Organization

In this research thesis, we will focus on designing the MeDLC structure for the TV application, and to fit with the lens- like distribution. In this case, we can make sure our design can project the image to the right direction and therefore result in a smaller crosstalk phenomenon. Final is to achieve a workable switching.

1.5 Organization

This thesis is organized as following: The principle of liquid crystal, the focusing formula, and the principle of conventional 2D/3D switching method is presented in Chapter 2. Additionally, this chapter also shows the crosstalk phenomenon. In Chapter 3, the fabrication process of the MeDLC will be introduced in detail, and the major instruments used to measure the MeDLC images. The design and simulation result of MeDLC and discussion will be shown in Chapter 4. Next the experimental result and summary will be presented in Chapter 5. Finally, the conclusions and future work of this thesis will be given inChapter 6.

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Chapter 2

Principle of Liquid Crystal Lens

First, this chapter covers the principles of how the refractive index changes when liquid crystal faced different direction. Second, principle of lens and GRIN lens, and the focal length proof of a cylindrical lenticular lens will be discussed. Third, in order to design a 2D/3D switching display, the conventional double electrode LC lens and, the principles of conventional 2D/3D switching method are introduced. Finally, a brief summary will be given.

2.1 Introduction to liquid crystal

Liquid crystal can be divided into two groups: positive dielectric anisotropy and negative dielectric anisotropy. For positive dielectric anisotropy is that if the component along the optical axis is greater than the component perpendicular to the axis (_//  _). These molecules align parallel to an applied field. If the reverse is true

(_//  _), than it is said to be negative dielectric anisotropy and the molecules will align perpendicular to an applied field. Either case can be apply for create a liquid crystal lens [24].

17

2.1.1 Liquid crystal orientation to match the index of refraction

Liquid crystals are characterized by an orientation order of their constituent rod like molecules. In nematic LC this orientation order has uniaxial symmetry, the axis of symmetry being parallel to a unit vector d called the director. Since the orientation of liquid crystal can be change when applying curtain voltage. This change will affect the direction of light when passing thru the liquid crystal layer because of the change of the refractive index in liquid crystal.

Refer to Fig. 2-1 assume light propagate in the z direction and polarized in x direction. When the indicatrix is not rotated the light will be affected by the refractionn_e. However when the indicatrix is rotated at an angle θ the same light will be affected by both n_e andn_o. The slice of the indicatrix can be expressed as Eq.2-1.

Θ

Z

X

no

ne

Θ ZΘ

XΘ

n^Θ

Fig. 2-1 Geometry to calculate change in index of refraction with liquid crystal molecule orientation.

18 Therefore, Eq.2-1 can be written as Eq.2-3.

1

Thus the equivalence equation of rotate angle can be derived as Eq.2-5.

thickness direction, a more useful form can be obtained to solve the n_ as Eq.2-6.

This expression gives the index of refraction for light polarized in the x direction and traveling in the z direction as a function of the angular orientation of the nematic liquid crystals in the x- z plane. Therefore, can be applied when the LC direction were no longer perpendicular or parallel to the x direction.

19

2.2 Introduction to Gradient Index Optics

A conventional lens explains how Gradient Index Lens (GRIN lens) is works: An incoming light ray is first refracted when entering the shaped lens surface this is caused by an abrupt change in the refractive index from air to the homogeneous material. The light passes the lens material in a direct way until it emerges through the exit surface of the lens where it is refracted again because of an abrupt index change from the lens material to air, as shown in Fig. 2-2.

Fig. 2-2 Conventional lens

GRIN is short for graded- index or gradient index which can refers to an optical element in which the refractive index varies. More specifically (from the Photonics Dictionary) a GRIN lens is a lens whose material refractive index varies continuously as a function of spatial coordinates in the medium. Also, a graded- index fiber describes an optical fiber having a core refractive index that decreases almost parabolically and radially outward toward the cladding as shown in Fig. 2-3. However the focal length of GRIN is fixed [26][27].

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Fig. 2-3 GRIN Lens

Fig. 2-4, shows the geometry of the focused GRIN lens. First we assume f >r and a center refractive index value of the center larger than the edge. And as the known formula, v=c/n, where c,n and v are the velocity in vacuum, index of refract ion and the velocity in medium. Using v=c/n we know the velocity is inverse proportional to the refractive index. Therefore, wave fronts will slow down when reaching the dense region and speed up in less dense regions. In this case the GRIN lens can be focused.

f d

n(r) r

n²

n¹

R

Fig. 2-4 The geometry of the focused GRIN Lens.

