OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 6 — Mar. 17, 2008
  • pp: 4302–4308
« Show journal navigation

Realization of liquid crystal lens of large aperture and low driving voltages using thin layer of weakly conductive material

M. Ye, B. Wang, and S. Sato  »View Author Affiliations


Optics Express, Vol. 16, Issue 6, pp. 4302-4308 (2008)
http://dx.doi.org/10.1364/OE.16.004302


View Full Text Article

Acrobat PDF (1738 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

We report a liquid crystal (LC) lens with a large aperture but low driving voltages with a thin layer of weakly conductive material. The effect of the conductive layer is demonstrated. A LC lens is realized with a diameter as large as 15 mm and with driving voltages as low as 11 Vrms. The properties of the LC lens are electrically tunable, and high optical quality is observed over the entire voltage range. The lens power decreases monotonically with the controlling voltage.

© 2008 Optical Society of America

1. Introduction

2. Liquid crystal lens with a circular-hole-patterned electrode

In a LC lens with a circular-hole-patterned electrode [6

6. M. Ye and S. Sato, “Optical properties of liquid crystal lens of any size,” Jpn. J. Appl. Phys. 41, L571–L573 (2002). [CrossRef]

, 11

11. M. Ye, B. Wang, and S. Sato, “Liquid crystal lens with a focal length that is variable in a wide range,” Appl. Opt. 43, 6407–6412 (2004). [CrossRef] [PubMed]

], a nearly symmetrical but spatial nonuniform electric field is formed in the LC layer when voltages are applied. The field reorients the LC directors and sets up a nearly symmetrical distribution of refractive index that is changeable with the applied voltages. If the geometrical structure of the cell, in particular the ratio of the hole diameter D to the distance d of the patterned electrode from the LC layer, is appropriate, the LC cell will have phase transformations close to those of conventional glass lenses at certain voltage ranges and becomes a lens with an electrically tunable focal length. The distance d of the hole-patterned electrode and the LC layer is kept by a dielectric material (usually a glass substrate) placed between them. If for a given diameter D the thickness of the glass substrate, that is the distance d, is suitable, a smoothly varying electric field will be distributed in the LC layer in a circular area (hole) approximately equal to that of the circular hole area in the electrode. A lens with almost the same aperture as that of the circular hole in the electrode then is possible. If, on the other hand, d is too large, the gradient of the electric field in the LC layer will become so small that the formation of a lens-like distribution of the refractive index in the LC layer will become impossible. If it is too small, the gradient electric field will be confined only near the boundary, and in the central part of the hole area there will be no necessary electric field to form the desired lens-like distribution of refractive index. There is an optimized ratio of D to d for lens operation. So to fabricate a LC lens with a large diameter, a thick substrate between the hole-patterned electrode and the LC layer is necessary.

The reorientation state of LC directors and hence lens properties are determined by potential differences across the two surfaces of the LC layer. If a thick glass substrate is used for a lens of large aperture, large portions of the applied voltages are dropped on the glass substrate. High driving voltages are needed to maintain required potential differences on the LC layer. For example, a distance d of approximately 800 µm between the patterned electrode and the LC layer and driving voltages of approximately 90 Vrms for a LC layer of 30 µm thickness are required for a LC lens with a diameter of D=2.0 mm [19

19. M. Ye, B. Wang, and S. Sato, “Effects of dielectric constant of glass substrates on properties of liquid crystal lens,” IEEE Photon. Technol. Lett. 19, 1295–1297 (2007). [CrossRef]

]. If D becomes 15.0 mm (the same as that of the cell used in this work), d of approximately 6 mm and voltages as high as approximately 270 Vrms for a LC layer of 75 µm thickness would be required.

