Femtosecond index change mechanisms and morphology of SiC crystalline materials

Femtosecond lasers have a unique ability of processing bulk transparent materials for various applications such as micromachining, waveguide manufacturing, and photonic bandgap structures just to name a few. These applications depend on the formation of micron or submicron size features that are known to be index modifications to the bulk substrate [H. Guo, H. Jiang, Y. Fang, C. Peng, H. Yang, Y. Li, Q. Gong, J. Opt. A: Pure Appl. Opt. 6 (2004) 787]. To the best of our knowledge the physical understanding of how these index-modified features are formed is still unknown, but many good theories exist such as Petite et al. [G. Petite, P. Daguzan, S. Guizard, P. Martin, in: IEEE Annual Report Conference on Electrical Insulation and Dielectric Phenomena, vol. 15, IEEE, 1995, pp. 40-44] or Tien et al. [A. Tien, S. Backus, H. Kapteyn, M. Murnane, G. Mourou, Phys. Rev. Lett. 82 (1999) 3883]. In this Letter the question on the physical cause for index changes is investigated by the combined efforts between Wright-Patterson AFB (WPAFB) and the University of Dayton (UD) using numerous imaging equipment such as TEM, AFM, NSOM, Nomarski microscopy, X-ray crystallography, Raman spectroscopy, and even diffraction efficiency experiments. With all the combined imaging equipment this research is able to present valuable data and deduce plausible theories of the physics of the index modification mechanism.