Nanophotonics or nano-optics is the study of the behavior of light on the nanometer scale, and of the interaction of nanometer-scale objects with light. It is a branch of optics, optical engineering, electrical engineering, and nanotechnology. It often involves dielectric structures such as nanoantennas, or metallic components, which can transport and focus light via surface plasmon polaritons. The term "nano-optics", just like the term "optics", usually refers to situations involving ultraviolet, visible, and near-infrared light (free-space wavelengths from 300 to 1200 nanometers).
Background Normal optical components, like lenses and microscopes, generally cannot normally focus light to nanometer (deep subwavelength) scales, because of the diffraction limit (Rayleigh criterion). Nevertheless, it is possible to squeeze light into a nanometer scale using other techniques like, for example, surface plasmons, localized surface plasmons around nanoscale metal objects, and the nanoscale apertures and nanoscale sharp tips used in near-field scanning optical microscopy (SNOM or NSOM) and photoassisted scanning tunnelling microscopy.
Application Nanophotonics researchers pursue a very wide variety of goals, in fields ranging from biochemistry to electrical engineering to carbon-free energy. A few of these goals are summarized below.
Optoelectronics and microelectronics If light can be squeezed into a small volume, it can be absorbed and detected by a small detector. Small photodetectors tend to have a variety of desirable properties including low noise, high speed, and low voltage and power. Small lasers have various desirable properties for optical communication including low threshold current (which helps power efficiency) and fast modulation (which means more data transmission). Very small lasers require subwavelength optical cavities. An example is spasers, the surface plasmon version of lasers. Integrated circuits are made using photolithography, i.e. exposure to light. In order to make very small transistors, the light needs to be focused into extremely sharp images. Using various techniques such as immersion lithography and phase-shifting photomasks, it has indeed been possible to make images much finer than the wavelength—for example, drawing 30 nm lines using 193 nm light. Plasmonic techniques have also been proposed for this application. Heat-assisted magnetic recording is a nanophotonic approach to increasing the amount of data that a magnetic disk drive can store. It requires a laser to heat a tiny, subwavelength area of the magnetic material before writing data. The magnetic write-head would have metal optical components to concentrate light at the right location. Miniaturization in optoelectronics, for example the miniaturization of transistors in integrated circuits, has improved their speed and cost. However, optoelectronic circuits can only be miniaturized if the optical components are shrunk along with the electronic components. This is relevant for on-chip optical communication (i.e. passing information from one part of a microchip to another by sending light through optical waveguides, instead of changing the voltage on a wire).
Solar cells Solar cells often work best when the light is absorbed very close to the surface, both because electrons near the surface have a better chance of being collected, and because the device can be made thinner, which reduces cost. Researchers have investigated a variety of nanophotonic techniques to intensify light in the optimal locations within a solar cell.
Controlled release of anti-cancer therapeutics Nanophotonics has also been implicated in aiding the controlled and on-demand release of anti-cancer therapeutics like adriamycin from nanoporous optical antennas to target triple-negative breast cancer and mitigate exocytosis anti-cancer drug resistance mechanisms and therefore circumvent toxicity to normal systemic tissues and cells.
Spectroscopy Using nanophotonics to create high peak intensities: If a given amount of light energy is squeezed into a smaller and smaller volume ("hot-spot"), the intensity in the hot-spot gets larger and larger. This is especially helpful in nonlinear optics; an example is surface-enhanced Raman scattering. It also allows sensitive spectroscopy measurements of even single molecules located in the hot-spot, unlike traditional spectroscopy methods which take an average over millions or billions of molecules.
Microscopy One goal of nanophotonics is to construct a so-called "superlens", which would use metamaterials (see below) or other techniques to create images that are more accurate than the diffraction limit (deep subwavelength). In 1995, John M. Guerra demonstrated this by imaging a silicon grating having 50 nm lines and spaces with illumination having 650 nm wavelength in air. This was accomplished by coupling a transparent phase grating having 50 nm lines and spaces (metamaterial) with an immersion microscope objective (superlens). Near-field scanning optical microscope (NSOM or SNOM) is a quite different nanophotonic technique that accomplishes the same goal of taking images with resolution far smaller than the wavelength. It involves raster-scanning a very sharp tip or very small aperture over the surface to be imaged. Near-field microscopy refers more generally to any technique using the near-field (see below) to achieve nanoscale, subwavelength resolution. In 1987, John M. Guerra (while at the Polaroid Corporation) achieved this with a non-scanning whole-field Photon tunneling microscope. In another example, dual-polarization interferometry has picometer resolution in the vertical plane above the waveguide surface.
Optical data storage Nanophotonics in the form of subwavelength near-field optical structures, either separate from the recording media, or integrated into the recording media, were first used by John M. Guerra and his team to achieve optical recording densities much higher than the diffraction limit allows. This work began in the 1980s at Polaroid Optical Engineering (Cambridge, Massachusetts), and continued under license at Calimetrics (Bedford, Massachusetts) with support from the NIST Advanced Technology Program.
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