21

A ray that traverses the lens on the optical axis passes along an optical path length (OPL) of (OPL )_axis  n(r)_{m ax}d ,whereas for a ray traversing at a height

2

r r , overlooking the slight bending of ray path, (OPL )_r  n(r)d . Since a planer wave front must be bend into a spherical wave fronts, the OPLs from one to the other, along any route must be equal as followed,

2 By substitution of Eq.2-7 and Eq.2-8can get,

d By substitution, we can get,

22

As the result, if the index of refraction drops off parabolically from its high along the central axis, the GRIN lens would focus at point F. However, GRIN lens can only be operated as a lens with fixed focal length. Hence, the application of LC lens became more useful. The LC lens can be acted as both GRIN lens with variable focal length and normal layer with certain refractive index by controlling the direction of LC direction. In this case, the ability of switching between 2D and 3D images can be achieved, which will be demonstrated detailly in next section.

2.3 Conventional LC lens

For a conventional liquid crystal lens [28] where the focal length is defined in Eq. 2-10. The LC direction can be controlled by the applied voltages between two parallel electrodes, owning to the fringing field effect as shown in Fig. 2-5. Passing through this lens, light encounters different refractive indexes so that the changes of phase retardation work as a lens, and refracts light to different angle. However, when applying a low operation voltage the electric field cannot a ffect the liquid crystal near the center efficiently and its equivalence lens results in a small numerical aperture (large focal length) as shown in Fig. 2-6 (a). So that the lens has to place in a long distance and cause a large volume of display which is not suitable for the market place.

Although a high operation voltage can cause a higher numerical aperture. However due to this high voltage the LC directory nea r the edge of the LC cell will show no distinct difference. Therefore, the equivalence lens can result in a multi focal length lens as shown in Fig. 2-6 (b). Which will result in a large beam size and therefore cause a high crosstalk phenomenon as shown in Fig. 2-7 (a) and Fig. 2-8 (a). For a comparison, an ideal lens which with smaller bean size will result in a small crosstalk

23

phenomenon as shown in Fig. 2-7 (b) and Fig. 2-8 (b).

This method not only solves the issue of mismatching of LC alignment, but also LC lens production incompatibility. Nevertheless, this method has a drawback of high operation voltage large beam size and result in high crosstalk.

ITO

Rubbing direction

LC layer

ITO _W

s

W_L

ITO

WE

d t

(a)

Glass

Fringing

field Electrode

(b)

Fig. 2-5 Structure of the traditional LC cylindrical lens (a) Top view and (b) Side view.

24

Fig. 2-6 The fringing field effect of a double electrode lens (a) an equivalence lens when applying low operation voltage (b) an equivalence lens when applying high operation voltage

f2

f1

. . .

Collimated light f

r (mm)

Normalizedintensity

0 1

LargeBeam size (at FWHM)

 Equivalence lens  Ideal lens

Collimated light

Fig. 2-7 The beam size of (a) equivalence lens of double electrode LC lens when applying large operation voltage (b) ideal lens

25

f2

f1

. . .

Left eye Right eye

High Crosstalk

f

Left eye Right eye

Low Crosstalk

(a) (b)

Fig. 2-8 The crosstalk of (a) equivalence lens of double electrode LC lens when applying large operation voltage (b) ideal lens

2.4 LC lens with 2D/3D switching

Among various types of 3D displays, switching methods can be divided into two categories such as the controls of transmittance and the control of light direction. For the latter case of controlling optical path, lens switching methods are introduced such as active LC lenticular lens and polarization activated microlens.

26

2.4.1 Active LC Lenticular lens

In the case of active LC lenticular lens as shown in Fig. 2-9. A fixed concave lens patterns were formed inside the cell and the LC were placed between the concave lens and bottom substrate, which only the refractive index of n_o is the same as the concave lens. If the light from the underlying LCD display is polarized perpendicular to the plane of the drawing, it encounters a refractive index transition from high to low within the cell, resulting in a net lens-action to form a 3D image.

If a voltage is applied over the cell, the extra-ordinary axis of the LC is aligned parallel to the lens axis, which is in the plane of the drawing. Therefore, the light encounters the lower refractive index corresponding to the ordinary axis of the LC.

Since the ordinary refractive index matches the refractive index of the concave lens, the lens is effectively switched off. In this case 2D images can be perceived.

Concave Lens

LC

^0

Fig. 2-9 Principle of active LC lenticular lens

2.4.2 Polarization activated microlens

For the polarization activated microlens as shown in Fig. 2-10. The light was polarized, and the light direction was changed after encounter with different refractive in the micro lens. The ability of switching between 2D and 3D images can be achieved by applying voltage to the polarization switch layer.

