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.

Liquid crystals for optical switches, smart windows, reconfigurable/tunable meta-structures, micro-resonator coupling and plasmonic nanostructures

“…11c] can also be performed by modulating the birefringence/index with a variety of mechanisms, including the ultrafast electronic optical nonlinearities mentioned previously [12,13,[85][86][87][88][89][90]. Theoretical simulations [86] with the same material parameters have shown the feasibility of extending such polarization switching and pulse modulation operations throughout the entire visible/near IR regime to the mid-infrared (λ ~ 5-μm) regime. Thicker and highly transmissive CLC's are likely to improve lasing action based on doping them with gain materials such as laser dyes [91,92].…”

“…It is important to emphasize here that the polarization rotation action of a 1-D chiral photonic crystal such as CLC is independent of the input vector orientation, and so CLC is ideally suited for orthogonal switching [Δφ = 90 o or /2 radian] of complex (azimuthal or radial) polarizations depicted in Figure 11b. CLC's of such thickness and beyond require special field-assisted techniques to fabricate; as reported in recent work [84][85][86], mm-thick CLC is capable of >360 degrees polarization rotation of CW and ps-fs pulsed lasers of conventional or complex polarization states in the entire visible to near-and mid-infrared (700-1700-5000 nm) regime. Dynamic switching of the polarization states and pulse modulations [self-compression, pump pulse induced complex laser vector beam switching, …etc., c.f.…”

“…which breaks up into two circular polarizations: left (σ−) and right (σ+) circular polarizations with respective refractive indices n− and n+. Due to the circular birefringence Δnc = n− − n+, a phase difference or retardation (Γ) is built up between the σ− and the right σ+ after traversing the CLC, and results in a rotation of the polarization vector by an angle Δφ = Γ/2 = ΔncπL/λ0 where L is the CLC thickness and λ0 is the optical wavelength in vacuum [84][85][86].…”