Using multidomain liquid-crystal structures to improve the optical characteristicsof microlenses

A. V. Morozova�

State Technical University, Novosibirsk

G. E. Nevskayab�

State Maritime Technical University, St. Petersburg�Submitted May 22, 2007�Opticheski� Zhurnal 75, 39–42 �February 2008�

It is proposed to use multidomain �MD� liquid-crystal �LC� structures in LC microlenses �LC-MLs�. The main drawback of nonsymmetric LCMLs with planar orientation is that they involvea prolonged transition process accompanied by the formation of a disclination. This paper pro-poses and implements a method of putting LCMLs into a working regime based on the applica-tion of an MD structure. As a result, the time to put them into the working regime was reducedby about an order of magnitude. It is demonstrated that MD structures can be used to form LClenses based on a Fresnel phase plate without using a complex electrode configuration.© 2008 Optical Society of America.

INTRODUCTION

Liquid-crystal microlenses �LCMLs� use the effect ofreorientation of nematic molecules in an inhomogeneouselectric field. The electric field configuration is specified by aspecial electrode geometry. In a spherical microlens, circularapertures are created in the electrodes to form an axiallysymmetric electric field. Two types of spherical microlenseshave been developed: nonsymmetric lenses have apertures inone of the electrodes, and symmetrical ones have apertures inregister in the two electrodes. To obtain a cylindrical micro-lens, slots of a definite size are created in the electrodes. Thefocal length of an LCML is varied by varying the amplitudeof the applied voltage. Microlenses are small and light inweight and are controlled from low-power sources. Thisopens up wide possibilities of using them in photonic de-vices: information displays, optical modulators, optical-focusing systems, vision-correction devices, etc. However,the optical properties of LCMLs are negatively affected bydefects that appear during the reorientation of the liquid-crystal �LC� molecules. This paper presents the results ofusing multidomain �MD� LC structures. It is shown that thisimproves the optical properties of the microlenses.

FIG. 1. Designs of LCMLs. �a� With homeotropic orientation, �b� with homo1—Glass, 2—chromium coating, 3—indium oxide coating, 4—LC, 5—poly

98 J. Opt. Technol. 75 �2�, February 2008 1070-9762/2008/0

USING MD STRUCTURES IN LCMLS WITH HOMEOTROPICORIENTATION

A characteristic feature of an LCML with homeotropicorientation �Fig. 1a� is the presence in the microlens of asatellite defect, which always appears.1,2 An example of suchan effect is shown in Fig. 2a.

The deformation caused by it causes the symmetry tobreak down and causes aberrations and astigmatism to ap-pear in microlenses. Moreover, as a result of the action ofthis defect, the curvature of the right and left branches of thephase profile becomes different,2 and the placement of thesatellite defect is not controlled. This increases the mutualinfluence of the microlenses on each other in rasters.

There have been studies directed toward finding methodsof reducing the influence of the satellite defect on homeotro-pic LCMLs. A method based on the use of an MD structurehas been developed that makes it possible to enclose a mi-crolens in a disclination ring. Based on this method, twodesigns of LCMLs with an MD structure of homeotrope-hybrid type have been proposed and implemented. In the firstdesign, the position of the boundary between the domainscoincides with the aperture on the control electrode �Fig. 1b�.

ous initial orientation, �c� with an MD structure of hybrid-homeotrope type.e film, 6—boundary of a domain pair.

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9820098-03$15.00 © 2008 Optical Society of America

In the second, the boundary between the domains is movedaway from the aperture into the depth of the cell �Fig. 1c�.Figures 2b and 2c show interference patterns observed whenLC molecules reorient in a homeotropic microlens with MDstructures. It can be seen that the character of the reorienta-tion of the nematic in the region of the aperture is similar toreorientation in a homeotropic microlens with homogeneousorientation of the molecules. It differs in that the satellitedefect is localized at the boundary between the homeotropic

FIG. 2. Visualization of a satellite defect in an LCML with homeotropicorientation. �a� With homogeneous initial orientation �Fig. 1a�, �b� with anMD structure of hybrid-homeotrope type �Fig. 1b�, �c� with an MD structureof hybrid-homeotrope type �Fig. 1c�.

99 J. Opt. Technol. 75 �2�, February 2008

and hybrid orientations. It has been established that, whenthe diameter of the homeotropic region is increased by 10%with respect to the diameter of the aperture, the influence ofthe satellite defect will be insignificant. The use of MD struc-tures in nonsymmetric LCMLs with homeotropic orientation�Figs. 1b and 1c� made it possible to reduce the defect levelof the microlenses, to reduce the LC-reorientation region be-yond the boundaries of the microlenses, to eliminate the mu-tual influence of the microlenses in the array, and to reducethe limiting distance between them.

