Open Access
Add to BibSonomy
Abstract
The liquid crystal (LC) geometrical phase optics, which is realized by the high-resolution control of the optical axis orientation in transparent micrometer-thin polymer films, is emerging as a next generation of planar optics. It features pronounced optical properties and stimuli-responsive behaviors, which could introduce appealing and new possibilities for photonic purposes. The development of fabrication techniques producing elements with large aperture sizes and arbitrarily varying molecular orientation is of significance in terms of practical utility. Here, we propose the pulsed polarization patterning technique to create large-aperture and defect-free LC geometrical phase elements. We investigated the capability of the azo photo-alignment material responding to nanosecond laser pulses and the corresponding anchoring behaviors to LCs. The threshold was reduced to one fourth of that under the continuous wave recording. The patterning resolution was found to be enhanced to around 0.71 µm, due to the ultra-fast interaction nature of the photo-alignment material with the polarized light field. We proposed the flying exposure mode to deliver high frequency modulated polarized laser pulses (8 kHz), with the precision stage moving in a uniform velocity for light-field stitching and the servo auto-focusing in the sample normal, enabling the stable and reliable polarization patterning for large aperture sizes. We further report on representative fabrication of LC polarization gratings with an aperture of 4 inch and 99.2% average diffraction efficiency.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Optical devices are common in civil and industry applications, from eyeglasses to telescope [1,2]. Traditional optics (reflection or refraction) modifies the wavefront of incident light through a surface profile in a media of specific refractive indices that can be engineered to control the optical path at each point. In this way, the phase and polarization changes are accumulated when propagating though the curved media. The thickness of traditional optics increases significantly when the lateral dimension expands, leading to high weight and volume of large aperture optical components, as well as time-consuming and high-cost manufacturing processes [3]. However, the aperture is important in optical devices [4] which defines the capability to collect light and is crucial in terms of various performance parameters such as the imaging quality, the angular resolution, the transmitting power, etc. al. The pressing need for optical systems with significantly reduced weight, size and driving power requirements promotes the development of next-generation optics [3,5–7]. In such background, the development and application of the planar optical technologies, such as the diffractive optics and metamaterial optics, are being intensively investigated [8–11]. The former utilizes a ∼1 µm surface relief layer to form a spatially varying optical response, where the relief structure was obtained by the folding the surface profile with integral multiple wavelengths. While the latter utilizes a ∼100 nm layer which contains arrays of subwavelength building blocks with spatially varying geometric parameters to mould the wavefront into shapes at will [12]. As the thickness of the functional layer of these two approaches is thin and is independent of the lateral dimension, the planar optical technologies are expected to achieve characteristics of lightweight and miniaturization, which may be further exploited to enable large aperture optics with unrivaled performance in terms of imaging resolution, field of view, transmitted power, etc., al. However, large aperture optical elements based on the diffractive optics and matamaterial optics face many technical bottlenecks [13,14], including the stringent requirements of the high-resolution and large-area nanofabrication technique, the uniform and large-area deposition of functional materials and the high fidelity and large-area etching process, which hinders the development of large aperture planar optics.
The combination of the geometric phase with liquid crystal (LC) materials offers an alternate platform for planar optics [15–17], which is achieved by spatially varying the optical axis orientation in the anisotropic material [18]. Apart from being a natural birefringent material to fulfill the phase retardance condition for geometric phase, LCs have unique superiorities and versatile capabilities [19,20]. Specifically, their pronounced optical properties (high transmittance, large birefringence and superior reliability) and stimuli-responsive behaviors introduce appealing and new possibilities for photonic purposes, enabling planar optics with advantages of light-weight, high efficiency, broadband operation, stimulus tunability/switchability, flexibility, and easy expansion of the aperture [21–23]. To bring this into reality, the high-resolution and continuous spatial regulation of LC domain directors, in addition to consistence over the whole device aperture is of paramount importance. To this regard, several photo-alignment techniques (to control the LC alignment with polarized light) have been proposed for the demonstration of LC geometric phase optical elements, such as the polarization holography [24], SLM digital polarization patterning [25], plasmonic photo-alignment [14,26] and polarization laser direct write technique [27–29]. One limitation of the aforementioned techniques lies in the fact that the continuous wave polarization recording in the photo-alignment material with a high exposure threshold (∼1 J/cm2, which is one order higher than the photoresist) would bring issues such as low fabrication efficiency, line bending, line-width broadening, low stitching quality, etc. al., as a result, the fabrication of large aperture and defect-free LC planar optics was still a great challenge.
