Nonlinear optics (NLO) describes the behaviour of light in nonlinear media, for which the dielectric polarization responds to the incoming light field in a nonlinear manner^{1, 2}. NLO has immense potential in applications dependent on nonlinear processes, for instance, frequency conversion^{3, 4}, multiple-photon absorption^{5, 6}, and self-phase modulation^{7, 8}. However, the generation of such optical nonlinearities typically requires very high light intensities and thus costly lasers. Crystals⁹, polymers^{3, 10}, and liquid crystals (LCs)¹¹ exhibiting high optical nonlinearity have been extensively studied with the aim of creating NLO effects using inexpensive, low-power lasers. The intensity-dependent refractive index, one of the useful NLO effects, evokes self-focusing, self-phase modulation, phase conjugation, and the generation of optical solitons. It thereby possesses great potential for use in a range of photonic applications, such as optical switching, data storage, and light-beam modulation^{12, 13, 14, 15}.

Here, we report that the light intensity needed to induce the self-focusing effect can be reduced to levels accessible with a common 1 mW handheld laser pointer in polymer-stabilized TR5-doped LC (PSLC)¹⁶. The key to this achievement is the use of hybrid molecular alignment instead of the commonly used homeotropic or planar LC alignment. Hybrid alignment refers to homeotropic (out-of-plane) alignment at one substrate of the LC cell, while homogeneous (in-plane) alignment occurs at the other substrate, as illustrated in Fig. 1¹⁷. Hybrid alignment has been investigated mostly from a theoretical perspective for issues such as LC molecular alignment¹⁸ and light propagation behaviour. We chose to use hybrid LC alignment because of the unique properties of hybrid-aligned cells. These include optical transparency, a bistable surface, and opposed surface anchoring, which might enhance reorientational optical nonlinearities in LC systems. The NLO response of the hybrid-aligned TR5-doped PSLC film was compared to homeotropic-aligned film with the same composition.

Figure 1. (a) Sketch of the geometry of a hybrid-aligned cell.
(b) Sketch of the geometry of a homeotropic-aligned cell.
(c) Photograph of a hybrid-aligned cell. (d) Photograph of a homeotropic-aligned cell. (e) Polarized optical micrograph of a hybrid-aligned cell on a rotatable stage. (f) Polarized optical micrograph of a homeotropic-aligned cell on a rotatable stage. Optically transparent film with hybrid alignment is confirmed from photograph and conoscopic images with a polarized optical microscope. ¹⁶

The threshold intensity of the hybrid-aligned PSLC cell to induce molecular reorientation based on nonlinear optical effect was at its lowest (0.4 W/cm²) in the homeotropic-0 arrangement, that is, when light having polarization parallel to the rubbing direction was incident on the sample from the homeotropic side. The threshold was then compared to that of the homeotropic-aligned PSLC sample with the same composition. The threshold intensity was markedly reduced in the hybrid-aligned cell by a factor of 8.5 compared with that of homeotropic-aligned PSLC (3.4 W/cm²). To the best of our knowledge, this is the lowest value obtained for the order-to-order type of self-focusing effect (from the original molecular alignment to the molecular director parallel to light polarization). Hence, the laser power required for nonlinear optical molecular reorientation could be in the sub-100 μW range and would be accessible with common low-power handheld laser pointers. As shown in Fig. 2, the diffraction ring patterns due to self-focusing and self-phase modulation immediately appeared in the hybrid-aligned cell merely by turning on the laser pointer.

Figure 2. (a) Sketch of the optical setup for the observation of the laser-pointer-driven nonlinear optical effect in a hybrid-aligned cell. (b) Typical laser-pointer-driven ring pattern generated from a hybrid-aligned cell (TR5: 0.03 mol%). A 1 mW commercially available laser pointer drives molecular reorientation, leading to self-focusing and self-phase modulation. ¹⁶

In summary, we demonstrated the nonlinear optical effect of hybrid-aligned olighothiophene-doped polymer-stabilized LC film. Self-focusing was enabled by photoinduced molecular reorientation at the lowest light intensity of 400 mW/cm², which is less by a factor of 8.5 than the intensity needed for homeotropic-aligned LC cells with the same composition. Even a battery-operated laser pointer induced the diffraction ring formation due to molecular reorientation. Furthermore, the film showed incident direction sensitivity, hence acting as a simple nonlinear optical diode. A large-area flexible film of the studied materials can be fabricated by display technology, which enables much wider applications of nonlinear optics.

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