Optically controlled bistable waveplates

All-optical switching of liquid crystals at terahertz frequencies enabled by metamaterials

Nematic liquid crystals integrated with metallic resonators (metamaterials) are intriguing hybrid systems, which not only offer added optical functionalities, but also promote strong light-matter interactions. In this report, we show with an analytical model that the electric field generated by a conventional oscillator-based terahertz time domain spectrometer is strong enough to induce partial, all-optical switching of nematic liquid crystals in such hybrid systems. Our analysis provides a robust theoretical footing for the mechanism of all-optical nonlinearity of liquid crystals, which was recently hypothesised to explain an anomalous resonance frequency shift in liquid crystal-loaded terahertz metamaterials. The integration of metallic resonators with nematic liquid crystals offers a robust approach to explore optical nonlinearity within such hybrid material systems in the terahertz range; paves the way towards increased efficiency of existing devices; and broadens the range of applications of liquid crystals in the terahertz frequency range.

Analysis of light diffraction by azobenzene-based photoalignment layers

Photoalignment materials, such as the azobenzene-based PAAD series studied here, are becoming increasingly important in liquid crystal-based optical devices and displays. Yet their properties and, in particular, their response to light, are still not fully understood. We investigate, experimentally and theoretically, the photoinduced birefringence, the order parameter and the formation of surface relief gratings, as well as the diffraction caused by them. We show that some of the azobenzene PAAD materials are suitable for the formation of surface relief gratings with high modulation depth, while others exhibit strong photoinduced birefringence. The two effects are inversely correlated: the stronger the surface relief grating is, the weaker is photoinduced birefringence. Analytical formulas based on the Raman-Nath approximation and numerical simulations of Maxwell's equations are used to quantify the diffraction caused by the induced diffraction gratings, showing excellent agreement between theory and experiment.

Efficient scattering model of multilayer systems with anisotropic films

We present an intuitive and efficient method for modeling light propagation in layered isotropic and anisotropic media, which we call the Iterated Ray Method. Considering a single layer sandwiched between semi-infinite layers, the infinite reflected and transmitted rays are summed to obtain effective Fresnel coefficients for the center layer. Thus, the system can be represented as two semi-infinite layers with an effective boundary. The model is coupled to a recursive algorithm to describe an arbitrarily large layered system in the same way. It is numerically stable in the presence of evanescent waves and computationally efficient, both in terms of operation counts and vectorization. We demonstrate its importance for the optical analysis and optimization of layered media, such as those used in photo-addressable liquid crystal cells, thin-film coatings, and Bragg gratings, by measuring the refractive index and thickness of a thin azobenzene dye photo-alignment layer, PAAD-22E, on an indium tin oxide coated glass slide.