  Biosynthesis of four samples of colloidal suspensions of gold nanoparticles
is achieved using hydroalcoholic extract and three different separated
compounds of the plant Bacopa procumbens. The nonlinear optical properties of
each sample are characterized with the Z-scan technique. In all cases, the
Z-scan curves indicate a negative or self-defocusing response, which is mainly
attributed to thermal effects. Among the four samples, the hydroalcoholic
extract was noted to have the highest nonlinear optical response and was
selected to demonstrate the formation of self-collimated beams (SCBs). This
kind of beams are obtained when a convergent CW laser, with only few tens of
milliwatts of optical power, is introduced into the sample and induces a
negative-lens effect that shifts the focal spot forward. As a result, the
otherwise highly focused beam propagate with little divergence over lengths of
up to 10mm. Moreover, an SCB is capable of controlling and steering a weak
probe beam of a different wavelength, since the probe experiences the lensing
induced by the pump. Noteworthy, the response time of the material was found to
be less than 0.07s, which makes it a plausible candidate for photonic
applications.
