Research Interests:
Plasmonics: manipulating the light!
The interaction of light with metallic
nanostructures is a very interesting phenomenon that can be used to
manipulate the light in many different ways, as much as that the
light may even start to behave in a completely unexpected and unpredicted
fashion. Plasmonics is all about controlling and manipulating light
by letting it interact with localized collection of free charge
carriers on the surface of a metallic nanostructure.
Plasmonic nanolens:
Imagining a curved-surfaced transparent
material, such as glass or
plastic, is quite natural when we talk about a lens that can image a
light-emitting object. However, with the discovery of metamaterials, it
has been shown that a slab of metamaterial, which is, in most cases,
an engineered metal structure, can focus the incident light and
hence can act as a lens that can image a light-emitting object. In this
arrangement, the light energy is converted to plasmon (collective free
charge carriers on the surface of a metal) energy, which travels
across the metal and finally converts back into light energy. Thus the
image is formed by the transfer of light energy through plasmons.
Instead of a flat slab of metal where plasmons travel perpendicular to
the radiation, one can also talk about an array of silver nanorods,
where the plasmons travel parallel to the radiation and subwavelength
image is formed through the plasmons. While theses structures are
capable of subwavelength imaging, they suffer through some major
restrictions. One of the important restrictions is that they can only
work with one particular wavelength, usually in the microwave region.
The other major shortcoming is related to plasmon propagation loss and
other losses, which prevent the image to be transferred to a long
distance for practical usage. Moreover, the size of the image remains
the same as the object (subwavelength range), and hence it is undetectable
in the far-field.
In order to overcome these limitations, we
have proposed a nanolens that is made of stacked array of silver
nanorods, which are arranged at tapered angles. The stacked arrangement
of silver nanorod arrays with precise dimensions provides the plasmonic
transfer of light energy at extremely low loss, making it possible to
have a long distance image transfer. Particularly, unlike many other
plasmonic devices, the near-field component of the source object in this
model is plasmonically transferred through and across the stacked
nanorods without long propagation of plasmons, because the plasmons
resonate within individual unit rods of length 50 nm. Therefore, the
propagation related losses are completely suppressed. At the same time,
this arrangement broadens the resonance bands to such an extent that a
large region of visible frequency resonates with the local plasmons,
providing the possibility of color imaging. Most importantly, the
tapered arrangement of nanorod arrays provides sufficient magnification
of the image, so that the image of a nanostructure can be detected in
the far field though usual optics and detectors, such as microscopes
and CCD cameras. This technique has the potential to be an
indispensable imaging tool, in particular, for bio-medical
applications, where individual viruses and other nano-entities of
different colors could be simultaneously imaged in far-field with usual
microscopes and detectors. The cartoon above portrays a color virus
being imaged by this plasmonic nanolens!
