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Abstract
A new type of electrically controlled liquid-crystal microlens matrix (EC-LCMM) with a nested electrode array for efficiently tuning and swinging focus, which means that the focus position can be adjusted in three dimensions, is proposed. The EC-LCMM is constructed by a 10 × 10 arrayed annular-sector-shaped aluminum electrode with a central microhole of 140μm diameter and three annular-sectors of 210μm external diameter and the period length of 280μm. To the arrangement of the patterned electrode, both the 10 × 10 LC microlens array based on the annular-sector-shaped aluminum electrode and the 9 × 9 LC microlens array based on an arrayed quasi-quadrilateral-ring-shaped electrode can be obtained. The 9 × 9 LC microlens array is formed by matching adjacent four annular-sector-shaped sub-electrodes in the 10 × 10 LC microlenses. The developed EC-LCMM can be used to electrically tune focus along the optical axis and also swing focus over a focal plane selected. The typical performances include: electrically tunable focusing in a driving voltage range of 3~7Vrms, the focal length in a range of 2~0.6mm, and the maximum focus swing distance being 16μm. For effectively describing the focus swing efficiency, the parameters of SF and SA are defined, which are the ratios between the focus swinging distance and the current focal length along the optical axis, and between the focus swinging extent and the external diameter of a single annular-sector-shaped aluminum electrode, respectively. The SF and SA of the EC-LCMM are ~16‰ and ~7.6%, respectively. It can be expected that the complex wavefront can be more efficiently measured and adjusted according to the EC-LCMM-based Shack-Hartmann wavefront measuring and adjusting architecture.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Conventional optical lenses can be utilized to achieve focusing operation by varying the functioned structural thickness to shape a convex-shaped profile using homogeneous refractive index material, which will introduce a needed light path difference between incident microbeams located at different position of the lens to construct a converging light-spot or focus over the focal plane. Recently, the electrically controlled liquid-crystal lenses are developed rapidly, which are usually a flat plate structure. Under the action of a spatial electric-field generated by a signal voltage applied, the liquid-crystal directors can be re-oriented along the electric-field direction so as to result in a needed refractive index distribution. Generally, a greater electric-field strength will stimulate a larger deflection angle of liquid-crystal directors, which means a greater variance of the refractive index of liquid-crystal materials. Based on the physical effect above, an adjustment of the equivalent refractive index at the same spatial region or along the same direction in the liquid-crystal layer can be realized effectively.
In 1979, the first electrically controlled liquid-crystal lens was proposed by Sato [1]. Ten years later, Nose and Sato proposed a single-hole liquid-crystal microlens with a symmetric patterned electrode by etching a circular through-hole into an indium tin oxide (ITO) glass substrate [2]. Continuously, Naumov et al. fabricated a spherical perfusion liquid-crystal microlens through arranging an annular-contacted spherical lenses inside an ITO flat panel liquid-crystal cell [3]. Recently, the liquid-crystal lens technology is already concentrated on simultaneously performing focus tuning and swing for various applications including typical complex wavefront measurement and adjustment. In 2006, Ye et al. proposed a type of liquid-crystal lens with focus movable on focal plane [4]. In the meantime, Ye and Sato devised a two-electrode liquid-crystal lens whose focus can be moved off the axis [5]. Kang et al. developed a liquid-crystal lens with an electrically tuning and swing focus by simple circular-hole-shaped electrode [6,7]. Although their liquid-crystal structures can be used to achieve focus adjustment, but the operation for measuring and compensating wavefront in a wide dynamic range [8–10] to minimize image distortion needs to drive hundreds of liquid-crystal microlenses. Currently, there are two ways to achieve the wavefront measurement and further regulation in a relatively large dynamic range. One is to find higher performance optical materials [11], and the other is to construct special functioned micro-architecture.