In this paper, we propose and demonstrate a high-throughput pulse polarization patterning technique to achieve large-aperture and defect-free LC planar optical elements. We first investigated and verified the ultra-fast response of the photo-alignment material to the nanosecond polarized laser pulses and its rewritable capabilities. We then proposed the flying exposure mode designed specifically for the pulsed polarization patterning, where the stage moves in a uniform speed in the designed route with shaped polarized laser pulse delivery on demand. With the aid of the imaging lens and the nanosecond recording process, the pulsed polarization pattering resolution reaches ∼0.71 µm, comparable to the plasmonic photo-alignment. The splicing error of different polarization patterns was eliminated by para-position superposition exposure. Finally, we fabricated 4-inch polarization gratings with average diffraction efficiencies beyond 99%. The grating periodicity was easily adjusted between 3 µm and 8 µm. The system completed writing each 4-inch LC polarization grating (PG) in 2 hours. The high-throughput pulsed polarization patterning technique would lead to geometrical phase LC planar optical elements with unprecedented performance characteristics, including the large aperture, the high texture quality and the arbitrarily designed phase front, which are of critical importance for future applications in opto-electronics, imaging, sensing, ultra-fast photonic technologies and beyond.
2. Setup of the pulsed polarization patterning system
The pulsed polarization patterning system consists of the light source, the digital micro-mirror device (DMD) modulator [30], the polarization modulator, the imaging part, the servo auto-focusing part and the precision stage (Fig. 1). The light source is an ultraviolet pulsed solid-state laser (355 nm, TALON-HE355-500SHR, MKS | Spectra-Physics) delivering trains of laser pulses of 20 ns at the frequency of 20 kHz. After being expanded and collimated, the laser beam irradiates the DMD device (1920 × 1080, pixel size 10.8 µm, refreshing frequency 8 kHz, Texas Instruments), which acts as the time and spatial modulator to allow the selected pulses to pass while refreshing the intensity pattern. Simultaneously, the polarization modulator alters the polarization direction of the linearly polarized light in accordance with the passed pattern. The first three components of the system provide an effective and flexible approach to modulate the laser pulses in terms of the pulse number, the intensity distribution of the laser pulse and the polarization direction of the pulse, allowing the ultrafast and versatile polarization patterning (Fig. 2(a)). The photo-alignment material molecule would be induced perpendicular to the polarization direction of the light, thereby regulating the ordered alignment of LC domains on demand [31,32]. The polarized pattern was projected onto the sample plane through the objective lens (20×, NA = 0.45, WD = 4.5 mm, TU Plan Fluor, Nikon), resulting in a theoretical patterning resolution of around 0.25 µm. The servo auto-focusing part consists of a servo motor in the sample normal, a continuous wave (CW) 650 nm laser (out of the absorption band of the photo-alignment material) and a fast response CCD camera. The auto-focusing part would guarantee the focusing of the imaging plane on the sample plane by a fast correction of the red laser spot as the conjugated image (at the rate of 1000 Hz with a precision of 5 nm). The fast-response auto-focusing part is paramount in ensuring high-quality long-term exposure for large aperture optics, which removes the influence of the unevenness of the large-sized substrate and the random environmental turbulence (ambient temperature and humidity). The precision stage consists of the linear encoders and the XY motors for precision positioning in the sample plane, which would allow the accurate polarization pattern stitching and positioned multiple exposures. The essence of the pulsed polarization patterning technique is to deliver laser pulses with an ultra-fast modulation in terms of the intensity, the number, the pattern and the polarization, while the precision stage moves in a uniform velocity for light-field stitching with the servo auto-focusing in the sample surface (the so-called flying exposure mode) (Fig. 2(b)). The pulsed polarization patterning yields several advantages. First, a high resolution and high line quality of polarization patterning would be expected. Each frame of the polarization pattern was recorded during single laser pulse (∼20 ns), i.e., the interaction time of the light with the photo-alignment material was very short [33]. The ultra-short recording time would provide resistance to ambient interference and prohibit the influence of the light-dragging effect during CW writing, resulting in defect-free LC optics. Second, the flying exposure mode in the pulsed polarization pattering technique would ensure a high fabrication efficiency (∼1 cm2/min), as the pattern stitching never stops during the whole recording process. The fabrication efficiency of the patterning technique directly determines the available aperture of the LC optical element, i.e., the geometric growth of the due time under a low fabrication efficiency would make the fabrication task impossible. These characteristics of the pulsed polarization patterning technique would lay a solid foundation for the realization of the large-aperture and defect-free LC planar optics.
