  Circular dichroism (CD) induced by spin angular momentum of light is vital to
investigate the chirality of microscopic objects such as molecules, proteins
and metamaterials. However, orbital angular momentum (OAM) of light failed to
directly implement an interaction with chiral molecules, which remains poorly
understood. Even with the enhancement of unique plasmonic nanoparticle
aggregates, the helical dichroism (HD) is still at a small level of ~0.6%.
Here, we experimentally report the direct observation of a giant HD at the
level of ~120% by matching the scales of chiral microstructures and OAM beams.
Our results reveal a strong interaction between OAM beams and chiral
microstructures in terms of their distinct reflectance, resulting in
sign-opposite HD values to distinguish their chirality. Such unique HD
phenomenon is investigated experimentally under various diameters and helical
pitches of chiral structures, showing the robust performance. This HD technique
can widely extend the responding areas of chiral spectroscopy in biology,
material science and nanotechnology.
