U.S. patent application number 11/973682 was filed with the patent office on 2008-04-24 for periodically poled optical crystals and process for the production thereof.
Invention is credited to Henry G. III Giesber.
Application Number | 20080095509 11/973682 |
Document ID | / |
Family ID | 35943212 |
Filed Date | 2008-04-24 |
United States Patent
Application |
20080095509 |
Kind Code |
A1 |
Giesber; Henry G. III |
April 24, 2008 |
Periodically poled optical crystals and process for the production
thereof
Abstract
Periodically poled crystals and a hydrothermal growth method for
making such are disclosed. Electronically periodically poled
crystals are employed as seed crystals in a hydrothermal growth
process in order to produce novel crystals having deep
ferroelectric domains.
Inventors: |
Giesber; Henry G. III;
(Charlotte, NC) |
Correspondence
Address: |
LEIGH P. GREGORY
PO BOX 168
CLEMSON
SC
29633-0168
US
|
Family ID: |
35943212 |
Appl. No.: |
11/973682 |
Filed: |
October 10, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11210590 |
Aug 24, 2005 |
7317859 |
|
|
11973682 |
Oct 10, 2007 |
|
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60604620 |
Aug 25, 2004 |
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Current U.S.
Class: |
385/129 ;
117/71 |
Current CPC
Class: |
C30B 29/30 20130101;
C30B 7/10 20130101 |
Class at
Publication: |
385/129 ;
117/071 |
International
Class: |
G02B 6/10 20060101
G02B006/10; C30B 7/10 20060101 C30B007/10 |
Claims
1. (canceled)
2. (canceled)
3. (canceled)
4. (canceled)
5. (canceled)
6. (canceled)
7. (canceled)
8. (canceled)
9. (canceled)
10. (canceled)
11. (canceled)
12. (canceled)
13. (canceled)
14. (canceled)
15. (canceled)
16. (canceled)
17. (canceled)
18. (canceled)
19. An optical crystal having periodically inverted domains and a
depth of at least 3 mm, wherein the inverted domains extend through
the entire depth of the crystal.
20. The crystal set forth in claim 19 wherein the crystal comprises
two distinct, periodically spaced domains.
21. The crystal set forth in claim 19 wherein the crystal comprises
three distinct, periodically spaced domains.
22. The crystal set forth in claim 19 wherein the crystal comprises
potassium titanyl phosphate.
23. The crystal set forth in claim 19 wherein the crystal is
selected from the group consisting of RbTiOPO.sub.4, KTiOAsO.sub.4,
RbTiOAsO.sub.4, Rb.sub.1-xK.sub.xTiOPO.sub.4,
Rb.sub.1-xK.sub.xTiOAsO.sub.4, Sr.sub.xBa.sub.(1-x)Ns.sub.2O.sub.6,
Ba.sub.2NaNb.sub.5O.sub.15, LiNbO.sub.3 and its isomorphs,
BaTiO.sub.3 and its isomorphs, LiTaO.sub.3, KnBO.sub.3, KTaO.sub.3,
NaTaO.sub.3, Pb(Ln)ZrO.sub.3, Pb(Ln)Zr(Ti)O.sub.3, ZnO, and
ZnS.
24. The crystal set forth in claim 19 having a depth greater than 5
mm.
Description
[0001] The present divisional application claims the benefit of
prior application U.S. Ser. No. 11/210,590, filed Aug. 24, 2005,
which claims the benefit of prior provisional application, U.S.
Ser. No. 60/604,620, filed Aug. 24, 2004.
FIELD OF THE INVENTION
[0002] The present invention is directed generally to periodically
poled crystals and specifically to a hydrothermal growth process
for producing such periodically poled crystals.
