Interaction Between UV Radiation and Cholesteric Liquid Crystals

Paper like cholesteric interferential mirror

A new type of flexible cholesteric liquid crystal mirror is presented. The simple and effective method for the deposition of a cholesteric mixture on a paper substrate and the particular design of the device give a homogeneous alignment of the cholesteric texture providing mirrors with an intense and uniform light reflectance. A desired polarization state for the reflected light, linear or circular, can be easily obtained varying the thickness and optical anisotropy of the polymer cover film. By using non-azobenzene based photosensitive materials a permanent array of RGB mirrors with high reflectivity can be obtained on the same device. Paper like reflective mirrors are versatile and they can find applications in reflective displays, adaptive optics, UV detectors and dosimeters, information recording, medicine and IR converters.

Photostimulated control of laser transmission through photoresponsive cholesteric liquid crystals

Cholesteric liquid crystals (CLCs) are selectively reflective optical materials, the color of which can be tuned via electrical, thermal, mechanical, or optical stimuli. In this work, we show that self-regulation of the transmission of a circularly polarized incident beam can occur upon phototuning of the selective reflection peak of a photosensitive CLC mixture towards the pump wavelength. The autonomous behavior occurs as the red-shifting selective reflection peak approaches the wavelength of the incident laser light. Once the red-edge of the CLC bandgap and incident laser wavelength overlap, the rate of tuning dramatically slows. The dwell time (i.e., duration of the overlap of stimulus wavelength with CLC bandgap) is shown to depend on the radiation wavelength, polarization, and intensity. Necessary conditions for substantial dwell time of the CLC reflection peak at the pump beam wavelength include irradiation with low intensity light (~1mW/cm²) and the utilization of circularly polarized light of the same handedness as the helical structure within the CLC. Monitoring the optical properties in both reflection and transmission geometries elucidates differences associated with attenuation of the light through the thickness of the CLC film.

Periodic structures generated by light in chiral liquid crystals

We discuss materials that reveal fundamental intercoupling of light and chirality in creation of complex structures. These materials are based on cholesteric liquid crystals (CLCs) photosensitized by azobenzene nematics. Transformation of the one-dimensional periodic structure of such CLCs into complex spatial patterns takes place on macroscopic scales, over the whole area of the CLC layer, under the influence of low power radiation including LED, ambient illumination, and sunlight. The obtained structures, with their origin in the strain of the CLC layers caused by trans-cis photoisomerization precede a shift in the bandgap position of the CLCs. The effect is observed both in red-shifting as well as blue-shifting CLCs.