
826 Part 4 Functional Materials
In the shortened notation (ooe, eoe, ... ), the fre-
quencies satisfy the condition ω
1
<ω
2
<ω
3
,i.e.
the first symbol refers to the longest-wavelength
radiation, and the last symbol refers to the shortest-
wavelength radiation. Here the ordinary beam, or
o-beam is the beam with its polarization normal to
the principal plane of the crystal, i. e. the plane con-
taining the wave vector k and the crystallophysical
axis Z (or the optical axis, for uniaxial crystals).
The extraordinary beam, or e-beam is the beam with
its polarization in the principal plane. The third-
order term χ
(3)
is responsible for the optical Kerr
effect.
Uniaxial Crystals
For uniaxial crystals, the difference between the refrac-
tive indices of the ordinary and extraordinary beams,
the birefringence ∆n, is zero along the optical axis (the
crystallophysical axis Z) and maximum in a direction
normal to this axis. The refractive index for the ordinary
beam does not depend on the direction of propagation.
However, the refractive index for the extraordinary beam
n
e
(θ), is a function of the polar angle θ between the
Z axis and the vector k:
n
e
(θ) = n
o
1+tan
2
θ
1+(n
o
/n
e
)
2
tan
2
θ
1/2
, (4.42)
where n
o
and n
e
are the refractive indices of the ordinary
and extraordinary beams, respectively in the plane nor-
mal to the Z axis, and are termed the principal values.
If n
o
> n
e
the crystal is called negative,andifn
o
< n
e
it is called positive. For the o-beam, the indicatrix of the
refractive indices is a sphere with radius n
o
, and for the
e-beam it is an ellipsoid of rotation with semiaxes n
o
and
n
e
. In the crystal, in general, the beam is divided into
two beams with orthogonal polarizations; the angle be-
tween these beams ρ is the birefringence (or walk-off )
angle.
Equations for calculating phase-matching angles in
uniaxial crystals are given in [4.1–4].
Biaxial Crystals
For biaxial crystals, the optical indicatrix is a bilayer
surface with four points of interlayer contact, which
correspond to the directions of the two optical axes.
In the simple case of light propagation in the principal
planes XY, YZ, and XZ, the dependences of the re-
fractive indices on the direction of light propagation are
represented by a combination of an ellipse and a circle.
Thus, in the principal planes, a biaxial crystal can be
considered as a uniaxial crystal; for example, a biaxial
crystal with n
Z
> n
Y
> n
X
in the XY plane is similar to
a negative uniaxial crystal with n
o
= n
Z
n
e
(ϕ) = n
Y
1+tan
2
ϕ
1+(n
Y
/n
X
)
2
tan
2
ϕ
1/2
, (4.43)
where ϕ is the azimutal angle. Equations for cal-
culating phase-matching angles for propagation in
the principal planes of biaxial crystals are given in
[4.3–6].
4.4.3 Guidelines for Use of Tables
Tables 4.4-3 – 4.4-21 are arranged according to piezo-
electric classes in order of decreasing symmetry (see
Table 4.4-2), and alphabetically within each class.
They contain a number of columns placed on two
pages, even and odd. The following properties are
presented for each dielectric material: density ,
Mohs hardness, thermal conductivity κ, static dielec-
tric constant ε
ij
, dissipation factor tan δ at various
temperatures and frequencies, elastic stiffness c
mn
,
elastic compliance s
mn
(for isotropic and cubic mater-
ials only), piezoelectric strain tensor d
in
, elastooptic
tensor p
mn
, electrooptic coefficients r
mk
(the lat-
ter two at 633 nm unless otherwise stated), optical
transparency range, temperature variation of the re-
fractive indices dn/ dT, refractive indices n (the
latter two at 1.064 µm unless otherwise stated), dis-
persion relations (Sellmeier equations), second-order
nonlinear dielectric susceptibility d
ij
, and third-order
nonlinear dielectric susceptibility χ
(3)
ijk
(for isotropic
and cubic materials only) (the latter two at 1.064 µm
unless otherwise stated). For isotropic materials, the
two-photon absorption coefficient β is also included
The numerical values of the elastic and elastooptic
constants are often averages of three or more meas-
urements, as presented in [4.7–11]. In such cases, the
corresponding Landolt–Börnstein volume is cited to-
gether with the most reliable (latest) reference. The
standard deviation of the averaged value is given in
parentheses. Vertical bars mean the modulus of
the corresponding quantity. The absolute scale for the
second-order nonlinear susceptibilities of crystals is
based on [4.12–14]. The second-order susceptibilities
for all crystals measured relative to a standard crys-
tal have been recalculated accordingly. In particular,
Part 4 4.3