Plastic and nonmetallic article shaping or treating: processes – Direct application of electrical or wave energy to work – Producing or treating inorganic material – not as pigments,...
Reexamination Certificate
2002-01-15
2004-10-05
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Direct application of electrical or wave energy to work
Producing or treating inorganic material, not as pigments,...
C264S435000
Reexamination Certificate
active
06800238
ABSTRACT:
FILED OF THE INVENTION
The invention relates to ferroelectric materials. More specifically, the invention relates to ferroelectric materials with patterned domain structures.
BACKGROUND OF INVENTION
Nonlinear materials are used in a variety of technologies including data storage, display and communications technologies. Nonlinear materials and their effects with interacting electromagnetic radiation is well documented. Nonlinear materials are used as harmonic generators. Most commonly, nonlinear materials are used to generated the second harmonic emission light wave &lgr;
e
of an interacting light source with a fundamental wavelength &lgr;
i
.
FIG. 1
, for example, shows a single pass second harmonic generator construction
100
. A solid state infrared laser
101
emits light with a fundamental wavelength
107
. The light wave
107
is focused with a confocal lense
103
on a crystal
104
that is formed from a nonlinear material. The emission second harmonic wavelength
109
is half of the fundamental wavelength
107
; equivocally the second harmonic output frequency is twice that of the fundamental input frequency. The nonlinear crystal
104
needs to be transparent to incident light with a wavelength
107
so that the light wave
107
can propagate through the crystal
104
. Further, the crystal
104
needs to be transparent the to second harmonic light with a wavelength
109
so that the second harmonic light wave
109
is emitted from the crystal
104
.
There are several factors that lead to inefficient conversion of the fundamental wave length
107
to the second harmonic wavelength
109
. Specifically, low nonlinear coefficient of crystal material, defects in the crystal structure, low transparency of the nonlinear material, and other geometric considerations of the crystal can all lead to inefficient conversion of the fundamental wavelength
107
to the second harmonic wavelength
109
. A crystal structure that is made from a material with a small nonlinear coefficient can in theory be compensated for by increasing the crystal pass length L. In practice, however, local defects and variations in refractive index throughout the crystal
104
begin to diminish any benefits gained from extending the crystal path length.
Even when the crystal
104
is formed from a material that exhibits a large nonlinear coefficient, the actual observed conversion efficiency of the fundamental wavelength
107
to its corresponding harmonic wavelength
109
is typically low. This is because light with a wavelength
107
and
109
exhibits different indices of refraction within the crystal
104
. Hence, the fundamental wavelength
107
and the harmonic wavelength
109
have different phase velocities as they propagate through the crystal
104
. Consequently, as the second harmonic wave
109
is locally generated in one portion of the crystal, it will be out of phase with the fundamental wavelength
107
and with the second harmonic wave
109
that is locally generated in a later part of the crystal
104
resulting in destructive interference and low output of the second harmonic light. To help overcome this problem, nonlinear materials are modified. Nonlinear materials are modified either so that the phase velocities of &lgr;
e
and &lgr;
i
are matched, a method referred to a bifringent phase matching, or alternatively the nonlinear materials are modified such that the sign of the nonlinear coefficient is periodically modulated by a distance corresponding to the coherence length of the light, a method referred to a quasi-phase matching (QPM) and described in an early work by J. A. Armstrong, N. Bloembergen, J. Ducuing and P. S. Pershan in “Interaction Between Light Waves in a Nonlinear Dielectric,” Phys. Rev., 127, 1918, 1962.
QPM is a method which compensates for the differences in the phase velocity between the fundamental wavelength of the interacting light source and the corresponding harmonic wavelength within the nonlinear crystal. In quasi-phase matching, the fundamental wave and the harmonic wave still have different phase velocities, but they are shifted &pgr; out of phase relative to one another over the coherence length. The coherence length is used to refer to the distance over which two traveling waves slip out of phase by &pgr; radians. The sign of the non-linear coefficient is reversed once every coherence length (or odd multiples of coherence lengths) causing a locally generated harmonic field within the nonlinear structure to transfer power to the harmonic beam. By compensating for the phase velocity mismatch between the fundamental wave and the harmonic wave in this way, all the elements of the crystal nonlinear tensor can be accessed throughout the entire transparency range of the crystal. This invention is directed to improved materials and methods for making quasi-phase matching structures preferably for use in non-linear optics.
SUMMARY OF THE INVENTION
The invention provides a method for domain patterning of nonlinear ferroelectric materials. The method is particularly useful for domain patterning of ferroelectric structures which exhibit low coercive fields and which exhibit charging with small changes in temperature. The method seeks to reduce the formation of random micro-domains that typically result during thermal cycling of ferroelectric materials and which lead to patterning defects and reduced efficiencies. According to the preferred method of the invention, a ferroelectric structure is provided with conductive layers on the top surface and the bottom surface of the structure which correspond to surfaces that are normal to the crystallographic polarization axis or z-polarization vectors. The conductive layer is a conductive polymer, a metal layer or a layer of conductive polymer composition. Preferably, the conductive layers are formed from a mixture of polyaniline salt, n-Methyl pyrrolidone and Isopropanol, available under the name of ORMECON™ D-1000 manufactured by Ormecon Chemie GmbH & Co. KG, Ferdinand-Harten-Str. 7, D-22949, Ammersbek, Germany.
A mask is provided over a patterning surface of the structure. For simplicity, the patterning surface is referred to herein as the top surface of the structure. The mask preferably substantially replicates the intended domain pattern. Portions of the conductive layer on the top surface of the structure are removed in accordance with the pattern of the mask, thus leaving a conductive domain template on the top surface of the structure. Subsequently, a sufficient bias voltage is applied to the conductive domain template and the conductive layer on the bottom surface of the structure, thereby producing a domain patterned ferroelectric structure. The conductive layer, the mask and the conductive domain template are then preferably removed from the structure. The resulting domain patterned ferroelectric structure is then relatively stable against charging effects due to temperature variations. A final protective conductive coating may be applied to provide additional long-term stability of the domain pattern.
The mask is preferably provided by lithographic techniques by using lithographic materials. Accordingly, a portion of the conductive layer on the top surface of the ferroelectric structure is coated with a photo-resist such by any suitable method. After the photo-resist is coated on the top conductive layer, the photo-resist is thermal cycled in accordance with the manufacturer's recommendations. The photo-resist is then exposed according to a predetermined pattern with a suitable light source and developed to form the mask.
During thermal cycling of the photo-resist, charging on the surfaces of the ferroelectric typically occurs leading to electron emission and random domain formation during cooling. In order to mitigate the charging of the structure during thermal cycling of photo-resist, it is preferable that the conductive layers on the top surface and the bottom surface are placed in electrical communication prior to-thermal cycling, thus reducing the charging. The top and bottom conductive layers are
Fiorilla Christopher A.
Okamoto & Benedicto LLP
Silicon Light Machines Inc.
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