  Recently, we proposed a metasurface design for chiral sensing that (i)
results in enhanced chiroptical signals by more than two orders of magnitude
for ultrathin, subwavelength, chiral samples over a uniform and accessible
area, (ii) allows for complete measurements of the total chirality (magnitude
and sign of both its real and imaginary part), and (iii) offers the possibility
for a crucial signal reversal (excitation with reversed polarization) that
enables chirality measurements in an absolute manner, i.e., without the need
for sample removal. Our design is based on the anisotropic response of the
metasurface, rather than the superchirality of the generated near-fields, as in
most contemporary nanophotonic-based chiral sensing approaches. Here, we derive
analytically, and verify numerically, simple formulas that provide insight to
the sensing mechanism and explain how anisotropic metasurfaces, in general,
offer additional degrees of freedom with respect to their isotropic
counterparts. We provide a detailed discussion of the key functionalities and
benefits of our proposed design and we demonstrate practical measurement
schemes for the unambiguous determination of an unknown chirality. Last, we
provide the design principles towards broadband operation - from near-infrared
to near-ultraviolet frequencies - opening the way for highly sensitive
nanoscale chiroptical spectroscopy.