Fig. 1. Setup of the pulsed polarization patterning system. (a) Schematic illustration of the optical setup of the patterning system which consists of the pulsed laser, the DMD modulator, the polarization modulator, the imaging part, the servo auto-focusing part and the precision stage. (b) Image of the self-build pulsed polarization patterning system which supports patterning a LC planar optical element up to 65-inch.
Fig. 2. Laser pulse modulation with multiple parameters and the flying exposure mode illustration. (a) The laser pulse from the pulsed laser was modulated by the DMD modulator and the polarization modulator. (b) Schematic illustration of the flying exposure mode combining the laser pulse modulation and servo auto-focusing functions.
3. Response behaviors of the photo-alignment material to nanosecond pulses
The high-resolution patterning of the LCs to obtain a spatially variant distribution of the optical axis of the anisotropic material is a prerequisite for the realization of the LC geometrical phase planar optics. Common generation of the uniaxial alignment of LCs was achieved by the intermolecular interactions of the rod-like molecules with substrate treated with rubbing. Although the modified micro-rubbing technique using the atomic force microscope stylus provides a feasible approach to achieve nanoscale alignment [34], the arbitrary alignment over macroscopic areas was impractical. The photo-alignment technique emerges as a feasible approach to solve the paradox. The irradiation of the linearly polarized light onto the photo-alignment material would induce a uniaxial orientation of molecules, resulting in dispersive surface forces to align the LCs in contact. The anisotropic response of the photo-alignment material to CW light have been severely investigated and applied to the fabrication of LC planar optical elements [18,27]. However, the exposure threshold of the photo-alignment material is high (∼J/cm2) and the recording time for each frame in this technique can exceed 5 minutes [35]. Reduction of the recording time from several minutes to nanoseconds would not only improve the preparation efficiency but also enhance the stability to environmental disturbance. The response behaviors of the photo-alignment material to nanosecond pulses have not been verified yet. To this regard, there are two critical concerns about the response behaviors which directly determine the characteristics of the proposed pulsed polarization pattering. The first is whether the photo-alignment material would respond to the polarized nanosecond pulses and be induced with the anisotropic interactions for LC alignment. The second is the sustainability of the photo-alignment material under the focused laser pulses with a very high transient power density (>105 W/cm2). For these purposes, we have irradiated the photo-alignment material with laser pulses of different dosage and polarizations and investigated the alignment performance of LC molecules placed on the patterned substrate.
The solution of the photo-alignment material sulfonate azo dye SD1 in dimethylformam (0.5 wt. %) was spin-coated (2500 rpm) onto the cleaned glass substrate to form a nanometer thickness alignment layer. The thin alignment layer was sufficient to provide anisotropic intermolecular interactions for uniaxial LC alignment while not affecting the optical transmittance of the optics. It was assumed that the azo dye SD1 molecules would experience an ultra-fast rotation to align perpendicular to the polarization direction of the light, in a time-scale comparable to the pulse duration of the laser (Fig. 3(a)). For demonstration, the self-developed pulsed polarization patterning system was programed to write an array of rectangles (100 µm*200 µm) with varying pulse energy from 0.05 J/cm2 to 0.8 J/cm2, while the polarization of the light was unchanged. The solution of the LC polymer OCM-A1 (Raito Materials Technology Co., China) in propylene glycol methyl ether acetate at a concentration of 30 wt. % was spin-coated onto the patterned substrate and was UV-cured to yield a uniform LC polymer film with a thickness of 1.1 µm. The film was then placed under the polarizing optical microscopy (POM) for texture observation where the sample exhibited dark and bright states with different rotations (Figs. 3(c)–3(d)). Clear patterned rectangles could be easily discriminated from the texture images, indicating the robust response of the photo-alignment material to the pulsed laser. In addition, the contrast between the bright state and the dark state increases as the exposure density increases, indicating a possible threshold of the photo-alignment material under pulsed laser irradiation. We further plotted the dependence of the order of the LC polymer on the pulse energy density, which indicated that the threshold light density was around 0.25 J/cm2 (Fig. 3(b)). It was worth to mention that a threshold density beyond 1.0 J/cm2 was commonly adopted in CW photo-alignment [35,36] to achieve sufficient anchoring energy of the SD1 film for the stable ordering of the LC. The threshold reduction using the pulsed laser was advantageous in terms of the fabrication efficiency. There are two possible reasons for the reduction in threshold. One is that some energy of the CW light was wasted on the thermal effect instead of inducting the rotation of the SD1 molecule. The other is that the thermal effect could accelerate molecular irregular motions which may disturb the ordering along the specific direction, leading to an increased threshold under CW pumping.