3. Lowering driving voltages using thin layer of weakly conductive material

The problem of high voltage is solved in this work by placing a layer of material of weak conductivity between the hole-patterned electrode and the LC layer. The nonuniform electric field induced by the hole-patterned electrode in the cell arouses a radial electric current that redistributes the electric charges in the weakly conductive layer. An electric field resulting from the charge redistribution then occurs in the conductive layer, the direction of which is opposite to that of the radial component of the nonuniform electric field created by the patterned electrode. As a result, the abruptly changing potential caused by the patterned electrode is smoothed by the conductive layer. Though the net charge in the conductive layer remains at zero, the redistribution of the charges in the cell caused by the layer flattens the profile of the potential difference across the LC layer. The radial current in the weakly conductive layer is so small that some time is required before the layer becomes equipotential. The symbol of the charges in the two electrodes changes alternately since the cell that is driven by AC voltages. The radial current then periodically changes the flowing direction. If the symbol change of the charges, the speed of which is determined by the frequency f of the driving voltages, takes place before the layer becoming equipotential, there is always a radial current in the conductive layer. A gradient potential distribution in the layer can then be maintained, leading to a gradual distribution of an electric field in the LC layer, which may form a lens-like distribution of refractive index. Then, an LC lens may be possible if amplitude and frequency of the applied voltage are properly adjusted.

Fig. 1. (a) Structure of the LC cell. (b) Structure of the patterned electrode.

4. Experiment

The LC lens used in our experiment is shown in Fig. 1. The cell is driven by two voltages, via indium tin oxide (ITO) electrodes, and its structure is similar to that reported in Ref. [19

19. M. Ye, B. Wang, and S. Sato, “Effects of dielectric constant of glass substrates on properties of liquid crystal lens,” IEEE Photon. Technol. Lett. 19, 1295–1297 (2007). [CrossRef]

] except that a chamber of 75 µm thickness between the hole-patterned electrode and the LC layer is formed by glass substrates 2 and 3. The patterned electrode on substrate 3 shown in Fig. 2(b) is composed of an outer part for the V 1 application and an inner part for the V 2 application. V 1 and V 2 are sine waves and are in phase. There is a circular hole of D=15 mm diameter in the center of the outer part. The inner part of the circular shape of 14 mm diameter and concentric with the circular hole is surrounded by the outer part. A thin ITO line of 0.5 mm width leads the inner part to an outside contact for voltage application. A homogeneously aligned LC (E44 from Merck) layer of 75 µm thickness is contained in the cell. As discussed previously, for the circular hole of D=15 mm diameter in the patterned electrode, usually a distance of d=6.0 mm between the hole-patterned electrode and the LC layer is needed for lens operation. In the present cell, d is only (300+100+75) µm=475 µm. Liquid glycerin with high electrical resistivity (approximately 104 Ωm) is injected into the chamber, and its effect is observed experimentally.

Fig. 2. (a) Interference pattern when the chamber is filled with air, and voltages of amplitudes V 1=15 Vrms and V 2=4 Vrms, and of frequency f=1 kHz are applied. (b) Photo of the cell captured by the interferometer when liquid glycerin is partly injected into the chamber and no voltages are applied. (c) Interference pattern when the chamber is partly filled with glycerin and voltages of amplitudes V 1=15 Vrms and V 2=4 Vrms, and of frequency f=1 kHz are applied.

The properties of the lens are examined by an interference method. The LC cell is placed between two crossed polarizers, with the rubbing direction of the cell making an angle of 45° with each polarizer. A laser beam of 405 nm wavelength is incident on the LC cell. It is divided by the first polarizer into ordinary and extraordinary waves. The ordinary wave experiences a spatially uniform phase shift, while the extraordinary wave experiences a spatially varying phase shift. The second polarizer recombines the two waves, and the interference between them takes place. The interference fringe patterns give the information of the phase shift experienced by the extraordinary wave [11

11. M. Ye, B. Wang, and S. Sato, “Liquid crystal lens with a focal length that is variable in a wide range,” Appl. Opt. 43, 6407–6412 (2004). [CrossRef] [PubMed]

]; the phase difference between adjacent fringes is 2π. Figure 2(a) shows the interference pattern at V 1=15 Vrms, V 2=4 Vrms, and f=1 kHz before the glycerin is injected into the chamber. Since d is too short for a hole as large as D=15 mm diameter, the gradient electric field is confined near the boundary of the hole area and so is the gradient refractive index. Thus, there are interference fringes only near the boundary of the hole area. Then glycerin is partly injected so that part of the chamber [lower part in Fig. 2(b)] is filled with glycerin, and the upper part is still filled with air. When the same voltages V 1=15 and V 2=4 Vrms of 1 kHz frequency are applied, interference fringes appear, as shown in Fig. 2(c). As discussed previously, the charge redistribution in the cell expands the gradient electric field to a wider area; it smoothes the spatially abrupt field in the LC layer. While the electric field in the LC layer is still confined near the boundary in the upper part of the cell, it expands to the whole area where there is glycerin. A gradient distribution of the refractive index then appears in the lower part of the cell, and one can see a lot of interference fringes there when V 1=15 Vrms and V 2=4 Vrms of 1 kHz frequency are applied.