In this paper, a new type of the electrically controlled liquid-crystal microlens matrix (EC-LCMM) is constructed by combining a 10 × 10 LC microlens array with an arrayed annular-sector-shaped electrode and a 9 × 9 LC microlens array with an arrayed quasi-quadrilateral-ring-shaped electrode formed by matching adjacent four annular-sector-shaped sub-electrodes in the 10 × 10 LC microlenses. The developed EC-LCMM can be used to electrically tuning focus along the optical axis and also swing focus on the focal plane selected. It can be expected that the complex wavefront can be more efficiently measured and adjusted according to the EC-LCMM-based Shack-Hartmann wavefront measuring and adjusting architecture.
2. Structure and principles
The detailed schematic of the EC-LCMM is shown in Fig. 1(a). The key functional structures of the EC-LCMM are both ~500μm glass substrates with different conductive film pre-coated on their inner surface. The inner surface of the bottom substrate is deposited with an indium tin oxide (ITO) film and also the inner surface of the top substrate with an aluminum (Al) film by common magnetron sputtering. To form a functional patterned Al electrode on the top substrate, a conventional ultraviolet photolithography and common wet-etching process are used. Firstly, a thin layer of positive photoresist (RZJ-304 of Suzhou Ruihong Electronic Chemical Co., Ltd.) is spin-coated on the surface of the aluminum film, and then a contacting type of ultraviolet photolithography with a photomask (Shenzhen Newway Photomask Making Co., Ltd.) is performed, as shown in Fig. 1(c). Subsequently, a positive photoresist developer (RZX-3038 of Suzhou Ruihong Electronic Chemical Co., Ltd.) is used to remove the denatured photoresist and perform wet-etching due to the aluminum corrosion of the developer used. Finally, the undenatured photoresist is removed using acetone and subjected to conventional cleaning. The structural parameters of the patterned Al electrode are shown in Fig. 1(b). As shown, the patterned Al electrode is formed by periodically arranging the same annular-sector-shaped electrode. Each annular-sector-shaped electrode with a central microhole of 140μm diameter and an annular-sector of 210μm external diameter, is divided into three independent annular-sector-shaped sub-electrodes with a ~100° sector angle, and thus a trapezoid-shaped gap between adjacent sub-electrodes is formed. The period length and array scale of the EC-LCMM are 280μm and 10 × 10, respectively. For better viewing, only an array of 4 × 4 is shown in Fig. 1(b). A layer of polyimide (PI) is continuously spin-coated on both the patterned Al and ITO electrodes of the EC-LCMM and then prebaked for 10min at 80°C and cured for 30min at 230°C, sequentially. The cured PI layer on the patterned Al and ITO electrodes acting as an alignment layer, are rubbed in the anti-parallel direction for homogeneously aligning LC molecules filled later. Glass microsphere spacers (HT 200 of Nano-Micro) with a diameter of 20μm and mixed with the adhesive, are deposited to separate two substrates. Finally, a layer of long rod-shaped nematic LC materials (Merck E44: no = 1.523 and ne = 1.778) is fully filled in the formed micro-cavity and then sealed carefully. Figure 2 shows the fabricated EC-LCMM.
Fig. 1 (a) The schematic of the EC-LCCM, and (b) the key structural parameters of the annular-sector-shaped electrode, and (c) the partial photomask used.
Since the current period length of the EC-LCMM being relatively small and the total number of the control sub-electrodes being very large, the conductive leads are required to be thin and densely distributed. For fabricating the EC-LCMM with a 10 × 10 scale, the main fabrication challenge is to make relatively thin conductive leads. Currently, the electrodes and conductive leads are shaped by common wet-etching, which means that the minimum line width should be controlled in a typical scale of ~10μm in our experiments. Figure 3 shows several typical patterned electrodes fabricated. As shown in Fig. 3(a), a fabricated top substrate with a general quality is already shaped and then the damaged conductive leads can be observed apparently in several regions. Figure 3(b) shows a fabricated top substrate with an excellent quality by optimizing the photolithography process and the photoresist thickness and the wet-etching technology. Figure 3(c) shows an ideal top substrate with a relatively small array size and sparsely distributing control leads formed by the same wet-etching process. It can be seen that although the conductive leads shown in Fig. 3(b) are not broken, an undesired voltage wastage will be across the uneven conductive leads, so as to reduce the electrical efficiency of the device. It should be noted that an effective method for fabricating the similar devices with the same or even larger size is to use dry-etching process.
