  In modern optics, materials with large birefringence ({\Delta}n, where n is
the refractive index) are sought after for polarization control (e.g. in wave
plates, polarizing beam splitters, etc.), nonlinear optics and quantum optics
(e.g. for phase matching and production of entangled photons),
micromanipulation, and as a platform for unconventional light-matter coupling,
such as Dyakonov-like surface polaritons and hyperbolic phonon polaritons.
Layered "van der Waals" materials, with strong intra-layer bonding and weak
inter-layer bonding, can feature some of the largest optical anisotropy;
however, their use in most optical systems is limited because their optic axis
is out of the plane of the layers and the layers are weakly attached, making
the anisotropy hard to access. Here, we demonstrate that a bulk crystal with
subtle periodic modulations in its structure -- Sr9/8TiS3 -- is transparent and
positive-uniaxial, with extraordinary index n_e = 4.5 and ordinary index n_o =
2.4 in the mid- to far-infrared. The excess Sr, compared to stoichiometric
SrTiS3, results in the formation of TiS6 trigonal-prismatic units that break
the infinite chains of face-shared TiS6 octahedra in SrTiS3 into periodic
blocks of five TiS6 octahedral units. The additional electrons introduced by
the excess Sr subsequently occupy the TiS6 octahedral blocks to form highly
oriented and polarizable electron clouds, which selectively boost the
extraordinary index n_e and result in record birefringence ({\Delta}n > 2.1
with low loss). The connection between subtle structural modulations and large
changes in refractive index suggests new categories of anisotropic materials
and also tunable optical materials with large refractive-index modulation and
low optical losses.
