3D observations and control of laser-induced refractive index changes in optical glasses using spatio-temporal adaptive optics
Résumé
Ultrafast laser radiation shows increasing potential for 3D functionalization of optical materials by processing embedded refractive index changes. Consequently photonic functions were demonstrated in various glasses of optical relevance. The mechanisms are based on the nonlinear confinement of laser energy in small volumes and the subsequent thermo-mechanical and structural rearrangement of the dielectric matrix. However, the irradiation outcome depends on the material relaxation paths as well as on the spatio-temporal characteristics of the writing beam. It can result in both positive and negative index variations, the latter being detrimental for waveguiding applications. These facts impose specific limitations to the photoinscription process. Additionally, if at low photon doses isotropic refractive index changes are induced via soft electronic alterations, in more energetic regimes corresponding to thermo-mechanical photoinscription effects, ultrafast laser radiation can generate an intriguing nanoscale spontaneous arrangement, leading to form birefringence and modulated index patterns. The formation dynamics of refractive index structures and the characteristics times of the driving factors can be visualised using time-resolved microscopy techniques. This determines a guideline for intelligently improving the irradiation outcome using tailored irradiation and regulated energy delivery. From a practical view, building up on the above mentioned observations, recently, new beam manipulation concepts were developed which allow a modulation of the energy feedthrough according to the material transient reactions, enabling thus a synergetic interaction between light and matter and, therefore, optimal results. We consequently discuss the possibility of controlling laser-induced physical phenomena employing automated temporal pulse shaping. Examples of adaptive design of refractive index changes in “thermal” glasses will be shown as well as insights into 3D parallel writing techniques for complex structures utilizing wavefront engineering. Moreover, using the birefringence properties and the associated anisotropic light scattering properties characteristic to the ordered nanostructures, polarization sensitive devices were designed and fabricated. The polarization sensitivity allows particular light propagation and confinement properties in three dimensional structures.