Biomedical Engineering Reference
In-Depth Information
shown greatly enhanced CD response with a large tunable spectral
window. Depending on their feature sizes, shape and compositions,
andsoon,thoseartificialmoleculescanworkinafrequencydomain
ranging from microwave to optical frequency. The enhanced CD
response and the large tunability are usually originated from the
metalcomponents,whichcarryalargeamountoffreeelectronsthat
reactstronglytoexternalelectromagneticradiation.Thesecollective
excitationsofthefreeelectronsonmetalsurfaces,knownassurface
plasmons (SPs), have been a subject of intense academic interest in
the past decades [2, 3]. By exciting localized SPs, electromagnetic
fieldcanbeconfinedintoasubwavelengthvolumearoundthemetal
structures. The use of SPs enables the scaling down of the artificial
chiral objects into the nanometer scale.
Two categories of chiral structures are intensively investigated.
One is two-dimensional (2D) planar chiral structures that usually
sit on a substrate, and the other is three-dimensional (3D) chiral
structures fabricated by either well-controlled top-down approach
[4] or bottom-up assembling of small metallic nanoparticles (NPs)
[5-7]. A 2D object, for example a spiral, is chiral in the sense
that it cannot be superimposed on its mirror image by in-plane
rotation or translations [8]. Asymmetric light propagation and
optical rotations can be observed in analogues to the Faraday
effect [9, 10]. On the contrary, the chirality of a 3D chiral object,
such as a helix, is apparent. The propagation of right-handed
and left-handed circularly polarized light through such helices is
significantly different. An array of such helices can function as an
ideal polarization-dependent filter [11]. For achiral structures, the
asymmetric transmission of light with different handedness can
also be externally accomplished by taking into account the incident
directionoflight[12].ButsuchkindofCDisnotanintrinsicproperty
of the structures and will vanish when the incident excitation is
averaged over the whole space.
Besides the structural chirality, chiral nanoplasmonics also
concerns the chirality of the local field associated with SPs, with
emphases on the local optical chirality and its influence on a
nearby chiral molecule [13, 14]. For a chiral structure, such as
a spiral, the near-fields of its SP resonances are chiral, which
have been confirmed by both experimental observations [15] and
