A Deformable Low-Threshold Optical Limiter with Oligothiophene-Doped Liquid Crystals

Inorganic Nanorods Enable the Memorization of Photoinduced Microlens Arrays in Dye-Doped Liquid Crystals

The photoinduced molecular reorientation of liquid crystals (LCs) caused by their nonlinear optical responses has attracted much attention due to their large refractive index change, leading to promising applications in optical devices. This reorientation is typically induced by light irradiation above a threshold intensity and is temporary, with the initial orientation recovering unless the LCs are polymerized and cross-linked. Our report highlights the memory effect of molecular reorientation in LCs. The high-intensity laser beam irradiation of dye-doped LCs containing polymer-grafted ZnO nanorods resulted in the molecular reorientation of the LCs. The effect was maintained even after the light was turned off. This memorized molecular orientation functioned as a polarization-dependent microlens due to the self-phase modulation and self-focusing effect of the propagating light. The polarization selectivity of the microlens allows for innovative optical applications, including security printing and anticounterfeiting.

Polarizability inversion suspension for nonlinear optical limiting with a low limiting threshold

We propose what we believe to be a novel nonlinear optical limiting (NOL) method with a low limiting threshold based on a light intensity-controlled polarizability inversion suspension (PIS). This suspension has negative polarizability under weak light, allowing stable propagation of weak light with a low loss. Nevertheless, the suspension reverses into positive polarizability due to the optical Kerr effect under strong light, resulting in enhanced scattering that rapidly attenuates the intense light. In a proof-of-concept experiment, PS (polystyrene)-CS2-CCl4 suspension is used as the example suspension. We experimentally verify the NOL performance of several samples. Among them, 4 g/L PS-CS2-CCl4 suspension with a volume ratio of 0.15 has the best optical limiting effect, with a high limiting capacity coefficient of 0.48 and a very low limiting threshold of 14.80 kW/cm2, which is an order magnitude lower than that of most common NOL materials. Therefore, the proposed method provides a new promising approach to achieve NOL of continuous wave laser with a low limiting threshold.

Broadband nonlinear modulation of incoherent light using a transparent optoelectronic neuron array

Nonlinear optical processing of ambient natural light is highly desired for computational imaging and sensing. Strong optical nonlinear response under weak broadband incoherent light is essential for this purpose. By merging 2D transparent phototransistors (TPTs) with liquid crystal (LC) modulators, we create an optoelectronic neuron array that allows self-amplitude modulation of spatially incoherent light, achieving a large nonlinear contrast over a broad spectrum at orders-of-magnitude lower intensity than achievable in most optical nonlinear materials. We fabricated a 10,000-pixel array of optoelectronic neurons, and experimentally demonstrated an intelligent imaging system that instantly attenuates intense glares while retaining the weaker-intensity objects captured by a cellphone camera. This intelligent glare-reduction is important for various imaging applications, including autonomous driving, machine vision, and security cameras. The rapid nonlinear processing of incoherent broadband light might also find applications in optical computing, where nonlinear activation functions for ambient light conditions are highly sought.

Effect of Host Structure on Optical Freedericksz Transition in Dye-Doped Liquid Crystals

The optical Freedericksz transition (OFT) can reversibly control the molecular orientation of liquid crystals (LCs) only by light irradiation, leading to the development of all-optical devices, such as smart windows. In particular, oligothiophene-doped LCs show the highly sensitive OFT due to the interaction between dyes and an optical-electric field. However, the sensitivity is still low for the application to optical devices. It is necessary to understand the factors in LCs affecting the OFT behavior to reduce the sensitivity. In this study, we investigated the effect of the host LC structure on the OFT in oligothiophene-doped LCs. The threshold light intensity for the OFT in trifluorinated LCs was 42% lower than that in LCs without fluorine substituents. This result contributes to the material design for the low-threshold optical devices utilizing the OFT of dye-doped LCs.