Fig. 3. Study on the response of nanosecond pulse to azo molecules and the performance of patterned alignment. (a) The schematic diagram of ultrafast rotation of azo molecules under the action of pulsed polarized laser at nanosecond scale. (b) The dependence of order degree of LC polymer on pulse energy density. (c)-(d) POM (10 X) images of patterned squares with different exposure dosage.
After verifying the alignment capability of the photo-alignment material azo dye SD1 to the polarized laser pulses, we further investigated the sustainability of the material under the pulsed pumping. Considering that the instantaneous power density irradiated onto the photo-alignment material was very high, the azo dye SD1 may be destroyed and decomposed during the interaction with the intense UV light, leaving “dead” sites inert to polarized light. The decomposition process may also leave unknown byproduct molecules that may disturb the anchoring interactions with the LC molecules. We initially assume that the photo-alignment material could withstand the hash pumping condition under the laser pulse and would further respond to subsequent laser pulses of different polarizations, i.e., the orientation of the SD1 molecule would be altered successively by a train of laser pulses of different polarizations (Fig. 4(a)). For experimental demonstration, the pulsed polarization patterning system was programed to write five successive graphs at the fixed location (Fig. 4(b)). These graphs are cut down one by one and internally tangent to each other to discriminate the pulse order. The pulse exposure density for each graph was set to be 0.5 J/cm2. The polarization direction was changed in the turn of 0°, 15°, 26°, 40° and 45°. Accordingly, the angle difference of the molecular direction in the first rectangle graph and that in the last sphere would also be 45°, considering the fact that the azo dye SD1 molecule would align perpendicular to incident polarization. After patterning, the LC polymer solution was spin-coated onto the substrate and followed by a UV curing process to obtain the solid film for inspection. The sample was placed under the POM (20 X) with the LC molecules in the first patterned rectangle perpendicular to the analyzing polarizer. With the successive patterned graphs, the outer regions of the second rectangle stayed dark while the following graphs became brighter and brighter as the graph size scaled down. The observed fact agrees well with the experimental design that the contrast between the first rectangle and the last sphere under the POM should be the highest with the 45° molecular orientation difference (Figs. 4(c)–4(d)). Thus, the azo dye SD1 is robust in receiving multiple laser pulses with a high transient peak power density and the repeatable orientation characteristics of the material was verified. This feature also provides important significance for the preparation of subsequent LC planar optical devices. The ability of rapid brushing and tolerance of SD1 oriented molecules can effectively improve the preparation efficiency of large-aperture LC planar optical devices.
Fig. 4. Sustainability of the photo-alignment material under the cyclic pulsed pumping. (a) The schematic diagram of the cyclic pulsed polarization patterning. (b) Pulsed polarization patterning of five successive graphs at the fixed location. (c)-(d) POM images of the cyclic photo-alignment pattern with a 45° orientation difference.
