  For a number of optical applications, it is advantageous to precisely tune
the refractive index of a liquid. Here, we harness a well-established concept
in optics for this purpose. The Kramers-Kronig relation provides physical
connection between the spectral variation of the (real) refractive index and
the absorption coefficient. In particular a sharp spectral variation of the
absorption coefficient gives rise to either an enhancement or reduction of the
refractive index in the spectral vicinity of this variation. By using bright
commodity dyes that fulfil this absorption requirement, we demonstrate the use
of the Kramers-Kronig relation to predictively dial-in refractive index values
in water solutions that are otherwise only attained by toxic specialised
liquids.
