Opto-Electronics
Review
25
(2017)
89–92
Contents
lists
available
at
ScienceDirect
Opto-Electronics
Review
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epage
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ww.journals.elsevier.com/opto-electronics
review
Electro-optic
coefficients
of
a
non-congruent
lithium
niobate
fabricated
by
vapour
transport
equilibration:
Composition
effect
J.-Q.
Xu
a,b
,
W.-Y.
Du
a,b
,
Q.
Sun
a,b
,
W.-H.
Wong
a,b,c
,
D.-Y.
Yu
a,b
,
E.Y.-B.
Pun
c
,
D.-L.
Zhang
a,b,c,∗
a
Department
of
Opto-electronics
and
Information
Engineering,
School
of
Precision
Instruments
and
Opto-electronics
Engineering,
Tianjin
University,
Tianjin
300072,
China
b
Key
Laboratory
of
Optoelectronic
Information
Technology
(Ministry
of
Education),
Tianjin
University,
Tianjin
300072,
China
c
Department
of
Electronic
Engineering
and
State
Key
Laboratory
of
Millimeter
Waves,
City
University
of
Hong
Kong,
83
Tat
Chee
Avenue,
Kowloon,
Hong
Kong,
People’s
Republic
of
China
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
7
February
2017
Received
in
revised
form
22
March
2017
Accepted
4
April
2017
Available
online
26
April
2017
Keywords:
LiNbO
3
single-crystal
Electro-optic
property
Composition
effect
Defects
a
b
s
t
r
a
c
t
Composition
effect
on
electro-optic
(EO)
properties
of
a
LiNbO
3
(LN)
single-crystal
has
been
investigated
in
a
Li
2
O-content
range
of
47.0–49.95
mol%.
Some
non-congruent
LN
crystals
with
different
Li
2
O-contents
were
prepared
by
performing
Li-deficient
or
Li-rich
vapour
transport
equilibration
treatments
on
as-
grown
congruent
LN
crystals.
Unclamped
EO
coefficients
13
and
33
of
these
samples
were
measured
by
a
Mach–Zehnder
interferometric
method.
The
measurements
show
that
in
the
Li-deficient
regime
both
13
and
33
increase
by
∼8%
as
Li
2
O-content
decreases
from
the
congruent
48.6
mol%
to
the
47.0
mol%
in
the
Li-deficient
regime.
The
feature
is
desired
for
the
EO
application
of
the
Li-deficient
crystal.
In
the
near-stoichiometric
regime,
both
13
and
33
reveal
a
non-monotonic
dependence.
As
the
Li
2
O-content
increases
from
the
48.6
mol%,
the
EO
coefficient
decreases.
Around
Li
2
O-content
49.5
mol%,
a
minimum
is
reached.
After
that,
the
EO
coefficient
recovers
slowly.
At
the
stoichiometric
composition,
it
recovers
to
a
value
close
to
that
at
the
congruent
point.
Comparison
shows
that
different
crystal
growth
methods
give
rise
to
different
defect
structure
features
and
hence
different
composition
effects.
©
2017
Association
of
Polish
Electrical
Engineers
(SEP).
Published
by
Elsevier
B.V.
All
rights
reserved.
1.
Introduction
LiNbO
3
(LN)
is
widely
investigated
due
to
its
good
acousto-optic,
electro-optic
(EO)
and
nonlinear
optical
properties.
By
utiliz-
ing
its
excellent
EO
property,
various
passive
[1–3]
and
active
devices
[1,4–7]
have
been
demonstrated.
As
the
composition
approaches
stoichiometry,
the
crystal,
called
near-stoichiometric
(NS)
LN
(Li
2
O-content
>49
mol%),
exhibits
a
number
of
attrac-
tive
features
such
as
larger
nonlinearity,
lower
coercive
field
for
domain
reversal,
largely
lowered
threshold
concentration
of
photorefractive-damage-resistant
dopants
such
as
Mg,
Zr,
Sc,
etc.
In
addition,
a
non-congruent,
Li-deficient
LN
may
be
used
to
develop
active
waveguide
devices
as
such
crystal
enables
to
promote
the
solubility
of
rare
earth
ion
and
hence
the
optical
gain
of
an
amplifier
or
laser
[8,9].
Alternatively,
a
Li-deficient/NS
LN
plate
may
be
fab-
∗
Corresponding
author
at:
Department
of
Opto-electronics
and
Information
Engi-
neering,
School
of
Precision
Instruments
and
Opto-electronics
Engineering,
Tianjin
University,
Tianjin
300072,
China.
E-mail
address:
dlzhang@tju.edu.cn
(D.-L.
Zhang).
ricated
by
performing
a
post
Li-deficient/Li-rich
vapour
transport
equilibration
(VTE)
treatment
on
a
congruent
LN
crystal.
To
design
and
implement
an
EO
device
based
on
an
NS/Li-
deficient
LN,
it
is
essential
to
know
the
composition
effect
on
the
EO
coefficients
of
an
LN
substrate.
Over
the
past
years,
people
has
stud-
ied
the
effects
of
various
dopants
on
the
EO
coefficients
of
LN.
These
include
Cr
3+
[10],
Hf
4+
[11],
Mg
2+
[12–14],
Ti
4+
[14,15],
Zr
4+
[16],
Fe
3+
[17],
Zn
2+
[18],
H
+
[19–21],
Er
3+
[22,23]
and
In
3+
[23].
Among
them,
the
Cr
3+
,
Zn
2+
and
H
+
dopants
have
a
relatively
large
effect
while
the
others
have
a
weak
effect.
In
addition,
people
have
also
studied
the
stoichiometry
effect
on
the
EO
property
of
LN
(mainly
in
the
NS
regime)
for
the
crystals
prepared
by
different
growth
methods
[10,13,24–27].
The
study
has
shown
that
the
crystal
com-
position
takes
a
large
effect,
and
different
crystal
growth
methods
cause
different
crystal
composition
effects
on
the
EO
coefficient.
For
the
Li-rich
VTE
preparation
method,
which
involves
Li
diffu-
sion
into
the
LN
crystal,
the
lattice
is
already
present
before
the
VTE
treatment.
The
ion
environment
in
cells
generated
after
the
VTE
may
be
distinct
from
that
after
the
crystal
growth.
It
is
unclear
what
composition
effect
is
caused
by
the
Li-rich
VTE.
It
is
neces-
sary
to
perform
an
independent
investigation
on
the
EO
properties
http://dx.doi.org/10.1016/j.opelre.2017.04.001
1896-3757/©
2017
Association
of
Polish
Electrical
Engineers
(SEP).
Published
by
Elsevier
B.V.
All
rights
reserved.