  Integrated photonic chips have significant potential in telecommunications,
classic computing, quantum systems, and topological photonics. Direct laser
writing offers unique capability for creating three-dimensional photonic
devices in an optical glass chip with quick prototyping. However, existing
laser writing schemes cannot create index-modified structures in glass that
precisely match the laser focal shape while also achieving high scanning speed
and high refractive index contrast. Here, we introduce the theory of a
refractive index modification scheme that combines the advantages of both
traditional non-thermal and thermal regime fabrication methods. We also propose
a model of waveguide formation that was verified through a thorough study on
the effects of phase aberrations on the laser focus. The presented new photonic
chip fabrication scheme uses a novel focal intensity distribution, where pulse
energy is relocated to the bottom of a laser focus by manipulating primary and
higher order spherical aberrations. The technique can produce index
modifications with high scanning speed (20 mm/s or higher), high index contrast
(16 x 10-3), and high precision to fabricate with arbitrary cross-sections.
This method has potential to expand the capabilities of photonic chips in
applications that require small-scale, high precision, or high contrast
refractive index control.
