Optical phase modulation using nematic dual-frequency liquid crystal materials (2f-LCM) has been investigated. The most interesting results are demonstrated using a specially developed 2f-LCM with following physical parameters (at T=21°C): values of dielectric anisotropies are +2.4 (at low frequency f = 1 kHz) and -2.6 (at high frequency f = 40 kHz), frequency of dielectric anisotropy sign inversion is 9 kHz, optical anisotropy is 0.25, transition temperatures from the nematic to isotropic phase and from the nematic to smectic one are 74-78°C and <-7°C, respectively. The phase retardation characteristics have been studied in both quasi-static and dynamic modes for both 0-aligned (homogeneous alignment) and p-aligned (director pretilt angle at counter surfaces of cell is of the opposite sign) liquid crystal (LC) cells. LC director pretilt angle of |Q| ~3-4° was defined by rubbed polyimide alignment layers.

Both 0-aligned and p-aligned LC cells can be used for 2f-LCM based phase modulators. However, to get fast and stable phase modulation in case of p-aligned cell the initial splay director configuration should at first be transferred into the bend one by applying an electric field. When the driving voltage is switched off this bend configuration relaxes inevitably into a topologically equivalent 180^o-twisted state. The 180^o-twisted director configuration is topologically non-equivalent to the initial splay configuration, which, however, is thermodynamically stable one. Finally, owing to presence of defects and thermal fluctuations, the 180^o-twisted state relaxes very slowly into the initial non-twisted state with the splay deformation of the director field.

We have established that the main obstacle in obtaining the fast phase modulation within a high dynamic range of 2p is due to by electrohydrodinamic (EHD) flows, which disturb the homogeneous (non-twisted) director reorientation. The effect of EHD flows have been confirmed both experimentally and by numerical simulations. EHD flows are not significant in case of a single-frequency driving (either by low frequency 1 kHz or by high frequency 35 kHz voltage driving). The EHD instability results from driving by periodic dual-frequency waveforms. The high frequency part of the driving waveform plays a key role in appearance of EHD instability. Using numerical simulation it was also shown that the electrohydrodinamic destabilization could be significantly suppressed by a proper choice of the Leslie coefficients ratio |a₂/a_4,5|.

Stable phase modulation at repetition frequencies of ~750 and ~500 Hz was realized respectively within the p and 2p dynamic range using LC layers of thicknesses higher than 7 mm. To obtain such phase modulation parameters, for example, for layers of thickness as large as 8 mm the rms magnitudes of the driving voltage of low (1 kHz) and high (35 kHz) frequency should be of 50 and 30 V, respectively.

The work was supported by CRDF (GAP) under Project RPO-1411-MO-03.