Fig. 3 The typical patterned electrodes fabricated by the wet-etching process. (a) A fabricated top substrate with a general quality, (b) a similar top substrate with an excellent quality by optimizing photolithography process and the photoresist thickness and the key wet-etching technology, and (c) an ideal top substrate with a relatively small array scale.
To the arrangement of the patterned electrode above, both the 10 × 10 LC microlens array based on the annular-sector-shaped aluminum electrode and the 9 × 9 LC microlens array based on an arrayed quasi-quadrilateral-ring-shaped electrode can be obtained, as shown in Figs. 4(a) and 4(b). The 9 × 9 LC microlens array is shaped automatically through matching adjacent four annular-sector-shaped sub-electrodes in the 10 × 10 LC microlens array, as indicated in Fig. 4(b), the corresponding part of the blue dotted box is three units of the 9 × 9 LC microlens array. It should be noted that the EC-LCMM is actually a nested device, which consists of two LC microlens arrays with different focus tuning and swing performances. Considering the geometric difference between the annular-sector-shaped electrode and the quasi-quadrilateral-ring-shaped electrode, the 9 × 9 LC microlens array will present a longer focal length than that of the 10 × 10 LC microlens array.
Fig. 4 Schematics of: (a) a 10 × 10 LC microlens array based on the annular-sector-shaped aluminum electrode, and (b) a 9 × 9 LC microlens array based on an arrayed quasi-quadrilateral-ring-shaped electrode, which is shaped automatically through matching adjacent four annular-sector-shaped sub-electrodes in the 10 × 10 LC microlens array.
Due to the excellent electro-optical response characteristics of LC materials, for example, the redistribution of LC molecules in the applied electric-field [4–7,12–26]), the focus position of the EC-LCMM can be adjusted in three dimensions, which will be a key structural component for constructing a Shack-Hartmann wavefront measuring and regulating architecture with a relatively large dynamic range. Figure 5 shows the schematic of the EC-LCMM for efficiently tuning and swinging focus, the partial red dotted frames are the control sub-electrodes of a single LC microlens in the 10 × 10 LC microlens array, which are numbered by EI, EII and EIII. When the voltage signals applied over all the control sub-electrodes are the same, the axisymmetric electric-field will drive the LC molecules to form an approximately axisymmetric distribution. Therefore, the formed refractive index profile will lead to a focal spot along the optical axis, and then the focal length can be tuned by adjusting the identical voltage signals applied. When the voltage signals applied over the EI, EII and EIII, are different, a non-axisymmetric electric field will be produced in the LC layer so as to make a non-axisymmetric refractive index distribution, and thus a focal spot can be formed over the focal plane. According to the experiments, the moving direction and distance of the focal spot can be controlled by simply adjusting the voltage signals applied over three sub-electrodes, respectively, which means that a focus swing function can be obtained.
3. Experiments and discussion
In order to acquire the optical performances of the fabricated EC-LCMM, an optical measurement system is constructed, as shown in Fig. 6. The key devices include: an AC generator, a laser a spectral range from 501nm to 561nm (Changchun New Industries Optoelectronics Tech. Co., Ltd.), a beam quality analyzer of the WincamD series (DataRay Corporation), a polarizer of OMPO30-LJ (Zolix Corporation), a beam expander of HB-20XAR (Newport Corporation). The polarizer is utilized to maintain a linear polarization of the beams incident upon the EC-LCMM having the same orientation with the friction direction of the PI alignment layer (i.e., LC aligning direction) fabricated over both the electrode plates of the LC micro-cavity. The EC-LCMM is driven by voltage signals of 1 kHz square-wave with different RMS values, which are exported form an AC generator. The focus tuning behaviors are measured by moving the beam quality analyzer along the optical axis, and then the focus swing behaviors by observing the focus distribution over a focal plane selected. The measurement light-path and the platform are demonstrated by Figs. 6(a) and 6(b), respectively.
