The increasing demand for efficient and user-friendly sensing technologies across various fields necessitates the development of novel sensing materials and platforms. Stimuli-responsive photonic crystals (PCs), particularly cholesteric liquid crystals (CLCs), are attractive candidates due to their self-assembled helical superstructures and ability to produce structural colors. However, existing CLC-based sensors often rely on visual perception of color changes or spectroscopic measurements, limiting their practical applications. This research addresses this limitation by introducing a visual sensing platform that utilizes geometric phase encoding to generate real-time visual patterns as sensing signals. The platform employs stimuli-responsive CLCP films encoded with q-plate patterns. Changes in external stimuli, such as humidity, alter the CLC helix pitch, resulting in changes in the reflected vortex beam patterns, providing a direct visual indication of the stimulus. The use of orbital angular momentum (OAM)-carrying laser beams further enhances the platform's capabilities for remote sensing due to the high directionality and long-range transmission of such beams. This approach provides a new level of practicality for CLC-based sensors, expanding their potential applications in diverse fields.

Existing literature highlights the responsiveness of CLCs to external stimuli like temperature, humidity, strain, and pH, leading to their use in various optical sensors. However, these sensors often rely on measuring wavelength changes using spectrometers, limiting their usability. Research on geometric phase (Pancharatnam-Berry phase) encoding in planar CLC devices has shown promise for creating active planar optical elements (POEs) such as deflectors, lenses, and optical vortex (OV) generators. These elements leverage the spin-orbit interaction of light and can manipulate the polarization state of reflected light to encode information. The integration of geometric phase encoding with stimuli-responsive CLCs offers a unique opportunity to create visual sensing platforms that directly translate stimulus changes into easily observable patterns, overcoming the limitations of wavelength-based sensing.

The study involved the fabrication of humidity-responsive CLCP films encoded with q-plate patterns. A humidity-responsive CLCP material system comprising reactive mesogens (RMs), a photoinitiator, and a chiral agent was used. The fabrication process involved patterning a cell coated with a photoalignment material (SD1) using 405 nm linearly polarized light to induce molecular alignment. The CLC mixture was then introduced, and polymerization was performed using UV light. The resulting CLCP film was treated with potassium hydroxide (KOH) solution to enhance hydrophilicity and create a hygroscopic CLCP salt. The films were characterized using polarizing optical microscopy and reflection spectroscopy to determine the relationship between the reflection band and relative humidity (RH). Humidity sensing was demonstrated using a single q-plate encoded CLCP film illuminated with a He-Ne laser. The reflected vortex beam patterns were captured using a charge-coupled device (CCD) camera, and the changes in pattern morphology with varying RH were analyzed. A dual-wavelength humidity monitoring system and a CLCP film encoded with a four-quadrant q-plate array pattern were also explored to extend the sensing range and improve real-time capture of humidity levels. Remote sensing capabilities were investigated by utilizing the high directionality of the laser beams.

The fabricated humidity-responsive CLCP films showed a significant redshift in their central reflection band with increasing RH, ranging from 581 nm to 719 nm as RH increased from 10% to 95%. The single q-plate encoded CLCP film demonstrated humidity-dependent modulation of the reflected vortex beam pattern. Below 40% RH, minimal changes were observed, while above 40%, the reflected light spot intensity increased with increasing RH. The quality of the vortex beam was highest between 73% and 86% RH. The proposed platform demonstrated the feasibility of real-time, visual humidity sensing using geometric phase encoding, eliminating the need for complex spectroscopic measurements. The use of OAM-carrying laser beams enabled remote sensing capabilities, overcoming limitations of traditional sensors in challenging environments.

The results demonstrate a novel approach to CLC-based sensing that offers significant advantages over existing methods. The ability to directly visualize the sensing signal through changes in the reflected vortex beam patterns eliminates the need for specialized equipment and simplifies the sensing process. The high directionality and long-range transmission properties of OAM-carrying laser beams make the platform suitable for remote sensing applications. The demonstrated humidity sensor serves as a proof-of-concept, and the platform's design is adaptable for other stimuli-responsive CLCs and sensing applications. Future research could focus on optimizing the CLCP material system for improved sensitivity and response time, exploring different q-plate patterns for enhanced sensing capabilities, and integrating the platform with wireless communication for fully autonomous remote monitoring.

This study successfully demonstrated a novel visual sensing platform using geometric phase-encoded stimuli-responsive CLCPs for real-time remote monitoring. The use of q-plate patterns and OAM-carrying laser beams enabled direct visualization of humidity changes through variations in the reflected vortex beam patterns. This approach offers a significant advancement in CLC-based sensing, providing a practical and user-friendly alternative to traditional methods. Future work should focus on expanding this platform to other stimuli and applications, further optimizing the material and design for enhanced performance.

The current study primarily focuses on humidity sensing as a proof-of-concept. Further research is needed to evaluate the platform's performance with other stimuli and to optimize the material system for improved sensitivity and response time across a wider range of environmental conditions. The long-term stability of the CLCP films under various environmental conditions also requires further investigation. The remote sensing capabilities are currently limited by the power of the laser source and atmospheric conditions.