  Zirconium fluoride (ZBLAN) glass, the standard material used in fiber-based
mid-infrared photonics, has been re-designed to enable the fabrication of high
index-contrast low-loss waveguides via femtosecond laser direct writing. We
demonstrate that in contrast to pure ZBLAN, a positive index change of close to
10$^{-2}$ can be induced in hybrid zirconium/hafnium (Z/HBLAN) glasses during
ultrafast laser inscription and show that this can be explained by an electron
cloud distortion effect that is driven by the existence of two glass formers
with contrasting polarizability. High numerical aperture (NA) type-I waveguides
that support a well confined 3.1 $\mu$m wavelength mode with a mode-field
diameter (MFD) as small as 12 $\mu$m have successfully been fabricated. These
findings open the door for the fabrication of mid-infrared integrated photonic
devices that can readily be pigtailed to existing ZBLAN fibers.
