Page 458 - Introduction to Information Optics

8.3. Optical Storage Media 443
switched by the absorption of light of /.j and A 2 . In contrast to photochromies,
the stimulated emission of / 3 from ET material is utilized as output instead of
the absorption of the reading light of A 2 . An example of an ET material is
SrS:Eu,Sm. The mechanism of light emission is as follows: Both the ground and
excited states of each impurity (Eu and Sm) exist within the bandgap of the
host material (SrS). Blue writing light (A { — 488 nm) excites an electron in the
2 +
ground state of Eu into its excited state. Some of the electrons at this higher
2+ 3+
energy level of the Eu tunnel to the Sm ions, where they remain trapped
2+
until stimulated by an infrared reading light of A 2, The Sm ions so formed
are thermally stable deep traps of about 1.1 to 1.2 eV. Such a material has to
be heated to about 250°C before the electrons are freed. Upon stimulation with
an infrared reading light (/t 2 = 1064 nm), trapped electrons then tunnel back
2+
to the Eu ions, resulting in the characteristic Eu emission {/ 3 = 615 nm)
when the electrons return to the ground state.
In addition to ET materials [19] that were originally developed as infrared
light sensors, an ET material using Eu-doped potassium chloride (KCl:Eu) has
also been reported [20]. The specimen of KCl:Eu was first irradiated with
ultraviolet light at about 240 nm. When the specimen was stimulated with
visible light of 560 nm, intense emission with a peak at about 420 nm was
observed.

8.3.9. TWO-PHOTON-ABSORPTION MATERIALS

The significance of two-photon absorption in optical storage has been known
for some time [21]. However, recent application of two-photon-absorption
materials to 3-D optical storage is usually associated with the work of
Parthenopoulos and Rentzepis [22]. Two-photon-absorption materials work
in a very similar way to ET materials. They require a write beam to record the
data, and a read beam to stimulate the excited parts of the material to radiate
luminescence. However, instead of only one photon, two-photon-absorption
materials require two photons for the transition from the ground state to the
stable excited state, and also another pair of photons to stimulate the excited
state to return to the ground state. These two photons could have the same
wavelength, although photons with different wavelengths are preferred. The
two-photon-absorption phenomenon is generally known as photon gating,
which means using a photon to control the behavior of another photon [23].
An example of a two-photon-absorption material is spirobenzopyran [22].
The mechanism of two-photon absorption is as follows: The molecule is
initially at the SI state. By simultaneously absorbing a photon at 532 nm and
another photon at 1064 nm, the molecule is first excited to the S2 state and
then stays at the stable S3 state. Upon absorption of two photons at the
reading wavelength of 1064 nm, the molecule is excited to the S4 state, which