Reactive mesogen retarders and applications

Billy Tran*, Tom Baur

Meadowlark Optics, 5964 Iris Parkway, Frederick, CO 80530

ABSTRACT

Reactive mesogen retarders or waveplates provide new opportunities for flexibility in the design of opticalsystems requiring polarization control. These true zero order retarders are less than 10 microns thick andcan be used either as a free standing film or they can be coated as a film on other optical elements such as

lenses or mirrors. The slow axis direction can be patterned to form small, even microscopic, discreteretarders or continuously varied to make radial or axial polarizers. We describe in detail the properties of

these new optical retarders.

Keywords: Polymer, photoalignment, UV-cured, patterned retarders, waveplate

1. INTRODUCTION

The use of reactive mesogen to produce optically birefringent films shows much promise. A homogenousorientation of the slow axis over the clear aperture can produce thin films on the order of 5 to 10 umthickness with 2J1 00 retardance uniformity. The true zero-order nature of the retarders allow for uniqueoptical characteristics such as a transmitted beam deviation of less the 5 arc-sec and almost no transmittedwavefront distortion.

Azo-benzene derived photoalignment material can be finely patterned. Using laser etching techniques andUV alignment, polarization, gratings, radial and axial retarders and polarizers can be made. Nersisyan andTabiryan have shown that photoaligiiment techniques produce controlled variation of slow axis direction [1].Custom patterning of polarizers and retarders has applications in photonics and in other industries.Combining a patterned half wave retarder with a linear polarizer yields a patterned polarizer.

We can make large aperture retarders by spin coating to apply the photoalignment and reactive mesogenlayers. Slow axis directions can be varied over spatial scales of a few microns by patterned exposure of thephotoalignment layer to polarized UV light. While a sizable literature exists on creating and using diffractivewaveplates or polarization gratings [2], [3], we describe here the manufacture and performance of reactivemesogen retarders with uniform slow axis direction. These are sold under the trade name of Raptor retarders.

Polymer Optics and Molded Glass Optics: Design, Fabrication, and Materials II, edited by David H. Krevor, William S. Beich,Michael P. Schaub, Stefan M. Bäumer, Proc. of SPIE Vol. 8489, 84890B © 2012 SPIE 000 code: 0277-786/12/$18

doi: 10.1117/12.929870

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2. MATERIALS

We make optical retarders by spin coating and cross-linking a reactive mesogen layer over an underlyingphotoalignment layer that is also spun coat onto a fused silica window. When patterning is not required thisalignment layer can also be a standard, mechanically buffed polyimide layer normally used for aligningnematic liquid crystals.

The photoalignment material is a derivative of azo-benzene [4], [5]. Azo compounds have wide uses inoptics and dyes industries. Using polarized UV light, the azo compounds will isomerize from the cis to thetrans orientation, with the trans conformation perpendicular to the axis of polarization. This orientation ofthe trans state causes alignment in the subsequently applied reactive mesogen material.

Reactive mesogens are liquid crystals that can be polymerized to form a high molecular weight polymer.They are monomers that have various side chains, though typically acrylates. Reactive mesogens coatedover a photoalignment layer or a buffed polyimide alignment layer photopolymerize in-situ to form thinoptically birefingent polymers. Cross-linking is achieved with unpolarized UV light.

3. PRODUCING THE RETARDER

We usually apply the photoalignment and reactive mesogen layers to uncoated fused-silica substrates. Thephotoalignment material is spun coated onto a round substrate. First we apply a layer of photoalignmentmaterial. The photoalignment coated substrate is heated and dried to remove excess solvent. We then alignthe photoalignment layer using polarized UV light source. The slow axis is in the direction that is parallel tothe polarization direction of the UV light that was incident on the photoalignment layer. When illuminateduniformly, the alignment direction is also uniform across the entire clear aperture of the retarder. Thealignment direction can be changed by changing the direction of linear polarization of the UV light.However, after a layer of reactive mesogen is coated and cured in the following steps, the photoalignmentdirection is locked and changes to the orientation of the slow axis are no longer possible.

It is at this step in the preparation that we can produce uniquely patterned retarders and polarizers. Radialand axial retarders can be produced using custom polarization patterning techniques with holographic ordirect write laser patterning [1]. A patterned polarizer is made by combining a half-wave retarder with alinear polarizer.

A solution of reactive mesogen (RM) is spun coated onto the photoaligned substrate. The deposited RIVI isheated to anneal it to the substrate. The annealing helps to align the RM to the alignment direction of thephotoalignment layer. The reactive mesogen coated substrate is then exposed to non-polarized UV light at awavelength of 354 nm. This produces an irreversible crosslinking of the reactive mesogen and produces anoptically birefringent polymer layer [6].

