Liquid Crystals

Chapter 9 - Thermal, Density, and Other Nonelectronic Nonlinear Mechanisms

9.1.   INTRODUCTION

In Chapter 5 we discussed how the electrostrictive effect gives rise to density fluctuations
in liquid crystals, which are manifested in frequency shifts and broadening in the
spectra of the scattered light. In absorbing media, density fluctuations are also created
through the temperature rise following the absorption of the laser. The nature of optical
absorption in liquid crystals, as in any other material, depends on the laser wavelength.

The (linear) transmission spectrum of a typical liquid crystal is shown in Figures 9.1
and 9.2, where dips in the curve correspond to strong single-photon absorption. The
linear absorption constant α is quite high at wavelengths near or shorter than the ultraviolet
(α≈102–103cm-1), where the absorption band begins; in the visible and
near-infrared regions, the absorption constant is typically small (with α≤1 cm-1 and
α≤10 cm-1, respectively); in the midinfrared and longer wavelength regions, the
absorption constant is higher (α≈10–102 cm-1).

Under intense laser illumination, two- and multiphoton absorption processes will
occur, as depicted in Figure 9.3. In this case a so-called nonabsorbing material in the
single-photon picture outlined previously could actually be quite absorptive, if the
two- or multiphoton process corresponds to a real transition to an excited state.

From the standpoint of understanding laser-induced temperature and density
changes in liquid crystals, these photoabsorption processes may be simply represented
as a means of transferring energy to the molecule. Figure 2.5 schematically
depicts the scenario following the absorption of the incoming photons by the liquid
crystalline molecules.

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