Liquid Crystals

Chapter 1.4.2 - Dye-Doped Liquid Crystals

1.4.2.   Dye-Doped Liquid Crystals

From the standpoint of optical properties, the doping of liquid crystals by appropriately
dissolved concentrations and types of dyes clearly deserves special attention.
The most important effect of dye molecules on liquid crystals is the modification of
their well-known linear, and more recently observed nonlinear, optical properties
(see Chapters 8 and 12).

An obvious effect of dissolved dye is to increase the absorption of a particular liquid
crystal at some specified wavelength region. If the dye molecules undergo some
physical or orientational changes following photon absorption, they could also affect
the orientation of the host liquid crystal, giving rise to nonlinear or storage-type optical
effects12 (see Chapter 8).

In linear optical and electro-optical applications, another frequently employed effect
is the so-called guest–host effect. This utilizes the fact that the absorption coefficients
of the dissolved dichroic dyes are different for optical fields polarized parallel or perpendicular
to the long (optical) axis of the dye molecule. In general, a dichroic dye molecule
absorbs much more for optical field polarization parallel to its long axis than for
optical field polarization perpendicular to its long axis. These molecules are generally
elongated in shape and can be oriented and reoriented by the host nematic liquid crystals.
Accordingly, the transmission of the cell can be switched with the application of
an external field (see Fig. 1.16).

Figure 1.16. Alignment of a dichroic dye-doped nematic liquid crystal: (a) before application of switching electric field; (b) switching field on.