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reportPhotonics
8.7.4 State of R&D
Waveguides and Photonic Crystals
Waveguides are typically structures in which a material with a high index of refraction is surrounded by a low index cladding material. The structure is therefore able to guide photons along its length.
The wider intention behind work in this area is to integrate optical and electronic components. At present, there is a fundamental tension between optical components, which offer very high data transfer speeds but which are limited in the extend to which they can be scaled down, and electronics components, many of which exist at sub 100nm length scales. One practical reason for the scaling limits of optical waveguides is that bending light typically requires high-radius turns.
With nanotechnology smaller waveguides can be created using photonic crystals. Photonic crystals are three dimensional periodic structures, which can be made to demonstrate total internal refraction, making them suitable for sharp waveguide bends. Photonic crystals admit or reflect specific photons depending on their wavelength and the design of the crystal.
Paul Braun of the University of Illinois at Urbana-Champaign developed photonic crystal waveguides in 2002, using a polymer etching technique to create specific pathways through the crystals.
However, it has not been possible to create a three dimensional photonic crystal that reflects visible light. Michael Bartl at the University of Utah has attempted to solve this by analysing the structure of the scales of a Brazilian beetle, which exhibits these desired optical properties.
A number of groups (e.g. Technical University of Denmark, IMEC) are using electron beam lithography and deep UV lithography and to fabricate nanophotonics structures from silicon. Nanoimprint lithography has also demonstrated some utility in this area.
IBM is also prioritising silicon-based nanophotonics, due in part to the compatibility of the material with existing manufacturing processes. The company has developed silicon waveguides, which, whilst being over 200nm, require nanometre scale tolerances on the waveguide surface.
Modulators are used to convert optical signals or light waves to electronic pulses. External Light Modulators adjust the parameters of a light beam. In December 2007, IBM also released details of a silicon Mach-Zehnder electro-optic modulator. This device was orders of magnitude smaller than previously demonstrated modulators.
Plasmonics
Plasmonics manipulates light with metals. Metals absorb light in contrast to general materials used in optics, such as fibers and plastics. Despite absorption and thus loss of light, use of metals can give benefits you would not get with other materials. The absorption property might also in some cases be useful.
Plasmons are ‘optically induced oscillations of free electrons on the surface of a metal'. They are essentially ‘density waves of electrons', generated when light strikes a metallic nanostructures. The existence and behaviour of plasmons can be controlled by nanoscale engineering of the metal surfaces involved.
Fundamental Research
Fundamental plasmonics research is trying to answer two key questions. The first question is related to how small you can shrink light. Based on the diffraction limit it is believed that we cannot make light smaller than half its wavelength (~500nm). Researchers are with the help of plasmonics trying to figure out how to focus light in a spot that is ~1 nm. The second question is related to the study of the electron movements in the metal particles. The timescale (10^-18 s) of this movements is also interesting as it opens possibilities for e.g. Attosecond-lasers pulses. In addition, research is focusing on the optimal metals to use - aluminium enables plasmons to travel furthest before dissipating. There are also potential issues with the heat generated in the metal. However, this fundamental research is still far from applications.
| Examples from research and ideas for applications |
| Surface plasmon polaritons What is it? A taper is used to concentrate light with a wavelength of 1500 nm to a "hot spot" with a diameter smaller than 100 nm. For what: For example to couple light into metallic nanowire waveguides or biosensors. Example: Haemoglobin measurements measuring the iron content in blood. How: Nanoneedles put nanodrop of blood on sensor, the iron content is measured optically, measuring how light is absorbed. If the drop is nanosize the light needs to be guided. |
| Surface plasmon devices What is it? Electrical source for plasmons. Solving problem: Plasmons are light waves need to be generated. Currently plasmons are generated by expensive lasers, light sources, which is not applicable to most future applications. When: Research is ready. Needs to be commercialised. |
| Surface plasmon cavities What is it? A way to trap light. Solving problem: Contrary to electrons that are stored in USB sticks etc, light cannot be stored. The light is trapped in the ring. For what? In addition to light storage in general, you could combine a Plasmon ring with a LED and make it emit light more efficiently. |
Challenges
Plasmonics itself was discovered a long time ago but it is only recently that research groups have started to apply it to solve current technology needs. Most of the ideas presented in this chapter are still very far from concept and a challenge is to prove that they work. Working with metals is a challenge in itself.
