Electrically tunable focusing liquid crystal (LC) lenses are reviewed in this paper. The distribution of the orientations of LC directors which is controlled by electric fields results in a distribution of refractive indices of LC directors. The incident light can be modulated by the electrically tunable lens-like phase difference of the LC lens. We introduce the basic operating principles of LC lenses and discuss the structures of LC lenses. The major challenges of LC lenses are also discussed. We believe this paper provides a guideline for basic understanding of LC lenses.

A polarization-independent liquid crystal phase modulation using polymer-network liquid crystals in a 90 twisted cell (T-PNLC) is demonstrated. T-PNLC consists of three layers. Liquid crystal (LC) directors in the two layers near glass substrates are orthogonal to each other and those two layers modulate two eigen-polarizations of an incident light. As a result, two eigen-polarizations of an incident light experience the same phase shift. In the middle layer, LC directors are perpendicular to the glass substrate and contribute no phase shift. The phase shift of T-PNLC is electrically tunable and polarization-independent. T-PNLC does not require any bias voltage for operation. The phase shift is 0.28 p rad for the voltage of 30 V rms . By measuring and analyzing the optical phase shift of T-PNLC at the oblique incidence of transverse magnetic wave, the pretilt angle of LC directors and the effective thickness of three layers are obtained and discussed. The potential applications are spatial light modulators, laser beam steering, and micro-lens arrays.