Radiant energy – Inspection of solids or liquids by charged particles
Reexamination Certificate
2001-11-30
2003-11-25
Lee, John R. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
C250S307000, C359S326000, C359S328000
Reexamination Certificate
active
06653630
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to ferroelectric materials, and specifically to a system for producing structured domains in the materials.
BACKGROUND OF THE INVENTION
FIG. 1
is a schematic diagram of two types of collinear ferroelectric, as are known in the art. A ferroelectric is a material which, by virtue of the material's underlying crystal structure, is able to maintain an electric polarization, or dipole moment, in the absence of an electric field. The ferroelectric may be in the form of a mono-domain
20
, wherein the ferroelectric has one polarization direction, or a multi-domain
22
, wherein the crystal has many domain regions, each domain having a different direction of polarization. Mono-domain ferroelectrics are well known as materials exhibiting useful properties such as piezoelectricity and electro-optical qualities. While irregular multi-domain ferroelectrics are not considered as particularly useful, multi-domain ferroelectrics where the multi-domains have a definite structure, termed domain engineered structures (DESs), have been found to have extremely useful properties.
The polarization of a single domain in collinear ferroelectrics may be in one of two directions, at 180° to each other. When a single domain is formed, the polarization of the domain will form spontaneously in one of the directions. The initial direction may be influenced during formation of the domain, for instance in a process termed poling wherein an electric field is applied as the ferroelectric material forms. Once formed, the polarization of the domain may be altered by further poling applications of the electric field. Typically, in the absence of an electric field, a multi-domain ferroelectric forms with the domains randomly oriented, giving an overall polarization close to or equal to zero, since this is the most stable energy state of the multi-domain.
FIG. 2
is a hysteresis curve for a multi-domain ferroelectric, plotting polarization P vs. electric field E, as is known in the art. As the electric field strength is increased, the domains of the ferroelectric start to align in a positive direction giving rise to a rapid increase in the overall polarization (OB). At very high field levels, the polarization reaches a saturation value (P
sat
), where all the multi-domains are substantially aligned in the positive direction. As the external field is reduced, the polarization reduces as some of the domains change alignment, but the polarization does not fall to zero when the external field is removed.
At zero external field, the domains remain aligned in the positive direction, hence the ferroelectric will show a remanent polarization P
r
, The ferroelectric cannot be completely depolarized until a field of magnitude OF is applied in the negative direction. The external field needed to reduce the polarization to zero is termed the coercive field strength E
c
. If the field is increased to a more negative value, the direction of polarization reverses, and if the field is increased sufficiently in the negative direction, the ferroelectric again reaches saturation. The value of the spontaneous polarization P
s
(OS) is obtained by extrapolating the saturation curve onto the polarization axes. P
s
is the polarization that the multi-domain ferroelectric would have, in the absence of an external field, if all the domains were aligned.
FIG. 3
is a schematic diagram of domain engineered structures and graphs of their properties, as are known in the art. DES
24
comprises two head-to-head domains formed in a rectangular plate of LiNbO
3
with dimensions (x, y, z). A graph
26
shows the acoustic impedance vs. frequency for acoustic vibrations of DES
24
. A graph
28
gives the impedance response for acoustic vibrations of a mono-domain crystal
25
of LiNbO
3
having the same dimensions as DES
24
, showing resonances dependent on the values of x, y and z. It is seen that these resonances are absent in DES
24
. Conversely, two “bending” resonances are present in DES
24
which are not present in the mono-domain crystal.
DES
30
has a linear periodic structure where alternating domains have opposite polarizations. Structures such as DES
30
allow, for example, second harmonic generation and optical parametric oscillation for electromagnetic waves incident on the structure, because of the non-linear properties of the alternating domains of the DES. More detailed descriptions of properties and methods of production of structures such as DES
24
and DES
30
are given in an article entitled “Ferroelectric Domain Engineering for Quasi-Phase-Matched Nonlinear Optical Devices” by Rosenman, Skliar, and Arie, published in
Ferroelectrics
1, N4, pp 1-64 (1998), which is incorporated herein by reference. Methods detailed therein, and others known in the art for producing domain engineered structures, are summarized below.
Domain engineered structures may be formed by altering the doping of a crystal during its growth. In an article entitled “Enhancement of second-harmonic generation in LiNbO3 crystals with periodic laminar ferroelectric domains” by Feng et al., published in
Applied Physics Letters
37, pg 607 (1980), which is incorporated herein by reference, the authors describe a method for varying the spontaneous polarization of growing LiNbO
3
crystals by changing the doping of the crystals. The doping was changed by periodically altering the temperature of the growing crystal, which in turn altered the concentration of yttrium which was used to dope the crystals. The variation in yttrium concentration caused a periodic reversal of the polarization of the crystals, the reversal appearing through the bulk of the crystals.
Diffusion at relatively high temperatures can be used to form DESs. For example, in an article entitled “Balanced phase matching in segmented KTiOPO
4
waveguides” by Bierlein et al., published in
Applied Physics Letters
56, pg 1725 (1990), which is incorporated herein by reference, the authors describe polarization in KTiOPO
4
(KTP) crystals. By immersing the KTP crystals in molten RbNO
3
/Ba(NO
3
)
2
, at a temperature of about 350° C. for approximately 1 hour, Rb
+
ions exchanged with K
+
ions of the KTP. The presence of the Ba
2+
ions caused polarization inversion of domains at the surface of the KTP. It will be appreciated that diffusion induced DESs are substantially surface structures.
Electron beam writing may be used to form DESs. For example, in an article entitled “Fabrication of Domain Reversed Gratings FOR SHG in LiNbO3 by Electron Beam Bombardment” by Keys et al., published in
Electronics Letters
26, pg 188 (1990), which is incorporated herein by reference, the authors describe domain polarization reversal on the negative face of a LiNbO
3
crystal. It will be appreciated since the electron beams penetrate no more than some microns in depth, DESs produced by electron beam writing must be of this order of thickness.
FIG. 4
is a schematic diagram of a poling system for fabrication of DESs, as is known in the art. A mono-domain ferroelectric
40
has a periodic dielectric photo-resist
42
applied to an upper surface of the ferroelectric. A first conductor
44
is overlaid on photo-resist
42
and the upper surface, and a second conductor
46
is applied to a lower surface of the ferroelectric. A high-voltage pulse is applied between the two electrodes. The pulse reverses the polarization of the ferroelectric in regions where the first conductor contacts the ferroelectric. This technique, and similar ones using liquid electrodes, have been used to form periodic DESs having thicknesses in a range of 0.5-3 mm, and with periods between 3.4 and 39 microns.
Scanning force microscopy (SFM) is a method known in the art for imaging surfaces, and also for modification of domain structures of thin films of ferroelectrics. A review of SFM is provided in an article entitled “Nanoscale Scanning Force Imaging of Polarization Phenomenon in Ferroelectric Thin Films” by Auciello et al., published in The Annual Review
Rosenman Gil
Rosenwaks Yossi
Urenski Ronen-Pavel
Bazerman & Drangel PC
Lee John R.
Leybourne James J
Ramot - University Authority for Applied Research & Industrial D
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