Fig. 6 Schematic diagram of the optical measurement system built by us: (a) The measurement light-path and (b) the testing platform.
The micro-beam variance behaviors of the fabricated EC-LCMM are observed under the condition of applying different voltage signals over the LC device, as demonstrated in Fig. 7. The initial case without any voltage signal over the LC device is shown in Fig. 7(a), where a partial aluminum electrode pattern can be seen clearly. After applying a ~2Vrms signal voltage, a slightly fuzzy micro-beam distribution circling each annular-sector-shaped electrode and the straight aluminum conductive leads can be observed, as shown in Fig. 7(b). The applied signal voltage being increased to ~3.96Vrms, a clear patterned electrode being similar with that in initial case, can be viewed again, as shown in Fig. 7(c). The electrode fuzzy phenomenon above can be attributed to a slight interference between both the o- and e-beams separated by the LC materials, in which the e-beams will exhibit a variable refractive index with the variance of the spatial electric-field intensity and direction.
Fig. 7 Typical microbeam distributions of the EC-LCMM driven by low signal voltages: (a) Initial case without any voltage signal, and (b) by ~2Vrms, and (c) by ~3.96Vrms.
Continuously increasing the signal voltage, an arrayed microbeam converging including focusing can be achieved, and further a defocusing tend can also be observed, as shown in Fig. 8. An arrayed focusing operation characterized by both the arrayed distributions of the focal spot and the sharp point spread function (PSF) is achieved at ~4.2Vrms, and also presents a focal length of ~1.85mm, as shown in Fig. 8(a). The defocusing trend can be exhibited by a gradually enlarged variance of the converging light-spot at ~5.3Vrms and ~5.85Vrms, as shown in Figs. 8(b) and 8(c).
Fig. 8 Microbeam converging and then defocusing characters as increasing the signal voltage applied over the EC-LCMM. (a-1) The 2D PSF of the EC-LCMM at ~4.2Vrms, and (a-2) the 3D PSF of the EC-LCMM at ~4.2Vrms, and (b) an enlarged converging light-spot distribution at ~5.3Vrms, and (c) a defocusing tend can be observed because of the converging light-spot being further enlarged at ~5.85Vrms.
It’s obvious that an arrayed converging light-spots with a relatively low intensity and smaller size, which are formed by the quasi-quadrilateral-ring-shaped electrodes, can also be observed, as demonstrated in Fig. 8(a). Currently, the average light-spot intensity or the height of the PSF is less than ~10% of that formed by the annular-sector-shaped electrodes. Increasing the signal voltage from ~4.2Vrms to ~5.85Vrms, the dim and small light-spots also present a diverging trend, where firstly gradually diverging along the y-direction in a range of ~4.2Vrms to ~5.3Vrms, and then rapidly dispersing along the x-direction in a range of ~5.3Vrms to ~5.8Vrms, so as to exhibit an asymmetric variance character along the optical axis.
The case that a converged light-spot would be located at the center of each quasi-quadrilateral-ring-shaped electrode according to the design, has already splitted two small light-spots and then distributed along the x-direction, can be attributed to an additional spatial electric-field stimulated by a straight conductive lead with a relatively large width. The patterned electrode and the two converged light-spot distribution are shown in Fig. 9, which is constructed by overlapping a partial patterned electrode image obtained at ~3.96Vrms and the corresponding focus distribution image obtained at ~4.2Vrms. Each small light-spot surrounded by a brown circle is arranged on both sides of a conductive lead indicated by a brown sign b. The case indicated by a red sign also presents a two small light-spot distribution due to an equivalent straight conductive lead, as shown by the right bottom diagram. It can be expected that a single light-spot positioned at the center of each quasi-quadrilateral-ring-shaped electrode can be shaped if the straight conductive lead being thin enough or even inexistence, as shown by the right upper diagram.
