In our Spotlight from a few weeks ago - Nanopyramids - temporary resting places for light - we wrote about things like Q-factors, qubits, stopping light and other fascinating concepts and emerging techniques that could lead to quantum computing. Some of the feedback we received could be summarized with "Huh? Q-what...?" So today we'll take another look at the Quality (Q) factor of photonic-crystal nanocavities and the context it is relevant in. These 'nanocages' for light are currently the focus of much interest of nanotechnology research in photonics because they can strongly confine photons in a tiny space. Just as semiconductor crystals control the flow of electrons (the basis for all electronics), photonic crystals are a unique material used to construct photonic devices and circuits for manipulating light, i.e. photons. A prominent example of a photonic crystal is the naturally occurring gemstone opal. Photons (behaving as waves) propagate through it - or not - depending on their wavelength. Wavelengths of light (stream of photons) that are allowed to travel throught he crystal are known as "modes". Disallowed bands of wavelengths are called photonic band gaps. What's so interesting for researchers is that, once you are able to fully control and manipulate photons, you could not only vastly improve existing applications like optical data storage, high-precision sensing and telecommunications, but develop exotic technologies like quantum computing.
Some 5,000 years ago, the megalomaniac rulers of Egypt built themselves the mothers of all tombstones - pyramids. Pyramids are the largest monuments constructed by mankind (by the way, the largest of them all, the Great Pyramid of Cholula in Mexico, has an estimated volume of 4.45 million cubic meters; which makes it almost one third larger than the Great Pyramid of Giza in Egypt; the Giza pyramid is taller, though). On the other end of the size spectrum, down to earth scientists in Germany have built some of the smallest pyramids - they are only a few hundred nanometers tall. Like their big brothers in the Arabian desert the nanopyramids' purpose is to 'entomb' something. Designed to work as a new class of microcavity optical resonator, these structures rely on internal reflection of light waves from the tilted pyramid facets to achieve strong confinement of light in all three spatial dimensions with low loss. Where the Egyptian pyramids were supposed to be a place of ascendance for the pharaoh buried inside, the German nanopyramids are 'temporary resting places' for light, giving raise to quantum optical phenomena that could provide the basis for future quantum computers.
Ever since Roman glass blowers made the Lycurgus cup, some 2,400 years ago, researchers and engineers have figured out to do all kinds of things with light, be it in optical fiber communications; the use of lasers for welding, cutting and surface modification of materials; photonic gyroscopes in aviation; or optical switches in computing. As a general field of science, photonics defines the knowledge devoted to the generation, transmission, detection, control and handling of light. One sub-domain, nanophotonics deals with the manipulation and emission of light using nanoscale material and devices. One of the sizzling hot topics within nanophotonics is plasmonics, which holds the promise of a class of subwavelength-scale optoelectronic components that could form the building blocks of a chip-based optical device technology that is scaleable to molecular dimensions. Here, we report on the latest achievements of a Spanish-French group of researchers that brings the exiting concept of an optically driven lab-on-a-chip closer.
Integrating biochemical analysis with micro- and nano-oelectromechanical systems (MEMS and NEMS) has led to the development of a new class of biomedical analytical devices called lab-on-a-chip. They combine a number of biological functions (such as enzymatic reactions, antigen-antibody conjugation, and DNA probing) with proper micro- or even nanofluidic laboratory components (such as sample dilution, pumping, mixing, metering, incubation, or separation) and detection in micron- and nanometer-sized channels and reservoirs into one single device. In order to reduce the size of these lab-on-chip devices even more, researchers increasingly are finding ways to turn micron-sized components into nanosized components. One problem they have been faced with so far is the issue of illumination. Today, many state-of-the-art lab-on-a-chip devices use external illumination sources, such as lasers or light emitting diodes (LEDs). Being able to fully integrate the excitation and detection mechanisms on lab-on-a-chip devices would allow further size reductions and increase the flexibility for using and handling them. Researchers at Cornell University have electrospun light-emitting nanofibers that, if they can be integrated with micro- and nanofluidic devices, could achieve excitation of light-induced fluorescence and detection within that same device.
For several years now, researchers have documented the many intriguing mechanical and electrical properties of carbon nanotubes (CNTs). Some researchers have focused on the optical properties of CNTs. Studying the passive optical response of CNTs they have revealed the manner in which CNTs' optical properties are related to shape and structure of CNTs. It was found that periodic CNT arrays exhibit Bragg diffraction, photonic bandgap properties, and plasmonic resonance; nonperiodic CNT arrays interact with light waves similarly to the way in which radio antennae interact with radio waves. In conventional radio antenna theory an antenna acts as a resonator of the external electromagnetic radiation. Scientists now have demonstrated that a single multiwall carbon nanotube (MWCNT) acts as an optical antenna, whose response is fully consistent with conventional radio antenna theory.
Along the way to all-optical devices in communication and information technology, photonic crystals play a significant role. They form a basis material for the future realization of optical components and circuits, and maybe even complex optical circuits or optical computers. Examples include complex waveguides, integrated microcavities, channel drop filters, optical switches and low-threshold lasers. All such devices depend on the inclusion of defect structures, non-linear materials and/or light-emitters into photonic bandgap material. The combination of several devices into one photonic crystal would allow to realize the optical equivalent of an electronic circuit. So far, the intentional inclusion of such combined structures was very difficult to realize in practice, however. A group of German and Italian researchers now present a powerful technique that allows to create such photonic circuits inside photonic crystals by controlled micro-infiltration of liquid substances with sub-micron resolution. This approach forms an enabling technology for the realization of all optical devices and circuits.
Artificial opals are gemstones that are of considerable scientific and technological interest as photonic crystals, as components of light sources, solar cells, and chemical sensors. They are conveniently made from periodic stackings of nanospheres. It would be exciting if one could fabricate optical cavities in these photonic crystals by removing, or adding high dielectric material to a single unit cell in the structure. These optical cavities would localize light that potentially enables the fabrication of high-resolution miniature on-chip sensors, or even qubits for quantum computers. Previously, such controlled modification of the nanostructure of a single colloid in an opal has not been achieved. Now, researchers in The Netherlands developed a method for realizing both single and arrays of material cavities, or defects, in individual colloids on the surface of silicon dioxide artificial opals by a focused ion beam milling technique. This research could ultimately lead to the fabrication of a photon-on-demand light source.
Photonic crystals are attractive optical materials for controlling and manipulating the flow of light. They can be engineered to produce a variety of optical filtering functions. The growing efforts of physicists and materials scientists to fabricate photonic (nano)crystals were motivated mainly by the potential application of these materials in optical computing, the manufacturing of more efficient lasers, and other exciting new phenomena, like those arising from the application of disturbances such as shock waves. The manufacturing of large-area photonic crystals operating in the visible spectrum is still a challenging and expensive task, given present-day laboratory techniques. However, as with so many other materials, nature has already found a solution. Because they are ready made, common in nature, and because they show a very high complexity, biological photonic-crystal structures will be an essential tool for building a useful knowledge of inhomogeneous optical media.