BACKGROUND OF THE INVENTION
[0003] There has been much work directed to providing frequency
conversion of the output from presently available laser and laser
diode sources to wavelengths not readily available from these
sources. The most attractive alternative for frequency conversion,
such as frequency doubling, sum frequency generation and difference
frequency generation, is quasi-phase matching (QPM) of an input
radiation beam or beams from laser or laser diode sources and their
harmonic waves in second order optical crystals. Typical second
order optical crystals for use in such applications include
inorganic crystals such as, for example, LiNbO.sub.3, LiTaO.sub.3
and KTP. In the case of such crystals, the QPM conditions must be
satisfied between the interacting waves in order to achieve
efficient nonlinear optical interaction.
[0004] QPM allows interactions between lightwaves or radiation
polarized such that the nonlinearity is maximized and allows
interactions to occur in the crystal for which birefringent phase
matching is not possible. Compared to birefringent phase matching,
QPM allows both access to new wavelengths and higher conversion
efficiencies. Since the refractive index of the crystal is
dependent upon wavelength of the light to be converted, it is
necessary to provide a periodic inverted domain structure (i.e.,
periodic poling) within the crystal so as to have domains in the
crystal of nonlinear optical coefficient of periodic inverted sign,
e.g., two or more regions or domains of different states of
ferroelectric polarization transverse to the path of light to be
converted. First order QPM requires sign reversals of the effective
nonlinear coefficient with a period equal to two coherence lengths.
The light waves produced by the nonlinear polarization periodic
pattern in the crystal are in phase at the given wavelength so that
the waves intensify each other.
[0005] To date, one frequency conversion that is highly desirable
is that which generates visible light in the "blue" radiation
spectrum, such as wavelengths in the range of about 390 nm to 492
nm, which has many applications such is in color display devices,
color projectors and color printers.
[0006] In practice, the ability to create finely spaced domains
with sufficiently accurate periodicity and well defined domain
walls in the crystal is a challenging, if not difficult, task to
accomplish, particularly on a continuous yield basis. So far, there
are presently several ways to form the periodic domain pattern of
desired spontaneous polarization in the nonlinear crystal, i.e.,
processing regions or domains having a ferroelectric polarization
direction that is dominant over all other possible directions.
These several ways may be classified, in part, as (1) inverted
domain patterns of differing composition, i.e., by surface impurity
diffusion or by ion exchange, (2) inverted domain patterns of same
composition, i.e., electric field treatment with or without heat,
(3) inverted domains through periodic modulation during crystal
growth, i.e., current bias or temperature fluctuation treatment
during crystal growth (e.g., by a modified Czochralski process) and
(4) electron beam treatment.
[0007] KTP is poled most typically by an ion exchange process
within the first classification. A chromium mask is evaporated onto
the surface of the crystal. Looking through the mask is somewhat
like looking through a black, plastic comb, with the teeth of the
comb representing the presence of chromium. The masked crystal is
placed in a melt of BaNO.sub.3 or RbNO.sub.3. Exchange of Ba or Rb
for the K in the KTP occurs only where the chromium is absent; the
chromium blocks the exchange where it is present. The mask is then
removed. The resultant, periodic stripes where ion exchange has
occurred have a different index of refraction than the pure KTP
stripes.
[0008] The second type of classification is generally achieved by
the application of a high voltage, electric field through the
employment of a pattern of electrodes formed on one major surface
of the crystal with a planar electrode formed on the opposite major
surface of the crystal forming the opposing field electrode. The
applied field is either pulsed or a continuous wave for a short
period of time and is generally accompanied with an applied
temperature such as above 100.degree. C. The permanent inversion of
the domains is accomplished by means of minute changes in ions in
the unit lattice of the crystal due to the application of the
electric field. By "permanent", what is meant is that the inverted
domain pattern will remain as long as the crystal is not
subsequently reheated to high temperature near the Curie
temperature of the crystal or subjected to any further high voltage
fields.
[0009] In about 1963, R. C. Miller recognized that inverted domains
could be formed in ferroelectric crystals by cycling an applied
electric field to switch the spontaneous polarization of the
crystal. U.S. Pat. No. 5,193,023 teaches periodic poling, using a
pattern of electrodes on one side of a crystal and a planar
electrode on the opposite side of the crystal across which an
electric field is applied. In the examples of U.S. Pat. No.
