Nanoplasmonics research focuses on the optical phenomena in the nanoscale vicinity of metal surfaces. In nanoplasmonics, researchers focus nanoscale light below the diffraction limit (which typically is half the width of the wavelength of light being used to view the specimen) by converting free photons into localized charge-density oscillations – so-called surface plasmons – on noble-metal nanostructures, which serve as nanoscale analogs of radio antennas and are typically designed by using antenna theory concepts.
Nanoplasmonic effects have been exploited throughout history in creating artful artifacts such as the Lycurgus Cup, which was created in ancient Rome some 1600 years ago. They are also visible in the vibrant hues of the great medieval stained-glass windows in European cathedrals and castles.
More recently, plasmon resonances have been used by engineers to develop new, light-activated cancer treatments and to enhance light absorption in photovoltaics and photocatalysis.
Nanoplasmonics in photovoltaics
Sunlight is composed of many wavelengths of light. In a traditional solar panel, silicon atoms are struck by sunlight and the atoms' outermost electrons absorb energy from some of these wavelengths of sunlight, causing the electrons to get excited. Once the excited electrons absorb enough energy to jump free from the silicon atoms, they can flow independently through the material to produce electricity. This is called the photovoltaic effect – a phenomenon that takes place in a solar panel's photovoltaic cells.
Although silicon-based photovoltaic cells can absorb light wavelengths that fall in the visible spectrum – light that is visible to the human eye – longer wavelengths such as infrared light pass through the silicon. These wavelengths of light pass right through the silicon and never get converted to electricity – and in the case of infrared, they are normally lost as unwanted heat.
Nanoplasmonic techniques that rely on nanostructured metal surfaces could harvest more of the sun's energy.
The use of plasmonic black metals – nanostructured metals are designed to have low reflectivity and high absorption of visible and infrared light – could someday provide a pathway to more efficient photovoltaics (PV) to improve solar energy harvesting.
Conventional pigments produce colors by selectively absorbing light of different wavelengths – for example, red ink appears red because it absorbs strongly in the blue and green spectral regions. The reliance on dyes and chemicals for colorants is fraught with problems such as the fading of dyes due to chemical reactions; the need for different dyes for different colors; and the effect of dye-related chemical waste on the environment.
A similar effect can be realized at a much smaller scale by using arrays of metallic nanostructures, since light of certain wavelengths excites collective oscillations of free electrons, known as plasmon resonances, in such structures.
Unlike color pigments which can be overlaid to generate new secondary colors, discrete metal nanostructures rely on size, shape and relative positioning to generate new colors. For instance, aluminum nanostructures can be engineered to produce a broad range of colors, thus making it suitable for ultra-high resolution color printing previously demonstrated in silver and gold. Furthermore, the process of color mixing in nanometallic structures results in 'purer' colors than the basic color palette (read more: "Photorealistic plasmonic printing with aluminum nanostructures").
Reproduction of Monet’s Impression, Sunrise using the expanded palette by color toning and mixing strategies. (A) Original input image adapted with permission from Musee Marmottan Monet, Paris, France / Giraudon / Bridgeman Images. (B) Reproduction using only the limited palette of “primary plasmonic colors” falls short of the original image. (C) Realistic reproduction of the artwork using an expanded palette of colors, allowing for the subtle variations in tone and color in the original to the replicated. (D) Higher magnification image of dotted box in (C) highlighting the brush strokes that are resolved in the plasmonic painting. (E) Tilted SEM image of pixels showing how the colors observed in the plasmonic painting are the manifestation of a predefined structural layout. (Reprinted with permission by American Chemical Society)
With the potential to revolutionize infection diagnostics, researchers have developed nanoplasmonic sensors capable of detecting live viruses. These label-free optofluidic-nanoplasmonic biosensor have been demonstrated to directly detect live viruses from biological media at medically relevant concentrations with little to no sample preparation.
And, approaching the nanotechnology holy grail in label-free cancer marker detection – single molecules – researchers have demonstrated the label-free detection of a single protein using a nanoplasmonic-photonic hybrid microcavity.