4. Alignment resolution of the pulsed polarization patterning technique
The resolution L and sample size D have always been the main parameters defining the capabilities of nanofabrication techniques. In the field of optics, the former parameter determines the performance characteristics including the diffraction angle, relative aperture and efficiency, while the latter links more with imaging resolution, angular resolution and transmitted power [4]. More importantly, the two parameters are in a close restrictive relation. For example, the data volume scales with D2/L, i.e., fine linewidth and large aperture would present a huge challenge concerning data processing capability in nanofabrication. The pulsed polarization patterning technique proposed here would achieve a balance concerning the two parameters, thus enabling delicate LC planar optics with large apertures. The ultra-fast response of the photo-alignment material to nanosecond laser pulses would eliminate the line-widening effect during CW writing (Fig. 5(a)), enabling a high polarization patterning resolution of 0.71 µm. In addition, the flying exposure mode specifically designed for pulsed polarization patterning would distribute polarized light fields at the rate of 1 kHz with ultra-high movement precision in X, Y and Z dimensions, providing a patterning efficiency of ∼1 cm2/min. The patterning system was programmed to write a traditional resolution plate pattern onto the substrate coated with the photo-alignment material SD1. After photo-alignment, the substrate was coated with the LC polymer with a film thickness of 500 nm (spin-coated from the LC polymer solution of 20 wt. % at 2500 rpm and UV cured under 365 nm LED for 1 minute). The thin LCP layer was chosen for the demonstration of the patterning resolution, and multiple coating process would achieve the half-wavelength retardation condition for high optical efficiency. The sample was then placed under the POM (10 X) for inspection. The resolution plate consists of interlaced radial regions with molecular orientation of 0° and 45°, and thus they appear opposite dark and bright states under the POM (Figs. 5(b)–5(c)). The patterning resolution was deduced from the narrowest resolvable line width in the center to be around 0.71 µm. It is worth to note that the typical photo-alignment resolution using the CW polarization direct write technique was around 3 µm [15,27,28], while it can be improved to 1 µm by evoking the capabilities of plasmonic materials in focusing light at subwavelength scales [14]. The measured polarization patterning resolution under the pulsed writing verified the potential of the technique in generating large aperture LC planar optics with fine features. The limitation of the patterning solution on smoothing the phase gradient may also be improved using a bifacial LC layer [24]. The patterning system further output a complex pattern of the Soochow University badge by inscribing the 45° orientated molecular regions in the uniform background of 0° orientated LC molecules (Figs. 5(d)–5(e)). The details of the complex Chinese characteristics are clearly recorded in the anisotropic material, indicating the high resolution of the technique in writing arbitrary LC molecular patterns.
Fig. 5. Alignment resolution of the pulsed polarization patterning technique. (a) Schematic diagram of line-widening effect elimination with nanosecond pulses. (b)-(c) Bright and dark state characterization of resolution plate under POM. (d)-(e) Bright and dark state characterization of the complex pattern of Suzhou University badge.
5. Fabrication of the large aperture and defect-free LC polarization grating (LCPG)
In the pulsed polarization patterning system, the high rate modulated light field pattern imaged from the DMD was utilized to write the polarization information into the photo-alignment material which subsequently directed the high-resolution alignment of LC for planar optics. The size of each frame of the light field pattern was around 200 µm. In order to obtain the large-aperture LC optical elements, we have proposed the flying exposure mode to enable the high-throughput stitching of the pulsed light field pattern. The essence of the technique is to deliver the arbitrarily modulated light field pattern pulses onto the substrate while the precision stage moves in the X-Y directions and the autofocusing part correcting the Z position error. After verifying the high patterning resolution of the pulsed polarization patterning system, we further fabricated several practical LCPGs for demonstration. The size of these LCPGs was 4 inch, which was sufficiently large for practical applications such as laser steering, beam splitting and polarization imaging. Parameters of these elements including the periodicity and the polarization gray scale can be easily adjusted with the aid of the flexibility of the patterning system. In addition, the high-quality polarization gratings resulting from the pulsed polarization patterning would be utilized as the templates for mass production by holo-printing [37].