5,193,023 where an electric field is employed, poling is
accomplished in an atmosphere containing oxygen with an applied
temperature in the range of 150.degree. C. to 1200.degree. C. and
an applied voltage field of several hundreds of volts per
centimeter or less. The field inversion in U.S. Pat. No. 5,193,023
is accomplished at relatively lower applied voltages, such as at
several hundreds of volts per centimeter (or several kilovolts per
centimeter when using pulse voltages) or less, since the crystal is
heated to a sufficiently high temperature during the applied
E-field process. However, it has been found that higher voltages
can be successfully employed at room temperature.
[0010] Examples of the third type of classification are,
respectively, the articles of A Feisst et al., "Current Induced
Periodic Ferroelectric Domain Structures in LiNbO.sub.3 Applied for
Efficient Nonlinear Optical Frequency Mixing", Applied Physics
Letters, Vol. 47(11), pp. 1125-1127, Dec. 1, 1985 and Duan Feng et
al., "Enhancement of Second Harmonic generation in LiNbO.sub.3
Crystals With Periodic Laminar Ferroelectric Domains", Applied
Physics Letters, Vol. 37(1), pp. 607-609, Oct. 1, 1980. Both of
these articles describe crystals grown by flux growth methods at
temperatures above the Curie temperature of the crystal.
[0011] An example for the fourth type of classification is the
article of H. Ito et al., "Fabrication of Periodic Domain Grating
in LiNbO.sub.3 by Electron Beam Writing for Application of
Nonlinear Optical processes", Electronic Letters, Vol. 27(14), pp.
1221-1222, Jul. 4, 1991.
[0012] Of all of the foregoing classifications, heretofore the
second type of classification has been found the most successful
from the standpoint of providing periodic domains that have
accurate periodicity and substantially vertically formed domain
walls creating the nonlinear periodic waveguide in the crystal. The
use of the applied electric field permits the formation of domains
that have accurate periodicity and the domains are formed through
the crystal forming domain walls that have some parallelism with
the z axis of the crystal. However, in the case of the second type
as well as all other types classified, the processing only provides
for shallow domain structures that do not effectively extend
through the crystal bulk and do not form vertical wall boundaries
for the formed inverted domains substantially parallel with the z
axis of the crystal. What is needed is a process that provides for
vertically formed domain walls that extend in the z axis direction
through the crystal bulk without walkoff, i.e., capable of
providing bulk frequency conversion, forming highly uniform
periodicity, laterally extending domain patterns which achieve
first order intervals over long crystal interaction lengths.
Heretodate, such domain patterns have only extended a maximum of
about 3 mm into the crystal depth.
[0013] Although not known as a means for producing periodically
poled crystals, hydrothermal techniques are an excellent and well
known route to high quality single crystals for a variety of
electro-optic applications. For example, all electronic grade
quartz is grown commercially by the hydrothermal method. Further,
KTP is grown by both flux and hydrothermal methods, and it is
widely acknowledged by those familiar with the art that the
hydrothermally grown product is generally of superior quality. The
hydrothermal method involves the use of superheated water (liquid
water heated above its boiling point) under pressure to cause
transport of soluble species from a nutrient rich zone to a
supersaturated growth zone. Generally, a seed crystal is placed in
the growth zone to control the growth and supersaturation is
achieved by the use of differential temperature gradients. The
superheated fluid is generally contained under pressure, typically
5-30 kpsi, in a metal autoclave. Depending on the chemical demands
of the system the autoclave can be lined with a noble metal using
either a fixed or floating liner. These general techniques are well
known to those of ordinary skill in the art and have been used for
the growth of a variety of electro-optic crystals.
