Liquid Crystals

Chapter 1.5.2 - Liquid Crystal Optical Slab Waveguide, Fiber, and Nanostructured Photonic Crystals

1.5.2.   Liquid Crystal Optical Slab Waveguide, Fiber,
and Nanostructured Photonic Crystals

Besides the bulk thin film structures discussed in the preceding section, liquid crystals
could also be fabricated into optical waveguides24-30 or nanostructured photonic
crystals.31

Both slab and cylindrical (fiber) waveguide structures have been investigated. A
typical liquid crystal slab waveguide24,25 is shown in Figure 1.19. A thin film (approximately
1 μm) of liquid crystal is sandwiched between two glass slides (of lower
refractive index than the liquid crystal), one of which has been deposited with an
organic film into which an input laser is introduced via the coupling prism. The laser
excites the transverse electric (TE) and/or transverse magnetic (TM) modes in the
organic film, which are then guided into the nematic liquid crystal region. Using such
optical waveguides, Whinnery et al.24 and Giallorenzi et al.25 have measured the scattering
losses in nematic and smectic liquid crystals and introduced electro-optical and
integrated optical switching devices. However, the large losses in nematics (about 20
dB/cm) and their relatively slow responses impose serious limitations in practical
integrated electro-optical applications. The scattering losses in smectic waveguides
are generally much lower, and they may be useful in nonlinear optical applications
(see Chapter 10).

Liquid crystal “fibers” are usually made by filling hollow fibers (microcapillaries)
made of material of lower indices of refraction.26,27 The microcapillaries are usually

Figure 1.19. Schematic depiction of a liquid crystal slab waveguide structure.


Figure 1.20. (a) Axial alignment of a nematic liquid crystal cored fiber; (b) mixed radial and axial alignments of a nematic liquid crystal cored fiber.


made of Pyrex or silica glass, whose refractive indices are 1.47 and 1.45, respectively.
It was reported26 that the scattering losses of the nematic liquid crystal fiber
core are considerably reduced for a core diameter smaller than 10 μm; typically, the
loss is about 3 dB/cm (compared to 20 dB/cm for a slab waveguide or bulk thin film).
Also, the director axis alignment within the core is highly dependent on the liquid
crystals–capillary interface interaction (i.e., the capillary material). In silica or Pyrex
capillaries the nematic director tends to align along the axis of the fiber (Fig. 1.20a),
whereas in borosilicate capillaries the nematic director tends to align in a radial
direction, occasionally mixed in with a thread of axially aligned material running
down the axis of the fiber (Fig. 1.20b).

Fabrications of such fibers with isotropic phase liquid crystals are much easier.
27,29 Because of the fluid property and much lower scattering loss, liquid crystal
fibers of much longer dimension have been fabricated and shown to exhibit interesting
nonlinear optical properties; high quality image transmitting fiber arrays28,29
have also been fabricated for passive pulsed laser limiting applications. Other optical
devices based on liquid crystal filled photonic crystal (holey) fibers have also
been reported.30

Recently, photonic crystals31 in one-, two-, and three-dimensional forms have
received intense research interest owing to the rich variety of possibilities in terms of

Figure 1.21. TiO2 inverse opal structure for liquid crystal infiltration.

material compositions, lattice structures, and their electronic as well as optical properties.
By using an active tunable material as a constituent, photonic crystals can
function as tunable filters, switches, and lasing devices. In particular, liquid crystals
have been employed in many studies involving opals and inverse opal structures (see
Fig. 1.21). In particular, Graugnard et al.31 has reported non-close-packed inverse
opals, consisting of overlapping air spheres in a TiO2 matrix, which were infiltrated
with liquid crystal. Because of the higher volume fraction for nematic liquid crystal
(NLC) infiltration, a larger electrical tuning range (>20 nm) of the Bragg reflection
peak can be achieved.

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