After superposition pulsed polarization patterning (See Supplemental Document), the glass substrates were coated the reactive LC polymer solution (35 wt.%, 3000 rpm) and were subsequently UV cured to fix the molecular orientation in the film. The reactive LC polymer solution coating and the UV curing processes were repeated twice to obtain a LC polymer film with a thickness of 2.1 µm, which fulfills the half wave retardation condition at the wavelength of 633 nm. The fabricated 4-inch LCPG appeared uniform rainbow-like colors under the illumination of the room lamp (Fig. 6(a)). We further prepared four LCPGs with periodicities of 3.04 µm, 3.80 µm and 6.07 µm and 8.00 µm (Fig. 6(b)). The LCPG periodicity resulting from the direct write technique in previous publications was beyond 20 µm [25,27,38]. This further demonstrated that the pulsed polarization patterning technique is advantageous in terms of the patterning resolution due to the ultra-fast interaction of the photo-alignment material. The due time for patterning these gratings with the system was around 2 hours. The expected task time was mainly determined by the size of the optical element and was weakly related with the linewidth and the polarization gray scale, as the size of the single light field pattern stayed the same. In order to characterize the diffraction performance of the prepared PG, the detecting light with right-handedness circular, left-handedness circular and linear polarizations was incident onto the samples (Fig. 6(c)). The quantitative diffraction efficiency was measured using the high precision energy meter (1936-C, Newport). The beam was diffracted into the specific single diffraction order under the circular polarizations while into the symmetric ±1 orders under the linear polarization, while the measured average diffraction efficiency was beyond 99% across the whole 4-inch LCPG.
Fig. 6. Fabrication of large aperture, defect-free LCPGs. (a) Image of the 4-inch LCPG under room illumination. (b) POM images of the LCPGs with varying periodicities. (c) Diffraction patterns of the LCPG with right-handedness circular, left-handedness circular and linear polarizations.
6. Conclusion
In this work, we have proposed the pulsed polarization patterning technique to obtain large aperture and defect-free LC planar optics. The alignment behaviors of the SD1 azo molecule under the polarization laser pulses were investigated. The azo molecule would rotate perpendicular to the polarization direction in the time-scale of 20 ns. The sustainability of the Azo-molecule responding to a train of laser pulses of varying polarizations was verified. It was also found that the alignment threshold under the pulsed pumping was reduced to one-fourth of that under CW pumping. The alignment resolution was improved to 0.71 µm due to the ultra-fast interaction nature of the pulsed patterning. We also proposed and successfully implemented the flying exposure mode to deliver ultra-fast modulated laser pulses in terms of the intensity, the number, the pattern and the polarization, while the precision stage moves in a uniform velocity for light-field stitching with the servo auto-focusing in the sample surface. The flying exposure mode was advantageous in improving the fabrication efficiency and robustness of the polarization patterning technique. We further prepared 4-inch LCPGs with a periodicity down to 3.04 µm and average diffraction efficiency beyond 99% with the aid of the pulsed patterning technique.
Funding
National Key Research and Development Program of China (2022YFB3606603); National Natural Science Foundation of China (62175170, 62275180); Priority Academic Program Development of Jiangsu Higher Education Institutions.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data that support the findings of this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
