LIQUID CRYSTAL LENSES FOR CORRECTION OF P ~S~YOP

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Estella Quinn
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1 LIQUID CRYSTAL LENSES FOR CORRECTION OF P ~S~YOP GUOQIANG LI and N. PEYGHAMBARIAN College of Optical Sciences, University of Arizona, Tucson, A , USA gli@ootics.arizt~ii~.e~i~ Correction of presbyopia has been increasingly important. An electro-active lens allows voltage controlled change of the focusing power across the entire aperture. Such a lens must have high light efficiency, relatively large aperture, fast switching time, low driving voltage, and power-failure-safe configuration. New switchable, flat, thin liquid crystal diffractive lenses that meet the above requirements will be presented. The operation principle is based on electrical tuning of the refractive index of a 5 p-thick layer of nematic liquid crystal using a circular array of photolithographic~y defined transparent electrodes. The effects of the gaps between the ring electrodes and the fringing field on the lens performance will be analyzed. Lenses with three different designs will he demonstrated: (1) All the ring electrodes for modulating the multi-level phase profile are patterned in one layer with a lpm gap between the neighboring electrodes. (2) In order to avoid the lateral gaps between the electrodes, a preliminary experiment with interleaved electrode pattern has been performed for a 4-level lens. (3) A robust design is given with three-layer electrode pattern and two-layer via structures for flexible interconnection and no-gap pattern. Designs 1 and 3 allow any even-numbered phase levels greater than 4 and provides the capability of correction for near-, intermediate-, and distance-vision. Such a lens has potential of revolutionizing the field of presbyopia correction. 1. Introduction Presbyopia is a condition of vision that the eye s ability to focus is diminished with aging, mainly due to the loss of elasticity of the crystalline lens. For example, a normal eye typically has an accommodation of 10 diopters at age 25 and at age 60, the accommodation is reduced to only about 1 diopter. For those with no refractive error, they usually have good distance vision but have difficulty in near vision such as reading and intermediate vision such as walking down steps. The first symptoms usually occur at mid-forties. The conventional bifocal and trifocal lenses for this correction have been around for more than 200 years and they have some drawbacks. They have a limited field of view for each vision task, requiring user to gaze down to accomplish near vision and in some cases causing dizziness and discomfort. Some users need three different eyewears for reading, computer, and driving. Our goal is to develop electro- active lenses1-2 that allow voltage controlled change of the focusing power across the entire aperture. Such a lens must have high light efficiency, relatively large aperture, fast switching time, low driving voltage, and power-failure-safe configuration3. New switchable, flat, thin liquid cr stal (LC) diffractive lenses 3-7 that meet the above requirements will be presented. 2. Design and results The LC lens structure is shown in Fig. 1 (a). A nematic LC layer is sandwiched between a ground electrode substrate and a patterned electrode substrate. The 3

$For higher diffraction efficiency and ease of control, the patterned electrodes 3 - Fig. 1 LC lens with patterned electrodes.$

2 4 ground electrode substrate contains a uniform conductive indium-tin-oxide (ITO) layer and the patterned electrodes are fabricated by photolithographic processing. For higher diffraction efficiency and ease of control, the patterned electrodes 3 - Fig. 1 LC lens with patterned electrodes. (a) Hat lens structure; (b) Top view of one-layer electrode pattern; (c) Cross section of the two-layer electrode pattern, where odd- and even-numbered electrodes are interleaved into two layers. have a ring shape defined by diffractive optics. The center wavelength is the peak of the human photopic response, 555 nm. The initial orientation of the molecules is parallel to the polarization of the incident beam, which is an extraordinary beam and its effective refractive index can be changed in the range from n, to no due to the reorientation of the LC molecule when a voltage is applied. The material has a positive dielectric anisotropy (>0.2), which provides enough phase modulation for the visible wavelength with a 5 ym-thick cell. If all the ring electrodes for tailoring the refractive index are patterned in one layer, there must be gaps between neighboring electrodes (Fig. l(b)). In order to eliminate the gaps between the electrodes, the odd- and even-numbered electrodes can be interleaved into two layers which are separated by a SiO2 insulator layer (Fig. l(c)). The effects of the gaps between the ring electrodes and the fringing field on the lens performance have been analyzed. Here, we just show an example. Assume the focal length is lm, and the diameter of the lens is 10mm. Figure 2 (a) depicts the normalized intensity distribution around the main focal point for no gap and gaps of various values. When the aperture of the lens is large, the gaps distort the phase profile and hence reduce the diffraction efficiency. The gaps should be kept small. Figure 2 (b) illustrates the deviation of the phase profile of one zone from the ideal case caused by the fringing field effect. Type B transition reduces the diffraction efficiency too.

