DWs optimized for a given wavelength range are produced by properly
choosing the optical anisotropy and the thickness of the LC layer.
of this description is indeed easily verified with the aid of elementary analysis
of light propagation through the waveplate using a Jones matrix approach.
The diffraction efficiency of DWs is
a smooth function of wavelength l, and
it exceeds a given value h in a bandwidth Dl | l( 1–h)1/2. A spectral range
Dl > 100 nm is thus achieved in the
visible even for a diffraction efficiency as
high as 95 percent. These components
were known in polarization holography
as “polarization gratings.” Regarding
them as diffractive waveplates allows
for an easier understanding of all other
features of these components that are not
found in Bragg gratings. As waveplates,
their diffraction spectrum can be further
broadened—and practically made achromatic—exceeding 200 nm for the visible
and 300 nm for the visible/near-infrared
spectral range. This is done by stacking
waveplates with certain angles between
their optical axis orientation according to
the method proposed by Pancharatnam.
Combining DWs into a full-wave
waveplate would eliminate the diffraction altogether. Such a system of DWs,
one of them switchable between diffractive and non-diffractive states, allows
switching the propagation of an incident
light beam from the 0th to a 1st order
without losses or distortions: Light propagates through the system as through a
transparent optical window when both
DWs are in the state of a half-wave waveplate, and the beam is fully deflected if
one of the DWs is switched off.
This can be done electrically in the
case of LCs and optically for both LCs
and LC polymers. Since the propaga-
tion direction of light depends on the
sign of the circular polarization, it can be
switched using an electrically controlled
phase retarder without switching the DW
itself. Optical switching was demon-
strated with single-nanosecond laser
pulses of low energy density ( 10 mJ/cm2)
as well as with low power ( 1 m W/cm2)
continuous-wave beams for DWs made of
azobenzene LC. Highly nonlinear optical
and photosensitive materials are typically
strongly absorptive. Due to the large opti-
cal anisotropy of LCs (Dn |0.3), the half-
wave condition is realized in thin material
layers without dramatically affecting the
overall transmission of the system.
Fabrication
Researchers claimed a DW with 100-per-
cent diffraction efficiency in 1987 using
a 10-mm-thick film of an azobenzene
dye-doped gel. The photoalignment of azo
dyes perpendicular to light polarization
is the basis of most DW developments.
DWs with a continuous optical-axis
orientation pattern are fabricated with a
holographic setup comprising orthogonal
circularly polarized laser beams. The effec-
tive polarization in the overlap region of
such beams is linear and rotating in the
recording plane in a cycloidal manner. A
highly versatile technique involves using
10-nm-thick azo-dye coatings with cycloi-
dal photoalignment patterns as orienting
layers for LCs or LC polymers. Cinna-
mates present another class of widely used
photoalignment materials.
Alignment layer
Transparent
electrode
The diffractive state of a LC DW (left) can be switched into a non-diffractive state due
to electrically induced transformation from cycloidal to homeotropic orientation (right).
DWs based on LC polymers can be switched only optically using a cw laser beam as
well as a short laser pulse.
Holographic exposure Holographic technique
Direct printing
Master DW
DW in print
Cycloidal
polarization pattern
Substrate
Photoalignment layer
Unpolarized UV
Diffracted
beams
Polarized UV
LC pre-polymer
UV polymerization
LC polymer
Key fabrication steps of LC polymer DW include depositing a 10- to 50-nm-thick photo-
alignment layer on a substrate, exposing it to overlapping orthogonal circularly polarized
beams to impart cycloidal orienting boundary conditions, and subsequently coating it with
an LC pre-polymer layer to obtain cycloidal orientation pattern in a thicker material layer
meeting the half-waveplate condition.
