t sounds too good to be true: a micron-thick optical component with diffrac-
tion efficiency as high as a Bragg grating but with a spectrum of wavelengths
and divergence angles that are two to three orders broader—plus electrical or
optical switchability. If such a component were feasible, it may become a valu-
able photonics tool that could give displays the ability to use all available light; enable
beam steering systems to become compact and tolerant to high power beams; and give
spectrometers broadband gratings. It could also make biological and chemical sensors
more sensitive and give spatial light modulators higher speed.
At this point, all sights are probably turning toward liquid crystal (LC) diffraction
gratings—HPDLCs (holographic polymer dispersed LCs), POLICRYPS (
polymer-liquid-crystal-polymer systems), and even CLCs (cholesteric LCs), which often occur
naturally. All of those can meet the condition for the Bragg regime and exhibit
high diffraction efficiency, and all can be switchable. HPDLCs are comprised of LC
microdroplets, and they can diffract light independent of polarization. For the same
reason, however, they scatter light and may be hazy.
POLICRYPs are a perfect sequence of polymer walls separated by well-aligned LC
layers. Therefore, they are well transparent but also diffract one component of polarization only, just like CLCs. Indeed, these components can be stacked for polariza-tion-independent functionality. However, stacking further increases the thickness of
the system. A thickness of tens of micrometers is typically sufficient for obtaining near
100-percent diffraction for these structures, which are based on highly optically anisotropic materials. This is not much from the mechanical standpoint, and these components are good for many uses. But complications arise when one tries to optimize their
optical properties. Thicker layers are associated with higher absorption and scattering
losses, higher electric fields or intensity of controlled light, and longer switching times.
But there may be another way to get to the micrometer scale while maintaining
all that LCs could offer (and possibly more than Bragg gratings could): with diffractive waveplates (DWs).
I
(Facing page) Broadband high efficiency diffraction of an unpolarized light by a diffractive
waveplate is revealed in the image of a hand taken through the waveplate. (Above) LC polymer
DWs fabricated on (a) a thin flexible plastic substrate, (b) a dielectric mirror, and (c) a concave
surface of a lens. DWs as large as two inches in diameter are demonstrated on (d) glass
and on (e) a polycarbonate substrate. LC polymer DWs may contain (f) quantum dots or
photoresponsive dyes such as azobenzene.
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