Massive, soft, and tunable chiral photonic crystals for optical polarization manipulation and pulse modulation

Abstract

Photonic crystals enable modulation of light waves in space, time, and frequency domains; in particular, chiral photonic crystals are uniquely suitable for polarization rotation and switching of complex vector fields. Current development of chiral photonic crystals, nevertheless, are still confronted with limitations of one form or the other such as large optical losses, limited or absence of tunability, narrow operation bandwidth, and/or insufficient optical thickness for practical implementation. In this work, we show that cholesteric liquid crystals as 1D tunable chiral photonic crystals are promising alternatives to not only address all these issues and deficiencies but also enable new photonic applications in wider temporal and spectral realms. Our work entails a detailed study of the dynamical evolution of cholesteric helical self-assembly and defect formation in the bulk of thick cholesteric liquid crystals under various applied electric field conditions and a thorough exploration of how applying fields of vastly different frequencies can eliminate and/or prevent the formation of unremovable defects and to control the alignment of cholesteric helices in the entire bulk. We have developed a dual-frequency field assembly technique that enables robust room-temperature fabrication of stable well-aligned cholesteric liquid crystals to unprecedented thickness (containing thousands of grating periods) demanded by many photonic applications. The resulting chiral photonic crystals exhibit useful much-sought-after capabilities impossible with other existing or developing chiral photonic crystals—compactness (single, flat, millimeter-thick optical element), high transmission, dynamic tunability, large polarization rotation, and various switching/modulation possibilities for ultrafast and continuous-wave lasers in the visible, near- and mid-infrared regimes.