface