We demonstrate patterned polarizers for visible wavelengths using dichroic dye in a liquid crystal polymer (LCP) host. Contact lithography is used to pattern a thin alignment layer, which subsequently transfers the pattern to the LCP. A gray dichroic dye mixture for the visible spectrum is optimized and implemented along with LCP to fabricate this polarizer. A peak extinction ratio of 41 was measured at a 633 nm wavelength, while simultaneously showing patterns as small as 3 μm. Finally, multi layer films are demonstrated by fabricating a two layer patterned circular polarizer consisting of a quarter-wave retarder and a color polarizer. Our process has applications in three-dimensional displays, interferometry, optical storage, and polarimeters.
©2010 Optical Society of America
Patterned polarizers have a variety of applications in polarimetry, interferometry, three-dimensional displays, and optical data storage. Linear micropolarizer arrays have been fabricated using a variety of techniques including etched dichroic polymers [1,2], wire-grid polarizers [3–5], liquid crystal (LC) arrays [5,6], and photoaligned liquid crystal polymers (LCP) [7,8]. Wire-grid polarizers are by far the most common commercial products for infrared applications; however, micropatterned wire-grid polarizers have limited spatial resolution, poor performance at visible wavelengths, require complicated lithographic processing, are susceptible to defects, and cannot be easily extended to non-linear polarizations [4,9]. An alternative and potentially simpler technique to create patterned polarizers is the photoalignment of absorbing materials which can produce micron sized polarizers of high efficiency and extinction for ultra violet (UV), visible, and near infrared (NIR) wavelengths.
Micropatterned LC alignment has been demonstrated using atomic force microscopes [10,11], ion beams , oblique angle gold deposition , and photoalignment with polarized laser light . While smaller resolution alignment has been demonstrated using the first two techniques, it is impractical for large areas. The use of linearly polarized ultraviolet (LPUV) radiation to align linearly photopolymerizable polymers (LPP) has been under intense research for many years as a noncontact replacement for traditional mechanical buffing. Photoalignment of LCPs has been demonstrated over large areas and with features as small as 2.5 μm . Many different LPP materials have been explored including azodye and dye doped polymers [5,6,13], polyimide [15,16], and cinnamoyl or coumarin side-chain polymers [17–20].
Previous demonstrations of liquid crystal polarizers included liquid crystal cells filled with dichroic LC materials, such as Merck ZLI-4714, and polymethacrylate based LC doped with dichroic dye [7,21]. In this paper, we demonstrate a two layer polarizer system, which can be generalized to more layers. One advantage of a two layer system is that the alignment exposure dose is independent of the final thickness and dye concentration of the polarizer. The first layer is a thin LPP layer that can be lithographically patterned. The second layer is a liquid crystal polymer (LCP) layer which adopts the alignment of the LPP layer, can be coated at a range of thicknesses and retardance , and can be doped with a different concentration of dichroic dyes to modulate its polarizing properties. Dichroic dyes are long anisotropic molecules that preferentially absorb light polarized either parallel or perpendicular to their molecular axis for positive and negative dichroism respectively. Cooperative reorientation of guest particles in a LCP host has previously been observed [22–24]. Additionally, we demonstrate the use of multiple layers of the LPP/LCP system to create more complex polarization elements such as color circular polarizers.
2. Experimental Methods and Materials
All the materials were obtained from commercial suppliers and used without further processing or purification. The LPP material, model LIA-01, was provided by Dainippon Ink and Chemical. The LCP material, model RMS03-001C was purchased from Merck and is delivered as a 30% (w/w) solution of propylene glycol monomethyl ether acetate (PGMEA). The LCP is a reactive mesogen that cures under UV light. Multiple dichroic dyes were purchased from Hayashibara Biochemical Laboratories, Inc. Glass soda lime wafers of 1.5” diameter were used as substrates. Norland Optical Adhesive 60 is used as a barrier layer and was purchased from Edmund Optics,
Hamamatsu Deuterium Fiber Optic Lamp with an intensity of 20 mW/cm2 was used for exposure, Transmission spectrums were measured using a Varian 5000 UV-VIS-NIR Spectrometer. A rotatable linear polarizer was included in the beam path for measuring polarizer transmittance. For measurement of visibility of the patterned images, a 633 nm Helium Neon laser was spatially filtered and expanded to a half inch beam. A reference linear polarizer was aligned with the polarization of the HeNe laser to increase the degree of polarizance (DOP) of the illumination. A one inch 0.50 numerical aperture aspherical objective lens from Edmund Optics was used to image the patterned polarizer onto a five-megapixel TCA CMOS CCD. Captured images were analyzed using Gwyddion  to extract intensity profiles for each feature size.
2.3 Polarizer Fabrication
A 1.5” diameter soda lime wafer was coated with LPP at 2000 rpm, dried at 95° C for 2 minutes. This LPP material is rewritable; therefore the entire substrate was first exposed with LPUV for 30 seconds at 0° with the deuterium fiber optic lamp. A dark field Air Force Resolution chrome mask from Edmund Optics was then placed in contact with the wafer. The wafer and mask assembly were rotated 90° and a second exposure was performed for 180 seconds resulting in pattern orthogonal to the substrate. A mixture of LCP and dichroic dye was spin coated on top of the patterned LPP substrates. The key to a uniform polarizing coating is the complete miscibility of the dichroic dye and the LC, which can have limited solubility and produce phase separation. In order to address this issue, a 10 mg/ml stock solution of the dichroic dyes in CHCl3 was prepared and this solution was mixed with an equal volume of LCP- RMS03-001C solution in PGMEA leading to very uniform solution. This solution of dye and LCP in CHCl3/PGMEA mixture was then spin coated on top of the aligned patterned substrate at 1000 rpm and then dried for 2 minutes at 55° C to remove residual solvent. As the solvent evaporates, the LC/dye mixture aligns to the LPP pattern in a nematic phase. The substrate was then exposed to unpolarized UV light with a 50 mW/cm2 intensity for six minutes in order to cure the material resulting in a durable thin film.
For generating multilayer element, such as circular polarizers, two successive layers of LPP/LCP are spin coated. The first layer is a patterned retarder (obtained by spin coating of undoped LCP) and the second a uniform linear polarizer (obtained by spin coating the mixture of dye and LCP). Norland Optical Adhesive 60 was used as a barrier layer between the two LPP/LCP films by spin coating the adhesive at 2500 rpm and UV curing for 5 minutes with an intensity of 50 mW/cm2 .
2.4 Polarizer Characterization
Polarizance and extinction ratio are two metrics used to assess the quality of a polarization optic. Polarizance describes the degree of polarization (DOP) of the transmitted light when unpolarized light is incident on the optic and is defined in Eq. (1). The extinction ratio is a metric of a polarizer’s attenuation efficiency for two orthogonal polarization states, such as horizontal and vertical or right and left circular polarization, and is defined in Eq. (2). For both Eq. (1) and (2), I1 and I2 are measured intensities from orthogonal polarization states.Eq. (3).
3.1 Dichroic Dye Characterization
In order to demonstrate different colored polarizers, three different individual dichroic dye molecules with varying visible absorption spectrums were exploited to fabricate polarizers as discussed in the experimental section. Table 1 shows each dye’s model number, visible color, peak absorbance wavelength in CHCl3, and peak absorbance in a polarizer at a concentration of 10 mg/mL CHCl3 with an equal volume of Merck RMS03-001C. All of the dyes used have positive dichroism. For simplicity each dye will be referred to by its respective color and not the dye number.Figure 1(a) shows the transmission spectrum for each color polarizer. Each polarizer is made with a dye concentration of 10mg/mL CHCl3 mixed with an equal part of Merck LCP. Figure 1(b) shows the three polarizers viewed through both a horizontal and vertical polarizer. Figure 1(c) shows each dye at a dilute concentration of 1 mg dye per 100 mL CHCl3. These three colors can be mixed together and diluted to obtain a wide range of colors. Figure 1(d) shows a transition of dye mixtures, which progresses from blue to purple to yellow and back to blue. The top, middle, and bottom correspond to dilutions of 2, 4, and 10 μg/mL. All of the colors were measured using a calibrated camera and plotted in the CIE xyY color scheme in Fig. 1(e). The black outline traces the CIE 1931 Standard Observer and shows that the dyes are capable of covering a large portion of the visible spectrum.
3.2 Single Dye Polarizer
Multiple concentrations of blue dye were dissolved in CHCl3 and added to an equal volume of Merck LCP. Figure 2(a) shows the extinction ratio across the visible spectrum. The dye has peak extinction ratios at 633 nm of 40.1, 9.8, and 3.1 for dye concentrations of 30, 20, and 10 mg/mL of CHCl3 respectively. The collected data shows that the extinction ratio increases exponentially with dye concentration. Figure 2(b) shows visibility as a function of feature size. The theoretical limit for the visibility is the polarizance of the polarizer. However, the visibility is limited by the quality of the contact printing and the length of the reorientation region in the LCP. Figure 2(c) shows a magnified view of the two smallest feature sets viewed with horizontal and vertical polarization incident.
3.3 Gray Dye Optimization
The optimum dye mixture for a broadband visible polarizer was determined by measuring the transmission spectrum for many ratios of the three dyes. The standard deviation of the transmission between wavelengths of 425 to 675 nm was used as the quantitative metric. By definition, a perfect gray dye has a flat transmission with zero standard deviation. The spectrum was limited to the areas in which the change in dye ratios had the most effect on the transmission and it also correlates to the peak response of the human eye. We performed a systematic study to determine an optimal dye ratio for a gray polarizer. Figure 3(a) shows the standard deviation of the transmission as a function of the purple dye ratio. Each curve represents a varying ratio of yellow dye, with the amount of blue dye held constant. A 2 μg/mL CHCl3 dilution was first prepared for each dye. The dilute dyes were then mixed and measured in a spectrometer. The optimum ratio was measured to be 10 Blue: 10 Yellow: 2 Blue, or 5:5:1 in reduced form. The standard deviation of the transmission for this dye ratio was 2.1% over the specified range. Figure 3(b) shows the transmission spectrum of the optimized gray mixture over the entire visible region. The lower right inset pictures the gray dye and the upper left shows a gray polarizer made with the same ratios, at a 10 mg/mL total dye weight to CHCl3 concentration.
3.4 Gray Dye Polarizer
Multiple concentrations of the optimized gray dye were dissolved in CHCl3 and then added to an equal volume of Merck LCP. Patterned polarizers were made with each concentration of dye. Figure 4(a) shows the extinction ratio and polarizance of each concentration across the visible spectrum. The dye has two absorbance peaks as shown in the extinction ratio; 50.7 at596 nm and 74.9 at 401 nm for 50 mg total dye per mL CHCl3. Magnified images of the polarizers were taken at a wavelength of 633 nm, which has an extinction ratio of 41.1. Figure 4(b) shows visibility of features as small as 5 microns. The visibility limit is equal to the polarizance of the polarizer and is shown by the respective solid lines. Figure 4(c) shows a magnified view of the two smallest feature sets viewed with horizontal and vertical polarization incident.
3.5 Circular Polarizer
One key advantage of the LPP/LCP system over existing wire-grid polarizers is the ability to coat multiple retardance and absorbance layers to form color or gray elliptical or circular polarizing filters. Using the aforementioned materials, we demonstrate a multilayer polarization element. Figure 5 shows a schematic of the film stack for a patterned circular polarizer. The first layer is a patterned LPP, with pattern orientations of 0° and 90°. LCP is next coated on top at a quarter-wave thickness. This layer serves as a quarter-wave plate and converts incident circular polarization into either 45° or 135° linear polarization. A buffer layer is then utilized to protect and separate the wave plate layers. We used Norland Optical Adhesive 60 because the material does not dissolve the LCP and is optically transparent. Next, a second LPP layer is coated and uniformly aligned at 45°. The final layer is LCP with dye coated to half-wave thickness, the same structure as the previous polarizers shown in Fig. 2 and 4. This layer transmits or blocks 45° or 135° linearly polarized light respectively. This design acts as a partial circular polarizer, in that it does not convert the light back into circular polarization. A third stage of LPP/LCP could be added to perform this function if it is required; however it is not necessary for Stokes vector measurement.
The diagramed film stack was fabricated with 25 mg/mL of blue dye for the polarizing element. The extinction ratio was measured by inserting a reference right and left circular polarizer into the spectrometer prior to the patterned circular polarizer. Figure 6(a) shows the extinction ratio and polarizance of the circular polarizer. By switching the reference circular polarizers to linear polarizers and reversing the patterned circular polarizer so that the beam is first incident on the linear polarizer, the extinction ratio of the uniform linear polarizer film can be measured. This allows us to determine how well the quarter wave plate convertscircular polarization into linear. The peak linear and circular extinction ratios are 12.0 and 9.2 respectively, which is a 23% loss in efficiency. Multiple factors contribute to this loss including deviation from the ideal quarter-wave thickness, angular misalignment between the two layers, and depolarization from the buffer layer.
5. Discussion and Conclusion
We demonstrate the ability to create linear polarizers using a two layer liquid crystal system. The highest extinction ratio measured for a blue polarizer was 40.1 at 633 nm. This polarizer consisted of a dye/LC solution of 30 mg/mL blue dye in CHCl3 and 1 mL of LCP. This concentration of blue dye was close to the solubility limit for the LCP in the PGMEA solvent. The solubility limit varies for each dye and is highly dependent on the solvent. For blue dye the limit in PGMEA solvent was near 2% by weight. Higher solubility can be achieved utilizing alternate solvents and/or other dichroic materials. Experimentally, we find that the extinction ratio is exponentially dependent on the dye concentration. Therefore, it is expected that extinction ratios over 1000 can be achieved by doubling the concentration of dye.
The smallest feature resolved when viewing the patterned polarizers is 3.1 μm. The resolution of the polarizers is primarily limited by the contact lithography. When patterning photoresist the edge profile of the resist can be adjusted by changing bake and development parameters and therefore very sharp profiles can be achieved. In the case of the alignment of the LPP materials, there is no equivalent development process and, therefore, edge effects have a larger impact on the pattern resolution in the LCP layer. In order to improve pattern resolution projection lithography or a laser writer system using polarized light could be employed.
The primary advantage of the two layer material system over a single layer dichroic polarizer is that the alignment and optical properties of the device are decoupled. Contact lithography resolution is highly dependent on the film thickness due to diffraction. In our system the lithography is performed on a 50 nm layer, which allows for both higher resolution and an optically active LCP layer of arbitrary thickness/retardance. There is no complication associated with finite depth of focus of the exposure because the LPP layer is so thin. Secondly, the dye concentration in a single layer system affects the exposure and alignment properties of the polarizer and in a two layer system identical exposures are performed for any concentration of dye in the LCP.
Finally, we have demonstrated the ability to produce multilayer patterned polarizer and/or retarder films. The demonstration of an integrated patterned circular polarizer allows for new imaging polarimeters which can measure all four Stokes parameters.
This research is partially funded by the Technology Research Infrastructure Fund (TRIF) and the Air Force Office of Scientific Research (AFOSR) MURI Program. The Authors thank Prof. Thomas Milster’s and Prof. Nasser Peyghambarian’s research groups for allowing us to utilize their equipment. The authors would also like to thank Dainippon Ink and Chemical for material donations. G. Myhre thanks M. Jungwirth, O.D. Herrera and J. Wohltmann for helpful comments on the manuscript.
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