Ultrafast Meets Ultrasmall: Where Are the Limits of Ultrafast Waveguide Writing?

Abstract

Femtosecond laser direct writing (FsLDW) of optical waveguides in dielectrics has the potential to enable the direct fabrication of extremely complex three-dimensional networks of optical waveguides in the volume of any transparent dielectric material. Such a capability is unique, suggesting that ultrafast lasers could be the enabling technology required to build the photonic circuitry of the approaching quantum era. However, at the present time, FsLDW has yet to supersede established conventional techniques, such as two-dimensional lithography or ion exchange, which offer intrinsically less flexibility. The main reason for this is that the unique features of FsLDW have tended to hamper its practical implementation and adoption. In the first section of this chapter, we present the mechanisms governing energy deposition during laser writing and review some practical strategies employed to optimize the energy coupling. In the second section, the pervasiveness of thermal processes in the formation of waveguides is discussed. We show the extent to which ultrafast lasers as pure heat sources constitute a mixed blessing but one which can be systematically managed.