ABSTRACT
An object is chiral when it is not identical to its mirror image. For
electromagnetic waves, circularly polarized plane waves are chiral
in the sense that each “enantiomer” carries an opposite spin angular
momentum. The interactions of chiral objects with left-or right-
handed chiral electromagnetic waves are different, giving rise to
the circular dichroism (CD) that can serve as a measurement of the
chirality of the object. For most natural molecules or biomolecules,
CD is typically about 10−2 (electronic) or 10−4 ∼ 10−6 (vibrational) compared with their parent signals [1]. Although the CD signals
of most natural molecules occur in the ultraviolet (UV) range,
rationally designed chiral “meta-atoms” and “meta-molecules” have
shown greatly enhanced CD response with a large tunable spectral
window. Depending on their feature sizes, shape and compositions,
and so on, those artificial molecules can work in a frequency domain
ranging from microwave to optical frequency. The enhanced CD
response and the large tunability are usually originated from the
metal components, which carry a large amount of free electrons that
react strongly to external electromagnetic radiation. These collective
excitations of the free electrons on metal surfaces, known as surface
plasmons (SPs), have been a subject of intense academic interest in
the past decades [2, 3]. By exciting localized SPs, electromagnetic
field can be confined into a subwavelength volume around the metal
structures. The use of SPs enables the scaling down of the artificial
chiral objects into the nanometer scale.