References
1. J. Xiong, E. L. Hsiang, Z. He, T. Zhan, and S. T. Wu, “Augmented reality and virtual reality displays: emerging technologies and future perspectives,” Light: Sci. Appl. 10(1), 216 (2021). [CrossRef]
2. T. Zhan, Y. H. Lee, G. Tan, J. Xiong, K. Yin, F. Gou, J. Zou, N. Zhang, D. Zhao, J. Yang, S. Liu, and S. T. Wu, “Pancharatnam-Berry optical elements for head-up and near-eye Displays,” J. Opt. Soc. Am. B 36(5), D52–D65 (2019). [CrossRef]
3. N. V. Tabiryan, D. E. Roberts, Z. Liao, J. Y. Hwang, M. Moran, O. Ouskova, A. Pshenichnyi, J. Sigley, A. Tabirian, R. Vergara, L. D. Sio, B. R. Kimball, D. M. Steeves, J. Slagle, M. E. McConney, and T. J. Bunning, “Advances in transparent planar optics: enabling large aperture, ultrathin lenses,” Adv. Opt. Mater. 9(5), 2001692 (2021). [CrossRef]
4. M. N. Miskiewicz, J. Kim, Y. M. Li, R. K. Komanduri, and M. J. Escuti, “Progress on large-area polarization grating fabrication,” Proc. SPIE 8395, 83950G (2012). [CrossRef]
5. J. Kim, M. N. Miskiewicz, S. Serati, and M. J. Escuti, “Nonmechanical laser beam steering based on polymer polarization gratings: Design optimization and demonstration,” J. Lightwave Technol. 33(10), 2068–2077 (2015). [CrossRef]
6. J. Kim, C. Oh, S. Serati, and M. J. Escuti, “Wide-angle, nonmechanical beam steering with high throughput utilizing polarization gratings,” Appl. Opt. 50(17), 2636–2639 (2011). [CrossRef]
7. K. Gao, H. H. Cheng, A. Bhowmik, C. McGinty, and P. Bos, “Nonmechanical zoom lens based on the Pancharatnam phase effect,” Appl. Opt. 55(5), 1145–1150 (2016). [CrossRef]
8. L. De Sio, D. E. Roberts, Z. Liao, J. Hwang, N. Tabiryan, D. M. Steeves, and B. R. Kimball, “Beam shaping diffractive wave plates,” Appl. Opt. 57(1), A118–A121 (2018). [CrossRef]
9. N. Yu and F. Capasso, “Flat optics with designer metasurfaces,” Nat. Mater. 13(2), 139–150 (2014). [CrossRef]
10. G. Zheng, H. Mühlenbernd, M. Kenney, G. Li, T. Zentgraf, and S. Zhang, “Metasurface holograms reaching 80% efficiency,” Nat. Nanotechnol. 10(4), 308–312 (2015). [CrossRef]
11. D. Lin, P. Fan, E. Hasman, and M. L. Brongersma, “Dielectric gradient metasurface optical elements,” Science 345(6194), 298–302 (2014). [CrossRef]
12. X. Luo, “Multiscale optical field manipulation via planar digital optics,” ACS Photonics 10(7), 2116–2127 (2023). [CrossRef]
13. M. Meem, S. Banerji, A. Majumder, F. G. Vasquez, B. Sensale-Rodriguez, and R. Menon, “Broadband lightweight flat lenses for long-wave infrared imaging,” Proc. Natl. Acad. Sci. U. S. A. 116(43), 21375–21378 (2019). [CrossRef]
14. Y. Guo, M. Jiang, C. Peng, K. Sun, O. Yaroshchuk, O. Lavrentovich, and Q. H. Wei, “High-resolution and high-throughput plasmonic photopatterning of complex molecular orientations in liquid crystals,” Adv. Mater. 28(12), 2353–2358 (2016). [CrossRef]
15. X. Xiang, J. Kim, and M. J. Escuti, “Far-field and fresnel liquid crystal geometric phase holograms via direct-write photo-alignment,” Crystals 7(12), 383 (2017). [CrossRef]
16. C. P. Jisha, S. Nolte, and A. Alberucci, “Geometric Phase in Optics: From Wavefront Manipulation to Waveguiding,” Laser Photonics Rev. 15(10), 2100003 (2021). [CrossRef]
17. K. Gao, H. H. Cheng, A. K. Bhowmik, and P. J. Bos, “Thin-film Pancharatnam lens with low f-number and high quality,” Opt. Express 23(20), 26086–26094 (2015). [CrossRef]
18. J. Kobashi, H. Yoshida, and M. Ozaki, “Planar optics with patterned chiral liquid crystals,” Nat. Photonics 10(6), 389–392 (2016). [CrossRef]
19. N. V. Tabiryan, S. V. Serak, D. E. Roberts, D. M. Steeves, and B. R. Kimball, “Thin waveplate lenses of switchable focal length–new generation in optics,” Opt. Express 23(20), 25783–25794 (2015). [CrossRef]