3 5 Fig. 2 Effects of the gaps between the neighboring ring electrodes and the fringing field. (a) Intensity distribution at the focal plane for no gap and gaps of various values. (b) Illustration of the phase profile caused by the fringing field. Dashed line, the ideal phase profile. Lenses with three different designs have been demonstrated: (1) All the ring electrodes for modulating the multi-level phase profile are patterned in one layer with a 1pm gap between the neighboring electrodes3. Over the patterned ITO, an electrically insulating layer of SiOz is sputtered and into which small via openings (conducting holes for vertical interconnections) were etched. An electrically conductive layer of IT0 is subsequently sputtered over the insulating layer to fill the vias and contact the electrodes and patterned to form independent electrical bus bars. Lenses with eight phase levels, 10 mm diameters and focal lengths of 1 m and 0.5 m (+1.0 diopter and +2 diopter of add power, respectively) have been demonstrated. (2) In order to avoid the lateral gaps between the electrodes and allow high diffraction efficiency, the odd- and even-numbered ring electrodes are separated

6 in two layers4. A preliminary experiment with interleaved electrode pattern has been performed for a 4-level, 15 mm-aperture, 2-diopter lens with the expected performance.

4 6 in two layers4. A preliminary experiment with interleaved electrode pattern has been performed for a 4-level, 15 mm-aperture, 2-diopter lens with the expected performance. (3) A robust design is given with three-layer electrode pattern and two-layer via structures for flexible interconnection and no-gap pattern5. The microfabricated transparent concentric ring electrodes are distributed in two layers and different voltages are applied to each electrode through bus lines in another layer. Connection between the electrodes and the bus lines is achieved by vias in the third dimension. This design makes it easier to fabricate lenses with higher-level phase steps and larger aperture and overcome the shorts between the electrodes. This method can be used for design of LC lens of any phase levels. It should be noted that, unlike the conventional binary optics, in this design the increase of the phase levels (e.g., to 16 levels) in each zone does not increase the fabrication steps. For vision correction of presbyopic eyes, polarization insensitive switchable lenses are needed. As homogeneously aligned nematic LC is polarization sensitive, two lenses with orthogonal buffing directions were integrated as a single polarization insensitive lens. A method for active alignment of the two lenses has been described'. To test the imaging properties of the lens, a model human eye was constructed and a double lens element was placed in front of the model eye to provide near vision correction. For demonstration, a lens is first tuned with a focal length of 50 cm (2-diopter add power), 4-level phase modulation, diffraction efficiency of 78% and then reconfigured to operate as a l-diopter &level lens with a diffraction efficiency of above 91%. The lens operates with low voltages (c 2 V~S). fast response (-130 ms), small aberrations (the RMS value of the higher-order aberrations is about O.O393L), and a power-failure-safe configuration. Figure 3 shows correction of a model eye using the switchable lens. The object is initially in the reading distance. When the LC lens is off, the model eye has insufficient power to form a sharp image (Fig. 3(a)). But by switching on the diffractive lens with 2-diopter add power, the image is brought into focus with excellent contrast (Fig. 3(b)). Figure 3(c) illustrates the number of the phase steps of the lens is tuned from 4-level to 8- level and correspondingly the focal length is adjusted from f to 2f. In this case, the object is moved to a farther distance. Dependence of the diffraction efficiency on the incidence angle is related to the field of view effect for normal use of the spectacle lenses. The diffraction efficiency decreases monotonically as the increase of the incidence angle. It drops about 4% when the lens is tilted 20' and it is acceptable for real application. This decrease results from the change of the phase profile for normal incidence light compared with light coming at an oblique angle. Such a lens provides the capability of correction for near-, intermediate-, and distance-vision.

7 Fig. 3 Hybrid imaging using the van-focal LC lens for demonstration of vision correction. The LC lens is (a) OFF and @) activated with 2-diopter power.

$Discussions and conclusions The approach demonstrated here can be extended to the design of lenses with multiple digital focal lengths while keeping the same diffraction efficiency.$ The focal length can be adjusted by controlling the zone period using individually addressed ring pattern.

