Wavefront correction based on a reflective liquid crystal wavefront sensor

Optimizing liquid crystal cell thickness in electro-optical Fresnel lenses through theoretical calculations and experimental validation

Tunable liquid crystal (LC) lenses have gained significant attention in recent decades due to their lightweight, low cost, and versatility in applications such as augmented reality, ophthalmic devices, and astronomy. Although various structures have been proposed to improve the performance of LC lenses, the thickness of the LC cell is a critical design parameter that is often reported without sufficient justification. While increasing the cell thickness can lead to a shorter focal length, it also results in higher material response times and light scattering. To address this issue, the Fresnel structure has been introduced as a solution to achieve a higher focal length dynamic range without increasing the cell thickness. In this study, we numerically investigate, for the first time (to our knowledge) the relationship between the number of phase resets and the minimum required cell thickness to achieve a Fresnel phase profile. Our findings reveal that the diffraction efficiency (DE) of a Fresnel lens also depends on the cell thickness. Specifically, to achieve a fast response Fresnel-structured-based LC lens with high optical transmission and over 90% DE using E7 as the LC material, the cell thickness should fall within the range of 13 to 23 µm.

Novel liquid crystal photonic devices enabled by two-photon polymerization [Invited]

In addition to displays, liquid crystals (LCs) have also found widespread applications in photonic devices, such as adaptive lens, adaptive optics, and sensors, because of their responses to electric field, temperature, and light. As the fabrication technique advances, more sophisticated devices can be designed and created. In this review, we report recent advances of two-photon polymerization-based direct-laser writing enabled LC devices. Firstly, we describe the basic working principle of two-photon polymerization. With this powerful fabrication technique, we can generate anchoring energy by surface morphology to align LC directors on different form factors. To prove this concept, we demonstrate LC alignment on planar, curvilinear surfaces as well as in three-dimensional volumes. Based on the results, we further propose a novel, ultra-broadband, twisted-nematic diffractive waveplate that can potentially be fulfilled by this technique. Next, we briefly discuss the current status of direct-laser writing on LC reactive mesogens and its potential applications. Finally, we present two design challenges: fabrication yield and polymer relaxation/deformation, remaining to be overcome.

Switchable Pancharatnam-Berry microlens array with nano-imprinted liquid crystal alignment

We report a rapid nano-imprinting technique to pattern the liquid crystal alignment of a Pancharatnam-Berry phase microlens array. Through implementing a single-side aligned cell, we demonstrate a switchable microlens array with fast response time and low operation voltage. Further investigation of focusing property as well as imaging capability ensure the good quality of the microlens array. Besides planar structures, this method is also promising for patterning liquid crystal alignment on curvilinear surfaces.

Adaptive liquid crystal microlens array enabled by two-photon polymerization

A tunable-focus liquid crystal microlens array is demonstrated and characterized. Using two-photon polymerization based direct-laser writing, a polymerized microlens array is fabricated on one substrate. Such a microlens array creates inhomogeneous electric field distribution and homogeneous-like liquid-crystal alignment, simultaneously. The phase profile and thus the focal length can be tuned dynamically by the applied voltage. We also further investigate the focusing property and the imaging capability of the fabricated sample. Using the adaptive microlens array as an example, we demonstrate that directly forming a curvilinear surface with liquid-crystal alignment is feasible. In addition to adaptive lens, this direct-laser writing method is also a powerful tool for making other tunable photonic devices.

Liquid crystal spherical microlens array with high fill factor and optical power

A novel liquid crystal spherical microlens array with high optical power and almost 100% of fill-factor is proposed and experimentally demonstrated. The combination of a specific structure and electrical waveforms applied to the electrodes generates an array of spherical microlenses with square aperture. The manufacturing process is simple (patterned electrodes) and the microlenses are reconfigurable by low voltage signals (the electrodes are in contact with the LC layer). This device could be a key for the next generation of autostereoscopic devices based on Integral Imaging technique.

Polymer-stabilized liquid crystal microlens array with large dynamic range and fast response time

We report a polymer-stabilized liquid crystal (LC) microlens array with a large dynamic range and fast response time. The top substrate has a planar indium-tin oxide (ITO) electrode, while the bottom substrate has two patterned ITO electrodes for generating a fringing field and uniform longitudinal field. The fringing field is utilized to create the desired gradient refractive index profile in the LC/monomer layer, which is later stabilized by UV curing to form polymer networks. To tune the focal length, we apply a longitudinal field to change the lens shape. This microlens array offers several attractive features, such as large dynamic range, fast response time, and good mechanical stability.

Reconfigurable wavefront sensor for ultrashort pulses

A highly flexible Shack-Hartmann wavefront sensor for ultrashort pulse diagnostics is presented. The temporal system performance is studied in detail. Reflective operation is enabled by programming tilt-tolerant microaxicons into a liquid-crystal-on-silicon spatial light modulator. Nearly undistorted pulse transfer is obtained by generating nondiffracting needle beams as subbeams. Reproducible wavefront analysis and spatially resolved second-order autocorrelation are demonstrated at incident angles up to 50° and pulse durations down to 6 fs.