Fig. 3. Interference patterns changing with the amplitude of V 2 when the chamber is completely filled with glycerin and voltages of amplitude V 1=11 Vrms and of frequency f=400 Hz are applied.

After the chamber is completely filled with glycerin, a gradient electric field spreads over the hole area, and a gradient distribution of refractive index is then formed. By adjusting the amplitudes and frequency of the voltages, one can get a lens-like distribution of refractive index. The experiment shows that at V 1=11 Vrms, f=400 Hz, the LC cell has lens-like phase transformations and becomes an LC lens, the properties of which change with V 2. Figure 3 shows the interference fringes at various values of V 2. The dashed circles represent the position of the circular hole in the patterned electrode. The interference patterns are composed of almost circular fringes. So the properties of the LC lens are nearly axially symmetrical. The number of the interference fringes decreases with V 2, meaning that the gradient of the phase profile of the extraordinary wave decreases with increasing V 2.

Fig. 4. Phase retardation at various values of V 2.

Figure 4 shows the phase retardations of the extraordinary wave at various values of V 2 derived from the interference patterns. The symbols are the measurements, and the curves represent the regression results using quadratic equations. It can be seen that the phase retardation is tunable by V 2, the LC lens shows high optical quality, and the quality is preserved over the whole range of V 2.

Fig. 5. Lens power at various values of V 2.

Figure 5 shows the lens power changing with V 2. Similar to the LC lenses driven by two voltages reported previously [11

11. M. Ye, B. Wang, and S. Sato, “Liquid crystal lens with a focal length that is variable in a wide range,” Appl. Opt. 43, 6407–6412 (2004). [CrossRef] [PubMed]

], V 2 flattens out the spatially varying electric field and hence the refractive index in the LC layer. As a result, the lens power decreases monotonically with the increasing V 2.

It is convenient for us to examine the mechanism for the reformation of the electric field by a layer of conductive material in an LC cell using liquid glycerin that is viscous and does not volatilize appreciably. However, the properties of liquids are usually variable; the conductivity of the liquid glycerin slightly changes with time. As a result, the performance of an LC lens containing liquid glycerin is unstable. Instead of a liquid film, an LC lens fabricated using a solid film with weak conductivity may have similar but much more stable properties.

5. Conclusion

The realization of an LC lens of large aperture but low driving voltages introducing a thin layer of weakly conductive material is reported. The redistribution of the charges in the weakly conductive material smoothes the spatially varying electric field, and even if the ratio of the diameter of the hole in a patterned electrode to the distance between the hole-patterned electrode and the LC layer is too large compared to the standard value found previously, the formation of a desired spatial slowly varying electric field in the LC layer, and hence lens-like distributions of refractive index, becomes possible. The effect of the weakly conductive layer is demonstrated. An LC lens is realized having a diameter as large as 15 mm and with driving voltages as low as 11 Vrms. The properties of the lens are electrically tunable, and over the entire voltage range high optical quality is preserved. The lens power decreases monotonically with the controlling voltage.

References and links

1.

S. Sato, “Liquid-crystal lens-cells with variable focal length,” Jpn. J. Appl. Phys. 18, 1679–1684 (1979). [CrossRef]

2.

S. T. Kowel, D. S. Cleverly, and P. G. Kornreich, “Focusing by electrical modulation of refraction in a liquid crystal cell,” Appl. Opt. 23, 278–289 (1984). [CrossRef] [PubMed]

3.

T. Nose and S. Sato, “A liquid crystal microlens obtained with a non-uniform electric field,” Liq. Cryst. 5, 1425–1433 (1989). [CrossRef]

4.