The focal length can be adjusted by controlling the zone period using individually addressed ring pattern.

If the zone period r: is increased to 2 rt by grouping every two neighboring subzones into one, i.e., applying the same voltage to the two neighboring electrodes, the focal length is changed to 2f with the same L- level phase modulation.

5 7 Fig. 3 Hybrid imaging using the van-focal LC lens for demonstration of vision correction. The LC lens is (a) OFF activated with 2-diopter power. (c) The lens is reconfigured from 4-level 1- diopter to 8-level 1-diopter case. (d) Hybrid imaging with the 1-diopter power. 3. Discussions and conclusions The approach demonstrated here can be extended to the design of lenses with multiple digital focal lengths while keeping the same diffraction efficiency. This provides more powerful ability in accommodating near-, intermediate-, and distance-vision. Individually addressable electrodes (subzones) would allow this capability. The focal length can be adjusted by controlling the zone period using individually addressed ring pattern. Assume the geometry of the electrode pattern is designed for an elementary focal length f with L-level phase modulation in each zone. If the zone period r: is increased to 2 rt by grouping every two neighboring subzones into one, i.e., applying the same voltage to the two neighboring electrodes, the focal length is changed to 2f with the same L- level phase modulation. Similarly, with the fixed electrode pattern, the focal length can be varied to kf (k is an integer) by increasing the zone period to k q2. This technique may also be applied to provide the capability of quasicontinuously changing the focal length, allowing correction for all the subjects with different accommodation requirements by using individually addressed ring pattern. For each desired focal length, a number of rings are grouped together to form each subzone. With current fabrication technology, the array of ring electrodes with a small feature size (less than 5 pm) can be made. For a lens of 15mm in diameter, the focal length can be continuously changed from 30 cm to

6 8 infinity. The proposed structure is easier to control than the spatial light modulator. The diffractive lens has opposite chromatic aberration as to the human eye, so they can cancel each other to some extent. The chromatic aberration of the diffractive lens can be reduced by using the concept of multi-order diffractive lens, where the phase jump at the zone boundaries is p2n (p>l, integer) for the design wavelength5. Assuming the brain is adapted to a certain degree of chromatic aberration, balancing the dispersion of the diffractive lens and the eye is less desirable. On the other hand, the brain can handle both balanced and imbalanced chromatic aberrations. Negative focusing powers can also be achieved with the same lenses by changing the sign of the slope of the applied voltages. Usually correction of presbyopia needs an add power less than 3 diopters. With the state-of-the-art facilities, it is feasible to make such lenses. For correcting a residual refractive error for myopia or hyperopia, a curved substrate can be used or the lens can be used together with a contact lens for eyes that need minor correction for distance vision. The other concern is the temperature dependence of the lens performance. The refractive indices of the LC change due to the temperature variation (n, has a larger change than no). A temperature sensor and a variable voltage circuit are needed for compensation. These results represent significant advance in the state-of-the-art in liquid crystal diffractive lenses for vision care and other applications. They have the potential of revolutionizing the field of presbyopia correction when it is combined with autofocus function. The authors thank J. McGinn, J. Haddock, M. Giridhar, D. Mathine, P. Valley, P. Ayras, J. Schwiegerling, G. Meredith, S. Honkanen, and B. Kippelen for help in this work. References 1. C.W. Fowler and E.S. Pateras, Liquid crystal lens review, Ophthal. Physiol. Opt. 10,186 (1990). 2. W. N. Chman, Candiffractive liquid crystal lenses aid presbyopes? Ophthal. Physiol- Opt. 13,427 (1993). 3. G. Li et al., Switchable electro-optic diffractive lens with high efficiency for ophthalmic applications, Proc. Natl. Acad. Sci. USA 103, 6100 (2006). 4. G. Li, P. Valley, M. S. Giridhar, D. Mathine, G. Meredith, J. Haddock, B. Kippelen, N. Peyghambarian, Large-aperture switchable thin diffractive lens with interleaved electrode pattern, Appl. Phys. Lett. 89, (2006). 5. G. Li, P. Valley, P. Ayrtis, D. Mathine, S. Honkanen, and N. Peyghambarian, Highefficiency switchable flat diffractive ophthalmic lens with three-layer electrode pattern and two-layer via structures, Appl. Phys. Lett. 90, (2007).

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