SUMMARY OF THE INVENTION
[0014] Accordingly, the present invention is directed to a method
for growing a periodically poled crystal which includes the steps
of: providing a pressure vessel having a growth region and a
nutrient region, providing a seed crystal having periodically
inverted domains, positioning the seed crystal in the growth region
of the pressure vessel, providing a medium comprising a nutrient
and a mineralizer in the nutrient region, and heating and
pressurizing the vessel such that a growth temperature is produced
in the growth region, a nutrient temperature is produced in the
nutrient region, and a temperature gradient is produced between the
growth region and the nutrient region, whereby growth of the
crystal is initiated, the growth temperature ranging from about
375.degree. C. to about 495.degree. C., preferably from about
450.degree. C. to about 475.degree. C., the nutrient temperature
ranging from about 495.degree. C. to about 650.degree. C.,
preferably from about 550.degree. C. to about 575.degree. C., and
the pressure ranging from about 3000 psi to about 35000 psi,
preferably from about 8000 psi to about 14000 psi. Preferably, the
seed crystal is electronically periodically poled.
[0015] In a preferred embodiment the seed crystal has two distinct,
periodically spaced domains. However, a seed crystal having three
or more distinct, periodically spaced domains may also be employed
in accordance with the present invention.
[0016] Preferably, the crystal grown by the present method is KTP,
but a variety of optical crystals may be grown by the present
method.
[0017] The present invention also is directed to a periodically
poled crystal having a depth of greater than 3 mm through which the
periodicity extends, made by a process which includes the steps of:
providing a pressure vessel having a growth region and a nutrient
region, providing a seed crystal having periodically inverted
domains, positioning the seed crystal in the growth region of the
pressure vessel, providing a medium comprising a nutrient and a
mineralizer in the nutrient region, and heating and pressurizing
the vessel such that a growth temperature is produced in the growth
region, a nutrient temperature is produced in the nutrient region,
and a temperature gradient is produced between the growth region
and the nutrient region, whereby growth of the crystal is
initiated, the growth temperature ranging from about 375.degree. C.
to about 495.degree. C., preferably from about 450.degree. C. to
about 475.degree. C., the nutrient temperature ranging from about
495.degree. C. to about 650.degree. C., preferably from about
550.degree. C. to about 575.degree. C., and the pressure ranging
from about 3000 psi to about 35000 psi, preferably from about 8000
psi to about 14000 psi. Preferably, the seed crystal is
electronically periodically poled.
[0018] As above, in a preferred embodiment the seed crystal has two
distinct, periodically spaced domains. However, a seed crystal
having three or more distinct, periodically spaced domains may also
be employed in accordance with the present invention.
[0019] Preferably, the periodically poled crystal of the present
invention is KTP, but a variety of optical crystals are within the
scope of the present invention.
[0020] Specifically, the present invention is directed to an
optical crystal having at least two periodically inverted domains
and having a depth of at least about 3 mm, wherein the periodically
inverted domains extend through the entire depth of the
crystal.
BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING
[0021] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate presently
preferred embodiments of the present invention and, together with
the general description given above and the detailed description of
the preferred embodiments given below, serve to explain the
principles of the present invention.
[0022] FIG. 1 schematically illustrates a silver tube with seed
crystals suspended from a ladder for the growth of larger crystals
in accordance with the present invention by a transport growth
technique.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] The present invention is directed to periodically poled
crystals and a hydrothermal growth method for making such crystals.
The present crystals are poled throughout their depth and may
exceed 5 mm in depth. Specifically, the present inventive crystals
are hydrothermally grown from seed crystals having inverted
domains. Preferably, the seed crystals have been periodically poled
electronically.
