Plasmonic nanolens!

(cartoon depicting virus imaging through a silver nanolens)
Ref: Nature Photonics 2, 438 - 442 (2008).

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!