N. A. Riza and M. C. DeJule, “Three-terminal adaptive nematic liquid-crystal lens device,” Opt. Lett. 19, 1013–1015 (1994). [CrossRef] [PubMed]

5.

A. F. Naumov, M. Yu. Loktev, I. R. Guralnik, and G. Vdovin, “Liquid-crystal adaptive lenses with modal control,” Opt. Lett. 23, 992–994, (1998). [CrossRef]

6.

M. Ye and S. Sato, “Optical properties of liquid crystal lens of any size,” Jpn. J. Appl. Phys. 41, L571–L573 (2002). [CrossRef]

7.

B. Wang, M. Ye, M. Honma, T. Nose, and S. Sato, “Liquid crystal lens with spherical electrode,” Jpn. J. Appl. Phys. 41, L1232–L1233 (2002). [CrossRef]

8.

N. A. Riza and S. A. Khan, “Programmable high speed polarization multiplexed optical scanner,” Opt. Lett. 28, 561–563, (2003). [CrossRef] [PubMed]

9.

H. Ren and S. T. Wu, “Tunable electronic lens using a gradient polymer network liquid crystal,” Appl. Phys. Lett. 82, 22–24 (2003). [CrossRef]

10.

B. Wang, M. Ye, and S. Sato, “Lens of electrically controllable focal length made by a glass lens and liquid-crystal layers,” Appl. Opt. 43, 3420–3425 (2004). [CrossRef] [PubMed]

11.

M. Ye, B. Wang, and S. Sato, “Liquid crystal lens with a focal length that is variable in a wide range,” Appl. Opt. 43, 6407–6412 (2004). [CrossRef] [PubMed]

12.

V. V. Presnyakov and T. V. Galstian, “Electrically tunable polymer stabilized liquid-crystal lens,” J. Appl. Phys. 97, 103101–103101-6 (2005). [CrossRef]

13.

G. Q. Li, D. L. Mathine, P. Valley, P. Ayras, J. N. Haddock, M. S. Giridhar, G. Williby, J. Schwiegerling, G. R. Meredith, B. Kippelen, S. Honkanen, and N. Peyghambarian, “Switchable electro-optic diffractive lens with high efficiency for ophthalmic applications,” Proc. Natl. Acad. Sci. USA 103, 6100–6104 (2006). [CrossRef] [PubMed]

14.

M. Ye, Y. Yokoyama, and S. Sato, “Liquid crystal lens prepared utilizing patterned molecular orientations on cell walls,” Appl. Phys. Lett. 89, 141112–141114-3 (2006). [CrossRef]

15.

B. Berge and J. Peseux, “Variable focal lens controlled by an external voltage: an application of electrowetting,” Eur. Phys. J. E 3, 159–163 (2000). [CrossRef]

16.

C. C. Cheng and J. A. Yeh, “Dielectrically actuated liquid lens,” Opt. Express 15, 7140–7145 (2007). [CrossRef] [PubMed]

17.

M. Ye, B. Wang, T. Takahashi, and S. Sato, “Properties of variable-focus liquid crystal lens and its application in focusing system,” Opt. Rev. 14, 173–175 (2007). [CrossRef]

18.

M. Ye, B. Wang, M. Kawamura, and S. Sato, “Image formation using liquid crystal lens,” Jpn. J. Appl. Phys. 46, 6776–6777 (2007). [CrossRef]

19.

M. Ye, B. Wang, and S. Sato, “Effects of dielectric constant of glass substrates on properties of liquid crystal lens,” IEEE Photon. Technol. Lett. 19, 1295–1297 (2007). [CrossRef]

OCIS Codes
(220.3630) Optical design and fabrication : Lenses
(230.3720) Optical devices : Liquid-crystal devices

ToC Category:
Optical Design and Fabrication

History
Original Manuscript: December 5, 2007
Revised Manuscript: January 9, 2008
Manuscript Accepted: January 15, 2008
Published: March 14, 2008

Citation
M. Ye, B. Wang, and S. Sato, "Realization of liquid crystal lens of large aperture and low driving voltages using thin layer of weakly conductive material," Opt. Express 16, 4302-4308 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-6-4302