[0024] Although KTP is preferred, a wide variety of periodically
poled crystals may be made in accordance with the present
invention. However, in order to grow a particular crystal by the
present method the crystal must be capable of being grown
hydrothermally, capable of being periodically poled, and have a
Curie temperature above its growth temperature. The Curie
temperature is the temperature above which a ferromagnetic material
loses its permanent magnetism. Thus, the periodic ferroelectric
domains are essentially lost and the magnetic properties of the
material become random. Thus, among the appropriate crystals for
growth as periodically poled crystals in accordance with the
present invention are KTP (KTiOPO.sub.4), RTP(RbTiOPO.sub.4), KTA
(KTiOAsO.sub.4), RTA (RbTiOAsO.sub.4), Rb:KTP
(Rb.sub.1-xK.sub.xTiOPO.sub.4), Rb:KTA
(Rb.sub.1-xK.sub.xTiOAsO.sub.4),
SBN(Sr.sub.xBa.sub.(1-x)Ns.sub.2O.sub.6) and
Ba.sub.2NaNb.sub.5O.sub.15, LiNbO.sub.3 and its isomorphs,
BaTiO.sub.3 and its isomorphs, LiTaO.sub.3, KnBO.sub.3, KTaO.sub.3,
NaTaO.sub.3, and Pb(Ln)ZrO.sub.3, Pb(Ln)Zr(Ti)O.sub.3 (PZT and
PLZT), ZnO, and ZnS.
[0025] An example of an appropriate apparatus for performing the
hydrothermal growth transport method is shown in FIG. 1, which
shows silver tube 10, preferably of dimensions 1.25 in by 15 in. A
silver baffle 12 with 1 or more holes in it is placed 3 in above
the bottom of the tube. Two single seed crystals are represented at
13. Holes are drilled in the crystals and they are hung by silver
thread 16 on a small silver ladder 18 placed within the tube. In a
preferred configuration, the two seed crystals are hung 13 in and
11 in above the bottom of the tube, respectively. Preferably, the
nutrient or feedstock and a mineralizer solution is added to the
tube and fills about 80% of the remaining volume of the tube. The
tube is welded shut and placed in an autoclave with a cold seal and
a 1.5 in by 16 in opening. An amount of water sufficient to occupy
up to 80% of the remaining free volume is added and the autoclave
sealed and placed in a growth station with band heaters affixed to
the autoclave. The autoclave is heated in such a way that a
temperature gradient is induced. After an extended period of time,
the autoclave is cooled, opened and the silver tube opened.
[0026] Looking specifically at KTP, a feedstock of K(TiO)(PO.sub.4)
is prepared via a melt of equal molar KH.sub.2PO.sub.4 and
TiO.sub.2. The starting feedstock can be placed in a precious metal
floating liner along with a suitable baffle approximately 2 inches
above the bottom of the feedstock. A ladder assembly containing
periodically poled KTP single crystal seeds is placed in the
floating liner. Approximately three-quarters of the remaining space
in the container is filled with a mineralizer solution. Typically
the mineralizer solution is 1-4M K.sub.2HPO.sub.4. The liner is
sealed and placed in a suitable autoclave that is then
counter-pressured so as to compensate for an internal pressure of
approximately 8-15,000 psi at temperature, depending on
concentration of mineralizer and growth temperature. Typically the
growth temperatures for periodically poled KTP boules are
550.degree. C. in the dissolving zone and 475.degree. C. in the
growth zone, although the temperatures can vary depending on the
desired growth rate and concentration of mineralizer. The thermal
gradient is always between 50.degree. and 100.degree.. The highest
temperatures should not exceed 700.degree. C. since the Curie point
of KTP is 936.degree. C. Under these growth conditions, between 0.3
and 1 mm per side per week of periodically poled KTP can be
transported to each patterned seed crystal. The autoclave is
maintained at these temperatures and pressures for 6-8 weeks to
obtain suitably sized boules with deep ferroelectric domains.
[0027] Additional illustrations of the present invention are
provided by the following specific examples for KTP and other
crystals.
Example 1
[0028] A feedstock of K(TiO)(PO.sub.4) is prepared via a melt of
equal molar KH.sub.2PO.sub.4 and TiO.sub.2 at 1200.degree. C. The
starting feedstock is placed in a precious metal floating liner
along with a suitable baffle approximately 2 inches above the
bottom of the feedstock. A ladder assembly containing periodically
poled KTP single crystal seeds is placed in the floating liner.
