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What is the Physics Behind
Blue-ray Laser Disc Players,
HD DVD and Ultimately
Ever Smaller Computer Harddrives?
R. A. Lukaszew
Physics and Astronomy Dept.
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.
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.
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
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.
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…
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).
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.
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
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.
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.
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.
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.
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.
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 !!!!
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.
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.
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.
•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.
Limits of Magnetic Recording
The new record
density of 20
gigabits!
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.