20. D. L. Tang, Z. L. Shao, X. Xie, Y. J. Zhou, X. H. Zhang, F. Fan, and S. C. Wen, “Flat multifunctional liquid crystal elements through multi-dimensional information multiplexing,” Opto-Electron. Adv. 6(4), 220063 (2023). [CrossRef]
21. R. K. Komanduri and M. J. Escuti, “High efficiency reflective liquid crystal polarization gratings,” Appl. Phys. Lett. 95(9), 091106 (2009). [CrossRef]
22. N. V. Tabiryan, S. V. Serak, S. R. Nersisyan, D. E. Roberts, B. Y. Zeldovich, D. M. Steeves, and B. R. Kimball, “Broadband waveplate lenses,” Opt. Express 24(7), 7091–7102 (2016). [CrossRef]
23. Y. X. Zhang, M. B. Pu, J. J. Jin, X. J. Lu, Y. H. Guo, J. X. Cai, F. Zhang, Y. L. Ha, Q. He, M. F. Xu, X. Li, X. L. Ma, and X. G. Luo, “Crosstalk-free achromatic full Stokes imaging polarimetry metasurface enabled by polarization-dependent phase optimization,” Opto-Electron. Adv. 5(11), 220058 (2022). [CrossRef]
24. Y. Wen, Q. Zhang, Q. He, F. Zhang, L. Xiong, F. Zhang, G. Fu, J. Xu, M. Pu, and X. Luo, “Shortening Focal Length of 100-mm Aperture Flat Lens Based on Improved Sagnac Interferometer and Bifacial Liquid Crystal,” Adv. Opt. Mater. 11(16), 2300127 (2023). [CrossRef]
25. Y. Li, Y. Liu, S. Li, P. Zhou, T. Zhan, Q. Chen, Y. Su, and S. T. Wu, “Single-exposure fabrication of tunable Pancharatnam-Berry devices using a dye-doped liquid crystal,” Opt. Express 27(6), 9054–9060 (2019). [CrossRef]
26. L. L. Huang, X. Z. Chen, H. Muhlenbernd, H. Zhang, S. M. Chen, B. F. Bai, Q. F. Tan, G. F. Jin, K. W. Cheah, C. W. Qiu, J. S. Li, T. Zentgraf, and S. Zhang, “Three-dimensional optical holography using a plasmonic metasurface,” Nat. Commun. 4(1), 2808 (2013). [CrossRef]
27. J. Kim, Y. Li, M. N. Miskiewicz, C. Oh, M. W. Kudenov, and M. J. Escuti, “Fabrication of ideal geometric-phase holograms with arbitrary wavefronts,” Optica 2(11), 958–964 (2015). [CrossRef]
28. H. Wu, W. Hu, H. C. Hu, X. W. Lin, G. Zhu, J. W. Choi, V. Chigrinov, and Y. Q. Lu, “Arbitrary photo-patterning in liquid crystal alignments using DMD based lithography system,” Opt. Express 20(15), 16684–16689 (2012). [CrossRef]
29. M. N. Miskiewicz and M. J. Escuti, “Direct-writing of complex liquid crystal patterns,” Opt. Express 22(10), 12691–12706 (2014). [CrossRef]
30. C. Sun, N. Fang, D. M. Wu, and X. Zhang, “Projection micro-stereolithography using digital micro-mirror dynamic mask,” Sens. Actuators, A 121(1), 113–120 (2005). [CrossRef]
31. M. Schadt, K. Schmitt, V. Kozinkov, and V. Chigrinov, “Surface-induced parallel alignment of liquid-crystals by linearly polymerized photopolymers,” Jpn. J. Appl. Phys. 31(7R), 2155–2164 (1992). [CrossRef]
32. W. Hu, A. Srivastava, F. Xu, J. T. Sun, X. W. Lin, H. Q. Cui, V. Chigrinov, and Y. Q. Lu, “Liquid crystal gratings based on alternate TN and PA photoalignment,” Opt. Express 20(5), 5384–5391 (2012). [CrossRef]
33. S. V. Serak, T. J. Bunning, and N. V. Tabiryan, “Ultrafast photoalignment: Recording a lens in a nanosecond,” Crystals 7(11), 338 (2017). [CrossRef]
34. M. Honma and T. Nose, “Liquid-crystal Fresnel zone plate fabricated by microrubbing,” Jpn. J. Appl. Phys. 44(1R), 287 (2005). [CrossRef]
35. O. Yaroshchuk, J. Ho, V. Chigrinov, and H. S. Kwok, “Azodyes as Photoalignment Materials for Polymerizable Liquid Crystals,” Jpn. J. Appl. Phys. 46(5A), 2995–2998 (2007). [CrossRef]
36. X. Li, V. M. Kozenkov, F. S. Y. Yeung, P. Xu, V. G. Chigrinov, and H. S. Kwok, “Liquid-Crystal Photoalignment by Super Thin Azo Dye Layer,” Jpn. J. Appl. Phys. 45(1A), 203–205 (2006). [CrossRef]
37. S. R. Nersisyan, N. V. Tabiryan, D. M. Steeves, and B. R. Kimball, “Characterization of optically imprinted polarization gratings,” Appl. Opt. 48(21), 4062–4067 (2009). [CrossRef]
38. M. N. Miskiewicz and M. J. Escuti, “Optimization of direct-write polarization gratings,” Opt. Eng. 54(2), 025101 (2015). [CrossRef]