Approximately three-quarters of the remaining space in the
container is filled with a 2M K.sub.2HPO.sub.4 mineralizer solution
at a pH of 9.5. The liner is sealed and placed in a suitable
autoclave that is then counter-pressured so as to compensate for an
internal pressure of approximately 12000 psi. The temperature is
550.degree. C. in the nutrient or feedstock zone and 475.degree. C.
in the growth zone, although the temperatures will vary somewhat
over the course of crystal growth. However, the thermal gradient is
always between 50.degree. and 100.degree.. Under these growth
conditions, between 0.3 and 1 mm per side per week of periodically
poled KTP can be transported to each patterned seed crystal. The
autoclave is maintained at these temperatures and pressures for 6-8
weeks to obtain suitably sized boules with deep ferroelectric
domains.
Example 2
[0029] A feedstock of K(TiO)(AsO.sub.4) is prepared via a melt of
equal molar KH.sub.2AsO.sub.4 and TiO.sub.2 at 1200.degree. C. As
in Example 1 above, the starting feedstock is placed in a precious
metal floating liner along with a suitable baffle approximately 2
inches above the bottom of the feedstock. A ladder assembly
containing periodically poled KTA single crystal seeds is placed in
the floating liner. Approximately three-quarters of the remaining
space in the container is filled with a 2M K.sub.2HAsO.sub.4
mineralizer solution at a pH of approximately 9. The liner is
sealed and placed in a suitable autoclave that is then
counter-pressured as for KTP growth so as to compensate for an
internal pressure of approximately 12000 psi. The temperature
550.degree. C. in the nutrient zone and 475.degree. C. in the
growth zone, although the temperatures may vary throughout the
course of crystal growth. The thermal gradient is always maintained
between 50.degree. and 100.degree.. For KTA, the highest
temperatures should not exceed 650.degree. C. since the Curie point
if KTA is near 850C. Under these growth conditions, between 0.3 and
1 mm per side per week of periodically poled KTA can be transported
to each patterned seed crystal. The autoclave is maintained at
these temperatures and pressures for 6-8 weeks to obtain suitably
sized boules with deep ferroelectric domains.
Example 3
[0030] A feedstock of Nb.sub.2O.sub.5 can be placed in a precious
metal floating liner along with a suitable baffle approximately 2
inches above the bottom of the feedstock. Alternatively, a
feedstock of LiNbO.sub.3 may be employed. A ladder assembly
containing periodically poled LiNbO.sub.3 single crystal seeds is
placed in the floating liner. Approximately three-quarters of the
remaining space in the container is filled with a 2M LiOH
mineralizer solution. The liner is sealed and placed in a suitable
autoclave that is then counter-pressured so as to compensate for an
internal pressure of approximately 25,000 psi. The temperatures for
periodically poled LiNbO.sub.3 boules are 650.degree. C. in the
nutrient zone and 550.degree. C. in the growth zone, although the
temperatures can vary by 150.degree. depending on the desired
growth rate and concentration of mineralizer. The thermal gradient
is typically around 100.degree. C. Under these growth conditions,
approximately 1.0 mm per side per week of periodically poled
LiNbO.sub.3 can be transported to each patterned crystal. The
autoclave is maintained at these temperatures and pressures for 6-8
weeks to obtain suitably sized boules with deep ferroelectric
domains.
[0031] Preferred embodiments of the invention have been described
using specific terms and devices. The words and terms used are for
illustrative purposes only. The words and terms are words and terms
of description, rather than of limitation. It is to be understood
that changes and variations may be made by those of ordinary skill
art without departing from the spirit or scope of the invention,
which is set forth in the following claims. In addition it should
be understood that aspects of the various embodiments may be
interchanged in whole or in part. Therefore, the spirit and scope
of the appended claims should not be limited to descriptions and
examples herein. Moreover, Applicants hereby disclose all
sub-ranges of all ranges disclosed herein. These sub-ranges are
also useful in carrying out the present invention.
* * * * *