  Optical anisotropy and nonlinearity are two tantalizingly important and
enticing properties of an optical crystal. Combining these two features will
have a miraculous effect. The up conversion can extend solid state laser
sources to the ultraviolet and deep ultraviolet (DUV) ranges through harmonic
generation and for down conversion needed for quantum information technology,
but only a few suitable materials are known as the medium because of the
combination of properties that are required. These include suitable band gaps,
moderate optical anisotropy for phase matching and strong nonlinear optical
(NLO) response. Fluorooxoborates are a new ideal platform for this effect in
DUV. Here we demonstrate that fluorooxoborate is the optimal framework for DUV
NLO material and show that the significance of the incorporation of fluorine in
borates. The NLO performance of fluorooxoborates is strongly improved in terms
of local crystal structure and distribution of electronic states. Importantly,
the role of fluorine is to control the structure, while maintaining high band
gaps but does not directly provide large contributions to birefringence and the
second harmonic generation as the conventional assumptions. This is a
consequence of the microscopic electron distribution and the energy position of
the fluorine states well below the valence band maxima. Based on our
understandings, we constructed two artificial structure and they all behave as
anticipated.