Fig. 9 Typical dual-light-spot distribution generated by the quasi-quadrilateral-ring-shaped electrodes and the straight conductive lead indicated by a brown sign b or an equivalent conductive lead indicated by a red sign a.
The relationship between the focal length with the driving voltage signal applied over the fabricated EC-LCMM is shown in Fig. 10. When the signal voltage is less than ~3Vrms, the generated electric-field is not enough to drive the LC molecules re-orientation. When the signal voltage is between ~4Vrms and ~7Vrms, the microbeam focusing can be achieved effectively and then the focal length will be reduced significantly as the increasing of the signal voltage applied. After exceeding ~7Vrms, the focal length of the EC-LCMM presents a slow decreasing tend and then the focal length being maintained between ~0.5mm and ~0.6mm. So, the tunable focus performance of the EC-LCMM is the best when the signal voltage is in a range from ~3 to ~7Vrms, and the focal length between ~2 to ~0.6mm.
Fig. 10 Relationship between the focal length and the driving signal voltage applied over the EC-LCMM developed.
The focus swing operation of the EC-LCMM is performed only considering the case of a single annulus-sector-shaped electrode, as shown in Fig. 11. Firstly, a constant signal voltage VIII of ~4.63Vrms is applied over the micro-region-EIII, and only the signal voltage applied over the micro-region-EI and -EII are changed. If keeping VI and let VII and VIII being ~2.53Vrms and ~4.63Vrms, respectively, it can be observed that the focus will swing towards the micro-region-EIII from the initial center of the cross-wire, as shown by Fig. 11(a). If let VII = VIII = ~4.76Vrms, the focus will return to the center, as shown by Fig. 11(b). If keeping VII and VIII and only increasing VI from ~5.07Vrms to ~7.83Vrms, the focus will swing towards the micro-region-EI, as shown in Figs. 11(c) to 11(f). According to experiments, a single LC microlens demonstrates a maximum range of ~16μm at the focal length of ~1.07mm at the optical axis direction. For effectively describing the focus swing efficiency, both parameters of SF and SA are defined, which are the ratios between the focus swinging extent and the current focal length along the optical axis, and also the focus swinging extent and the external diameter of annular-sector-shaped aluminum electrode, respectively. Currently, the parameters of SF and SA are ~16‰ and ~7.6%, respectively. The focus swing behaviors corresponding to a single quasi-quadrilateral-ring-shaped electrode is similar with the case above.
Fig. 11 Typical focus swinging characters of a single LC microlens in the EC-LCMM driven by applying three independent signal voltages over each annular-sector-shaped sub-electrode. The RMS signal voltages are shifted gradually according to the arrangement of [VI,VII,VIII]: (a) [4.63,2.53,4.63] and (b) [4.63,4.76,4.63] and (c) [5.07,4.76,4.63] and (d) [6.01,4.76,4.63] and (e) [6.61,4.76,4.63] and (f) [7.83,4.76,4.63].
4. Summary
A novel EC-LCMM with a nested annular-sector-shaped electrode array for effectively tuning and swing focus, is constructed successfully. Although both parameters of SF and SA of the EC-LCMM are only ~16‰ and ~7.6%, the current research about the EC-LCMM highlights the continuous development of the similar functioned devices and micro-optics architectures, so as to lay a solid basis for practical utilization in several particular fields, for example, the typical dual-mode (including: light intensity, wavefront) imaging detection of small and weak radiation objects in complicated circumstance. It can be expected that the complex wavefront can be more efficiently measured and further adjusted according to the LC-based Shack-Hartmann wavefront architecture because the wavefront of objects can be efficiently detected and re-constructed in a larger dynamic range.
Funding
National Natural Science Foundation of China (61432007 and 61176052), the Major Technological Innovation Projects in Hubei Province (2016AAA010), and the China Aerospace Science and Technology Innovation Fund (CASC2015).
Acknowledgments
The authors would like to thank the Analytical and Testing Centre of Huazhong University of Science and Technology for their valuable help.
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