Liquid Crystals

Chapter 1.2.2 - Visible and Infrared Absorption

From the preceding discussion, one can see that, in general, liquid crystals are quite
absorptive in the UV region, as are most organic molecules. In the visible and near-
infrared regimes (i.e., from 0.4 to 5 μm), there are relatively fewer absorption bands,
and thus liquid crystals are quite transparent in these regimes.

As the wavelength is increased toward the infrared (e.g., ≥ 9 μm), rovibrational
transitions begin to dominate. Since rovibrational energy levels are omnipresent in
all large molecules, in general, liquid crystals are quite absorptive in the infrared
regime.

The spectral transmission dependence of two typical liquid crystals is shown in
Figures 1.7a and 1.7b. The absorption coefficient α in the ultraviolet (~ 0.2 μm)
regime is on the order of 103 cm-1; in the visible (~ 0.5 μm) regime, α≈ 100 cm-1;
in the near-infrared (~ 10 μm) regime, α ≤ 102 cm-1; and in the infrared (~ 10 μm)
regime, α ≤ 102 cm-1. There are, of course, large variations among the thousands of
liquid crystals “discovered” or engineered so far, hence it is possible to identify
liquid crystals with the desired absorption/transparency for a particular wavelength
of interest.

Outside the far-infrared regime, e.g., in the microwave region, there have also
been active studies.6 At the 20–60 GHz region, for example, liquid crystals continue
to exhibit sizable birefringence. Studies have shown that for a typical liquid crystal
such as E7, the dielectric permittivities for extraordinary and ordinary waves are

Figure 1.7. Transmission spectra of nematic liquid crystals: (a) 5CB and (b) MBBA.

εe = 3.17 (refractive index ne = 1.78) and ε0 = 2.72 (refractive index n0 = 1.65),
respectively, i.e., a birefringence of Δn ~ 0.13.

UNLIMITED FREE ACCESS TO THE WORLD'S BEST IDEAS

close
Already an Engineering360 user? Log in.

This is embarrasing...

An error occurred while processing the form. Please try again in a few minutes.

Customize Your Engineering360 Experience

close
Category: Liquid Crystal Display (LCD) Modules
Privacy Policy

This is embarrasing...

An error occurred while processing the form. Please try again in a few minutes.