27 Polarisation

Switch Polariser

Fig. 2-10 Principle of polarization switch LC lens

However these two methods have some disadvantages, such as mismatching of LC alignment at the interface of concave lens, as well as incompatible with current LC displays production process and brightness.

28

2.4.3 Electric-field driven LC lens (ELC lens)

Fig. 2-11 shows the basic concept of electric- field driven LC lens for one lens pitch. At the voltage off state, incident light passes through ELC lens cell without change of propagation direction. At the voltage on state, local electric fields are formed. The electric field at the part of lens edge is much stronger than electric field at center of lens. This non uniform distribution of electric field cause non uniform of tilt angle of LC director and the refractive index changes accordingly. Therefore, a phase difference was achieved and cause the change of propagation direction of the light. In this case, a 3D image can be perceived.

LC director

Pixels LC

Lens

2D mode 3D mode

Fig. 2-11 Concept of electric- field driven LC lens at (a) lens off state for a 2D mode (b) lens on state for a 3D mode.

For a short summary the conventional double electrode LC lens have the drawback high operation voltage, large beam size and result in a high crosstalk of 3D display. And the methods of Active LC Lenticular Lens and Polarization activated microlens have the issue of mismatching of LC alignment and incompatible of LCD current production.

29

2.5 Crosstalk phenomenon of LC-lens on 3D displays.

In our opinion the use of an LCD equipped with lenticular lenses is a viable route to achieve multi- view 3D display. By using a switchable LC lenticular lens, we can have a display that shows natural 3D images as well as high-resolution 2D material.

And thus the most dominating characteristic affecting on the perceived stereo image quality in 3D displays is the 3D crosstalk. The crosstalk affects not only to how easily the image can be fused and how smooth the transition between adjacent views is, but also to the overall image quality, being in the worst case a source of visual discomfort.

3D crosstalk can be defined as the leakage of the left eye image data to the right eye.

This also indicating the images for each eyes must be well project. As for the application of using lens to provide 3D as shown in Fig. 2-12. The crosstalk phenomenon can be minimized when the image can be projected to the exact eyes, under the condition of the having high focusing ability of the lens. If a LC lens with low focusing ability than the left-eye image data will leak to the right–eye and vice versa, therefore these wrong image signal can cause high crosstalk phenomenon. And the crosstalk value definition is as shown in Eq.2-13 [19].

Fig. 2-12 Examples of 3D displays working principle of lenticular lens.

30 been discussed briefly in this chapter. We present the fundamental theory of refractive index difference when liquid crystal was no longer parallel or perpendicular to the light direction. This can give us the idea of how to let the incoming light to project to different direction. Followed by the basic focusing principle of a cylindrical lens can show the formula of focal length. Third the conventional LC lens was discussed.

These give us the idea of how to operate the liquid crystal lens. Finally, the definition of crosstalk for 3D images was illustrated.

From prior reports, active LC-lens plays an important role in 3D-display technology not only it can be used in 2D/3D images switching but also can be combined with a head tracking device. In this case a high resolution and wide viewing angle 3D images can be perceived.

In this thesis, these fundamental work principles are employed for designing the new LC lens for enhance the mismatching of LC alignment, production process, effective aperture, and the crosstalk phenomenon.

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Chapter 3

Fabrication and measurement Instruments

3.1 Introduction

The process to fabricate LC lens were described in this chapter. The commercial available indium-tin oxide (ITO) glass which thickness is 550m and the resistance is 20Ω/□ was cleaned by standard process in advance. After that, the semiconductor process including spin coating, lithography and etching was utilized in order to obtain the desired pattern. Followed by the spin coating and rubbing techniques to make the alignment layer. Next, the top and bottom glass was assembled which the cell gap was controlled about 60m by spacers and fill in the LC. Finally, wires for various voltage driving will be connected to each electrode.

3.2 Fabrication process

This part will describe the cell fabrication process which include s spin coating, lithography, wet etching, rubbing, assembling. The detailed fabrication steps are listed below and as shown in Fig. 3-1. Also the ITO patterning is shown schematically in Fig.

3-4.

32

Glass Cleaning

Etching

Etching Solution Preparation Shadow Mask

Preparation

Lithography

Patterned Glass Cleaning

Coating

Assembling Soldering OM Checking

Rubbing

Fig. 3-1 Fabrication steps.

