Applied research into the candle would have never led to the lightbulb. As a species, our fascination with light has led to many amazing discoveries and inventions that have fundamentally changed how we live our lives. Many industries have been disrupted by optics: communications, where nearly all of our data is in the form of light (Internet, wireless networking, etc.); and lighting, where we’ve gone from candle, to incandescent, to LEDs in no time at all.

Yet this is only the start. The beauty of light is that it’s fundamentally fast, massless, and we can manipulate its properties. Developments in light-based technologies can be essentially summarized as increasing our control over light, but at smaller and smaller length scales. To put things into perspective, the first Ruby laser was about the size of a dustbin. They’re now about the size of a grain of sand.

The squeezing of light down to incredibly small scales is attractive in many areas. In particular, computation: Researchers believe the barriers presented with developing electronics to keep up with Moore’s Law, in the form of bandwidth and energy dissipation, may be overcome using light-based technologies. Recently however, we’ve begun to approach length scales comparable to the fundamental size of the wavelength of light.

One would naturally assume we could just keep going, but diffraction puts a limit on this. In classical optics, Abbe’s diffraction limit is a barrier whereby the manipulation of light at a scale less than the wavelength of light doesn’t work. Nanophotonics is the field in which researchers overcome this limit, and aim to control light at scales sub-wavelength. In electronics, the wavelength of the fundamental carrier (the electron) is a few nanometers, yet with light, the visible photon is many hundreds of nanometers. By trying to shrink the photon to scales fast approaching that of the electron, many amazing fundamental breakthroughs and spectacularly novel devices have been made. This is also expected to continue: In a recent review, scientists have identified some exciting key developments in this field.

The first is photonic crystals (Fig – right), whereby periodic features the size of the wavelength of light produce a photonic bandgap analogous to that of an energy-bandgap in a semiconductor. These structures can be used for waveguides in 2D on-chip all-optical integrated circuits, yet these are still fairly large. Going smaller, where metals behave a little differently than bulk, we enter the field of plasmonics. Because metals have a negative dielectric constant at optical frequencies (which makes them shiny), if we have an interface of an insulator-metal, light can propagate in the form of localized surface waves called plasmon polaritons (a hybrid wave of both a photon and electron charge oscillation). The effective wavelength here is smaller than the incident wavelength of light, thus even smaller circuit dimensions can be achieved, and researchers can couple light into tinier devices.

Now, if the insulator-metal structure is made to be a finite structure, e.g. nanoparticle or similar, the plasmons have an extra degree of confinement and form a resonating mod, where the most fundamental mode is dipolar – analogous to an antenna – hence an antenna for light. What this means is that by fabricating sub-wavelength metallic structures, and changing their geometry, we can control the directivity of far-field radiation in the optical regime, opening a completely new regime for light-matter interaction for devices. For example, think of radio antenna arrays, but for light you can see.

When we go sub-wavelength, we can create entirely new optical materials, which exhibit unusual optical responses. In effect, materials are created whereby artificial atoms (small metallic nanostructures) are created to interact with light in a specific way such that the bulk response resembles that of a material with new properties. These are called metamaterials, and require feature sizes many times smaller than the optical wavelength.

Metamaterials have been shown to produce some interesting features, such as negative refraction, non-linear optical responses, and perfect absorption. Thanks to the fabrication difficulty of 3D nanostructures, researchers tend to design 2D structures (or metasurfaces) in which bulk properties can be approximated from a thin layer of structured metal. Sharp, disruptive, sub-wavelength phase shifts can be created in order to completely control the propagation of light in entirely new ways. For smaller structures where we begin to approach sizes of the atomic unit cell, quantum effects begin to dominate. This regime may enable ultrafast quantum circuit integration with single photons, however fabrication issues come into play.

So nanophotonics applications are vast, but it is unknown what kinds of future devices may be created. Thus far, some key results have shown that imaging far beyond the diffraction limit is possible with nanophotonic devices: LED emission can be strongly enhanced through periodic arrays of metallic nanoparticles, solar cell efficiencies can be increased significantly through similar structures, and plasmon array lasers with tunable wavelengths are possible.

It is difficult to envisage how nanophotonics will develop over the coming years, thanks to the unpredictability of research. However, with the goal to have ultimate control over all properties of light and hybridization with electrons, phonons, and electron spins, it is easy to see how many fields will be affected by future nanophotonics developments, from energy harvesting to light emission and communications.

Research: http://www.sciencemag.org/content/348/6234/516.full.pdf?sid=b05394c7-14a5-4012-a0ab-e390bde22e2f – “Nanophotonics: Shrinking light-based technology”