A field of study for electronics purposes considers the use of plasmons as a signal carrying medium. Here the challenge lies in the great attenuation of the signal. Nontheless, on nanoscale an attenuation of millimetres might not be considered a problem.
Metamaterials
In the nanophotonics context, a metamaterial is a novel material with a negative index of refraction. They are typically inhomogeneous composites with embedded nanoparticles, in which the structural elements are much smaller than the wavelength.
A variety of different metamaterial types have been developed. Researchers at Boston University have developed a metamaterial which consists of a 200nm gold layer on a polyimide substrate. At Princeton University, researchers have tuned a material to work in the infrared spectrum. The material consists of alternating layers of indium gallium arsenide and aluminium indium arsenide.
One of the most exciting applications of metamaterials involves the use of their negative refraction properties to produce what are essentially cloaking devices. David Smith of Duke University demonstrated such a device in 2006, cloaking a metal cylinder from microwaves by surrounding it with metal wires and split rings printed onto fibreglass circuit boards.
The challenge in applying this technology to cloak objects from visible light is that a metamaterial would need to be developed which could negatively refract the whole spectrum of visible light. Two papers released in 2008 identified metamaterials that approached negative refraction of the visible light frequency band. Researchers at UC Berkeley used stacked layers of silver and magnesium fluoride, which negatively refracted near-infrared wavelengths. Coming closer to the visible spectrum, another group at UC Berkeley used nanowires grown inside porous aluminium oxide, and demonstrated negative refraction at 660 nanometres.
Another application of metamaterials could be to produce lenses which are able to provide very precise focusing control, adjusted by controlling the composition of the metamaterial. A similar approach could be used to concentrate light, having applications in solar energy.
Near-field scanning optical microscopy
Near Field Scanning Optical Microscopy(NSOM) is, as the name suggests, an optical microscopy technique which is capable of resolving images at less than the wavelength of visible light (possibly down to 50nm).
NSOM devices employ a point light source, which can be obtained by using an optical fibre with a small aperture at one end, or by using an AFM tip with a hole at the point. This light source is then placed very close to the surface which is being analysed, and is scanned over the whole sample. Information about the surface is obtained by i) analysing light which passes through the sample (transmission mode), ii) analysing light that is reflected from the surface, iii) using the probe to collect light rather than to transmit it, and iv) by using the probe tip to both light and collect information about the sample.
The parameter which is measured is typically the intensity of the light, although other information can be gathered such as the refraction index, whether there is excitation of molecules, magnetic properties, and several other factors.
Lasers
A laser emits usually spatially coherent light through a process called stimulated emission. Coherent light means that the source produces light waves that have the same frequencies and identical phase. Most other light sources emit incoherent light. Lasers are capable of emitting light with a narrow wavelength spectrum, i.e. e.g. monochromatic light. There are several different types of lasers. The most commonly known are Gas, Chemical, Solid-state, Fiber-hosted, Photonic crystal and Semiconductor lasers. Semiconductor lasers are most commonly used in Telecommunications and Photonic crystal lasers are lasers based on nano-structures.
Lasers for materials processing
The materials processing laser market is currently worth around 1.5 billion Euros, not including telecommunications diode lasers. Lasers can be divided into three categories:
- Microtype lasers
- Include Microsecond pulses (for e.g. medical technology), Nanosecond pulses (applied to many areas depending on wavelength) and picoseconds pulses (for e.g. frail materials)
- Competes with existing technologies in medical technology, electronics, and semiconductors: production of discs and displays: chemical etching of thin films. Has applications in industries where components so small that traditional material processing is not possible, and enables higher quality processing.
- Macrotype lasers
- Includes e.g. continuous lasers
- Competes with existing (metals) industry: welding, cutting of thick metal, and glading. Also some military applications.
- Growth of market will come when traditional methods are replaced.
- Marking Lasers
- Includes nanotype lasers
- Competes with printing technology, making marking cheaper
- Can be divided either by material type or size of print
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Visits: 777, Published on: April, 29th 2009, 01:21 PM, Last edit: June, 18th 2010, 02:02 PM Size: 10 KByte
Tags: Nanophotonics, ict, Technology Analysis, State of R&D