33

I. Glass cleaning

The detailed sequence is as follow:

Step 1: Place the glasses on the Teflon carrier, and put them into a container with acetone as shown in Fig. 3-2. Ultrasonic vibrates 20 minutes to remove the organic contamination on the glass.

Step 2: Rinsed by DI water 1 minute.

Step 3: Rubbing the glass with hand by detergent.

Step 4: Place the glasses on the Teflon carrier, and put them into a container with DI water as shown in Fig. 3-2. Ultrasonic vibrates 40 minutes to remove the remained particle and detergent on the glass.

Step 5: Use N₂ purge to dry the ITO glasses; Place them into a glass container with a cover.

Step 6: Put the glass container into an oven with 110℃ for 30 minutes.

DI water Acetone

Fig. 3-2 Schematic picture of step1 and step 4.

34

II. ITO patterning

The detailed sequence is as follow and also can refer to Fig. 3-4.

Step 1: For the display application, the glass is widely used as a substrate. In the fabrication, the ITO glass of about 0.55 mm thick was chosen. Before the lithography process, ITO glass was cleaned by step I.

Step 2: Put the glasses on the metal holder and put into the HMDS oven. The purpose is to eliminate the surplus steam and HMDS can improve the adhesion between organic photoresist and the glass, to let the phototresist can be uniformly coating on the substrate.

Step 3: A positive photoresist was applied and coated on substrate.

Step 4: Soft bake for one and a half minutes, to eliminate most of the solvent of the photoresist, to enhance the adhesion.

Step 5: Expose the ITO glass with ultra-violet (UV) light source through shadow mask for 40 second. Consequently, the pattern on the mask was transformed to the positive photoresist after developing. The MASK is as shown in Fig.

3-3.

Step 6: After exposure and development, the substrate was etched. And check by OM to see if there is any broken line.

Step 7: Remove the photoresist by acetone.

Electrode width=116um

Fig. 3-3 (a)The MASK pattern (b)(c)(d) is the pattern after etching.

35

(a)

(b)

(c)

(d)

(e)

ITO

Photoresist ITO

ITO

ITO

ITO

Photoresist

Photoresist

Fig. 3-4 Flow of fabricating ITO electrodes.

(a) ITO glass,

(b) spin-coating Positive photoresist uponthe ITO surface, (c) using lithography technique to obtain the latent image, (d) etching to produce the desired ITO pattern, and

(e) eliminating the remaining photomask by acetone.

36

III. Coating , Rubbing & Assembling

Step 1: Cleaning the patterned and bottom glass with the steps discussed previous.

Step 2: Put the glasses into the O-zone machine for 20 minutes, to eliminate the organic on the glasses, this step can make the coating more efficiently.

Step 3: Coating the solvent to make the PI adhesion more efficiently.

Step 4: Coating the PI.

Step 5: Put the glass container into an oven with 220℃ for 60 minutes.

Step 6: Rubbing the glasses and make the mark of the rubbing direction.

Step 7: Stick the plastic spacer on the bottom glass.

Step 8: Put the bottom glass on the top of the patterned glass and fixed with a tape.

Step 9: Put a heavy metal on the top of the glasses and glued.

Step 10: Fill in the LC

Step 11: Seal with non-bubbles glue and curing.

Next, the ultrasonic solder was soldered on the electrode to enhance the adhesive of the solder and smeared with AB glue to avoid the wires coming off. Finally, the Multi- Electrically Driven Cylindrical Liquid Crystal (MeDLC) will be measured.

Electrode width=116um

Slit width=116um

Fig. 3-5 Prototype of MeDLC

37

3.3 Measurement system

After the fabrication of MeDLC structure. The optical properties of the devices were measured by CCD; the setup is as shown in Fig. 3-6 and Fig. 3-7. First, the bright and dark lines can be recorded by the camera with cross polarizer. These lines were caused by the phase difference, and the profile of refractive index difference can be reconstructed, than the ideal curve and simulation result can be compared to check for the trend and performance of the MeDLC.

Fig. 3-6 Experiment setup

Fig. 3-7 Experiment setup

38

The method of tuning the voltage to an optimized value was shown as Fig. 3-8.

First we transfer the Δn profile into x axis only as show in Fig. 3-8(b). Each dot represents a phase difference in 2π which also means the position when two bright or dark lines occur. Final is tried to find an optimized value of voltage that consist with

First we transfer the Δn profile into x axis only as show in Fig. 3-8(b). Each dot represents a phase difference in 2π which also means the position when two bright or dark lines occur. Final is tried to find an optimized value of voltage that consist with

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