The Physics Behind Blu-ray, HD DVD, and Computer Hard-drives

1. What is the Physics Behind Blue-ray Laser Disc Players, HD DVD and Ultimately Ever Smaller Computer Hard-drives? R. A. Lukaszew Physics and Astronomy Dept.

2. LIGHT • When we talk about lasers, we are also talking about light. • We all know how important light is. • Without light, we couldn't see and plants wouldn't grow.

3. Photons • All light, including laser light, is made up of little packets called photons. • Photons aren't all alike, they can be different colors or contain different amounts of energy.

4. Spectrum • When we talk about light we usually mean visible light.That is the light which people can see and this is the light that lasers use

5. Lasers…? • The process which makes lasers possible, Stimulated Emission, was proposed in 1917 by Albert Einstein. • No one realized the incredible potential of this concept until the 1950's, when practical research was first performed on applying the theory of stimulated emission to making lasers.

6. LASERS • Lasers are a kind of light that is different than light from the sun, or from a light bulb. How is it different? The name has some clues…

7. So, what is a laser diode? • LASER: Light Amplification by Stimulated Emission of Radiation • It consists of a light amplifier (LED) combined with a resonant cavity (a geometrical feature).

8. LEDs • As special kind of materials have the possibility of exciting atoms by application of a voltage so that when the electrons go back to normal they emit light. • These are called light emitting diodes or LEDs. LEDs are a basic component in lasers.

9. Stimulated emission If an electron is already excited and there is an incoming photon with adequate energy it can "stimulate" the production of another photon of the same energy and in phase with the first one. Thus the produced light is coherent

10. Laser CD-DVD • The key device inside a compact- disc or CD-ROM player is a tiny but potent laser, which serves as a sharp optical stylus or tip. It enables the player to “read” information stored on the CD’s surface in the form of tiny pits.

11. The wavelength of the laser light limits the number of pits--and so the amount of data--that can be stored on the disc: the shorter the wavelength (i.e. the different color), the smaller the pits it can read.

14. Current CD players • The lasers in most CD players are made from a particular material (gallium arsenide – GaAs and related semiconductors– compounds) that, once energized, emit light having a wavelength of approximately 820 nanometers (billionths of a meter). • This infrared light can read pits no smaller than about a micron in size, which is roughly one fiftieth the diameter of a human hair.

15. Blue diode lasers • But blue diode lasers--emitting light at a wavelength of 460 nanometers--could do even better, because they could read far smaller pits. • Marked with these smaller pits, an audio CD could store, say, all nine of Beethoven’s symphonies, instead of just one. • Multimedia applications would also stand to benefit enormously.

16. What about DVDs? • A DVD is very similar to a CD, but it has a much larger data capacity. A standard DVD holds about seven times more data than a CD does. • This huge capacity means that a DVD has enough room to store a full-length, MPEG-2-encoded movie, as well as a lot of other information.

17. Blue Laser • Blue lasers are made with a special kind of semiconductor material: GaN • The technology for this is still complicated and therefore the price is high. Price: $1,999.99!!!!

20. HD-DVD • The HD DVD disc is designed to be the successor to the standard DVD format. • It can store about three times as much data as its predecessor. (15 GB per layer instead of 4.7 GB) • The basic physical mechanisms are common to all these devices. The only difference is the architecture of the actual media.

21. How about hard-drives? • Hard drives use a different type of technology that involves magnetic materials. • Magnetic materials exhibit interesting properties that can be exploited for this.

22. FERROMAGNETISM Ferromagnetic materials tend to stay magnetized to some extent even in the absence of an external magnetic field. Thus, bits of information can be encoded in a small region of magnetic material.

23. Modern disk drives use magnetoresistive (MR) or giant magnetoresistive (GMR) heads. Here the bits are written in magnetic media and later read by separate elements in a recording head that senses changes in resistance when on top of a bit. • A writing element writes bits onto the disk and a reading element reads the bits by detecting the presence of their magnetic fields. • Electronic circuits encode data from the computer's processor prior to writing, and decode the bit pattern after reading.

26. Limits of Magnetic Recording The new record density of 20 gigabits!

27. Our research at UT • We are studying important issues in the development of new materials with magnetic properties suitable for nano-fabrication useful in new generation media. • We are also working on new schemes for sensing element to detect the magnetization state of individual bits.