Additional layers of reactive mesogen can be spun coated onto the substrate to increase the opticalretardance. Each successive layer is aligned to the previous layer. Thus the photoalignment layer wouldonly need to be aligned once and each succeeding application of the RIVI material will align to the originalpattern.

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4. RESULTS

Using the process outlined above, we have produced thin film retarders with retardances ranging from 005waves 532nm to half wave 1550mm The value of retardance can be adjusted by diluting the reactivemesogen solution and modifying the spin coat parameters.

We measure the thickness and index of refraction of the retarder and alignment films by measuring thespacing and amplitude, respectively, of the etalon fringes produced in the transmission spectrum of thefilms. For a half wave retarder at a wavelength of 532nm, the thickness is calculated to be 3 microns. This 3micron thickness is comprised of a 1 micron thick layer of photoalignment, and a 2 micron thick layer ofreactive mesogen. Additional layers of RIVI increase thickness and therefore increase retardance. Retardancevalues can be controlled to ±lOnm by varying spin coat parameters and reactive mesogen dilution level. Thebirefringence of the crosslinked reactive mesogen layer is approximately 0.13 at the visible wavelengths.The dispersion of birefringence is similar to that of oriented polycarbonate films and is shown in Figure 1.The index of refraction of the reactive mesogen is approximately 1.4 in the visible wavelengths.

The transmission of the retarder, which is comprised of a layer of photoalignment material and a layer ofRM with a homogenous alignment, is show below in Figure 2. Raptor polymer shows excellent transmissionfrom 400nm to at least 3300nm. There is a weak absorption band at 2700nm, typical of the -OH absorption.

O.18 ,

01300 G0O 900 1200 1500 180C

Wave length (nm}

Figure 1. Measured birefringence of reactive mesogen as a function of wavelength.

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iC10L

9

3 0

50C

c- U.(1)

E40

0%

0 500 1000 1500 2000 2500 .000 .500Wavelength [nm]

Figure 2.Spectral scan of reactive mesogen (Raptor) polymer.

A cured layer of reactive mesogen used in the Raptor retarders loses less than 5% of its initial retardance at atemperature of 80°C in 500 hours of exposure. We find that a reactive mesogen retarder can sustaintemperatures of 100°C for a period of time with minimal loss of retardance as shown in Table 1 below. TheRaptor retarder tested had an initial retardance of 2J2 @ 633nm. The retarder is not laminated to a secondsubstrate and is left bare for this the temperature tolerance tests. Initial retardance values were measuredusing an ellipsometric technique at 5 different sites within the clear aperture of the Raptor retarder. Theretarder is then placed in an environmental chamber and brought up to 100°C from room temperature at arate of 1°C per minute. The temperature remained constant and the relative humidity was set to 0%. After 5days of continuous heating, we took the part out of the chamber and measured the retardance at the 5different sites. We then put the part back into the environmental chamber and brought the temperature backto 100°C and 0% relative humidity. We tested the part for another 5 days. We measure the retardance of thepart after the end of the second 5 day test period.

.rs

1

__0A813 0A792 0A780

2 0A709 0A704 04690

3 0A770 0A715 0A695

4 0.4756 04743 04722

5 04783 0.4754 04743

Average 047662 0A7416 04726

Change 000246 000156bare polymer retarder. Retardance is measured at 5 dif]I held at that temnerature for the duration of the exnerir

Table 1. Change in retardance of erent sites on the clear aperture.The part was heated to 100°C am nent.

Figure 3 shows the wavefront distortion on a typical Raptor retarder layer with a diameter of 35mm. Wemeasured the transmitted wavefront distortion to be 007 1 waves at 633 nm, peak to valley, with a ZygoGPT-XP interferometer. The wavefront distortion of the fused silica substrate was measured and used as a

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baseline. We then applied the photoalignment layer and reactive mesogen layers using the standardprocedure described in Section 2. We then measured the wavefront distortion of the completed Raptorretarder.

--0.03070

-

/

-

/

rn

I T I T . - 2..

cm 10.1

Figure 3. Surface plot of the distorted wavefront measurement of a Raptor layer

Laser damage testing shows that the Raptor layer can withstand a power of 035 J/cm2, peak fluence. TheRaptor test part was irradiated at 035 J/cm2 with no damage in the 10 sites tested. The test was done at1 064nm wavelength with a linear polarization and pulse width of 1 Onsec. Visible damage was observed witha 150x Nomarski brightfield microscope. Table 2 shows the change in damage for different irradiance levels.

Irradiance[J/cm2]

Non-damaged Sites

II

Damaged Sites

I

_IJ1I II I

II I

!III

-MIIITable 2. Laser damage testing results. Testing was done with a 1064nm laser.

