Andrey Iljin
Institute of Applied Physics, University of Münster, Münster, Germany
Institute of Physics, National Academy of Sciences of Ukraine, Kyiv, Ukraine

Liquid crystals, nonlinear optics and holography

Liquid crystals (LC), owing to their large birefringence and strong responsiveness to external fields appear nearly omnipresent in modern optoelectronic and photonic technologies and devices. Almost all known nonlinear optical phenomena have been observed in LC media due to their extraordinarily large optical nonlinearities that span over multiple time scales and a wide spectral range.
For the time being, light-induced realignment of the LC director with the associated reorientation of the optical axis has been regarded the principal mechanism engaged in the vast majority of nonlinear optical experiments. The elastic forces of LC medium, however, spread vastly outside the illuminated region making the nonlinear optical response substantially nonlocal and concealing the recorded optical information. Wherein, as far as holography needs are concerned, a strict one-to-one correspondence between the light intensity and refractive index modulation is required.
On absorption of light energy, a molecule sitting in a liquid crystalline (LC) medium firstly interacts with its closest neighbours, the orientational ordering of which is being greatly affected. A phenomenological model of Light-Induced Order Modification (LIOM) has been developed that accounts for the changes of the refractive indices of a thin LC layer resulted from the light-stimulated modulation of the LC order. It appears to be in a good agreement with and explains well the experimentally observed large optical nonlinearities. The LIOM-type mechanism does not depend on the cell thickness, works for the whole range of light wavelengths and runs by far faster than the LC director reorientation. Besides, since the optical read-out is spectrally independent from the pumping, such a mechanism could be useful for fast control of optical signals of very high intensity, for instance, in the IR range and beyond.
Despite being strictly local in material response the LIOM-mechanism nevertheless implies a fine tuning of a phase shift between an interference pattern and a refractive index grating it stimulated. Such a photorefractive non-locality could be widely used for the amplification of laser beams and optical images, creation of phase delay lines, in beam steering, phase conjugation, shaping of pulses, etc. There are the expectations, at least.