  Conventional linear optical activity effects are widely used for studying
chiral materials. However, poor contrast and artifacts due to sample anisotropy
limit the applicability of these methods. Here we demonstrate that nonlinear
second-harmonic-generation circular dichroism spectral microscopy can overcome
these limits. In intact collagenous tissues, clear spectral resonance is
observed with sub-micrometer spatial resolution. By performing gradual protein
denaturation studies, we show that the resonant responses are dominantly due to
the molecular chirality.
