  We provide the first demonstration of nonlinear self-trapping of light in
solutions of food coloring dyes, leading to dirt cheap soliton waveguides. The
optical self-focusing nonlinearity observed in such solutions is attributed to
the optical absorption force acting on dye molecules, as the thermal effect at
relatively higher power results in a self-defocusing nonlinearity. We found the
nonlinear response increases when the laser wavelength is switched to near the
peak absorption for all food dyes tested. Furthermore, the absorption-dependent
nonlinearity can be tuned by making purposive samples of different food dyes.
In particular, with appropriately mixing of dye solutions, nonlinear response
arises for otherwise inactive wavelengths, leading to interaction between two
beams of different wavelengths and formation of a coupled soliton-like pair. At
milliwatt low power levels, self-trapping achieved at one wavelength in
food-dye solutions can serve as low-cost waveguides for optical beams at other
wavelengths.