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. S. Sato, "Liquid-crystal lens-cells with variable focal length," Jpn. J. Appl. Phys. 18, 1679-1684 (1979). [CrossRef]
  2. S. T. Kowel, D. S. Cleverly, and P. G. Kornreich, "Focusing by electrical modulation of refraction in a liquid crystal cell," Appl. Opt. 23, 278-289 (1984). [CrossRef] [PubMed]
  3. T. Nose and S. Sato, "A liquid crystal microlens obtained with a non-uniform electric field," Liq. Cryst. 5, 1425-1433 (1989). [CrossRef]
  4. N. A. Riza and M. C. DeJule, "Three-terminal adaptive nematic liquid-crystal lens device," Opt. Lett. 19, 1013-1015 (1994). [CrossRef] [PubMed]
  5. A. F. Naumov, M. Yu. Loktev, I. R. Guralnik, and G. Vdovin, "Liquid-crystal adaptive lenses with modal control," Opt. Lett. 23, 992-994, (1998). [CrossRef]
  6. M. Ye and S. Sato, "Optical properties of liquid crystal lens of any size," Jpn. J. Appl. Phys. 41, L571-L573 (2002). [CrossRef]
  7. B. Wang, M. Ye, M. Honma, T. Nose, and S. Sato, "Liquid crystal lens with spherical electrode," Jpn. J. Appl. Phys. 41, L1232-L1233 (2002). [CrossRef]
  8. N. A. Riza and S. A. Khan, "Programmable high speed polarization multiplexed optical scanner," Opt. Lett. 28, 561-563, (2003). [CrossRef] [PubMed]
  9. H. Ren and S. T. Wu, "Tunable electronic lens using a gradient polymer network liquid crystal," Appl. Phys. Lett. 82, 22-24 (2003). [CrossRef]
  10. B. Wang, M. Ye, and S. Sato, "Lens of electrically controllable focal length made by a glass lens and liquid-crystal layers," Appl. Opt. 43, 3420-3425 (2004). [CrossRef] [PubMed]
  11. M. Ye, B. Wang, and S. Sato, "Liquid crystal lens with a focal length that is variable in a wide range," Appl. Opt. 43, 6407-6412 (2004). [CrossRef] [PubMed]
  12. V. V. Presnyakov and T. V. Galstian, "Electrically tunable polymer stabilized liquid-crystal lens," J. Appl. Phys. 97, 103101-103101-6 (2005). [CrossRef]
  13. G. Q. Li, D. L. Mathine, P. Valley, P. Ayras, J. N. Haddock, M. S. Giridhar, G. Williby, J. Schwiegerling, G. R. Meredith, B. Kippelen, S. Honkanen, and N. Peyghambarian, "Switchable electro-optic diffractive lens with high efficiency for ophthalmic applications," Proc. Natl. Acad. Sci. USA 103, 6100-6104 (2006). [CrossRef] [PubMed]
  14. M. Ye, Y. Yokoyama, and S. Sato, "Liquid crystal lens prepared utilizing patterned molecular orientations on cell walls," Appl. Phys. Lett. 89, 141112-141114-3 (2006). [CrossRef]
  15. B. Berge and J. Peseux, "Variable focal lens controlled by an external voltage: an application of electrowetting," Eur. Phys. J. E 3, 159 -163 (2000). [CrossRef]
  16. C. C. Cheng and J. A. Yeh, "Dielectrically actuated liquid lens," Opt. Express 15, 7140-7145 (2007). [CrossRef] [PubMed]
  17. M. Ye, B. Wang, T. Takahashi, and S. Sato, "Properties of variable-focus liquid crystal lens and its application in focusing system," Opt. Rev. 14, 173-175 (2007). [CrossRef]
  18. M. Ye, B. Wang, M. Kawamura, and S. Sato, "Image formation using liquid crystal lens," Jpn. J. Appl. Phys. 46, 6776-6777 (2007). [CrossRef]
  19. M. Ye, B. Wang, and S. Sato, "Effects of dielectric constant of glass substrates on properties of liquid crystal lens," IEEE Photon. Technol. Lett. 19, 1295-1297 (2007). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited