3 research outputs found
Microscopic Origin of Surface-Enhanced Circular Dichroism
Circular dichroism (CD), the difference
in absorption of two opposite
circularly polarized light sources by chiral molecules, can be significantly
enhanced when molecules are adsorbed on the surface of nanostructures.
We present a theory based on Poynting’s theorem adapted for
chiral media to analyze the surface-enhanced CD of a chiral molecule/nanostructure
coupled system. Our theory clarifies the microscopic origin of surface-enhanced
CD signals by showing that the enhanced CD has two forms, inherent
and induced. The inherent CD is the direct molecular CD that becomes
enhanced due to the strongly localized optical helicity density near
the nanostructure. The induced CD, previously ignored, derives from
asymmetric excitation and absorption of electromagnetic fields inside
the nanostructures surrounded by chiral molecules upon the injection
of two oppositely circularly polarized light sources. Moreover, it
is demonstrated that the induced CD can contribute significantly to
the CD signals measured by surface-enhanced chiroptical spectroscopy
Limitations and Opportunities for Optical Metafluids To Achieve an Unnatural Refractive Index
Optical
metafluids have held a special position among the platforms
of metamaterials, because other than the lithography-based hard approaches,
the soft fluidity-based solution process not only enables their immediate
practical utility but also allows for reconfigurable and adaptable
nanophotonic systems. However, the fundamental limits of the available
effective parameters of optical metafluids are not yet clearly defined.
Of particular interest is the accessible range of the refractive index
under a practically available volume fraction Ï• and the structural
motifs of building blocks. In addition, previously reported theoretical
works are based on an effective medium theory that excludes dipolar
coupling between building blocks. Using these initial approaches,
the interaction between the building blocks at a relatively higher
Ï• was not accurately rationalized. In this work, we advance
an effective medium theory by using the 3D dressed polarizability.
Then, we successfully rationalize the dipolar coupling between each
of the building blocks and systematically exploit the fundamental
limits of optical metafluids in terms of accessible effective parameters.
Also, for the first time, we discuss both the phase transition of
metafluids and uniaxial characteristics of fluidic crystals in terms
of engineering effective parameters. Thereby, the practically available
range of effective parameters from the concept of an optical metafluid
is realistically defined. It is revealed that an unnaturally near-zero
refractive index and an ultrahigh refractive index can be attainable
through optical metafluids. Given the fundamental limits defined by
3D dressed polarizability, a comprehensive perspective of the limits
and merits of optical metafluids is provided
Optically Patternable Metamaterial Below Diffraction Limit
We report an optically patternable
metamaterial (OPM) for ultraviolet
nanolithography below the diffraction limit. The OPM features monolayered
silver nanoislands embedded within a photosensitive polymer by using
spin-coating of an ultrathin polymer, oblique angle deposition, and
solid-state embedment of silver nanoislands. This unique configuration
simultaneously exhibits both negative effective permittivity and high
image contrast in the ultraviolet range, which enables the surface
plasmon excitation for the clear photolithographic definition of minimum
feature size of 70 nm (≲ λ/5) beyond the near-field zone.
This new metamaterial provides a new class of photoresist for ultraviolet
nanolithography below the diffraction limit