The Need for Photonic Integration

This wiki is largely drawn from a presentation by Larry Dalton delivered July 21, 2008. The steaming version of the persentation is available here.

One of the things that we all face as scientist is deciding what topics we want to pursue for our research career. Two things are useful one is the professional society which hold special symposia on hot topics such as high temperature super conductivity. An ever better avenue is that funding agencies hold workshops on emerging topics. In 2007 both the NSF and the Defense Science Board which make recommendations for defense spending, held workshops on photonics integration. So this is perceived as a hot research topics.

Types of photonic integration

There are two type of photonic integration. Type one refers to simply taking the photonic devices such as lasers, modulators and optical circuitry and attaching it by conventional means like wires and fibers to electronic technology. Type two is the more exciting aspect ,that is actually photonic and electron function on the same chip.

Particles suitable for telecommunication

In the universe there four types of particles involved in the communication of information.

Electrons—Negatively charged particles and the lightest of the three particles (electrons, protons, and neutrons) that make up atoms. The electron is most easily perturbed by the application of an electric field. If you take an single atom the energy levels are quantized. The highest occupied orbital is bound state so electrons are localized. But in metals, electrons in the highest occupied band conists of electrons that are not tightly bound to any specific atom known as the conduction band. That movement can be used convey information.

Photons—The smallest (quantized) units of light.

Phonons—Quantized vibrations or quantized sound waves however these are short ranged so we will ignore them. We have convert phonons into electrical or optical signals in order to convey them over distance.

Plasmons—Quantized waves in metals excited by light. These are short ranged so we will ignore. They are useful for engineering extremely small scale circuits connected by nanowires carrying plasmons.

Trends for Data in Telecommunications

Signaling with Mance heliograph, 1910

The history of modern telecommunication by wire and wireless dates back to the telephone in which we transduce the phonon vibrations of the human voice into electrical signals that were carried over copper wire. The wireless telecommunication date back to the heliograph which uses mechancial modulation of light. These original devices had very limited bandwidth because you could only run a shutter so fast. Copper wire was ok for the amount of data that a voice stream demands. Telecommunications has evolved to involve "machine-to-machine" telecommunications. This has placed dramatically larger demands on bandwidth– the amount of information we send per unit time. We also want more capability in smaller and smaller devices. Copper has given away to optical fiber. Wireless frequencies has extended to 100 gigahertz carriers, eg gGigabeam inc. You only have to look at the advertisements for bandwidth and integration for consumer devices to release how important these have become as drivers.

Moore’s Law- the History of Integration

The size of features has been decreasing.

Nano-engineering has been around for a long time. For example silicon CMOS technology has built smaller and smaller transistors and placed more transistors on a chip. Moore’s law shows feature size decreasing over the years and has allowed us to place 10 billion transistors on a chip in 2010 with a bandwidth of 30 GHz. This has resulted in a great reduction in cost. You can buy a computer today for about the same cost as 30 years ago but the functionality has exponentially increased. Tools such as reactive ion etching (RIE) and e-beam lithography have enabled nano-engineering.

File:700px-Moore Law diagram (2004).png

The number of transistors on a chip are steadily increasing.

Impact of Information Technology

We take for granted the dramatic impact that these changes have had on how we live. Information technology is one of the three largest component (energy and agriculture are the other two ) and is the fastest growing component of the world economy. One third to one half of the world’s population will be employed in information technology by the year 2010. It will have a greater impact in the next 20 years because of electronic/photonic integration and nano-engineered materials. Moore’s law talks specifically about electronic device integration. But photonics play a major role about everything we know about the universe, for example your eyes are photodetectors. We are not only seeing top down nano-engineering with electronic integration but we are also having “bottom up engineering”– starting with molecules and building up to structures.

Phototonic information / energy processing

Photonics applications

Photons are important. They can be used to;

• Sense- photodetectors are used in cameras and imaging.
• Compute, process- including photonic time domain processing, optical code readers
• Display- The computer display, calculator or digital watch use photons.
• Energize- light energy can be changed into electrical energy in solar cells.
• Actuate- high power laser light can be used to weld and cut, or perform delicate medical procedures
• Transport-Information over the telephone is carried by photons on optical fiber.
• Store- optical memories such as CDs and DVD are created and read by photons.

What is the best information carrier, electrons or photons?

Electrons are fermions. They are strongly interacting with matter. If you want to send them down a copper wire. Conduction electrons are equally well positioned by any metal atom along the wire once we apply an electric field as a driving force to send them in a direction. However they scatter off nuclei and as you increase temperature the vibrational energy of nuclei increases so the scattering becomes worse; the resistivity to the electrons moving down a wire increases with temperature. Electrons also interact strongly with each other, so it very difficult to propagate electrons over long distances through metal wire Photons are bosons. They are weakly interacting particles. They have nearly unlimited bandwidth. The transmission through optical fiber is very low loss, we can send signals around the world by fiber. When you want bandwidth and to transport over “large” distances, use photons; when other factors such as computing are important use electrons.