USING MD STRUCTURES IN LCMLS WITH PLANARORIENTATION

The main drawback of nonsymmetric LCMLs with pla-nar orientation is the formation of a disclination,3,4 whichdegrades its optical characteristics. We have studied how thepreslope of the LC molecules in nonsymmetric microlensesaffects the process of the appearance and the dynamics of thebehavior of a disclination. Planar orientation of the nematicmolecules was achieved by prerubbing substrates coatedwith a polyimide layer. The angle of the initial preslope ofthe nematic molecules on the substrate surface was 4°–5°.An estimate of the preslope angle of the director, which weproposed earlier, is described in Ref. 5. Two types of planarLC cells have been fabricated with different relative place-ment of the molecules on the substrates. In the case of par-allel assembly of the cell, a disclination appears at the for-mation stage of the microlens and is stable. In the case of acell with antiparallel assembly, the process of forming themicrolenses is accompanied by the appearance of a disclina-tion, which disappears in a small time interval. After theprocess of forming a microlens is complete, any change ofthe control voltage does not result in its repeated appearance,

FIG. 3. Design of an LCML with a short transition-process time based on anMD structure. 1—Opaque chromium coating, 2—domain with planar orien-tation, 3—domain with hybrid orientation, 4—transparent ITO coating.

FIG. 4. Interference patterns ob-served in a planar nonsymmetricLCML �diameter 200 �m� with anMD structure when a control voltagewith frequency 2 Hz and amplitude7.5 V is supplied for 1 �a�, 10 �b�, 20�c�, and 40 sec �d�.

99A. V. Morozov and G. E. Nevskaya

except for the case of returning the LC to the initial planarorientation. The formation time � of a microlens with anti-parallel assembly depends on the ratio H /d �H is the lensdiameter and d is the thickness of the LC layer� and thecontrol voltage. An increase of � is observed when theseparameters are increased. In the best case, the time for anLCML with diameter 200 �m to appear was 3–5 min, whilethat for one with diameter 400 �m was 10–15 min. An MDstructure �Fig. 3� is used to decrease the time of reaching thedisclination-free regime. A wedge-shaped region with hybridorientation is created at the boundary of the microlens, wherethe disclination is formed. The presence of an MD structurechanges the reorientation character of the LC molecules inthe microlens. As shown in Fig. 4, when the control voltageis supplied, two disclination regions are formed, which dis-appear in a short time interval. The time to form a microlensis 15 sec for a control voltage of 4.5 V and 40 sec for 7.5 V.

USING MD STRUCTURES IN CONTROLLABLE FRESNELPHASE PLATES

A simple method has been proposed for forming LClenses using MD structures, based on Fresnel phase platesand without a complex electrode configuration. AnnularFresnel zones are created by using regions with various ori-entations of the LC molecules. Figure 5 shows a lens thatuses an MD structure of planar-hybrid type. The radii of thezone boundaries vary proportionally to the square root of asequence of whole numbers n: r=�n�f , where f is the focallength, and � is the wavelength of the incident light. Such adesign makes it possible to form a lens in the region wherethe control electrodes overlap without creating a complextopology of the conductive layers. The same control voltageis applied to all the Fresnel zones. The absence of interelec-trode gaps ensures that there are no uncontrolled regions. Asan example, let us consider a hybrid-planar type of structurebased on an LC with positive dielectric anisotropy. When acontrol voltage is supplied to the electrodes, the domains willlie in a homogeneous electric field. Since the hybrid andplanar domains possess different volt-phase responses, thephase difference � between the Fresnel zones will changewhen control voltage U is decreased, as shown in Fig. 6 �U0

is the threshold voltage�.The proposed method can be used when creating phase

FIG. 5. Design of an LC lens based on a Fresnel phase plate using an MDstructure of planar-hybrid type. 1—Spacers that adjust the thickness of theLC, 2—MD structure, 3—continuous conducting ITO layer, 4—interdomainboundary.

diffraction gratings. In Ref. 6, phasing of the zones is accom-

100 J. Opt. Technol. 75 �2�, February 2008

plished by means of a set of transparent electrodes. Such adesign possesses all the drawbacks of the lenses consideredabove with a set of individually addressed electrodes. Whenthe proposed method is used, the complex design of the elec-trodes can be eliminated, and a sharp boundary can beachieved between the Fresnel zones.

CONCLUSION

Two designs of nonsymmetric homeotropic microlenseshave been proposed and implemented, using an MD structureof homeotrope-hybrid type. This made it possible to signifi-cantly restrict the size of the satellite defect and to reduce itsinfluence on the microlens. The optimum design of such amicrolens was selected. A method of rapidly forming a non-symmetrical LCML with planar orientation when a controlvoltage was applied was proposed and implemented. It isbased on the use of an MD structure of hybrid-planar type.

The design of an LCML based on Fresnel phase platesusing MD structures has been proposed. An advantage ofsuch a design is that there is no complex topological patternof the conductive layers and no uncontrolled regions of theLCs in the interelectrode gaps.

a�Email: kof@ref.nstu.rub�Email: nevskayag@mail.ru

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FIG. 6. Variation of the phase difference between the Fresnel zones as afunction of the control voltage for an LC lens with an MD structure ofplanar-hybrid type �the LC layer is 50 �m thick, ZhKM-1282�.

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100A. V. Morozov and G. E. Nevskaya