We can remove the Raptor polymer from the substrate window to create a thin, free standing polymerretarder. The free standing polyer retarder has a thickness that is determined by the amount of reactivemesogen and desired value of retardance. The thickness of a free-standing, half-wave Raptor retarder at532mn is 3 microns. Handling of the free-standing polymer is difficult because it is so thin. However, at aretardance value of half-wave at 1 550nm, the free-standing polymer has sufficient thickness to be properlyhandled. The thickness of the half-wave 1550mn free-standing retarder is approximately 9 microns. At thiswavelength, Raptor is suitable for use in telecommunications applications. Figure 4 show a Raptor polymerfilm supported by a polycarbonate ring.

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II

1 t.3 4

*n*adouilarkpalar*tf3I

Figure 4. Raptor polymer that has been lifted off its substrate. The Raptor polymer is attached to acircular ring of polycarbonate plastic. Retardance is X/2 @ 1550nm. Thickness is approximately 9microns.

We can apply Raptor retarder as a thin film on existing optics such as lenses and mirrors. The application ofRaptor to curved surfaces requires changes to the spin coat parameters. Using techniques similar to theapplication of Raptor to flat surfaces, we have produced Raptor on lenses. Figure 5 below shows an exampleof Raptor that is applied to a lens. In addition to coating onto curved surfaces, the Raptor retarder can also becoated on non-circular substrates. Various polygonal substrates, such as squares and octagons, have beencoated with Raptor. However, due to the nature of the spin coating process, the clear aperture of the Raptorretarder is circular, regardless of the finished shape of the substrate.

Figure 5. Raptor applied to 1-inch diameter lens, shown between back-illuminated crossed polarizers. The imageon the left is with the slow axis parallel to the transmission axis of the polarizer. The slow axis direction is rotated450 in the right image.

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Large aperture retarders can also be made using the spin coat process as shown in Figure 6. These largeaperture retarders have the same properties of the standard two inch diameter retarders. They are true zeroorder, therefore providing low angular sensitivity.

Figure 6. A 150 cm diameter Raptor retarder viewed between crossed polarizers.The retardance is half wave at a wavelength of 532nm.

Figure 7 shows that we can use a CO2 laser to pattern the Raptor retarder by removing the reactive mesogenlayer in selected areas. With the laser, it is possible to remove layers of the reactive mesogen and effectivelyremove optical retardance. The laser allows for precise removal of the reactive mesogen with no score marksin the fused silica substrate and no changes to the retardance in the remaining retarding area.

4

rnecidowlørk riptks-Figure 7. Example of Raptor retarder that was patterned using a laser system. It is shown herebetween back-illuminated crossed polarizers. The layer of reactive mesogen is removed using aCO2 laser.

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5. DISCUSSION

We have successfully produced thin layer reactive mesogen polymer retarders on a variety of substrates. Theretarders are true zero order and have both excellent optical and cosmetic qualities. Fully cured reactivemesogen retarders have high heat tolerances and can be used in optic systems in difficult environments.Large aperture retarders, such as the 6-inch diameter piece described above, have applications in largeaperture astronomical telescopes. This is the largest size we have made to date and is limited only by thesize of our spin coating equipment. The retarders have good transmission in the infrared.

The Raptor retarder can be used in a wide variety of applications and can be applied on most smooth, flatsurfaces, with the limiting factor being whether or not the substrate can be subjected to the spin coatingprocess. With the correct modifications to the spin coating process, Raptor polymer can also be applied tocurved surfaces. This provides an option for thin retarders and polarizers that are needed in optical systems.

The properties of the azo-benzene photalignment layer allows for custom patterned retarders and polarizers.We have used photoaligiiment to make radial polarizers as described by Tabiryan[ 1]. Using a laser markingsystem, we will produce retarders and polarizer with finely marked patterns.

The free-standing Raptor is an ultra-thin retarder. We can produce half-wave @1 550nm free-standingpolymers that can be cut to specific sizes for use in optical telecommunications.

ACKNOWLEDGEMENTS

The authors would like to thank the support of the engineers and technicians of Meadowlark Opticsin Frederick, Colorado.

REFERENCES:

[1] Tabiryan, Nelson V, Nersisyan, Sank R, et al, "The promise of diffractive waveplates" OPN Optics &Photonics News (March 2010).

[2] Escuti et al. Polarization gratings in mesogenic films" US Patent 7,692,759 B2. 6 April 2010.

[3] Komanduri, Ravi K, et al. "A high throughput liquid crystal light shutter for unpolarized light usingpolymer polarization gratings" SPIE. Vol. 8052 (201 1).

[4] Nersisyan, S, et al., "Study of azo dye surface command photoalignment material for photonicsapplications," Applied Optics Vol. 49, No. 10 (April 2010).

[5] Kurioz, Y, et al, "Orientation of a reactive mesogen on photosensitive surface" SID Digest, (2007).

[6]Tabiryan, Nelson V., Nersisyan, Sank R., et al. "Fabrication of liquid polymer axial waveplates for UV-JR wavelengths." Optics Express. Vol. 17, No. 14 (July 2009).

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