Coherent interaction of optical radiation beams with...

Details Coherent interaction of optical radiation beams with... Coherent interaction of optical radiation beams with...

2000-05-03

2002-06-18

Spyrou, Cassandra (Department: 2872)

Optical: systems and elements

Holographic system or element

Having particular recording medium

C359S003000, C359S483010, C365S119000

Reexamination Certificate

active

06407831

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to the coherent interaction of optical radiation beams with ions or molecules in solids, and to the choice of propagation direction and light polarization relative to the crystal symmetry axes of the solid, and more particularly to optimize the optical-electronic interaction effects in materials with generalized crystal symmetry.
DESCRIPTION OF THE RELATED ART
A variety of optical-electronic applications are based on the coherent interaction of optical radiation beams or fields with ion-doped or molecular crystals of various types; these interactions include optical coherent transients, spectral hole burning, and spatial-spectral holography (also called time- and space-domain holography). Devices based on these concepts are used in optical data storage, real-time optical signal processing, quantum computers, and other coherent computers where the coherent interaction of multiple radiation beams is enhanced, enhanced data erasure in coherent computers, and optical data routing and have applications to computers, communications networks, the Internet and other networks, time delays in RADAR, and numerous other applications.
Natural and synthetic optical materials have a wide range of potential crystal lattice symmetries. (A well-known catalog of all crystal space groups is the International Tables for Crystallography, Edited by Theo Hahn, published by the International Union of Crystallography, D. Reidel Publishing Co.) Within these materials, active ions or molecules occupy crystal ‘lattice sites’ that can be cataloged into subsets, with members of each subset having identical surroundings and having similar resonant frequencies for coupling to optical radiation (the members of each subset are said to be ‘crystallographically-equivalent’); each crystallographically-equivalent subset of lattice sites may contain ions or molecules with a finite number of different spatial orientations. The optical transitions of electrons in the ions or molecules can be described by two quantum energy levels and a transition dipole moment. In general, these transition dipole moments have a plurality of different spatial orientations, according to the different orientations of the crystallographically-equivalent sites noted above. Light beams, on the other hand, must have single optical propagation directions and polarization states relative to the crystal, with the consequence that the light polarization will have a plurality of different spatial relationships with otherwise identical ions or molecules.
When resonant coherent interactions occur, the interaction of the optical field and the two-level quantum systems can be characterized by the optical Rabi frequency:
ω
R
=
p
_
·
E
O
_
ℏ
where {overscore (p)} is the electric dipole moment with components p
i
=<1|p
i
|2> and {overscore (E)}
0
is the optical electric field vector. (Similar expressions apply for magnetic dipoles and magnetic optical fields. In extreme situations a power-broadened version of the above equation applies.)
The Rabi frequency is determined not, only by the magnitudes of the transition dipole moment and of the optical field, but also by the projection of one onto the other (vector projection or scalar product). Consequently, when arbitrarily polarized radiation is propagated through such materials, the coherent interaction of the field and the crystalline matter will induce macroscopic polarization oscillations at a plurality of different optical Rabi frequencies.
The presence of multiple optical Rabi frequencies generally reduces the effectiveness of the optical-electronic device due to consequent complex transient material polarization behavior and the optical interference or beating of the associated optical signal amplitudes radiated by the material. Such interference, for example, can in turn limit the optical-electronic system bandwidth and hence the response time and data handling capability in the optical-electronic application. The interference may also reduce the optical diffraction efficiency, i.e., the signal selection or deflection efficiency in such devices as optical data routers for optical communications networks and wavelength-division multiplexing systems.
To avoid the deleterious effects of this multiple frequency interference, while still being able to optimize other system parameters, it is necessary to design a procedure for obtaining ‘single-Rabi-frequency’ behavior in a generalized situation. The small group of materials that have only a single site orientation can readily exhibit single frequency behavior. Here, However, we show that a wider range of materials, including multi-site materials, can exhibit single-Rabi-frequency behavior under the conditions that we have discovered.
The crystalline compound, Y
3
Al
5
O
12
(yttrium aluminum garnet—“YAG”), which has been used by several research groups for device demonstrations, is a particularly complicated optical material, and its behavior serves to illustrate the problems arising from the interference effects described above, and also to illustrate our invention. In past applications, light was propagated along the so-called crystallographic <111> direction of YAG, a propagation direction that does not yield single-Rabi-frequency behavior. There are one hundred sixty (160) ions per unit cell of the YAG lattice (the unit cell is the fundamental building block of the crystal). When rare earth ions are substituted as active ions for yttrium at the dodecahedral lattice sites, there are six (6) crystallographically-equivalent sites, each with a differently-oriented local environment. Hence, there are six different directions for the individual transition dipoles of the active rare earth ions. For an arbitrary optical propagation direction and for arbitrary optical polarization, there will be six different Rabi frequencies.
In principle, one could eliminate the degradation in performance arising from the presence of multiple Rabi frequencies by choosing a different material with an appropriately high symmetry that restricts the sites to a single orientation. In general, though, that high-crystal-symmetry approach to obtaining a single Rabi frequency does not work for device applications, since one must simultaneously optimize many other material properties, including the optical coherence or dephasing time, inhomogeneous optical line broadening, transition probability, persistence of spectral hole burning, dependence of all of these properties on the applied magnetic field, and dependence of all of these properties on temperature. Satisfying all of these demands in a single material is at best a difficult challenge, even when no other restrictions on material selection exist (such as the restriction to a single site orientation). This ‘high-symmetry’ approach has so far proven to be impractical. Conventionally, the optical material had to be chosen from a small subset of available materials, most or all of which do not have a single set of identically-aligned and oriented crystallographically-equivalent dipoles; that represents a sacrifice in potential bandwidth, diffraction efficiency, and performance.
The following example shows the difficulty of the high-symmetry single-site-orientation approach. The choice of a single-site material would make it relatively simple to achieve a single optical Rabi frequency in the transmission of a radiation beam or field, and could thereby increase the effectiveness of the device, but it also significantly reduces the optical transition probability in many cases and even reduces it identically to zero in many cases of interest. It also restricts the choice of materials that may be used to a very small fraction of the totality of optical materials that might be otherwise considered for use in the particular application. Since many interesting optical materials with multiple site orientations have other characteristics that are superior to those of materials with the high crystal symmetry necessary to giv

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